Equinox

UTC date and time of solstices and equinoxes[1]
year Equinox
Mar
Solstice
June
Equinox
Sept
Solstice
Dec
day time day time day time day time
2004 20 06:49 21 00:57 22 16:30 21 12:42
2005 20 12:33 21 06:46 22 22:23 21 18:35
2006 20 18:26 21 12:26 23 04:03 22 00:22
2007 21 00:07 21 18:06 23 09:51 22 06:08
2008 20 05:48 20 23:59 22 15:44 21 12:04
2009 20 11:44 21 05:45 22 21:18 21 17:47
2010 20 17:32 21 11:28 23 03:09 21 23:38
2011 20 23:21 21 17:16 23 09:04 22 05:30
2012 20 05:14 20 23:09 22 14:49 21 11:12
2013 20 11:02 21 05:04 22 20:44 21 17:11
2014 20 16:57 21 10:51 23 02:29 21 23:03
2015 20 22:45 21 16:38 23 08:20 22 04:48
2016 20 04:30 20 22:34 22 14:21 21 10:44
2017 20 10:28 21 04:24 22 20:02 21 16:28
Illumination of the Earth by the Sun on the day of an 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 (white).
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)

An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. It may be better understood to mean that latitudes +L and -L north and south of the Equator experience nights of equal length.

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: classically, the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth's equator where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

Contents

Names

Length of equinoctial day and night

On a day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly 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 when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the center of the Sun is below the horizon, its 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 cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the Equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.

The date at which the time between sunset and sunrise crosses 12 hours is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the Equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.

Heliocentric view of the seasons

The Earth's seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honor, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North and South Poles of the Earth are just on the terminator and day and night are divided equally between the hemispheres.

The table above gives the dates and times of equinoxes and solstices over several years. A few remarks can be made about the equinoxes:

Geocentric view of the seasons

In the half year centered on the June solstice, the Sun rises and sets towards the north, which means longer days with shorter nights for the Northern Hemisphere and shorter days with longer nights for the Southern Hemisphere. In the half year centered on the December solstice, the Sun rises and sets towards the south and the durations of day and night are reversed.

Also on the day of an equinox, the Sun rises everywhere on Earth (except the Poles) at 06:00 in the morning and sets at 18:00 in the evening (local time). These times are not exact for several reasons, one being that the Sun is much larger in diameter than the Earth that more than half of the Earth could be in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line); other reasons are as follows:

Day arcs of the Sun

Some of the above statements can be made clearer when picturing the day arc (i.e. the path the Sun tracks along the celestial dome in its diurnal movement). The pictures show this for every hour on equinox day. In addition, some 'ghost' suns are also indicated below the horizon, up to 18° down. The Sun in this area still causes twilight. The pictures can be used for both Northern and Southern hemispheres. The observer is supposed to sit near the tree on the island in the middle of the ocean; the green arrows give cardinal directions.

The following special cases are depicted:

Celestial coordinate systems

The vernal point (vernal equinox) — the one the Sun passes in March on its way from south to north — is used as the origin of some celestial coordinate systems:

Because of the precession of the Earth's axis, the position of the vernal point changes with respect to the celestial sphere over time and as a consequence, both the equatorial and the ecliptic coordinate systems change over time. Therefore, when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date.[2]

The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.

The upper culmination of the vernal point is considered the start of the sidereal day for the observer. The hour angle of the vernal point is, by definition, the observer's sidereal time.

For western tropical astrology, the same thing holds true; the vernal equinox is the first point (i.e. the start) of the sign of Aries. In this system, it is of no significance that the fixed stars and equinox shift compared to each other due to the precession of the equinoxes.

Cultural aspects of the equinox

March equinox commemorations

Bas-relief in Persepolis - a symbol Iranian/Persian Nowruz - on the day of an equinox, the power of an eternally fighting bull (personifying the Earth) and that of a lion (personifying the Sun) are equal.
Chichen Itza pyramid during the spring equinox - Kukulkan, the famous descent of the snake

September equinox commemorations

Equinoxes of other planets

When the planet Saturn is at equinox, its rings pick up almost no light, as seen in this image from above by Cassini in 2009.

Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its normally majestic ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above—a view seen by humans during an equinox for the first time from the Cassini space probe in 2009—they receive very little sunshine, indeed more planetshine than light from the Sun.

This lack of sunshine occurs once every 14 years, 266 days, and can last a few weeks before and after the exact equinox. The most recent exact equinox for Saturn was on August 11, 2009. Its next equinox will take place on April 30, 2024.

Myths, fables and facts

See Also

Solstice

References

External links