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"UT date and time of
equinoxes and "solstices on Earth[1]
event "equinox "solstice "equinox "solstice
month March June September December
year
day time day time day time day time
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 08:21 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
2018 20 16:15 21 10:07 23 01:54 21 22:23
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:44 22 13:31 21 10:02

An equinox is commonly regarded as the moment when the "plane of "Earth's "equator passes through the center of the "Sun's disk,[2] which occurs twice each year, around 20 March and 23 September. This simplified, but incorrect, understanding of "Earth's orbital motion can lead to errors of up to 69 seconds from the actual time of equinox.

The instants of the equinoxes are currently defined to occur when the "ecliptic longitude of the Sun is either 0° or 180°.[3] As the true motion of the Earth in its orbit oscillates around the center of mass of the Terra–Luna binary, there are tiny (up to 1¼ "arcsecond) variations of the Sun's "ecliptic latitude (discussed in section below) that may mean the Sun's center is not precisely over the equator at the moment of equinox.

On the day of an equinox, "daytime and "nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the "angular size of the Sun and "atmospheric refraction. The word is derived from the "Latin aequinoctium, from aequus (equal) and nox ("genitive noctis) (night).

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The Sun on the equinox as seen from the site of Pizzo Vento, "Fondachelli-Fantina, "Sicily

Contents

Equinoxes on Earth[edit]

General[edit]

The equinoxes are the only times when the "solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern "hemispheres are equally illuminated. The word comes from Latin equi or "equal" and nox meaning "night".

In other words, the equinoxes are the only times when the "subsolar point is on the equator, meaning that the Sun is "exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.

The equinoxes, along with "solstices, are directly related to the "seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of "spring in most cultures and is considered the New Year in the Persian calendar or "Iranian calendars as "Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn.[4]

Date[edit]

When "Julius Caesar established the "Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year is longer than the "tropical year by about 11.3 minutes on average (or 1 day in 128 years), the calendar "drifted" with respect to the two equinoxes — such that in "AD 300 the spring equinox occurred on about 21 March, and by AD 1500 it had drifted backwards to 11 March.

This drift induced "Pope Gregory XIII to create a modern "Gregorian calendar. The Pope wanted to continue to conform with the edicts concerning the "date of Easter of the "Council of Nicaea of AD 325, which means he wanted to move the vernal equinox to the date on which it fell at that time (21 March is the day allocated to it in the Easter table of the Julian calendar). However, the leap year intervals in his calendar were not smooth (400 is not an exact multiple of 97). This causes the equinox to oscillate by about 53 hours around its mean position. This in turn raised the possibility that it could fall on 22 March, and thus Easter Day might theoretically commence before the equinox. The astronomers chose the appropriate number of days to omit so that the equinox would swing from 19 to 21 March but never fall on the 22nd (although it can in a handful of years fall early in the morning of that day in the Far East).

Names[edit]

Length of equinoctial day and night[edit]

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Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12 hours of daylight at all latitudes during the equinoxes

Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles.["citation needed] On the 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, so night and day are about the same length. Sunrise and sunset can be defined in several ways, but a widespread definition is the time that the top limb of the sun is level with the horizon.[14] With this definition, the day is longer than the night at the equinoxes:[2]

  1. From the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. "Sunrise, which begins daytime, occurs when the top of the Sun's disk rises above the "eastern horizon. At that instant, the disk's centre is still below the horizon.
  2. The Earth's atmosphere "refracts sunlight. As a result, an observer sees daylight before the top of the Sun's disk rises above the horizon.

In sunrise/sunset tables, the assumed semidiameter (apparent "radius) of the Sun is 16 "arcminutes and the "atmospheric refraction is assumed to be 34 arcminutes. Their combination means that when the upper limb of the Sun is on the visible horizon, its centre is 50 arcminutes below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer.[15]

These 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,[16] actually occurring a few days towards the winter side of each equinox.

The times of sunset and sunrise vary with the observer's location ("longitude and "latitude), so the dates when day and night are equal also depend upon the observer's location.

