In astronomy, equinox can have two meanings:

• The moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment—see below.
• The time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system—see Equinox (celestial coordinates)

An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.

More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.

There is either an equinox (autumn and spring) or a solstice (summer and winter) on approximately the 21st day of the last month of every quarter of the calendar year. On a day which has an equinox, the center of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night).

In reality, the day is longer than the night at an equinox. Commonly the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the center of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its center is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox in the northern hemisphere.

Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight).

The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the equator (blue).

Diagram of the Earth's seasons as seen from the north. Far right: December solstice

Diagram of the Earth's seasons as seen from the south. Far left: June solstice

Day arc at 0° latitude, equator

Day arc at 20° latitude

Day arc at 50° latitude

Day arc at 70° latitude

Day arc at 90° latitude, pole