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

The point in the orbit of a satellite around the Moon closest to the Moon; opposite of → apocynthion.

→ peri- + Gk. Cynthia "goddess of the Moon;" → Cynthian.

Pirâmâh, from pirâ-, → peri-, + mâh→ moon.

The point in an object's orbit around a galaxy when the object lies closest to the galactic center; opposite of → apogalacticon.

→ peri- + → galaxy.

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.

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.

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.

In a charge-coupled device, the detection of charge collected by the transport register rather than by the image-sensing elements.

→ peripheral; → response.

In optics, the ability to see over large angles of view.

→ peripheral; → vision.

The act of permitting. Authorization granted to do something.

verbal noun of → permit; → -tion.

A transition between two quantum mechanical states that does not violate the quantum mechanical selection rules.

→ permitted; → transition.

Math.: A rearrangement of the elements of a set in a particular order. The number of permutations of n objects is equal to n! (→ factorial n). For example, there are 24 permutations of letters A, B, C, and D (4! = 1 × 2 × 3 × 4). The number of permutations of n objects taken r at a time is denoted by ⁿP_r and equals n! / (n - r)!. For example, the number of permutation of A, B, C, and D taken two at a time is 12. If n objects are of k different kinds, with r₁ alike of one kind, permutations of n objects equals n! / r₁! r₂! ... r_k!, where r₁ + r₂ + ... r_k = n.

Verbal noun of → permute.

Permuteš, verbal noun of → permute.
Jâygašt , from jâ "place" (from Mid.Pers. giyag "place;" O.Pers. ā-vahana- "place, village;" Av. vah- "to dwell, stay," vanhaiti "he dwells, stays;" Skt. vásati "he dwells;" Gk. aesa (nukta) "to pass (the night);" Ossetic wat "room; bed; place;" Tokharian B wäs- "to stay, wait;" PIE base ues- "to stay, live, spend the night") + gašt past tense of gaštan, gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- "to turn, bend").

The motion of a hypothetical machine which, once set in motion, will go on for ever without any losses due to → friction or other forms of → dissipation of energy and without receiving any external energy.

→ perpetual; → motion.

1) A human being, whether man, woman, or child as distinguished from an animal or a thing.
2) Philosophy: A self-conscious or rational being.
3) Grammar: Any one of three forms of verbs and pronouns used to denote the speaker, the person addressed, or somebody else being referred to.
4) Law: A living human being or a group, either or both having legal rights and responsibilities.
See also:
→ personal, → personal equation, → personality.

M.E. persone, from O.Fr. persone "human being," from L. persona "human being," originally "character in a drama, mask," possibly borrowed from Etruscan phersu "mask," from Gk. prosopa "face; mask" + -na a suffix.

Tanum, from O.Pers. and Av., related to Mod./Mid.Pers. tan "body, person," O.Pers. tanūš "body," tanūm [acc.sg.] "(to) onself;" Av. tanū- "body, person, self," tanūm [acc.sg.]; cf. Skt. tanūš- "body, self;" PIE base *ten-uh- "body."

1) A person of distinction or importance.
2) A dramatic, historical, or fictional → character in a play, story, etc.

M.E. "form, appearance, stature, figure, air, and the like, of a person," from O.Fr. personage "size, stature; a dignitary," from M.L. personagium, from persona, → person.

Tanumsâ, from tanum, → person, + -sâ, contraction of -âsâ, suffix of "form, type, similarity."