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Equinox
Semi-annual astronomical event where the Sun is directly above the Earth's equator

A solar equinox occurs twice a year, around 20 March and 23 September, when the Sun appears directly above the equator and rises due east and sets due west. This moment marks when Earth's rotation axis is perpendicular to the Sun-Earth line, resulting in nearly equal daytime and night worldwide. In the Northern Hemisphere, the March equinox is the vernal equinox, while the September equinox is autumnal; reversed in the Southern Hemisphere. Equinox timings are defined by the Sun’s ecliptic longitude at 0° and 180°. Equinoxes alternate with solstices, with dates varying slightly due to factors like leap years. For detailed dates and times, see the UT equinox and solstice calendar.

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Equinoxes on Earth

Main article: Sun path

See also: Equinox (celestial coordinates)

General

Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the duration of the day and the night are practically equal and the word "equinox" comes from Latin aequus, meaning "equal", and nox, meaning "night".

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 or Iranian calendars,14 while the autumnal equinox (September) marks the beginning of autumn.15 Ancient Greek calendars too had the beginning of the year either at the autumnal or vernal equinox and some at solstices. The Antikythera mechanism predicts the equinoxes and solstices.16

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.

For the same reason, this is also the time when the Sun rises for an observer at one of Earth's rotational poles and sets at the other. For a brief period lasting approximately four days, both North and South Poles are in daylight.17 For example, in 2021 sunrise on the North Pole is 18 March 07:09 UTC, and sunset on the South Pole is 22 March 13:08 UTC. Also in 2021, sunrise on the South Pole is 20 September 16:08 UTC, and sunset on the North Pole is 24 September 22:30 UTC.1819

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.

Date

When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox;20 this was already the starting day of the year in the Persian and Indian calendars. 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 – so that in 300 AD the spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.21

This drift induced Pope Gregory XIII to establish the modern Gregorian calendar. The Pope wanted to continue to conform with the edicts of the Council of Nicaea in 325 AD concerning the date of Easter, 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), and to maintain it at around that date in the future, which he achieved by reducing the number of leap years from 100 to 97 every 400 years. However, there remained a small residual variation in the date and time of the vernal equinox of about ±27 hours from its mean position, virtually all because the distribution of 24 hour centurial leap-days causes large jumps (see Gregorian calendar leap solstice).

Modern dates

The dates of the equinoxes change progressively during the leap-year cycle, because the Gregorian calendar year is not commensurate with the period of the Earth's revolution about the Sun. It is only after a complete Gregorian leap-year cycle of 400 years that the seasons commence at approximately the same time. In the 21st century the earliest March equinox will be 19 March 2096, while the latest was 21 March 2003. The earliest September equinox will be 21 September 2096 while the latest was 23 September 2003 (Universal Time).22

Names

  • Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver = spring, and autumnus = autumn). These are the historically universal and still most widely used terms for the equinoxes, but are potentially confusing because in the southern hemisphere the vernal equinox does not occur in spring and the autumnal equinox does not occur in autumn. The equivalent common language English terms spring equinox and autumn (or fall) equinox are even more ambiguous.232425 It has become increasingly common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox.2627
  • March equinox and September equinox: names referring to the months of the year in which they occur, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month (as they do not in the Islamic calendar and Hebrew calendar, for example).28 Although the terms have become very common in the 21st century, they were sometimes used at least as long ago as the mid-20th century.29
  • Northward equinox and southward equinox: names referring to the apparent direction of motion of the Sun. The northward equinox occurs in March when the Sun crosses the equator from south to north, and the southward equinox occurs in September when the Sun crosses the equator from north to south. These terms can be used unambiguously for other planets. They are rarely seen, although were first proposed over 100 years ago.30
  • First point of Aries and first point of Libra: names referring to the astrological signs the Sun is entering. However, the precession of the equinoxes has shifted these points into the constellations Pisces and Virgo, respectively.31

Length of equinoctial day and night

On the date 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 day32 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.33 With this definition, the day is longer than the night at the equinoxes:34

  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 may be visible. Sunrise, which begins daytime, occurs when the top of the Sun's disk appears 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 appears above the horizon.

In sunrise/sunset tables, the atmospheric refraction is assumed to be 34 arcminutes, and the assumed semidiameter (apparent radius) of the Sun is 16 arcminutes. (The apparent radius varies slightly depending on time of year, slightly larger at perihelion in January than aphelion in July, but the difference is comparatively small.) 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.35

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,36 actually occurring a few days towards the winter side of each equinox. One result of this is that, at latitudes below ±2.0 degrees, all the days of the year are longer than the nights.37

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 viewer standing on the sea shore to 160 arcminutes for a mountaineer on Everest.38 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.

