March 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. The March equinox is known as the vernal equinox in the Northern Hemisphere and as the autumnal equinox in the Southern Hemisphere.
File:Earth-lighting-equinox EN.png|thumb|right|300px|Illumination of Earth by the Sun on the day of an equinox
On the Gregorian calendar at 0° longitude, the northward equinox usually occurs on March 20. However, it can occur as early as March 19, and it can occur as late as March 21. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from March 20 each year.
The March equinox may be taken to mark the beginning of astronomical spring and the end of astronomical winter in the Northern Hemisphere but marks the beginning of astronomical autumn and the end of astronomical summer in the Southern Hemisphere.
In astronomy, the March equinox is the zero point of sidereal time and, consequently, the right ascension and ecliptic longitude. It also serves as a reference for calendars and celebrations in many cultures and religions.

Constellation

The point where the Sun crosses the celestial equator northwards is called the First Point of Aries. However, due to the precession of the equinoxes, this point is no longer in the constellation Aries, but rather in Pisces. By the year 2600 it will be in Aquarius. The Earth's axis causes the First Point of Aries to travel westwards across the sky at a rate of roughly one degree every 72 years. Based on the modern constellation boundaries, the northward equinox passed from Taurus into Aries in the year −1865, passed into Pisces in the year −67, will pass into Aquarius in the year 2597, and will pass into Capricornus in the year 4312. It passed by a 'corner' of Cetus at 0°10′ distance in the year 1489.

Apparent movement of the Sun

In its apparent motion on the day of an equinox, the Sun's disk crosses the Earth's horizon directly to the east at sunrise; and again, some 12 hours later, directly to the west at sunset. The March equinox, like all equinoxes, is characterized by having an almost exactly equal amount of daylight and night across most latitudes on Earth.

Culture

Calendars

The Persian calendar begins each year at the northward equinox, observationally determined at Tehran.
The Indian national calendar starts the year on the day next to the vernal equinox on March 22 with a 30-day month, then has 5 months of 31 days followed by 6 months of 30 days.

Julian calendar

The Julian calendar reform lengthened seven months and replaced the intercalary month with an intercalary day to be added every four years to February. It was based on a length for the year of 365 days and 6 hours, while the mean tropical year is about 11 minutes and 15 seconds less than that. This had the effect of adding about three quarters of an hour every four years. The effect accumulated from inception in 45 BC and by the 16th century, the northern vernal equinox fell on March 10 or 11. Since the date of the equinox is critical to calculation of the date of Easter, its drift from astronomical reality was of grave concern to the Catholic church. It was for this reason that the Gregorian calendar reform was introduced in 1582, to reinstate the date to about 21 March and to minimise any further drifting. Since 1900, the calendars differ by 13 days: from 1 March 2100, the difference will become 14 days.

Commemorations

Abrahamic tradition

  • The Jewish Passover usually falls on the first full moon after the Northern Hemisphere vernal equinox, although occasionally it will occur on the second full moon.
  • The Christian Churches calculate Easter as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is March 21. The Eastern Orthodox Churches use the older Julian calendar, while the western churches use the Gregorian calendar, and the western full moons currently fall four, five or 34 days before the eastern ones. The result is that the two Easters generally fall on different days but they sometimes coincide. The earliest possible western Easter date in any year is March 22 on each calendar. The latest possible western Easter date in any year is April 25.

    Iranian tradition

  • The northward equinox marks the first day of various calendars including the Iranian calendar. The ancient Iranian peoples' new year's festival of Nowruz can be celebrated March 20 or March 21. According to the ancient Persian mythology Jamshid, the mythological king of Persia, ascended to the throne on this day and each year this is commemorated with festivities for two weeks. Along with Iranian peoples, it is also a holiday celebrated by Turkic people, the North Caucasus and in Albania. It is also a holiday for Zoroastrians, adherents of the Baháʼí Faith and Nizari Ismaili Muslims irrespective of ethnicity.

    West Asia and North Africa

  • In many Arab countries, Mother's Day is celebrated on the northward equinox.
  • Sham el-Nessim is a modern celebration which is claimed by some to have been celebrated in ancient Egypt but with little evidence. It is one of the public holidays in Egypt. It is assumed by some that sometime during Egypt's Christian period the date moved to Easter Monday, but before then it coincided with the vernal equinox.

    South and Southeast Asia

According to the sidereal solar calendar, celebrations which originally coincided with the March equinox now take place throughout South Asia and parts of Southeast Asia on the day when the Sun enters the sidereal Aries, generally around April 14.