An Etymological Dictionary of Astronomy and Astrophysics

Anatomy: The inner of the two bones of the leg, that extend from the knee to the ankle and articulate with the femur and the talus; shinbone (Dictionary.com).

Etymology (EN): From L. tibia “shinbone,” also “pipe, flute,” of unknown origin.

Etymology (PE): Dorošt-ney, literally “large reed,” from dorošt “large,” → macro-, + ney “reed, pipe, flute.”

Anatomy: The inner of the two bones of the leg, that extend from the knee to the ankle and articulate with the femur and the talus; shinbone (Dictionary.com).

Etymology (EN): From L. tibia “shinbone,” also “pipe, flute,” of unknown origin.

Etymology (PE): Dorošt-ney, literally “large reed,” from dorošt “large,” → macro-, + ney “reed, pipe, flute.”

1. A small dot, mark, check, or electronic signal, as used to mark off an item on a list, serve as a reminder, or call attention to something (Dictionary.com).
   

2. To mark with a tick or ticks (usually followed by off).

Etymology (EN): M.E. tek “little touch,” akin to Du. tik “a touch, pat,”
M.H.G. zic.

Etymology (PE): Sof, sofidan, related to sufâr “the groove at the end part of an arrow,” → nock, on the model of Fr. coche “notch, score.”

1. A small dot, mark, check, or electronic signal, as used to mark off an item on a list, serve as a reminder, or call attention to something (Dictionary.com).
   

2. To mark with a tick or ticks (usually followed by off).

Etymology (EN): M.E. tek “little touch,” akin to Du. tik “a touch, pat,”
M.H.G. zic.

Etymology (PE): Sof, sofidan, related to sufâr “the groove at the end part of an arrow,” → nock, on the model of Fr. coche “notch, score.”

Of, pertaining to, characterized by, or subject to → tides.

See also:
→ tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal disruption event (TDE), → tidal disruption flare, → tidal dwarf galaxy, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching, → tidal tail.

Etymology (EN): A hybrid formation from → tide + Latin-derived suffix → -al.

Etymology (PE): Adj. from kašand, → tide.

Of, pertaining to, characterized by, or subject to → tides.

See also:
→ tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal disruption event (TDE), → tidal disruption flare, → tidal dwarf galaxy, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching, → tidal tail.

Etymology (EN): A hybrid formation from → tide + Latin-derived suffix → -al.

Etymology (PE): Adj. from kašand, → tide.

The physical process that slows the → Earth’s rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its
rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth’s → day and → month are equal, at which point the Moon will never seem to move in Earth’s sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

See also: → tidal; → braking.

The physical process that slows the → Earth’s rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its
rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth’s → day and → month are equal, at which point the Moon will never seem to move in Earth’s sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

See also: → tidal; → braking.

Either of the two swells of land or water (on Earth) created by the pull of another object (Moon) orbiting around it. The → gravitational attraction between the → Earth and the → Moon is strongest on the side of the Earth that happens to be facing the Moon. This attraction causes the water on this “near side” of Earth to be pulled toward the Moon. As gravitational force acts to draw the water closer to the Moon, → inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the Moon, causing a “bulge” of water on the near side toward the Moon.

On the opposite side of the Earth, or the “far side,” the gravitational attraction of the Moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge. In this way the combination of gravity and inertia creates two bulges of water (Ross, D.A. 1995. Introduction to Oceanography. New York, NY: Harper Collins. pp. 236-242).

See also: → tidal; → bulge.

Either of the two swells of land or water (on Earth) created by the pull of another object (Moon) orbiting around it. The → gravitational attraction between the → Earth and the → Moon is strongest on the side of the Earth that happens to be facing the Moon. This attraction causes the water on this “near side” of Earth to be pulled toward the Moon. As gravitational force acts to draw the water closer to the Moon, → inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the Moon, causing a “bulge” of water on the near side toward the Moon.

On the opposite side of the Earth, or the “far side,” the gravitational attraction of the Moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge. In this way the combination of gravity and inertia creates two bulges of water (Ross, D.A. 1995. Introduction to Oceanography. New York, NY: Harper Collins. pp. 236-242).

See also: → tidal; → bulge.

A process in which two stars remain → bound after their → close encounter, leading to the formation of a → binary system. Tidal capture becomes possible when two stars pass each other so closely (within a few stellar radii) that their → tidal forces are able to absorb the excess energy of → unbound → orbital motion. The process was originally suggested by Fabian et al. (1975) to explain the origin of → low-mass X-ray binary systems observed for the first time in → globular clusters.

See also: → tidal; → capture.

A process in which two stars remain → bound after their → close encounter, leading to the formation of a → binary system. Tidal capture becomes possible when two stars pass each other so closely (within a few stellar radii) that their → tidal forces are able to absorb the excess energy of → unbound → orbital motion. The process was originally suggested by Fabian et al. (1975) to explain the origin of → low-mass X-ray binary systems observed for the first time in → globular clusters.

See also: → tidal; → capture.

In a system composed of one celestial body orbiting another, the synchronization of the orbital and rotational motions of the two bodies under the action of → tidal forces. For example, Pluto is tidally coupled to its moon Charon. As for the → Earth-Moon system,
billions of years from now, the Earth and the Moon will have the same period of rotation, and these will also exactly equal the orbital period of the Moon around the Earth. → tidal friction.

See also: → tidal; → coupling.

In a system composed of one celestial body orbiting another, the synchronization of the orbital and rotational motions of the two bodies under the action of → tidal forces. For example, Pluto is tidally coupled to its moon Charon. As for the → Earth-Moon system,
billions of years from now, the Earth and the Moon will have the same period of rotation, and these will also exactly equal the orbital period of the Moon around the Earth. → tidal friction.

See also: → tidal; → coupling.

The water current brought about by the → tides.

See also: → tidal; → current.

The water current brought about by the → tides.

See also: → tidal; → current.

