An Etymological Dictionary of Astronomy and Astrophysics
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Of a physical law, invariance with respect to a → Lorentz transformation.

→ Lorentz; → invariance.

A repeated electromagnetic force on an electrically charged ring particle, nudging the particle in the same direction and at the same point in its orbit. Lorentz resonances are especially important for tiny ring particles whose charge-to-mass ratio is high and whose orbit periods are a simple integer fraction of the rotational period of the planet's magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ Lorentz; → resonance.

A set of linear equations that expresses the time and space coordinates of one → reference frame in terms of those of another one when one frame moves at a constant velocity with respect to the other. In general, the Lorentz transformation allows a change of the origin of a coordinate system, a rotation around the origin, a reversal of spatial or temporal direction, and a uniform movement along a spatial axis. If the system S'(x',y',z',t') moves at the velocity v with respect to S(x,y,z,t) in the positive direction of the x-axis, the Lorentz transformations will be: x' = γ(x - vt), y' = y, z' = z, t' = γ [t - (vx/c²)], where c is the → velocity of light and γ = [1 - (v/c)²]^-1/2. For the special case of velocities much less than c, the Lorentz transformation reduces to → Galilean transformation.

→ Lorentz; → transformation.

A spectral profile in which the intensity distribution follows a specific mathematical function (Lorentz or Cauchy probability). Compared to the normal or Gaussian profile, Lorentzian has a pointed peak and more important wings.

→ Lorentz; → profile.

A redshift characterizing a near-by receding object.

→ low; → redshift.

The quality of an instrument that lacks sufficient resolution for a specific observation. This is a relative quality, but presently a resolution below about 1 arcsecond.

→ low; → resolution.

Same as → LINER.

→ low; → ionization; → nuclear; → emission; → line; → region.

Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the → troposphere and lower → stratosphere); hence used in contrast to the common meaning for the → upper atmosphere. In other contexts, the term implies the lower troposphere (Meteorology Glossary, American Meteorological Society).

→ lower; → atmosphere.

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

→ luminosity; → size; → relation.

A galaxy that emits most of its energy in the infrared and whose infrared luminosity (in the 8-1000 µm range) is more than 10¹¹ solar luminosities. → ultraluminous infrared galaxy.

→ luminous; → infrared; → galaxy.

A stellar explosion thought to be caused by the → merger of stars in a → binary system. They are characterized by a distinct red color, and a → light curve that lingers with resurgent brightness in the → infrared. The luminosity of the explosion is between that of a → supernova and a → nova.

→ luminous; → red; → nova

An extremely thin gathering of gas surrounding the → Moon. It is made up of → atoms and → ions generated at the Moon's surface by interaction with → solar radiation, → plasma in the Earth's → magnetosphere, or → micrometeorites.

→ lunar; → exosphere.

An area on the surface of the → Moon that appears darker and smoother than its surroundings. Once thought to be seas, lunar maria are now known to be basaltic basins created by volcanic → lava floods; plural maria.

→ lunar; L. mare "sea," plural form maria, because Galileo thought the dark featureless areas on the Moon were → seas.

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet. The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon's gravity pulls on the bulge and slows down the → Earth's rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon's orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth's own rotation rate will match the Moon's orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon's own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon's average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon's distance will increase so much that it will be to far away to produce total eclipses of the Sun.

→ lunar; → recession.

The loose, fragmentary material on the Moon's surface. The lunar regolith has resulted from → meteorite collisions all along the Moon's history. It is the → debris thrown out of the → impact craters. The composition of the lunar regolith varies from place to place depending on the rock types impacted. Generally, the older the surface, the thicker the regolith. Regolith on young → maria may be only 2 meters thick; whereas, it is perhaps 20 meters thick in the older → highlands.

→ lunar; → regolith.

Same as → sidereal month.

→ lunar; → sidereal; → orbital; → period.

→ precession of the equator.

From luni-, from → lunar, + → solar; → precession.

The appearance of many differentially → redshifted→ Lyman alpha lines in → absorption in a → quasar's → spectrum, caused by intervening → hydrogen clouds along our → line of sight to the quasar.

→ Lyman; → forest.

The dividing point in a galaxy's spectrum at wavelengths shorter than the → Lyman limit. Galaxies contain large amounts of → neutral hydrogen which is very effective at absorbing radiation shortward of 912 Å. Hence galaxies are virtually dark at these wavelengths.

→ Lyman; → break.

A star-forming galaxy at → high redshift affected by the → Lyman break. Such a galaxy is detected in the red (R, → photometric band) but not in the blue (U and B bands). At those high redshfits (above 2.5), the → Lyman limit at 912 Å is shifted between the U and B bands.

→ Lyman; → break; → galaxy.