A third correction for the visual observation of a sunrise (or sunset) is the angle between the apparent horizon as seen by an observer and the geometric (or sensible) horizon. This is known as the dip of the horizon and varies from 3 arcminutes for a sunbather stood on the sea shore to 160 arcminutes for a mountaineer on Everest.[17] The effect of a larger dip on taller objects (reaching over 2½° of arc on Everest) accounts for the phenomenon of snow on a mountain peak turning gold in the sunlight long before the lower slopes are illuminated.

At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the poles, the equinox marks the transition from 24 hours of nighttime to 24 hours of daylight (or vice versa).["citation needed]

The word equilux is sometimes (but rarely) used to mean a day when the durations of light and darkness are equal.[18][note 1]

Geocentric view of the astronomical seasons[edit]

In the half-year centered on the June solstice, the Sun rises north of east and sets north of west, 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 south of east and sets south of west 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 about 06:00 and sets at about 18:00 (local solar time). These times are not exact for several reasons:

Day arcs of the Sun[edit]

Some of the statements above can be made clearer by picturing the day arc (i.e., "the path along which the Sun "appears to move across the sky). 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 and the southern hemispheres. The observer is understood to be sitting near the tree on the island depicted in the middle of the ocean; the green arrows give cardinal directions.

The following special cases are depicted:

Celestial coordinate systems[edit]

The vernal equinox occurs in March, about when the Sun crosses the celestial equator 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 the origin of some "celestial coordinate systems:

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Diagram illustrating the difference between the Sun's "celestial longitude being zero and the Sun's "declination being zero. The Sun's "celestial latitude never exceeds 1.2 "arcseconds, but is exaggerated in this diagram.

Strictly speaking, at the equinox the Sun's ecliptic longitude is zero. Its latitude will not be exactly zero since the Earth is not exactly in the plane of the ecliptic. Its declination will not be exactly zero either. (The ecliptic is defined by the center of mass of the Earth and Moon combined). The modern definition of equinox is the instants when the Sun's apparent geocentric longitude is 0° (northward equinox) or 180° (southward equinox).[19][20][21] See the adjacent diagram.

Because of the "precession of the Earth's axis, the position of the vernal point on the "celestial sphere changes over time, and the equatorial and the ecliptic coordinate systems change accordingly. Thus 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.[22]

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.

Using the current official "IAU constellation boundaries – and taking into account the variable precession speed and the rotation of the celestial equator – the equinoxes shift through the constellations as follows[23] (expressed in "astronomical year numbering when the year 0 = 1 BC, −1 = 2 BC, etc.):

Cultural aspects[edit]

The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional ("harvest) festivals are celebrated on the date of the equinoxes.

Effects on satellites[edit]

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]

Equinoxes on other planets[edit]

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When the planet "Saturn is at equinox, its "rings reflect little sunlight, as seen in this image by "Cassini in 2009.

Equinoxes occur on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places its "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 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.[24] This phenomenon occurs once every 14.7 years on average, and can last a few weeks before and after the exact equinox. Saturn's most recent equinox was on 11 August 2009, and its next will take place on 6 May 2025.[25]

Mars's most recent equinox was on 5 May 2017 (northern spring), and the next will be on 22 May 2018 (northern autumn).

See also[edit]

Notes[edit]

  1. ^ This meaning of "equilux" is rather modern (c. 1985 to 1986) and unusual. Technical references since the beginning of the 20th century (c. 1910) use the terms "equilux" and "isophot" to mean "of equal illumination" in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance John William Tudor Walsh, Textbook of Illuminating Engineering (Intermediate Grade), I. Pitman, 1947. The earliest confirmed use of the modern meaning was in a post on the "Usenet group net.astro dated 14 March 1986 net.astro – Spring Equilux Approaches, which refers to "discussion last year exploring the reasons why equilux and equinox are not coincident". Use of this particular pseudo-latin "protologism can only be traced to a extremely small (less than six) number of predominently US American people in such online media for the next 20 years until its broader adoption as a "neologism (c. 2006), and then its subsequent use by more mainstream organisations (c. 2012) The Equinox and Solstice, UK Meteorological Office.