The disruption of an extended astronomical object under the action of the → tidal forces exerted by another nearby object.

See also: → tidal; → disruption.

The disruption of an extended astronomical object under the action of the → tidal forces exerted by another nearby object.

See also: → tidal; → disruption.

The process in which a star is torn apart by the → tidal forces of a → supermassive black hole. About 50% of the star’s mass is eventually → accreted by the → black hole, generating a flare, which, in extreme cases of very high (→ super-Eddington) mass → accretion rates, can result in a → relativistic jet. TDEs have been proposed as sources of → ultra-high-energy cosmic rays and suggested as sources of high energy astrophysical → neutrinos (W. Winter and C. Lunardini, 2021, Nature Astronomy, arXiv:2005.06097 and references therein).

See also: → tidal; → disruption; → event.

The process in which a star is torn apart by the → tidal forces of a → supermassive black hole. About 50% of the star’s mass is eventually → accreted by the → black hole, generating a flare, which, in extreme cases of very high (→ super-Eddington) mass → accretion rates, can result in a → relativistic jet. TDEs have been proposed as sources of → ultra-high-energy cosmic rays and suggested as sources of high energy astrophysical → neutrinos (W. Winter and C. Lunardini, 2021, Nature Astronomy, arXiv:2005.06097 and references therein).

See also: → tidal; → disruption; → event.

A luminosity enhancement in the → light curve of a galaxy observed in X-rays or ultraviolet surveys supposed to be associated with the → tidal disruption of a star that has passed close to a → supermassive black hole in the core of a → host galaxy. An → accretion disk forms after the tidal disruption. The flare event marks the beginning of the accretion process onto the black hole.

See also: → tidal; → disruption; → flare.

A luminosity enhancement in the → light curve of a galaxy observed in X-rays or ultraviolet surveys supposed to be associated with the → tidal disruption of a star that has passed close to a → supermassive black hole in the core of a → host galaxy. An → accretion disk forms after the tidal disruption. The flare event marks the beginning of the accretion process onto the black hole.

See also: → tidal; → disruption; → flare.

A self-gravitating entity which has been formed from tidal material expelled during interactions between larger galaxies. TDGs are typically found at the tip of tidal tails at distances between 20 and 100 kpc from the merging galaxies, of which at least one should be a gas-rich galaxy. They are gas-rich objects that can be as massive as the Magellanic Clouds, form stars at a rate which might be as high as
in blue compact dwarf galaxies and seem dynamically independent from their parent galaxies.

See also: → tidal; → dwarf;
→ galaxy.

A self-gravitating entity which has been formed from tidal material expelled during interactions between larger galaxies. TDGs are typically found at the tip of tidal tails at distances between 20 and 100 kpc from the merging galaxies, of which at least one should be a gas-rich galaxy. They are gas-rich objects that can be as massive as the Magellanic Clouds, form stars at a rate which might be as high as
in blue compact dwarf galaxies and seem dynamically independent from their parent galaxies.

See also: → tidal; → dwarf;
→ galaxy.

The → gravitational force exerted on an extended body
as a result of the difference in the strength of gravity between near and far parts of the body. The ocean tides on Earth result from the varying gravitational force of the Moon exerted on the Earth’s oceans closest and farthest from the Moon. Tidal force, which is the → gradient of the gravitational force, varies as 1/r³. More specifically, F_tidal = dF/dr = (2GMm)/r³, where M is mass of the → primary body,
m is mass of the → secondary body, r is distance between objects, and G the → gravitational constant. The total tidal force experienced across a body is equal to the tidal force (force per unit distance) multiplied by the diameter of that body:

F_tt = F_tidal x 2R (provided that radius
R is much smaller than r). It is obvious that the tidal force experienced by Earth at Moon’s → perigee is larger than that at the → apogee. If the tidal force is stronger than a body’s cohesiveness, the body will be disrupted. The minimum distance that a secondary comes to a primary before it is shattered by tidal force is called its → Roche limit. Tidal forces create → tidal heating.

See also: → tidal; → force.

The → gravitational force exerted on an extended body
as a result of the difference in the strength of gravity between near and far parts of the body. The ocean tides on Earth result from the varying gravitational force of the Moon exerted on the Earth’s oceans closest and farthest from the Moon. Tidal force, which is the → gradient of the gravitational force, varies as 1/r³. More specifically, F_tidal = dF/dr = (2GMm)/r³, where M is mass of the → primary body,
m is mass of the → secondary body, r is distance between objects, and G the → gravitational constant. The total tidal force experienced across a body is equal to the tidal force (force per unit distance) multiplied by the diameter of that body:

F_tt = F_tidal x 2R (provided that radius
R is much smaller than r). It is obvious that the tidal force experienced by Earth at Moon’s → perigee is larger than that at the → apogee. If the tidal force is stronger than a body’s cohesiveness, the body will be disrupted. The minimum distance that a secondary comes to a primary before it is shattered by tidal force is called its → Roche limit. Tidal forces create → tidal heating.

See also: → tidal; → force.

The → friction exerted on a → primary body (Earth) because of the → phase lag between the → tides and the → gravitational attraction of the → secondary body (Moon).
The Earth’s → rotation is faster than the Moon’s orbital motion; therefore the Earth’s → tidal bulges lead the Moon on its orbit. This has two important effects: The Earth is being pulled slightly “back” from its sense of rotation. So the Earth’s rotation slows (by about 1 second every 50,000 years). Moreover, the Moon is being pulled slightly “forward” on its orbit. So it is harder for the Earth to hold it in place, and it moves further away from the Earth (by about 3-4 cm per yr). Tidal friction tends to synchronize the rotation period of a close-in companion with the period of its orbital motion around the primary. → tidal coupling.

See also: → tidal; → friction.