The date on which the day and night are exactly the same is known as an equilux; the neologism, believed to have been coined in the 1980s, achieved more widespread recognition in the 21st century.39 At the most precise measurements, a true equilux is rare, because the lengths of day and night change more rapidly than any other time of the year around the equinoxes. In the mid-latitudes, daylight increases or decreases by about three minutes per day at the equinoxes, and thus adjacent days and nights only reach within one minute of each other. The date of the closest approximation of the equilux varies slightly by latitude; in the mid-latitudes, it occurs a few days before the spring equinox and after the fall equinox in each respective hemisphere.40

Auroras

Mirror-image conjugate auroras have been observed during the equinoxes.41

Cultural aspects

Main articles: March equinox § Culture, and September equinox § Culture

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.

People in countries including Iran, Afghanistan, Tajikistan celebrate Nowruz which is spring equinox in northern hemisphere. This day marks the new year in Solar Hijri calendar.

Religious architecture is often determined by the equinox; the Angkor Wat Equinox during which the sun rises in a perfect alignment over Angkor Wat in Cambodia is one such example.42

Catholic churches, since the recommendations of Charles Borromeo, have often chosen the equinox as their reference point for the orientation of churches.43

Effects on satellites

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.)44

Satellites in geostationary orbit also experience difficulties maintaining power during the equinox because they have to travel through Earth's shadow and rely only on battery power. Usually, a satellite travels either north or south of the Earth's shadow because Earth's axis is not directly perpendicular to a line from the Earth to the Sun at other times. During the equinox, since geostationary satellites are situated above the Equator, they are in Earth's shadow for the longest duration all year.45

Equinoxes on other planets

Equinoxes are defined 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. 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, they receive more planetshine than light from the Sun.46 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 2025.47

Mars's most recent equinoxes were on 12 January 2024 (northern autumn), and on 26 December 2022 (northern spring).48

See also

Footnotes

Wikimedia Commons has media related to Equinox. Look up equinox in Wiktionary, the free dictionary. Wikiquote has quotations related to Equinox.

References

  1. In this article, dates before 15 October 1582 are given in the Julian calendar while more recent dates are given in the Gregorian calendar. Dates before 1 March 8 AD are given in the Julian calendar as observed in Rome; there is an uncertainty of a few days when these early dates are converted to the proleptic Julian calendar. /wiki/Proleptic_Julian_calendar ↩

  2. "Equinoxes". Astronomical Information Center. United States Naval Observatory. 14 June 2019. Archived from the original on 21 August 2019. Retrieved 9 July 2019. On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level regionSunrise and sunset times in 90°00'N, 0°00'E (North Pole), timeanddate.com https://web.archive.org/web/20190821111011/https://aa.usno.navy.mil/faq/docs/equinoxes.php ↩

  3. "ESRL Global Monitoring Division - Global Radiation Group". NOAA. www.esrl.noaa.gov. U.S. Department of Commerce. Retrieved 9 July 2019. https://www.esrl.noaa.gov/gmd/grad/solcalc/glossary.html#equinox ↩

  4. Astronomical Almanac. United States Naval Observatory. 2008. Glossary. /wiki/United_States_Naval_Observatory ↩

  5. Grieser, Justin (22 September 2014). "Autumn arrives: The fall equinox explained in six images". The Washington Post. Archived from the original on 8 June 2021. Retrieved 29 June 2024. https://www.washingtonpost.com/news/capital-weather-gang/wp/2014/09/22/autumn-arrives-the-fall-equinox-explained-in-six-images/ ↩

  6. Plait, Phil (22 September 2023). "The Equinox Is Not What You Think It Is". Scientific American. Retrieved 29 June 2024. https://www.scientificamerican.com/article/the-equinox-is-not-what-you-think-it-is/ ↩

  7. Yallop, B.D.; Hohenkerk, C.Y.; Bell, S.A. (2013). "Astronomical Phenomena". In Urban, S.E.; Seidelmann, P. K. (eds.). Explanatory supplement to the astronomical almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 506–507. ISBN 978-1-891389-85-6. 978-1-891389-85-6 ↩

  8. Astronomical Applications Department of USNO. "Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion". Retrieved 1 August 2022. /wiki/USNO ↩

