Vernal equinox. Also called FIRST POINT OF ARIES
Related Terms 
AIR ALMANAC
1. A periodical publication of astronomical data designed primarily for air navigation, but often used in marine navigation. 2. A joint publication of the U.S. Naval Observatory and H. M. Nautical Almanac Office, Royal Greenwich Observatory, designed primarily for air navigation. In general the information is similar to that of the Nautical Almanac, but is given to a precision of 1' of arc and 1s of time, at intervals of 10m (values for the sun and Aries are given to a precision of 0.1').

LONGITUDE OF THE ASCENDING NODE
1. The angular distance in the ecliptic from the vernal equinox to the ascending node of the orbit. 2. The angular distance, always measured eastward, in the plane of the celestial equator from Greenwich through 360°.

CELESTIAL LONGITUDE
Angular distance east of the vernal equinox, along the ecliptic; the arc of the ecliptic or the angle at the ecliptic pole between the circle of latitude of the vernal equinox at the circle of latitude of

LOCAL SIDEREAL NOON
Zero hours local sidereal time, or the instant the vernal equinox is over the upper branch of the local meridian. Local sidereal noon at the Greenwich meridian is called Greenwich sidereal noon.

LONGITUDE OF THE MOON’S NODES
The angular distance along the ecliptic of the moon’s nodes from the vernal equinox; the nodes have a retro grade motion, and complete a cycle of 360° in approximately 19 years.

VERNAL
Pertaining to spring. The corresponding adjectives for summer, fall, and winter are aestival, autumnal, and hibernal.

EQUINOCTIAL COLURE
The great circle of the celestial sphere through the celestial poles and the equinoxes; the hour circle of the vernal equinox

GREENWICH SIDEREAL NOON
Local sidereal noon at the Greenwich meridian; zero hours Greenwich sidereal time, or the instant the vernal equinox is over the upper branch of the Greenwich meridian.

RIGHT ASCENSION
Angular distance east of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of the vernal equinox and the hour circle of a point on the celestial sphere, measured eastward from the hour circle of the vernal equinox through 24 hours. Angular distance west of the vernal equinox, through 360°, is SIDEREAL HOUR ANGLE.

SIDEREAL TIME
Time defined by the daily rotation of the earth with respect to the vernal equinox of the first point of Aries. Sidereal time is numerically measured by the hour angle of the equinox, which represents the position of the equinox in the daily rotation. The period of one rotation of the equinox in hour angle, between two successive upper meridian transits, is a sidereal day. It is divided into 24 sidereal hours, reckoned at upper transit which is known as sidereal noon. The true equinox is at the intersection of the true celestial equator of date with the ecliptic of date; the time measured by its daily rotation is apparent sidereal time. The position of the equinox is affected by the nutation of the axis of rotation of the earth, and the nutation consequently introduces irregular periodic inequities into the apparent sidereal time and the length of the sidereal day. The time measured by the motion of the mean equinox of date, affected only by the secular inequalities due to the precession of the axis, is mean sidereal time. The maximum difference between apparent mean sidereal times is only a little over a second and its greatest daily change is a little more than a hundredth of a second. Because of its variable rate, apparent sidereal time is used by astronomers only as a measure of epoch; it is not used for time inter val. Mean sidereal time is deduced from apparent sidereal time by applying the equation of equinoxes.