The → friction exerted on a → primary body (Earth) because of the → phase lag between the → tides and the → gravitational attraction of the → secondary body (Moon).
The Earth’s → rotation is faster than the Moon’s orbital motion; therefore the Earth’s → tidal bulges lead the Moon on its orbit. This has two important effects: The Earth is being pulled slightly “back” from its sense of rotation. So the Earth’s rotation slows (by about 1 second every 50,000 years). Moreover, the Moon is being pulled slightly “forward” on its orbit. So it is harder for the Earth to hold it in place, and it moves further away from the Earth (by about 3-4 cm per yr). Tidal friction tends to synchronize the rotation period of a close-in companion with the period of its orbital motion around the primary. → tidal coupling.

See also: → tidal; → friction.

The heating of the → interior of a → planet or → satellite due to the → friction caused by → tidal forces. For example, the huge tidal forces by → Jupiter heat its close satellite → Io, making it a seismically very active body.

See also: → tidal; → heating.

The heating of the → interior of a → planet or → satellite due to the → friction caused by → tidal forces. For example, the huge tidal forces by → Jupiter heat its close satellite → Io, making it a seismically very active body.

See also: → tidal; → heating.

The process whereby the → rotation period of a → primary body becomes identical to the → orbital period of a → secondary body. Tidal locking results from → tidal braking and leads to → synchronous rotation.

See also: → tidal; → lock; → -ing.

The process whereby the → rotation period of a → primary body becomes identical to the → orbital period of a → secondary body. Tidal locking results from → tidal braking and leads to → synchronous rotation.

See also: → tidal; → lock; → -ing.

Same as → Roche limit.

See also: → tidal; → radius.

Same as → Roche limit.

See also: → tidal; → radius.

The stretching of an object under → tidal force. Tidal stretching results from a difference in the gravitational pull felt on two sides of a body. It is proportional to the inverse cube of the distance to the source of gravity (1/r³). As a consequence, nearby objects, even small ones like the Moon, raise high tides, whereas distant giants like Jupiter do not produce much of an effect.

See also: → tidal; → stretching.

The stretching of an object under → tidal force. Tidal stretching results from a difference in the gravitational pull felt on two sides of a body. It is proportional to the inverse cube of the distance to the source of gravity (1/r³). As a consequence, nearby objects, even small ones like the Moon, raise high tides, whereas distant giants like Jupiter do not produce much of an effect.

See also: → tidal; → stretching.

The phenomenon whereby gas and stars are ripped out from a gravitationally → bound system, such as a galaxy or → globular cluster, by the action of → tidal forces from an external, more massive object. See also → ram pressure stripping.

See also: → tidal; → strip.

The phenomenon whereby gas and stars are ripped out from a gravitationally → bound system, such as a galaxy or → globular cluster, by the action of → tidal forces from an external, more massive object. See also → ram pressure stripping.

See also: → tidal; → strip.

A long stream of stars and gas, often in the form of a spectacular tail, thrown off a galaxy when it collides with another galaxy. → interacting galaxies; → merger. Two tidal tails form in each galaxy, and they are more spectacular when the masses of the two galaxies are comparable, and when their relative orbit is in the same sense as the rotation inside each spiral galaxy.

See also: → tidal; → tail.

A long stream of stars and gas, often in the form of a spectacular tail, thrown off a galaxy when it collides with another galaxy. → interacting galaxies; → merger. Two tidal tails form in each galaxy, and they are more spectacular when the masses of the two galaxies are comparable, and when their relative orbit is in the same sense as the rotation inside each spiral galaxy.

See also: → tidal; → tail.

The description of a → system of two bodies undergoing → tidal locking.

See also: → tidal; → -ly; → lock.

The description of a → system of two bodies undergoing → tidal locking.

See also: → tidal; → -ly; → lock.

Describing a → stellar system that has undergone → tidal stripping.

See also: → tidal; → -ly; → strip.

Describing a → stellar system that has undergone → tidal stripping.

See also: → tidal; → -ly; → strip.

1. The periodic rising and falling of the waters of the ocean and its inlets. The tides result from the → gravitational attraction of the → Moon and → Sun acting upon the rotating → Earth. See also: → ebb tide, → high tide, → low tide, → neap tide, → spring tide, → tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal current, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.
   

2. → tidal force.

Etymology (EN): M.E.; O.E. tid “time, hour” (cf. O.S. tid, Du. tijd, O.H.G. zit, Ger. Zeit “time”).

Etymology (PE): Kešand, from Mod./Mid.Pers. kešidan/kašidan “to draw, protract, trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
Av. karš- “to draw; to plow,” karša- “furrow;” Proto-Iranian *kerš-/*xrah- “to draw, plow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow.”

1. The periodic rising and falling of the waters of the ocean and its inlets. The tides result from the → gravitational attraction of the → Moon and → Sun acting upon the rotating → Earth. See also: → ebb tide, → high tide, → low tide, → neap tide, → spring tide, → tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal current, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.
   

2. → tidal force.

Etymology (EN): M.E.; O.E. tid “time, hour” (cf. O.S. tid, Du. tijd, O.H.G. zit, Ger. Zeit “time”).

Etymology (PE): Kešand, from Mod./Mid.Pers. kešidan/kašidan “to draw, protract, trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
Av. karš- “to draw; to plow,” karša- “furrow;” Proto-Iranian *kerš-/*xrah- “to draw, plow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow.”

Firmly or closely fixed in place. → compact.

Etymology (EN): M.E. thight, from O.N. thettr “watertight, close in texture, solid” (cf. second element in O.E. metethiht “stout from eating;” M.H.G. dihte “dense, thick,” Ger. dicht “dense, tight,” O.H.G. gidigan, Ger. gediegen “genuine, solid, worthy”), from PIE base *tenk- “to become firm, curdle, thicken;” cf. Ir. techt “curdled, coagulated,” Lith. tankus “close, tight;” cognate with Pers. tang “tight,” as below.