  9. "Solstices and Equinoxes: 2001 to 2100". AstroPixels.com. 20 February 2018. Retrieved 21 December 2018. http://www.astropixels.com/ephemeris/soleq2001.html ↩

  10. Équinoxe de printemps entre 1583 et 2999 http://www.imcce.fr/newsletter/docs/Equinoxe_printemps_1583_2999.pdf ↩

  11. Solstice d’été de 1583 à 2999 http://www.imcce.fr/newsletter/docs/Solstice_ete_1583_2999.pdf ↩

  12. Équinoxe d’automne de 1583 à 2999 http://www.imcce.fr/newsletter/docs/Equinoxe_automne_1583_2999.pdf ↩

  13. Solstice d’hiver http://www.imcce.fr/newsletter/docs/Solstice_hiver_1583_2999.pdf ↩

  14. The year in the Iranian calendar begins on Nowruz, which means "new day". /wiki/Iranian_calendar ↩

  15. "March Equinox – Equal Day and Night, Nearly". Time and Date. 2017. Retrieved 22 May 2017. https://www.timeanddate.com/calendar/march-equinox.html ↩

  16. Freeth, T., Bitsakis, Y., Moussas, X., Seiradakis, J. H., Tselikas, A., Mangou, H., ... & Allen, M. (2006). Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism. Nature, 444(7119), 587-591. ↩

  17. This is possible because atmospheric refraction "lofts" the Sun's apparent disk above its true position in the sky. /wiki/Atmospheric_refraction ↩

  18. "Equinoxes". Astronomical Information Center. United States Naval Observatory. 14 June 2019. Archived from the original on 21 August 2019. Retrieved 9 July 2019. On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level regionSunrise and sunset times in 90°00'N, 0°00'E (North Pole), timeanddate.com https://web.archive.org/web/20190821111011/https://aa.usno.navy.mil/faq/docs/equinoxes.php ↩

  19. Sunrise and sunset times in 90°00'S, 0°00'E (South Pole), timeanddate.com https://www.timeanddate.com/sun/@-90,0 ↩

  20. Blackburn, Bonnie J.; Holford-Strevens, Leofranc (1999). The Oxford companion to the year. Oxford University Press. p. 135. ISBN 0-19-214231-3. Reprinted with corrections 2003. 0-19-214231-3 ↩

  21. Richards, E. G. (1998). Mapping Time: The Calendar and its History. Oxford University Press. pp. 250–251. ISBN 978-0192862051. 978-0192862051 ↩

  22. Yallop, B.D.; Hohenkerk, C.Y.; Bell, S.A. (2013). "Astronomical Phenomena". In Urban, S.E.; Seidelmann, P. K. (eds.). Explanatory supplement to the astronomical almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 506–507. ISBN 978-1-891389-85-6. 978-1-891389-85-6 ↩

  23. Skye, Michelle (2007). Goddess Alive!: Inviting Celtic & Norse Goddesses Into Your Life. Llewellyn Worldwide. pp. 69ff. ISBN 978-0-7387-1080-8. 978-0-7387-1080-8 ↩

  24. Curtis, Howard D. (2013). Orbital Mechanics for Engineering Students. Butterworth-Heinemann. pp. 188ff. ISBN 978-0-08-097748-5. 978-0-08-097748-5 ↩

  25. Grewal, Mohinder S.; Weill, Lawrence R.; Andrews, Angus P. (2007). Global Positioning Systems, Inertial Navigation, and Integration. John Wiley & Sons. pp. 459ff. ISBN 978-0-470-09971-1. 978-0-470-09971-1 ↩

  26. Bowditch, Nathaniel (2002). The American practical navigator: An epitome of navigation. National Imagery and Mapping Agency. Paradise Cay Publications. pp. 229ff. ISBN 978-0-939837-54-0. 978-0-939837-54-0 ↩

  27. Exploring the Earth. Allied Publishers. 2016. pp. 31ff. ISBN 978-81-8424-408-3. 978-81-8424-408-3 ↩

  28. La Rocque, Paula (2007). On Words: Insights into how our words work – and don't. Marion Street Press. pp. 89ff. ISBN 978-1-933338-20-0. 978-1-933338-20-0 ↩

