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

→ autumnal equinox.

→ autumn; → equinox.

One of the two points where the → ecliptic crosses the → celestial equator. At the autumnal equinox the Sun appears to be moving across the equator from the northern celestial hemisphere to the southern celestial hemisphere. The instant of the event.

→ autumn; → -al; → equinox.

The difference between → apparent sidereal time and → mean sidereal time. It is due to the nutation of the Earth's polar axis of rotation about its precessional motion. It ranges from +0.8 to +1.2 seconds. Also known as → nutation in right ascension.

→ equation; → equinox.

1) One of the two points on the → celestial sphere where the → celestial equator intersects the → ecliptic, that is when the apparent → ecliptic longitude of the Sun is 0° or 180°.
2) Either of the times at which the center of the Sun's disk passes through these points. → autumnal equinox; → vernal equinox.
At equinox, the length of the day and the night are equal all over the globe. The equinox is not a fixed point; it moves due to → precession and → nutation. If only precession is considered, we deal with the → mean equinox of date. If nutation is also taken into account, then we are concerned with the → true equinox.

M.E., from O.Fr. équinoxe, from M.L. equinoxium "equality of night (and day)," from L. æquinoctium, from æquus, "→ equal" + nox "→ night" (gen. noctis). In Gk. isimeria "equal day," from isos "equal," → iso-, + hemera "day."

From hamug, → equal, + -ân suffix denoting time and place.

The intersection of the planes of ecliptic and celestial equator.

→ line; → equinox.

A fictitious equinox whose position is that of the vernal equinox at a particular epoch with the effect of nutation removed.

→ mean; → equinox.

The slow motion of the equinoxes along the ecliptic, resulting from the combined motion of the equator (→ precession of the equator) and the ecliptic (→ precession of the ecliptic), or in other words the precession of the Earth's axis of rotation. Also know as → general precession. The First Point of Aries moves westward along the ecliptic at 50.38 arcseconds per year (1 degree every 71.6 years), causing the equinoxes to occur about twenty minutes earlier each sidereal year. See also → nutation.

→ precession; → equinox.

Same as → true equinox.

→ real; → equinox.

→ vernal equinox.

→ spring; → equinox.

The intersection of the → ecliptic with the → true celestial equator for a given epoch. It is derived from the → mean equinox accounting for the → nutation.

→ true; → equinox.

The point of intersection between the ecliptic and the celestial equator at which the Sun passes from south to north of the celestial equator during its apparent annual motion. The instant of this event. It occurs on March 20, 21 or rarely 19. At the vernal equinox, as with the → autumnal equinox, night and day are equal in length world over. Several thousands years ago the vernal equinox was in Aries, but because of precession it has now slid west into Pisces. Right ascension and celestial longitude are measured from the vernal equinox. Also known as spring equinox. → First Point of Aries.

→ vernal; → equinox.

The time interval between two successive passages of the Sun, when the true longitude of the Sun is considered. In other words, the interval during which the Sun's true longitude increases by 360 degrees. Its mean length for the epoch J2000.0 is 365.24236460 real solar days (approximately 365.2424 days). The vernal-equinox year, on which the Iranian calender is based, should not be confused with → tropical year. See also: A concise review of the Iranian calendar. → Iranian calendar

→ vernal; → equinox; → year.