Etymology (PE): Tang “tight; narrow, straight; tight,” also “horse girth, a strap for fastening a load” (Mid.Pers.
tang “tight, narrow”), tanjidan “to squeeze, press, pull together;”
cf. Skt. tanákti “draws together, contracts;” cognate with E. tight, as above; PIE base *tenk- “to become firm, curdle, thicken.”

Firmly or closely fixed in place. → compact.

Etymology (EN): M.E. thight, from O.N. thettr “watertight, close in texture, solid” (cf. second element in O.E. metethiht “stout from eating;” M.H.G. dihte “dense, thick,” Ger. dicht “dense, tight,” O.H.G. gidigan, Ger. gediegen “genuine, solid, worthy”), from PIE base *tenk- “to become firm, curdle, thicken;” cf. Ir. techt “curdled, coagulated,” Lith. tankus “close, tight;” cognate with Pers. tang “tight,” as below.

Etymology (PE): Tang “tight; narrow, straight; tight,” also “horse girth, a strap for fastening a load” (Mid.Pers.
tang “tight, narrow”), tanjidan “to squeeze, press, pull together;”
cf. Skt. tanákti “draws together, contracts;” cognate with E. tight, as above; PIE base *tenk- “to become firm, curdle, thicken.”

A cluster of stars in which members are closely situated so that high resolution observations are required to distinguish them individually.

See also: → tight; → star cluster.

A cluster of stars in which members are closely situated so that high resolution observations are required to distinguish them individually.

See also: → tight; → star cluster.

→ close binary star.

See also: → tight; → bound system; → binary star.

→ close binary star.

See also: → tight; → bound system; → binary star.

Optics: A deviation in the propagation direction of a beam of light.
Tilt quantizes the average slope in both the X and Y directions of a → wavefront or phase profile across the pupil of an optical system.

Etymology (EN): M.E. tylten “to upset, tumble,” from tealt “unsteady” (cf. O.N. tyllast “to trip,” Swed. tulta “to waddle,” Norw. tylta “to walk on tip-toe,” M.Du. touteren “to swing”).

Etymology (PE): Gerâ, present stem of gerâyidan “to incline toward; to intend; to make for.”
Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

Optics: A deviation in the propagation direction of a beam of light.
Tilt quantizes the average slope in both the X and Y directions of a → wavefront or phase profile across the pupil of an optical system.

Etymology (EN): M.E. tylten “to upset, tumble,” from tealt “unsteady” (cf. O.N. tyllast “to trip,” Swed. tulta “to waddle,” Norw. tylta “to walk on tip-toe,” M.Du. touteren “to swing”).

Etymology (PE): Gerâ, present stem of gerâyidan “to incline toward; to intend; to make for.”
Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

The angle a rocket makes with the vertical as it curves along its trajectory.

See also: → tilt; → angle.

The angle a rocket makes with the vertical as it curves along its trajectory.

See also: → tilt; → angle.

1. A non-spatial sequential relation in which events occur in apparently irreversible succession from the past through the present to the future. → time’s arrow.
   

2. A limited period or interval, as between two successive events.

Etymology (EN): M.E.; O.E. tima “limited space of time,” from P.Gmc. *timon “time” (cf. O.N. timi “time,” Swed. timme “an hour”), akin to L. tempus (genitive temporis) “time” (Fr. temps, Sp. tiempo, It. tempo); maybe related to Pers. Tabari tum, tomon, temen “time;” Aftari ton “time.”

Etymology (PE): Zamân “time,” from Mid.Pers. zamân, jamân “time,” zamânak “period, epoch;”
loaned into Aramaic and Ar., loaned into Arm. žam, žamanak “time;” prefixed Sogdian nγm “time, moment, hour;” Proto-Iranian *gām- “to go, to come;”
cf. Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
“step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE base *g^wem- “to go, come.”
Gâh “time; place;” Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Vaqt, pronounced vaxt (وخت), but written vaqt (وقت),
is a Pers. word meaning “portion (of time)”. Its variants and related words in Mod./Mid.Pers. are: baxt “what is allotted, fate, fortune,” baxš “portion, part, division,” baxšidan, baxtan “to divide, distribute, grant,” Av. base bag- “to attribute, allot, distribute,” baxš- “to apportion, divide, give to,”
baxta- “what is allotted (luck, fortune),” baxədra- “part, portion,” baγa- “master, god,” O.Pers. bāji- “tribute, tax,” cf. Skt. bhaj- “to share, divide, distribute, apportion,” bhájati “divides,” bhakta- “allotted; occupied with; a share; food or a meal, time of eating?,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion.”
Tâmen “time,” from Tabari temen, tumun, tum “time,” pərtəmən “long time;” Lori temen “age, length of life;” Aftari ton; Lâri, Garâši taim “time span” (related to L. tempus?).

1. A non-spatial sequential relation in which events occur in apparently irreversible succession from the past through the present to the future. → time’s arrow.
   

2. A limited period or interval, as between two successive events.

Etymology (EN): M.E.; O.E. tima “limited space of time,” from P.Gmc. *timon “time” (cf. O.N. timi “time,” Swed. timme “an hour”), akin to L. tempus (genitive temporis) “time” (Fr. temps, Sp. tiempo, It. tempo); maybe related to Pers. Tabari tum, tomon, temen “time;” Aftari ton “time.”

Etymology (PE): Zamân “time,” from Mid.Pers. zamân, jamân “time,” zamânak “period, epoch;”
loaned into Aramaic and Ar., loaned into Arm. žam, žamanak “time;” prefixed Sogdian nγm “time, moment, hour;” Proto-Iranian *gām- “to go, to come;”
cf. Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
“step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE base *g^wem- “to go, come.”
Gâh “time; place;” Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Vaqt, pronounced vaxt (وخت), but written vaqt (وقت),
is a Pers. word meaning “portion (of time)”. Its variants and related words in Mod./Mid.Pers. are: baxt “what is allotted, fate, fortune,” baxš “portion, part, division,” baxšidan, baxtan “to divide, distribute, grant,” Av. base bag- “to attribute, allot, distribute,” baxš- “to apportion, divide, give to,”
baxta- “what is allotted (luck, fortune),” baxədra- “part, portion,” baγa- “master, god,” O.Pers. bāji- “tribute, tax,” cf. Skt. bhaj- “to share, divide, distribute, apportion,” bhájati “divides,” bhakta- “allotted; occupied with; a share; food or a meal, time of eating?,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion.”
Tâmen “time,” from Tabari temen, tumun, tum “time,” pərtəmən “long time;” Lori temen “age, length of life;” Aftari ton; Lâri, Garâši taim “time span” (related to L. tempus?).

