Equinox

An equinox occurs twice a year (around 20 March and 22 September), 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, night and day are about 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 (RA = 00^h 00^m 00^s and longitude = 0º) and the autumnal point (RA = 12^h 00^m 00^s and longitude = 180º). By extension, the term equinox may denote an equinoctial point.

The equinoxes are the only times when the subsolar point is on the Equator. This point (the place on the Earth's surface where the center of the Sun can be observed exactly overhead) crosses the Equator moving northward at the March equinox and crosses the Equator moving southward at the September equinox. (Since the sun's ecliptic latitude isn't exactly zero it isn't exactly above the equator at the moment of the equinox, but the two events usually occur less than 30 seconds apart.)

The equinoxes are the only times when the terminator is inclined 90° to the Earth's Equator (while at solstices, that inclination reaches its minimum of 66.5°, corresponding to 90° minus Earth's axial tilt).^[2]

Another meaning of equinox is the date at which day and night are of equal length.^[3] Because times of sunset and sunrise, unlike the phenomenon described in preceding paragraphs, vary with an observer's geographic location (longitude and latitude), these dates likewise depend on location and do not exist for locations sufficiently close to the Equator. To avoid this ambiguity, the term equilux is sometimes used in this sense.^{[4][note 1]}

Equinoxes on the Earth

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  Illumination of Earth by the Sun at the March equinox.

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  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).

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

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

Date

When Julius Caesar established his calendar in 45 BC, he set March 25 as the spring equinox. Since a Julian year (365.25 days) is slightly longer than an actual year the calendar drifted with respect to the equinox, such that the equinox was occurring on about 21 March in AD 300 and by AD 1500 it had reached 11 March.

The shift to March 11 induced Pope Gregory XIII to create a modern Gregorian calendar. The Pope was moved by the desire to restore the edicts concerning the date of Easter of the Council of Nicaea of AD 325. (Incidentally, the date of Easter itself is fixed by an approximation of lunar cycles used in the Hebraic calendar, but according to the historian Bede the English name "Easter" comes from a pagan celebration by the Germanic tribes of the vernal (spring) equinox.) So, the shift in the date of the equinox that occurred between the 4th and the 16th centuries was annulled with the Gregorian calendar, but nothing was done for the first four centuries of the Julian calendar. The days of 29 February of the years AD 100, AD 200, AD 300, and the day created by the irregular application of leap years between the assassination of Caesar and the decree of Augustus re-arranging the calendar in AD 8, remained in effect. This moved the equinox four days earlier than in Caesar's time.

Names

• Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver = spring and autumnus = autumn).
• March equinox and September equinox: a usage becoming the preferred standard by technical writers choosing to avoid Northern Hemisphere bias (implied by assuming that March is in the springtime and September is autumnal—true for those in the Northern Hemisphere but exactly opposite in the Southern Hemisphere).
• Northward equinox and southward equinox: names referring to the apparent motion of the Sun at the times of the equinox.
• Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and autumnal equinox respectively (again, the seasonal attribution is that of the Northern Hemisphere).
• First point (or cusp) of Aries and first point of Libra are names formerly used by astronomers and now used by navigators and astrologers. Navigational ephemeris tables record the geographic position of the First Point of Aries as the reference for position of navigational stars. Due to the precession of the equinoxes, the astrological signs of the tropical zodiac where these equinoxes are located no longer correspond with the actual constellations once ascribed to them. The equinoxes are currently in the constellations of Pisces and Virgo. In sidereal astrology (notably Hindu astrology), by contrast, the first point of Aries remains aligned with Ras Hammel "the head of the ram", i.e. the Aries constellation.

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.

Geocentric view of the astronomical seasons

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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 at 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, so 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:

