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

The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.

→ active; → sun.

The → true Sun as seen by an observer on Earth. The term "apparent Sun" is used in contrast to → mean Sun, which refers to an average of the Sun's position). See also: → apparent solar time and → mean solar time.

→ apparent; → Sun.

The contradiction between a colder Sun (about 30% less luminous) some 4 billion years ago, as predicted by models, and the warm ancient Terrestrial and Martian climates derived from geological evidence.

→ faint; → early; → sun; → paradox.

A hypothetical Sun that moves along the ecliptic at a uniform rate equal to the average motion of the real Sun.

→ mean; → sun.

The phenomenon occurring when the Sun is visible above the horizon at midnight. This phenomenon can be seen at positions north of the Arctic Circle and south of the Antarctic Circle when the Sun is circumpolar (around the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere respectively).

→ midnight, → sun.

The Sun at its protostellar formation stage, before becoming a main sequence star, nearly 5 billion years ago. The protosun was more luminous than today and larger, with a radius comparable to that of the orbits of the inner planets

→ proto- + → sun.

The Sun when the 11-year cycle of → solar activity is at a minimum.

Quiet, M.E., from from O.Fr. quiete, from L. quies (genitive quietis) "rest, quiet;" → Sun.

Xoršid, → Sun; ârâm "quiet, rest, tranquility," → rest.

The image of the Sun obtained from its electromagnetic radiation in radio frequencies. The apparent size of the radio Sun depends of the frequency of the signal, since different radio frequencies originate from various atmospheric layers of the Sun.

→ radio; → sun.

Same as → Wolf number and → sunspot number.

→ relative; → sunspot; → number.

An average of 13 monthly → sunspot numbers, centered on the month of concern. The 1st and 13th months are given a weight of 0.5.

→ smooth; → sunspot; → number.

The star that governs the solar system. It is a yellow main-sequence star of spectral type G2, shines with apparent magnitude -26.74, and has an absolute magnitude of +4.83. The Sun is 4.6 billion years old and lies 27,000 light-years from the Galactic center.

O.E. sunne; cf. O.N., O.S., O.H.G. sunna, M.Du. sonne, Du. zon, Ger. Sonne, Goth. sunno; cognate with Pers. xor, hur, as below.

Xoršid "sun," originally "sunlight," from xor "sun," variant hur; Mid.Pers. xwar "sun;" Av. hū-, hvar- "sun;" cf. Skt. surya-, Gk. helios, L. sol, cognate with E. sun, as above; PIE base *sawel- "sun" + šid "light, sunlight;" from Mid.Pers. šêt "shining, radiant, bright;" Av. xšaēta- "shining, brilliant, splendid, excellent."

→ light pillar.

→ sun; → pillar.

A comet that passes extremely close to the Sun's → surface, in some cases within a few thousand kilometres of the Sun's surface. The Great Comet of 1965, Ikeya-Seki, was a member of the sun-grazer family, coming within about 650,000 km of the Sun's surface. Passing so close to the Sun, sun-grazers are subjected to destructive → tidal forces along with intense solar heat which can completely evaporate them during such a → close approach.

→ Sun; → grazer.

An instrument for showing apparent solar time by the position of the shadow cast by an indicator. → gnomon.

From → Sun + -dial M.E. instrument for telling time by the Sun's shadow, presumably from M.L. dialis "daily," from L. dies "day;" → diurnal.

Sâ'at-e âftâbi, from sâ'at, → clock, + âftâb, → Sun.

The light of the Sun.

→ sun; → light.

Âftâb, "sun(shine);" Mid.Pers. âftâp; Proto-Iranian *abi-tap-, from *abi- "to, upon, against" (O.Pers./Av. abiy-/aiwi- "to, upon, against;" Skt. abhi-, Gk. amphi-) + *tap- "to shine" (Mod.Pers. tâbidan, variants tâftban "to shine," tafsidan "to become hot;" Mid.Pers. tâftan "to heat, burn, shine;" taftan "to become hot;" Parthian t'b "to shine;" Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth;" cf. Skt. tap- "to heat, be/become hot; to spoil, injure, damage; to suffer," tapati "burns;" L. tepere "to be warm," tepidus "warm;" PIE base *tep- "to be warm").

The time at which the apparent upper limb of the rising Sun is on the astronomical horizon, that is when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun semidiameter.

→ Sun; → rise.

The time at which the apparent upper limb of the setting Sun is on the astronomical horizon, that is when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun semidiameter.

→ Sun; → set.

An area seen as a dark patch on the Sun's surface. Sunspots appear dark because they are cooler (of about 4000 °C) than the surrounding → photosphere (about 6000 °C). They range in size from a few hundred kilometers to several times the Earth's diameter and last from a few hours to a few months. Very small sunspots are called → pores. The number of sunspots varies from maximum to minimum in about 11 years, the → sunspot cycle. Their appearance during a cycle follows the → Sporer law. A typical spot has a central → umbra surrounded by a → penumbra, although either features can exist without the other. Sunspots are associated with strong magnetic fields of 0.2 to 0.4 → tesla. A given sunspot has a single magnetic → polarity. The opposite polarity may be found in other sunspots or in the bright and diffuse → facular region adjacent to the sunspot. The first recorded naked-eye sightings of sunspots were by Chinese astronomers in the first century B.C. Johannes Fabricius (1587-1617) was the first to argue that sunspots are areas on the solar surface.

→ Sun; → spot.

→ solar cycle.

→ sunspot; → cycle.

Periods of time when the → relative sunspot number is low. These periods of time occur approximately every 11 years and represent the minimum in the → sunspot cycle.

→ sunspot; → minimum.