The assignment of telescope time by an expert panel to proposals after evaluating the merits of the observation projects.

See also: → time; → allocation.

The assignment of telescope time by an expert panel to proposals after evaluating the merits of the observation projects.

See also: → time; → allocation.

Th speed of response of a detector, usually measured as 1/(2πν), where ν is the chopping frequency at which the responsivity fails to 1/√2 of its maximum value.

See also: → time; → constant.

Th speed of response of a detector, usually measured as 1/(2πν), where ν is the chopping frequency at which the responsivity fails to 1/√2 of its maximum value.

See also: → time; → constant.

1. The amount of time required for a → signal to travel from one point to another in an → electric circuit.
   

2. → gravitational lensing time delay.

See also: → time; → delay.

1. The amount of time required for a → signal to travel from one point to another in an → electric circuit.
   

2. → gravitational lensing time delay.

See also: → time; → delay.

A distance-like quantity derived from → gravitational lensing time delay. It is given by a combination of three angular diameter distances in a strong lens system:

D_Δt = (1 + z_L)[D_A(EL)D_A(ES) / D_A(LS)],

where z_L is the → redshift of the → gravitational lens, while D_A(EL), D_A(ES), and D_A(LS) are the angular diameter distances from the Earth to the lens, from the Earth to the source, and from the lens to the source, respectively. As each of the distance is proportional to the inverse of H₀, D_Δt is proportional to 1/H₀.

See also: → time; → delay; → distance.

A distance-like quantity derived from → gravitational lensing time delay. It is given by a combination of three angular diameter distances in a strong lens system:

D_Δt = (1 + z_L)[D_A(EL)D_A(ES) / D_A(LS)],

where z_L is the → redshift of the → gravitational lens, while D_A(EL), D_A(ES), and D_A(LS) are the angular diameter distances from the Earth to the lens, from the Earth to the source, and from the lens to the source, respectively. As each of the distance is proportional to the inverse of H₀, D_Δt is proportional to 1/H₀.

See also: → time; → delay; → distance.

A phenomenon related to special and general relativity.

1. In → special relativity, the apparent shortening of time that occurs at speeds approaching that of light. A clock moving relative to a stationary observer will
   appear to slow down by a factor √(1- v²/c²), where v is the velocity and c the speed of light. → twins paradox.
   

2. In → general relativity, a clock in a stronger gravitational field runs more slowly. The dilation factor is given by: √(1- 2GM/rc²), where G is the gravitational constant, M the mass of the object creating the gravitational field, r a radial coordinate of the observer, which is analogous to the classical distance from the center of the object, and c the speed of light.

Etymology (EN): → time; dilation, verbal noun of dilate, from
M.E. dilaten, from O.Fr. dilater, from L. dilatare “make wider, enlarge,” from → dis-
“apart” + latus “wide.”

Etymology (PE): Farâxeš, → dilation; zamân, → time.

A phenomenon related to special and general relativity.

1. In → special relativity, the apparent shortening of time that occurs at speeds approaching that of light. A clock moving relative to a stationary observer will
   appear to slow down by a factor √(1- v²/c²), where v is the velocity and c the speed of light. → twins paradox.
   

2. In → general relativity, a clock in a stronger gravitational field runs more slowly. The dilation factor is given by: √(1- 2GM/rc²), where G is the gravitational constant, M the mass of the object creating the gravitational field, r a radial coordinate of the observer, which is analogous to the classical distance from the center of the object, and c the speed of light.

Etymology (EN): → time; dilation, verbal noun of dilate, from
M.E. dilaten, from O.Fr. dilater, from L. dilatare “make wider, enlarge,” from → dis-
“apart” + latus “wide.”

Etymology (PE): Farâxeš, → dilation; zamân, → time.

One of the → orbital elements, the time when the → secondary body reaches → periapsis.

See also: → time; → periapsis; → passage.

One of the → orbital elements, the time when the → secondary body reaches → periapsis.

See also: → time; → periapsis; → passage.

Same → temporal resolution.

See also: → time; → resolution.

Same → temporal resolution.

See also: → time; → resolution.

A transformation operating on time in the equations of motion of a dynamical system in which t is replaced by -t.

See also: → time; → reversal.

A transformation operating on time in the equations of motion of a dynamical system in which t is replaced by -t.

See also: → time; → reversal.

A measure of duration of a specific process, such as → crossing time, → dynamical time scale,
→ evolutionary time scale, → Kelvin-Helmholtz time scale, → nuclear time scale, → photon escape time, → relaxation time,
→ star formation time scale.

See also: → time; → scale.

A measure of duration of a specific process, such as → crossing time, → dynamical time scale,
→ evolutionary time scale, → Kelvin-Helmholtz time scale, → nuclear time scale, → photon escape time, → relaxation time,
→ star formation time scale.

See also: → time; → scale.

A → sequence of values of a → variable in successive time order, usually at fixed intervals of time.

See also: → time; → series.

A → sequence of values of a → variable in successive time order, usually at fixed intervals of time.

See also: → time; → series.

Any of the 24 zones on the Earth surface delimited by → meridians at approximately 15° intervals. In each time zone a common standard time is used, and the time is one hour earlier than the zone immediately to the east.

See also: → time; → zone.

