An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The point in the orbit of a body revolving around the Earth at which it is nearest to the Earth; opposite of → apogee.

From Fr. périgée, from L. perigæum, from Gk. perigaion "near of the earth," from → per- "near" + gaia/ge "earth."

Pirâzam, from pirâ-, → peri-, + Av. zam- "the earth," Mid.Pers. zamig, Mod.Pers. zami, zamin "the earth;" cf. Skt. ksam, Gk. khthôn, khamai "on the ground," L. homo "earthly being" and humus "the earth" (as in homo sapiens or homicide, humble, humus, exhume); PIE root *dh(e)ghom "earth".

A solar or lunar eclipse that takes place when the Moon is at the → perigee of its orbit. The maximum duration of a solar perigee eclipse is 5h 14m (between first and the fourth contact). The maximum duration of a lunar perigee eclipse, between the two exterior contacts of the Moon with the penumbra, is 5h 16m, the maximum totality being 1h 40m (M.S.: DSE).

→ perigee; → eclipse.

The → full Moon when our natural satellite is at its closest approach to the Earth. Perigee full Moons are as much as 14% larger and 30% brighter than → apogee full Moons. Also called perigee-syzygy full Moon, super full Moon, and → supermoon. The Supermoon on November 14, 2016, was the closest (356,523 km) a Full Moon has been to Earth since January 26, 1948. The next time a Full Moon is even closer to Earth (356,448 km) will be on November 25, 2034.

→ perigee; → full; → moon.

→ perigee full Moon.

→ perigee-; → syzygy; → full; → moon.

The nearest point to the Sun in an orbit around the Sun; opposite of → aphelion.

Perihelion, from L. perihelium, from → peri- + helios "sun," cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun; PIE *sawel- "sun".

Pirâhur, from pirâ-, → peri-, + hur "sun," as above;

→ advance of perihelion.

→ advance of perihelion.

The distance between the → Sun and an → object in orbit around it when they are at their closest approach.

→ perihelion; → distance.

→ advance of perihelion.

→ perihelion; → precession.

The → point in the → orbit of a → satellite of → Jupiter that is nearest to Jupiter's center. → apojove.

→ peri- + jove, from L. Jovius "Jupiter," → Jovian.

1) Exposure to injury, loss, or destruction; grave risk; jeopardy; danger.
2) Something that causes or may cause injury, loss, or destruction (Dictionary.com).

M.E., from O.Fr. peril "danger, risk," from L. periculum "an attempt, trial; risk, danger," related to experiri "to try," cognate with Gk. peria "trial, attempt," empeiros "experienced;" O.Ir. aire "vigilance;" Goth. ferja "watcher;" O.E. fær "danger; fear."

Sij "trouble;" Mid.Pers. sêj "danger, trouble," sêjômand "dangerous;" Av. iθyejah- "trouble, danger," iθyejahvant- "full of danger, hazardous;" cf. Skt. tyajas- "difficulty, danger."

The point in a lunar orbit that is nearest to the moon. Same as → pericynthion.

From → peri- + lune, from L. luna "moon;" → lunar.

Pirâmâh, → apocynthion.

The total outer boundary of a two-dimensional figure.

→ peri- + → -meter.

Physics: The duration of one complete cycle of an oscillation; the reciprocal of the frequency.
Astro.: The amount of time an astronomical body takes to complete an orbit.

From M.E. periode, from M.Fr., from M.L. periodus "recurring portion, cycle," from L. periodus "a complete sentence," from Gk. periodos "rounded sentence, cycle, circuit, period of time," literally "going around," from → peri- "around" + hodos "way, journey;" cognate with L. cedere "to go, yield."

Dowré, from dowr, from Ar. daur "age, time; revolution."

The rate at which the rotation period of a → pulsar changes over time. This quantity, dP/dT, can range from as small as 0.05 picoseconds per year (1.5 x 10^-21 seconds per second) to as large as about 10 milliseconds per year (4.2 x 10^-10 seconds per second). For the → Crab pulsar, the period derivative is 4.2 x 10^-13 s s^-1, implying a decrease in the star's → rotation energy of about 4.5 x 10³⁸ erg s^-1. Period derivative is a very important parameter for the determination of the pulsar age.

→ period; → derivative.

A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud. Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.

→ period; → luminosity; → relation.

A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)^1/2, where ρ is the mean density, γ the ratio of → specific heats (C_p/C_v), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs.

→ period; → mean; → density; → relation.

Recurring at regular intervals of time.

Adjective of → period.

A comet with a period of less than 200 years. Also called short-period comet.

→ periodic; → comet.

A function f(x) if for all x, f(x + P) = f(x), where P is a positive constant. The least value of P > 0 is called the period of f(x).

→ periodic; → function.

Any motion that recurs in identical forms at equal intervals of time.

→ periodic; → motion.