Equinox

autumnal equinoxautumn equinoxequinoxesSpring Equinoxvernal equinoxequinoctialvernal pointFirst Point of Librafirst point of Aries*
An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun.wikipedia
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Equator

equatorial planeThe Equator
An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun.
In the cycle of Earth's seasons, the equatorial plane runs through the Sun twice per year: on the equinoxes in March and September.

Nowruz

NovruzNorouzPersian New Year
In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar, Hindu and the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn.
Nowruz is the day of the vernal equinox, and marks the beginning of spring in the Northern Hemisphere.

Spring (season)

springspringtimespring season
In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar, Hindu and the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn.
At the spring (or vernal) equinox, days and nights are approximately twelve hours long, with day length increasing and night length decreasing as the season progresses.

Gregorian calendar

GregorianN.S.NS
This drift induced Pope Gregory XIII to create the modern Gregorian calendar.
The calendar was developed as a correction to the Julian calendar, shortening the average year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.

March equinox

vernal equinoxspring equinoxNorthward equinox
This occurs twice each year: around 20 March and 23 September. In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox.
The March equinox or Northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth.

Sun

solarSolThe Sun
An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun.
Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt; Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumnal equinoxes.

September equinox

autumnal equinoxSouthward equinox23 September
This occurs twice each year: around 20 March and 23 September. In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox.
Due to differences between the calendar year and the tropical year, the September equinox can occur at any time from September 21 to 24.

Earth's orbit

orbitEarth orbitsEarth-Sun distance
Since the Moon (and to a lesser extent the planets) causes Earth's orbit to slightly vary from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination.
By astronomical convention, the four seasons are determined by the solstices (the two points in the Earth's orbit of the maximum tilt of the Earth's axis, toward the Sun or away from the Sun) and the equinoxes (the two points in the Earth's orbit where the Earth's tilted axis and an imaginary line drawn from the Earth to the Sun are exactly perpendicular to one another).

Position of the Sun

Sun's positionpositiondeclination
Since the Moon (and to a lesser extent the planets) causes Earth's orbit to slightly vary from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination.
In right-handed rectangular equatorial coordinates (where the X axis is in the direction of the vernal point, and the Y axis is 90° to the east, in the plane of the celestial equator, and the Z axis is directed toward the north celestial pole

Earth

Earth's surfaceterrestrialworld
An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun.
By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular.

Terminator (solar)

terminatorterminator linelunar terminator
The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator.
The terminator path also varies by time of year due to Earth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes, and its maximum angle is approximately 23.5° to the pole during the solstices.

Virgo (constellation)

VirgoVirgo constellationconstellation of Virgo
Due to the effects of precession, the First Point of Libra, (also known as the autumn equinox point) lies within the boundaries of Virgo very close to β Virginis.

First Point of Aries

sun passes into the constellation Aries
Named for the constellation of Aries, it is one of the two points on the celestial sphere at which the celestial equator crosses the ecliptic, the other being the First Point of Libra, located exactly 180° from it.

Julian calendar

Old CalendarJulianO.S.
When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox; this was already the starting day of the year in the Persian and Indian calendars.
As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons.

Libra (astrology)

LibraBalanceLibra/Tula
Under the tropical zodiac, the Sun transits this area on average between (northern autumnal equinox) September 23 and October 22, and under the sidereal zodiac, the sun currently transits the constellation of Libra from approximately October 31 to November 22.

Hebrew calendar

Jewish calendarHebrew monthHebrew
Since Passover is required to be celebrated in the spring, it should fall around, and normally just after, the vernal (spring) equinox.

Southern Hemisphere

SouthernaustralS. Hemisphere
In the Southern Hemisphere, the reverse is true.
September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox.

Orbital eccentricity

eccentricityeccentriceccentricities
Since the Moon (and to a lesser extent the planets) causes Earth's orbit to slightly vary from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination.
Orbital mechanics require that the duration of the seasons be proportional to the area of the Earth's orbit swept between the solstices and equinoxes, so when the orbital eccentricity is extreme, the seasons that occur on the far side of the orbit (aphelion) can be substantially longer in duration.

Northern Hemisphere

NorthernNorth HemisphereNorthern Hemispheric
In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox.
Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically on 23 September UTC).

Axial precession

precession of the equinoxesprecessionprecession of equinoxes
Secondly, the position of the Earth in its orbit around the Sun at the solstices, equinoxes, or other time defined relative to the seasons, slowly changes.

Taurus (constellation)

TaurusTaurus constellationToro
It is one of the oldest constellations, dating back to at least the Early Bronze Age when it marked the location of the Sun during the spring equinox.

Equinox (celestial coordinates)

equinoxvernal equinoxMarch equinox
That reference time is called the equinox of date.
In contrast to the common usage of spring and fall, or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

Sun path

apparent pathsun-pathcomplex path of the Sun
On the equator, the sun will be straight overhead and a vertical stick will cast no shadow at solar noon on the equinoxes.

Twilight

civil twilightastronomical twilightnautical twilight
Within the polar circles, twenty-four-hour daylight is encountered in summer, and in regions very close to the poles, twilight can last for weeks on the winter side of the equinoxes.

Equatorial coordinate system

equatorial coordinatesequatorialCoordinates
, (lower case "alpha", abbreviated RA) measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the object.