Any of the 24 zones on the Earth surface delimited by → meridians at approximately 15° intervals. In each time zone a common standard time is used, and the time is one hour earlier than the zone immediately to the east.

See also: → time; → zone.

The sequence of all natural processes in which the → entropy increases. In other words, the fact that these processes all move in one direction in time and are → irreversible. The past is distinctly different from the future; things always grow older, never younger.

Etymology (EN): → time; arrow, M.E. arewe, arwe, from O.E. arwan, earh “arrow,” from P.Gmc. *arkhwo (cf. Goth. arhwanza), from PIE base *arku- “bow and/or arrow,” source of Latin arcus, → arc.

Etymology (PE): Peykân “arrow,” → Sagitta; zamân, → time.

The sequence of all natural processes in which the → entropy increases. In other words, the fact that these processes all move in one direction in time and are → irreversible. The past is distinctly different from the future; things always grow older, never younger.

Etymology (EN): → time; arrow, M.E. arewe, arwe, from O.E. arwan, earh “arrow,” from P.Gmc. *arkhwo (cf. Goth. arhwanza), from PIE base *arku- “bow and/or arrow,” source of Latin arcus, → arc.

Etymology (PE): Peykân “arrow,” → Sagitta; zamân, → time.

Of, pertaining to, or describing an → event belonging to the interior of the → light cone.

See also: → time; → like.

Of, pertaining to, or describing an → event belonging to the interior of the → light cone.

See also: → time; → like.

The → space-time interval between two → events if it is real, i.e. ds² > 0.

See also: → timelike; → interval.

The → space-time interval between two → events if it is real, i.e. ds² > 0.

See also: → timelike; → interval.

Any mechanical, electric, or electronic device, such as a clock or watch, designed to measure and display the passage of time.

Etymology (EN): → time; → piece.

Etymology (PE): Zamân-šomâr, literally “time counter,” from zamân, → time, + šomâr “counter,” from šomârdan “to count,” from Mid.Pers. ôšmârtan, ôšmurtan “to reckon, calculate, enumerate, account for,” from Av. base (š)mar- “to have in mind, remember, recall,” pati-šmar- “to recall; to long for,” hišmar-, cf. Skt. smar- “to remember, become aware,” smarati “he remembers,” L. memor, memoria, Gk. mermera “care,” merimna “anxious thought, sorrow,” martyr “witness.”

Any mechanical, electric, or electronic device, such as a clock or watch, designed to measure and display the passage of time.

Etymology (EN): → time; → piece.

Etymology (PE): Zamân-šomâr, literally “time counter,” from zamân, → time, + šomâr “counter,” from šomârdan “to count,” from Mid.Pers. ôšmârtan, ôšmurtan “to reckon, calculate, enumerate, account for,” from Av. base (š)mar- “to have in mind, remember, recall,” pati-šmar- “to recall; to long for,” hišmar-, cf. Skt. smar- “to remember, become aware,” smarati “he remembers,” L. memor, memoria, Gk. mermera “care,” merimna “anxious thought, sorrow,” martyr “witness.”

A metallic chemical element; symbol Sn (L. stannum for → alloys containing → lead). → Atomic number 50; → atomic weight 118.69; → melting point 231.9681°C; → boiling point 2,270°C; → specific gravity 5.75 (gray), 7.3 (white).
The element was known in prehistoric times.

Etymology (EN): M.E., O.E. tin; cf. M.Du., Du. tin, O.H.G. zin, Ger. Zinn, O.N. tin; related to Fr. étain?

Etymology (PE): Arziz “tin,” from Mid.Pers. arziz “tin, lead,” arus “white, bright;” Av. ərəzata- “silver,” auruša- “white;” cf. Skt. arjuna- “white, shining,” rajata- “silver;” Gk. argos “white,” arguron “silver,” L. argentum “silver,” arguere “to make clear,” argmentum “argument;” PIE *arg- “to shine, be white, bright, clear.”
Qal’y of unknown origin.

A metallic chemical element; symbol Sn (L. stannum for → alloys containing → lead). → Atomic number 50; → atomic weight 118.69; → melting point 231.9681°C; → boiling point 2,270°C; → specific gravity 5.75 (gray), 7.3 (white).
The element was known in prehistoric times.

Etymology (EN): M.E., O.E. tin; cf. M.Du., Du. tin, O.H.G. zin, Ger. Zinn, O.N. tin; related to Fr. étain?

Etymology (PE): Arziz “tin,” from Mid.Pers. arziz “tin, lead,” arus “white, bright;” Av. ərəzata- “silver,” auruša- “white;” cf. Skt. arjuna- “white, shining,” rajata- “silver;” Gk. argos “white,” arguron “silver,” L. argentum “silver,” arguere “to make clear,” argmentum “argument;” PIE *arg- “to shine, be white, bright, clear.”
Qal’y of unknown origin.

Any of the several → absorption bands due to the molecule → titanium oxide that are prominent in the spectra of cool → K and → M stars.

See also: → titanium oxide; → band.

Any of the several → absorption bands due to the molecule → titanium oxide that are prominent in the spectra of cool → K and → M stars.

See also: → titanium oxide; → band.

1. The top, summit, or apex.
   

   2. To tilt or cause to tilt; overturn, upset, or overthrow.

Etymology (EN): 1) M.E. tip, from M.L.G. or M.Du. tip “utmost point, extremity” (cf. Ger. zipfel, a diminutive formation).

2. From, tip noun from tip (v.) “to overturn, upset,” from M.E. typen “to upset, overturn.”

Etymology (PE): 1) Nok “tip,” variant tok.

1. The top, summit, or apex.
   

   2. To tilt or cause to tilt; overturn, upset, or overthrow.

Etymology (EN): 1) M.E. tip, from M.L.G. or M.Du. tip “utmost point, extremity” (cf. Ger. zipfel, a diminutive formation).