• Most places on Earth use a time zone which is unequal to the local time, differing by up to an hour or even as much as two hours, if daylight saving time (summer time) is included. In that case, the Sun may rise at 08:00 and set at 20:00, but there still appears to be 12 hours of daylight.
• Even those people who have their time zone equal to the local time will not see sunrise and sunset at 06:00 and 18:00 respectively. This is due to the variable speed of the Earth in its orbit, and is described as the equation of time. It has different values for the March and September equinoxes (+8 and −8 minutes respectively).
• Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least one minute before the center appears, and likewise, the upper limb sets one minute later than the center of the solar disk. Also, when the Sun is near the horizon, atmospheric refraction shifts its apparent position above its true position by a little more than its own diameter. This makes sunrise more than another two minutes earlier and sunset the equal amount later. These two effects add up to almost seven minutes, making the equinox day 12 h 7 min long and the night only 11 h 53 min.
• The night includes twilight. When dawn and dusk are added to the daytime instead, the day would be almost 13 hours.
• The above numbers are only true for the tropics. For moderate latitudes, the discrepancy increases (for example, 12 minutes in London); and closer to the Poles it becomes very much larger (in terms of time). Up to about 100 km from either Pole, the Sun is up for a full 24 hours on an equinox day.
• Height of the horizon on both the sunrise and sunset sides changes the day's length. For an observer so situated, going up into the mountains will lengthen the day, while standing in a valley with hilltops on the east and the west can shorten the day significantly.

Day arcs of the Sun

Some of the statements above can be made clearer by 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° below it; the Sun in such areas still causes twilight. The depictions presented below can be used for both the Northern Hemisphere and the Southern Hemisphere. The observer is understood to be sitting near the tree on the island depticted in the middle of the ocean; the green arrows give cardinal directions.

• In the Northern Hemisphere, north is to the left, the Sun rises in the east (far arrow), culminates in the south (right arrow), while moving to the right and setting in the west (near arrow).
• In the Southern Hemisphere, south is to the left, the Sun rises in the east (near arrow), culminates in the north (right arrow), while moving to the left and setting in the west (far arrow).

The following special cases are depicted:

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  Day arc at 0° latitude (Equator)
  The arc passes through the zenith, resulting in almost no shadows at high noon.

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  Day arc at 20° latitude
  The Sun culminates at 70° altitude and its path at sunrise and sunset occurs at a steep 70° angle to the horizon. Twilight still lasts about one hour.

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  Day arc at 50° latitude
  Twilight lasts almost two hours.

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  Day arc at 70° latitude
  The Sun culminates at no more than 20° altitude and its daily path at sunrise and sunset is at a shallow 20° angle to the horizon. Twilight lasts for more than four hours; in fact, there is barely any night.

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  Day arc at 90° latitude (Pole)
  If it were not for atmospheric refraction, the Sun would be on the horizon all the time.

Celestial coordinate systems

The vernal equinox occurs when the Sun crosses the celestial equator in March on its way from south to north. The term "vernal point" is used for the time of this occurrence and for the direction in space where the Sun is seen at that time, which is used as the origin of some celestial coordinate systems:

• in the ecliptic coordinate system, the vernal point is the origin of the ecliptic longitude;
• in the equatorial coordinate system, the vernal point is the origin of the right ascension.

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.^[5]

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.

The same is true in western tropical astrology: 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 equinoxes shift over time with respect to the fixed stars.

Using the current official IAU constellation boundaries — and taking into account the variable precession speed and the rotation of the ecliptic — the equinoxes shift through the constellations as follows^[6] (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):

• The March equinox passed from Taurus into Aries in year −1865, passed into Pisces in year −67, will pass into Aquarius in year 2597, will pass into Capricornus in year 4312. It passed along (but not into) a 'corner' of Cetus on 0°10' distance in year 1489.
• The September equinox passed from Libra into Virgo in year −729, will pass into Leo in year 2439.

Cultural aspects

A number of traditional spring and autumn (harvest) festivals are celebrated on the date of the equinoxes.

Neopaganism

• Wiccans and many other Neopagans hold religious celebrations of Ostara on the spring equinox, and Mabon on the autumnal equinox.

Equinoxes of other planets

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When the planet Saturn is at equinox, its rings pick up almost no light, as seen in this image 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.^{[citation needed]}

This lack of sunshine occurs once every 14 years and 266 days. It 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.^{[citation needed]}

One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station antenna) for a short period each day. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration "Sun outage" windows.^{[citation needed]}

See also

• Nowruz
• Solstice

Notes

References

External links

• Details about the Length of Day and Night at the Equinoxes. U.S. Naval Observatory. Naval Meteorology and Oceanography Command
• Calculation of Length of Day (Formulas and Graphs)
• Equinoctial Points — The Nuttall Encyclopædia
• Table of times for Equinoxes, Solstices, Perihelion and Aphelion in 2000–2020
• Table of times of Spring Equinox for a thousand years: 1452–2547