  29. Popular Astronomy. 1945. https://books.google.com/books?id=CcEzAQAAIAAJ ↩

  30. Notes and Queries. Oxford University Press. 1895. https://archive.org/details/notesandqueries06whitgoog ↩

  31. Spherical Astronomy. Krishna Prakashan Media. pp. 233ff. GGKEY:RDRHQ35FBX7. https://books.google.com/books?id=9KFRhcsn8-UC&pg=PA233 ↩

  32. Here, "day" refers to when the Sun is above the horizon. ↩

  33. Forsythe, William C.; Rykiel, Edward J.; Stahl, Randal S.; Wu, Hsin-i; Schoolfield, Robert M. (1995). "A model comparison for day length as a function of latitude and day of year" (PDF). Ecological Modelling. 80 (1): 87–95. Bibcode:1995EcMod..80...87F. doi:10.1016/0304-3800(94)00034-F. https://www.ikhebeenvraag.be/mediastorage/FSDocument/171/Forsythe+-+A+model+comparison+for+daylength+as+a+function+of+latitude+and+day+of+year+-+1995.pdf ↩

  34. "Equinoxes". Astronomical Information Center. United States Naval Observatory. 14 June 2019. Archived from the original on 21 August 2019. Retrieved 9 July 2019. On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level regionSunrise and sunset times in 90°00'N, 0°00'E (North Pole), timeanddate.com https://web.archive.org/web/20190821111011/https://aa.usno.navy.mil/faq/docs/equinoxes.php ↩

  35. Seidelman, P. Kenneth, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 32. ISBN 0-935702-68-7. 0-935702-68-7 ↩

  36. "Sunrise and Sunset". 21 October 2002. Retrieved 22 September 2017. http://www.cso.caltech.edu/outreach/log/NIGHT_DAY/sunrise.htm ↩

  37. Team, GML Web. "Solar Calculator - NOAA Global Monitoring Laboratory". gml.noaa.gov. https://gml.noaa.gov/grad/solcalc/calcdetails.html ↩

  38. Biegert, Mark (21 October 2015). "Correcting Sextant Measurements for Dip". Math Encounters (blog). Retrieved 22 September 2017. http://mathscinotes.com/2015/10/correcting-sextant-measurements-for-dip/ ↩

  39. Prior to the 1980s there was no generally accepted term for the phenomenon, and the word "equilux" was more commonly used as a synonym for isophot.[33] The newer meaning of "equilux" is modern (c. 1985 to 1986), and not usually intended: Technical references since the beginning of the 20th century (c. 1910) have used the terms "equilux" and "isophot" interchangeably to mean "of equal illumination" in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance Walsh (1947).[34] The earliest confirmed use of the modern meaning was in a post on the Usenet group net.astro,[35] 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 an extremely small (less than six) number of predominantly U.S. 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).[36] https://en.wiktionary.org/wiki/isophot ↩

  40. "On the equinox, are day and night equal?". earthsky.org. 19 March 2024. Retrieved 23 June 2024. https://earthsky.org/astronomy-essentials/equal-day-and-night-on-the-equinox-march/ ↩

  41. Davis, Neil (1992). The Aurora Watcher's Handbook. University of Alaska Press. pp. 117–124. ISBN 0-912006-60-9. 0-912006-60-9 ↩

  42. DiBiasio, Jame (15 July 2013). The Story of Angkor. Silkworm Books. ISBN 978-1-63102-259-3. 978-1-63102-259-3 ↩

  43. Johnson, Walter (18 November 2011). Byways in British Archaeology. Cambridge University Press. ISBN 978-0-521-22877-0. 978-0-521-22877-0 ↩

  44. "Satellite Sun Interference". Intelsat. Retrieved 20 March 2019. http://www.intelsat.com/tools-resources/library/satellite-101/satellite-sun-interference/ ↩

  45. Abrahamian, David (17 April 2018). "How satellites are affected by the spring and autumn equinoxes". Viasat, Inc. Retrieved 20 March 2019. https://news.viasat.com/blog/scn/how-satellites-are-affected-by-the-spring-and-autumn-equinoxes/ ↩

  46. "PIA11667: The Rite of Spring". Jet Propulsion Laboratory, California Institute of Technology. Retrieved 21 March 2014. https://photojournal.jpl.nasa.gov/catalog/PIA11667 ↩

  47. Lakdawalla, Emily (7 July 2016). "Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets". The Planetary Society. Retrieved 31 January 2017. /wiki/Emily_Lakdawalla ↩

  48. "Mars Calendar". The Planetary Society. http://www.planetary.org/explore/space-topics/mars/mars-calendar.html ↩