References[edit]

  1. ^ United States Naval Observatory (21 September 2015). "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2025". Retrieved 9 December 2015. 
  2. ^ a b "Equinoxes". USNO Astronomical Information Center FAQ. Retrieved 4 September 2015. 
  3. ^ United States Naval Observatory (2006). Astronomical Almanac 2008.  Glossary Chapter.
  4. ^ "March Equinox – Equal Day and Night, Nearly". Time and Date. 2017. Retrieved 22 May 2017. 
  5. ^ Michelle Skye (2007). Goddess Alive!: Inviting Celtic & Norse Goddesses Into Your Life. Llewellyn Worldwide. pp. 69–. "ISBN "978-0-7387-1080-8. 
  6. ^ Howard D Curtis (5 October 2013). Orbital Mechanics for Engineering Students. Butterworth-Heinemann. pp. 188–. "ISBN "978-0-08-097748-5. 
  7. ^ Mohinder S. Grewal; Lawrence R. Weill; Angus P. Andrews (5 March 2007). Global Positioning Systems, Inertial Navigation, and Integration. John Wiley & Sons. pp. 459–. "ISBN "978-0-470-09971-1. 
  8. ^ Nathaniel Bowditch; National Imagery and Mapping Agency (2002). The American practical navigator : an epitome of navigation. Paradise Cay Publications. pp. 229–. "ISBN "978-0-939837-54-0. 
  9. ^ Exploring the Earth. Allied Publishers. pp. 31–. "ISBN "978-81-8424-408-3. 
  10. ^ Paula LaRocque (2007). On Words: Insights Into How Our Words Work – And Don't. Marion Street Press. pp. 89–. "ISBN "978-1-933338-20-0. 
  11. ^ Popular Astronomy. 1945. 
  12. ^ Notes and Queries. Oxford University Press. 1895. 
  13. ^ Spherical Astronomy. Krishna Prakashan Media. pp. 233–. GGKEY:RDRHQ35FBX7. 
  14. ^ Forsythe, William C; Rykiel, Edward J; Stahl, Randal S; Wu, Hsin-i; Schoolfield, Robert M (1995). "A model comparison for daylength as a function of latitude and day of year". Ecological Modelling. 80: 87. "doi:10.1016/0304-3800(94)00034-F. 
  15. ^ Seidelman, P. Kenneth, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 32. "ISBN "0-935702-68-7. 
  16. ^ "Sunrise and Sunset". 21 October 2002. Retrieved 22 September 2017. 
  17. ^ Biegert, Mark (21 October 2015). "Correcting Sextant Measurements For Dip". Math Encounters (blog). Retrieved 22 September 2017. 
  18. ^ Owens, Steve (20 March 2010). "Equinox, Equilux, and Twilight Times". Dark Sky Diary (blog). Retrieved 31 December 2010. 
  19. ^ United States Naval Observatory (2006). Astronomical Almanac 2008.  Glossary Chapter.
  20. ^ Meeus, Jean (1997). Mathematical Astronomy Morsels. 
  21. ^ Meeus, Jean (1998). Astronomical Algorithms, Second Edition. 
  22. ^ Montenbruck, Oliver; Pfleger, Thomas. Astronomy on the Personal Computer. Springer-Verlag. p. 17. "ISBN "0-387-57700-9. 
  23. ^ J. Meeus; Mathematical Astronomical Morsels; "ISBN "0-943396-51-4.
  24. ^ "PIA11667: The Rite of Spring". Jet Propulsion Laboratory, California Institute of Technology. Retrieved 21 March 2014. 
  25. ^ "Lakdawalla, Emily (7 July 2016). "Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets". "The Planetary Society. Retrieved 31 Jan 2017. 

External links[edit]

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