Equinox

In this view, the two equinoxes are shown as the intersection of the celestial equator and the ecliptic. The Sun, in its apparent movement for this, is north or south of the equatorial plane, cause of the succession of seasons. Astronomically, the first point of Aries is a fundamental reference.

Zodiacal Sun Clock of the Church of the Assumption of Catí (Castellón, Spain). In addition to the hours, it shows the equinoxes and the solstices.

The equinoxes (from the Latin aequinoctium (aequus nocte), "equal night") are the moments of the year in which the Sun is located in the plane of the celestial equator. That day and for an observer at the terrestrial equator, the Sun reaches the zenith (the highest point in the sky in relation to the observer, which is just above his head, that is, at 90 °). The declination parallel of the Sun and the celestial equator then coincide.

Occurs twice a year: between March 19 and 21 and between September 21 and 24 of each year.

As the name suggests, on the dates of the equinoxes, the day lasts approximately the same as the night all over the planet.

The equinoxes are used to determine the beginning of spring and autumn in each terrestrial hemisphere.

The equinox as a reference for astronomy

The equinoxes occur when the Sun is at the first point of Aries or the first point of Libra. The first is the point on the celestial equator where the Sun in its apparent annual movement along the ecliptic passes from south to north with respect to the equatorial plane, and its declination changes from negative to positive. At the first point of Libra the opposite happens: the Sun appears to pass from north to south of the celestial equator, and its declination changes from positive to negative.

None of the equinoxes are actually in the constellation that names them, due to precession: the first point of Aries is in Pisces, and the first point of Libra is in Virgo. The equatorial coordinates of each equinox are: a) for the vernal equinox, right ascension and zero declination; b) for the first point of Libra, right ascension, 12 hours, and null declination.

Specifically, the equinoctial point that is located towards the constellation of Pisces is 8 degrees from the border with that of Aquarius.

Astronomically it is the first point of Pisces, and the meridian that passes through it is the first reference celestial meridian to determine the right ascension (RA) coordinate of celestial objects (Sun, Moon, planets, stars, nebulae and galaxies). The equinoctial point was at the end of the constellation Aries 20 centuries and 50 years ago, and it will reach the border of the constellation Aquarius in less than six centuries.

First Point of Aries

As an astronomical reference, equinox is synonymous with the first point of Aries (also: Aries point): point of the celestial sphere of right ascension and null declination. It is the point where the Sun in its apparent annual movement along the ecliptic passes from south to north of the celestial equator, and its declination changes from negative to positive. This point or node is also often called the vernal equinox.

Thus, for example, sidereal time is measured from the local meridian to the March equinox in a retrograde sense, and the right ascension of a body in the celestial sphere is taken from the point Aries to the hour circle of the object, in a sense straight.

Now, the equinox is not a fixed point (neither of the two equinoxes, of course), but moves progressively due to precession and nutation. The first assumes an angular displacement of about 50.3” each year.

• True equinox is the intersection of the ecliptic with the true equator that moves through the precession and nutation.
• Average equinox or average date equinox. It's free from the nut. The equinox moves evenly due to only the precession.

The equinox as a change of season

Illumination of the Earth by the Sun on the day of equinox

From this point of view, the equinoxes are the instant (or the date, in a more general sense) in which certain seasonal changes occur, opposite for the northern and southern hemispheres:

March Equinox

On 20 March (approximately):

At the North Pole, it begins a day that will last six months.

In the northern hemisphere, spring begins, at the time called the spring or vernal equinox.

In the southern hemisphere, the autumn begins at the time called the autumn equinox.

At the South Pole, it begins a night that will last six months.

September Equinox

On 22-23 September (approximately):

At the North Pole, it begins a night that will last six months.

In the northern hemisphere, the autumn begins, at the time called the autumn equinox.

In the southern hemisphere, spring begins, at the time called spring equinox.

At the South Pole, it begins a day that will last six months.

The two equinoxes such as the intersection of the celestial equator and the ecliptic, and the solstices, moments of the year in which the Sun reaches its maximum southern or boreal position; the four situations in which the four seasons of the year begin in the northern hemisphere.

The equinoxes are really a particular moment in the calendar, an instant of time that occurs at a certain time, instead of a whole day (although we usually call the day in which this moment occurs an equinox or equinoctial day).

The extreme dates of the equinoxes for the 21st century are as follows:

Year	Equinox March	Equinox of September
2003{displaystyle 2003}	21d01h00m{displaystyle 21^{d}01^{h}00^{m}	23d10h47m{displaystyle 23^{d}10^{h}47^{m}
2096{displaystyle 2096}	19d14h03m{displaystyle 19^{d}14^{h}{h}{m}}	21d22h55m{displaystyle 21^{d}22^{h}55^{m}

Diurnal movement of the Sun at the equinoxes

In the equinoxes the Sun exits exactly east and is placed exactly west, being the duration of the day equal to the duration of the night. In the diurnal half circumference movement occurs above the horizon (day) and the other half below (night). The figure shows the trajectory of the Sun according to the latitude of the observer, located at the point C from your local horizon.

From the equator -latitude 0º-, the Sun apparently follows a vertical trajectory, from its rising in the East until it sets in the West, reaching the observer's zenith (yellow) at noon.

On the contrary, from the poles, either the north or the south (blue), the Sun does not rise above the horizon, but describes a grazing circle. Regardless of refraction, only half of the solar disk will be visible throughout the day: it neither rises, nor culminates nor sets.

Regarding the middle latitudes (orange) the observer will see the Sun rise in the east and set in the west, but its culmination will be different depending on whether we are in the northern or southern hemisphere:

• From the Northern Hemisphere (0o tariff rate) the Sun culminates in the southern point
• From the Southern Hemisphere (–90°F) the Sun culminates in the northern point

There is also another difference: Northern Hemisphere observers see the Sun "move" from east to west in a retrograde or clockwise direction, while from the southern hemisphere the Sun appears to move equally from east to west, but in a direct or counterclockwise direction.

Altitude of the Sun in equinox

Place	Altitude of Culmination	Notes
North Pole	0°	The Sun follows the circle of the horizon.
Arctic polar circle	23rd (North)	The sun culminates 67° south of the cenit.
Tropic of Cancer	67° (North)	The sun culminates 23° south of the cenit.
Ecuador	90°	The sun describes a semi-circle passing through the cenit.
Tropic of Capricorn	67° (South)	The sun culminates 23rd north of the cenit.
Antarctic polar circle	23rd (South)	The sun culminates 67° north of the cenit.
South Pole	0°	The sun follows the circle of the horizon.