2. From, tip noun from tip (v.) “to overturn, upset,” from M.E. typen “to upset, overturn.”

Etymology (PE): 1) Nok “tip,” variant tok.

A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (M_I = -4.1 ± 0.1).

See also: → tip; → red giant; → branch; → method.

A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (M_I = -4.1 ± 0.1).

See also: → tip; → red giant; → branch; → method.

A rapidly moving → mirror used in → adaptive optics to correct overall movements of the incoming → wavefront of light caused by → atmospheric turbulence. The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts of the wavefront in two dimensions. This is done by tipping and tilting the mirror rapidly in response to overall changes in position of a reference star. See also → deformable mirror.

Etymology (EN): From, tip noun from tip (v.) “to overturn, upset,” from M.E. typen “to upset, overturn” + tilt noun from tilt (v.)
“to cause to lean, incline, slope, or slant,” → tilt; → mirror.

Etymology (PE): Âyené, → mirror; kaj “turned aside; crooked, bent” (cf. Skt. kubja- “hump-backed, crooked,” Pali kujja- “bent,” L. gibbus “hump, hunch,” Lith. kupra “hump”) + -o- “and”

• râst→ right + -gar agent noun suffix → -or.

A rapidly moving → mirror used in → adaptive optics to correct overall movements of the incoming → wavefront of light caused by → atmospheric turbulence. The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts of the wavefront in two dimensions. This is done by tipping and tilting the mirror rapidly in response to overall changes in position of a reference star. See also → deformable mirror.

Etymology (EN): From, tip noun from tip (v.) “to overturn, upset,” from M.E. typen “to upset, overturn” + tilt noun from tilt (v.)
“to cause to lean, incline, slope, or slant,” → tilt; → mirror.

Etymology (PE): Âyené, → mirror; kaj “turned aside; crooked, bent” (cf. Skt. kubja- “hump-backed, crooked,” Pali kujja- “bent,” L. gibbus “hump, hunch,” Lith. kupra “hump”) + -o- “and”

• râst→ right + -gar agent noun suffix → -or.

Exhausted of strength and energy.

Etymology (EN): Past participle of tire “to weary; become weary,” M.E. tyren, O.E. teorian, of unknown origin.

Etymology (PE): Xasté “tired; hurt, wounded;” Mid.Pers. xastan, xad- “to injure, wound;” Av. vīxaδ- “to crush;” Proto-Iranian *xad- “to wound, hurt.”

Exhausted of strength and energy.

Etymology (EN): Past participle of tire “to weary; become weary,” M.E. tyren, O.E. teorian, of unknown origin.

Etymology (PE): Xasté “tired; hurt, wounded;” Mid.Pers. xastan, xad- “to injure, wound;” Av. vīxaδ- “to crush;” Proto-Iranian *xad- “to wound, hurt.”

The hypothesis that photons from distant objects lose energy during their intergalactic journey to us, thereby increasing in wavelength and becoming redshifted. This would provide an alternative to the → Big Bang model in accounting for the → redshifts of distant galaxies. However, there is no evidence for any such tired-light effect. First discussed by F. Zwicky (1929, Proceedings of the National Academy of Sciences, 15, 773).

See also: → tired; → light.

The hypothesis that photons from distant objects lose energy during their intergalactic journey to us, thereby increasing in wavelength and becoming redshifted. This would provide an alternative to the → Big Bang model in accounting for the → redshifts of distant galaxies. However, there is no evidence for any such tired-light effect. First discussed by F. Zwicky (1929, Proceedings of the National Academy of Sciences, 15, 773).

See also: → tired; → light.

In celestial mechanics, a combination of orbital elements
commonly used to distinguish between comets and asteroids. Objects whose Tisserand’s parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand’s invariant.

See also: Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).

In celestial mechanics, a combination of orbital elements
commonly used to distinguish between comets and asteroids. Objects whose Tisserand’s parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand’s invariant.

See also: Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).

The largest and the sixth moon of → Saturn discovered by Christiaan Huygens in 1655. Called also Saturn VI. Titan has a diameter of 5,150 km, about half the size of Earth and almost as large as Mars. It orbits Saturn at a mean distance of 1,221,830 km every 15.945 days.
It is the only moon known to have an → atmosphere.

Its surface temperature is -179 °C, which makes water as hard as rocks and allows → methane to be found in its liquid form.

Its surface pressure is slightly higher than Earth’s pressure (1.6 bars against 1 bar at sea level).

The Huygens probe released from → Cassini-Huygens landed on Titan on December 25, 2004.

From the data obtained by Cassini-Huygens, we know that Titan is a world with lakes and seas composed of liquid methane and → ethane near its poles, with vast, arid regions not made of silicates as on Earth, but of solid water ice coated with → hydrocarbons that fall from the atmosphere. Titan’s icy dunes are gigantic, reaching, on average, 1 to 2 km wide, hundreds kilometers long and around 100 m high.

Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years.
The Huygens probe made the first direct measurements of Titan’s lower atmosphere. Huygens also directly sampled → aerosols in the atmosphere and confirmed that → carbon and → nitrogen are their major constituents.

Cassini followed up Huygens’ measurements from orbit, detecting other chemicals that include → propylene and poisonous → hydrogen cyanide, in Titan’s atmosphere.

Cassini’s gravity measurements of Titan revealed that this moon is hiding an internal, liquid water and → ammonia ocean beneath its surface. Huygens also measured radio signals during its descent that strongly suggested the presence of an ocean 55 to 80 km below the moon’s surface.

See also: In Gk. mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

The largest and the sixth moon of → Saturn discovered by Christiaan Huygens in 1655. Called also Saturn VI. Titan has a diameter of 5,150 km, about half the size of Earth and almost as large as Mars. It orbits Saturn at a mean distance of 1,221,830 km every 15.945 days.
It is the only moon known to have an → atmosphere.

Its surface temperature is -179 °C, which makes water as hard as rocks and allows → methane to be found in its liquid form.

