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

A number that can be represented by a point on a line. The set of real numbers includes all rational and irrational numbers, but not the imaginary numbers.

→ real; → number.

In an optical system, a collection of points which actually serves as a source of light rays.

→ real; → object.

The operation of a computer during the actual time that the related physical processes take place so that the results can be used to guide the physical processes.

→ real; → time; → operation.

Data processing that takes place instantaneously upon data entry or receipt of a command.

→ real; → time; → processing.

1) An inclination or concern for the actual or real, as distinguished from the abstract or speculative.
2) Philo.: The doctrine that material objects exist independently of our perception of them.
3) Art, literature: The attempt to describe the objects or social conditions as they actually are, without idealization or presentation in abstract form.

→ real; → -ism.

1) A person who tends to view or represent things as they really are; an adherent of → realism.
2) Of or pertaining to realism or to a person who embodies its principles or practices.

→ real; → -ist.

1) Interested in, concerned with, or based on what is real or practical.
2) Philo.: Of or pertaining to realists or → realism.

From → real + -istic, from L. -isticus, Gk. -istikos, from -ikos, → -ic.

1) The state or quality of being real. A real thing or fact.
2) Philo.: The totality of what is, as opposed to what merely seems to be.

→ real + → -ity.

In reality; actually; indeed.

→ real; → -ly.

Of, relating to, or determined by or from stars.

From M.Fr. sidereal, from L. sidereus "starry, astral," from sidus (genitive sideris) "star, constellation."

Axtari, pertaining to axtar "star," probably a variant of setâré→ star.

The period of → Earth's rotation around its axis, the mean value of which is about 23h 56m 04.092s (23.934 470 hours). In other words, the interval between two successive passages of a star across a given → meridian. The sidereal day is 3 minutes 56 seconds shorter than the → mean solar day. The reason is that Earth moves a little less than a degree around the Sun during the time it takes for one full axial rotation. So, for the Sun to appear on the same meridian in the sky again after one full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is also why the stars rise and set about 4 min earlier each day.

→ sidereal; → day.

The angle on the celestial sphere measured westward from the hour circle of the vernal equinox to that of the celestial body.

→ sidereal; → hour angle.

The time taken by the Moon to complete one → revolution around the Earth with respect to a background, → fixed star. The average time is 27.321 661 days (27d 7h 43m 11.5s). Same as → lunar sidereal orbital period.

→ sidereal; → month.

→ sidereal revolution period; → sidereal rotation period.

→ sidereal; → period.

The time taken by a planet or satellite to complete one revolution about its primary with respect to stars. For Earth, same as → sidereal year. Sidereal periods of the solar system planets, interms of the sidereal year, are as follows: Mercury 0.240846 (87.9691 days); Venus 0.615 (225 days); Earth 1 (365.25636 solar days); Mars 1.881; Jupiter 11.86; Satrurn 29.46; Uranus 84.01; and Neptune 164.8. That of the Moon is 0.0748 (27.32 days) and for → Sedna 12050.

→ sidereal; → revolution; → period.

The rotation period of a celestial body with respect to fixed stars. For Earth, same as → sidereal day.

→ sidereal; → rotation; → period.

The time based upon → Earth's rotation with respect to the stars, with the → sidereal day as the unit of measurement. At the moment when the → vernal equinox crosses the → meridian in → upper culmination, sidereal time is equal to zero hours for that observing position. The → hour angle of the vernal equinox is equal to sidereal time.

→ sidereal; → time.

The interval between two successive passages of the Sun, in its apparent → annual motion around the → celestial sphere, through a particular point relative to stars. It is equal to 365.256356 days for the J2000.0 epoch and is 20m 24.5s longer than the → tropical year.

→ sidereal; → year.

The → sidereal time with respect to the → true equinox.

→ true; → sidereal; → time.