Its surface pressure is slightly higher than Earth’s pressure (1.6 bars against 1 bar at sea level).

The Huygens probe released from → Cassini-Huygens landed on Titan on December 25, 2004.

From the data obtained by Cassini-Huygens, we know that Titan is a world with lakes and seas composed of liquid methane and → ethane near its poles, with vast, arid regions not made of silicates as on Earth, but of solid water ice coated with → hydrocarbons that fall from the atmosphere. Titan’s icy dunes are gigantic, reaching, on average, 1 to 2 km wide, hundreds kilometers long and around 100 m high.

Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years.
The Huygens probe made the first direct measurements of Titan’s lower atmosphere. Huygens also directly sampled → aerosols in the atmosphere and confirmed that → carbon and → nitrogen are their major constituents.

Cassini followed up Huygens’ measurements from orbit, detecting other chemicals that include → propylene and poisonous → hydrogen cyanide, in Titan’s atmosphere.

Cassini’s gravity measurements of Titan revealed that this moon is hiding an internal, liquid water and → ammonia ocean beneath its surface. Huygens also measured radio signals during its descent that strongly suggested the presence of an ocean 55 to 80 km below the moon’s surface.

See also: In Gk. mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

The fourteenth and largest of → Uranus’s known satellites. It has a diameter of 1578 km and orbits its planet at a mean distance of 436,270 km. Titania was discovered by Herschel in 1787. Also called Uranus IV.

See also: Titania is the Queen of the Fairies and wife of Oberon in Shakespeare’s Midsummer-Night’s Dream.

The fourteenth and largest of → Uranus’s known satellites. It has a diameter of 1578 km and orbits its planet at a mean distance of 436,270 km. Titania was discovered by Herschel in 1787. Also called Uranus IV.

See also: Titania is the Queen of the Fairies and wife of Oberon in Shakespeare’s Midsummer-Night’s Dream.

A dark-gray or silvery, very hard, light metallic element, occurring combined in various minerals; symbol Ti. Atomic number 22; atomic weight 47.88; melting point 1,675°C; boiling point 3,260°C; specific gravity 4.54 at 20°C. It is used in metallurgy to remove oxygen and nitrogen from steel and to toughen it.

Etymology (EN): It was originally discovered by the English clergyman William Gregor in the mineral ilmenite (FeTiO₃) in 1791. It was rediscovered in 1795 by the German chemist Martin Heinrich Klaproth, who called it titanium because it had no characteristic properties to use as a name; from Titan + -ium.

Etymology (PE): Titan, loan from Fr., as above.

A dark-gray or silvery, very hard, light metallic element, occurring combined in various minerals; symbol Ti. Atomic number 22; atomic weight 47.88; melting point 1,675°C; boiling point 3,260°C; specific gravity 4.54 at 20°C. It is used in metallurgy to remove oxygen and nitrogen from steel and to toughen it.

Etymology (EN): It was originally discovered by the English clergyman William Gregor in the mineral ilmenite (FeTiO₃) in 1791. It was rediscovered in 1795 by the German chemist Martin Heinrich Klaproth, who called it titanium because it had no characteristic properties to use as a name; from Titan + -ium.

Etymology (PE): Titan, loan from Fr., as above.

A → diatomic molecule made up of → titanium and → oxygen atoms. See → TiO band.

See also: → titanium; → band.

A → diatomic molecule made up of → titanium and → oxygen atoms. See → TiO band.

See also: → titanium; → band.

The empirical rule relating the approximate distances of the → solar system  → planets from the → Sun. The original formulation was: a = (n + 4) / 10,
where a is the mean distance of a planet from the Sun in → astronomical units
and n = 0, 3, 6, 12, 24, 48, 96, 192 (doubling for each successive planet). The planets were seen to fit this sequence quite well, provided the → asteroids between → Mars and → Jupiter are counted as one planet, as did → Uranus discovered in 1781. However, → Neptune and the ex-planet → Pluto do not conform to the rule. The question of whether there is any physical significance to the “law,” i.e. some dynamical reason that will explain planetary orbit spacing has led to much discussion during the past two centuries. Today, many astronomers are very skeptical and consider this “laws” to be numerical coincidence.

See also: Named after the German mathematician Johann Titius (1729-1796), who
first found the law in 1766, and the German astronomer Johann Elert Bode (1747-1826), who published it in 1772; → law.

The empirical rule relating the approximate distances of the → solar system  → planets from the → Sun. The original formulation was: a = (n + 4) / 10,
where a is the mean distance of a planet from the Sun in → astronomical units
and n = 0, 3, 6, 12, 24, 48, 96, 192 (doubling for each successive planet). The planets were seen to fit this sequence quite well, provided the → asteroids between → Mars and → Jupiter are counted as one planet, as did → Uranus discovered in 1781. However, → Neptune and the ex-planet → Pluto do not conform to the rule. The question of whether there is any physical significance to the “law,” i.e. some dynamical reason that will explain planetary orbit spacing has led to much discussion during the past two centuries. Today, many astronomers are very skeptical and consider this “laws” to be numerical coincidence.

See also: Named after the German mathematician Johann Titius (1729-1796), who
first found the law in 1766, and the German astronomer Johann Elert Bode (1747-1826), who published it in 1772; → law.

The distinguishing name of a book, poem, picture, piece of music, or the like (Dictionary.com).

Etymology (EN): M.E., from O.Fr. title and in part from O.E. titul, both from L. titulus “inscription, label, heading; honorable appellation,” of unknown origin.

Etymology (PE): Sarâl, from sar, → head, + noun suffix -âl, → -al.

The distinguishing name of a book, poem, picture, piece of music, or the like (Dictionary.com).

Etymology (EN): M.E., from O.Fr. title and in part from O.E. titul, both from L. titulus “inscription, label, heading; honorable appellation,” of unknown origin.

Etymology (PE): Sarâl, from sar, → head, + noun suffix -âl, → -al.