What is an equinox?
On our annual journey around the Sun we experience changing seasons because Earth's axis is tilted by 23.5 degrees. This means that day length varies during the year. When one pole is tilted by the maximum 23.5 degrees into the Sun, this is a solstice. If it's the north pole, then the northern hemisphere has its summer solstice (longest day). Since the south pole is tilted 23.5 degrees away from the Sun, it has its winter solstice (shortest day).
You can see from the diagram that twice a year neither pole is pointed at the Sun. This is when an equinox occurs. The vernal equinox in the northern hemisphere is on or around March 20, as is the southern autumnal equinox. However, astronomically speaking, an equinox is more specific than the calendar day. It's the time that the Sun crosses the celestial equator, which is an imaginary line in the sky coordinate system that corresponds to Earth's equator. On this day, everywhere on the planet has approximately twelve hours of day and twelve of night. However local conditions cause slight variations.
Vernal equinox and New Year
In some cultures the vernal equinox has also been the start of the year.
Nowruz originated three thousand years ago as a Zoroastrian festival. The Zoroastrian solar year begins on the vernal equinox. Now, in addition to the Zoroastrian observances, it's a major celebration in Iran and for millions of people in many countries. Homes get a thorough cleaning, there are new clothes, presents for the children, special foods, and many local traditions. The United Nations recognizes International Nowruz Day for its contribution to peace and cultural diversity.
The Chinese Spring Festival (also known as the Lunar New Year), like Nowruz, involves getting new clothes and giving the house a good cleaning. However spring begins in late January or early February, not at the equinox. On the old Japanese calendar the new year was the first day of spring, which was in February, midway between the winter solstice and the vernal equinox.
There used to be four quarter days on the English calendar. They occurred near the solstices and equinoxes. Lady Day was on March 25 and for a long time this was the start of the new year. Lady Day ended up on April 6 after Britain adopted the Gregorian calendar in 1752. The British tax year still starts on this day, but it's a new year that isn't much celebrated.
Vernal equinox festivals
Easter isn't observed on the vernal equinox, but the date of Easter is derived from it. Easter is on the first Sunday after the first full moon that occurs on or after the March equinox. This is why its date can vary between March 22 and April 25.
In the early days of Christianity, Easter was its central festival, as it commemorated the resurrection of Jesus Christ. However, as with other Christian holidays, older traditions got mixed in over time. Easter eggs, for example, are a remnant of fertility festivals. Even the name comes from Eostre, the Germanic goddess of spring.
Although the vernal equinox isn't the first day of spring in Japan, Vernal Equinox Day (Shunbun no hi) is a national holiday. It's a family day and a time to appreciate nature. Some people will follow the old customs of visiting and tidying family graves. The holiday falls on the day when the vernal equinox occurs in Japan, so its date is declared only the year before so that the relevant astronomical measurements are up to date.
Archaeology and the equinox
There is archaeological evidence for the importance of the equinox to past civilizations. For example, in Ireland a mound in Loughcrew, built over five thousand years ago, is aligned to the equinoxes. At sunrise on the equinox, the Sun illuminates stone carvings that are otherwise in darkness. And in Egypt, the Great Sphinx faces due east, looking straight at the rising Sun on an equinox.
The pyramid El Castillo in the ruins of Chichen Itza in Mexico is the temple of the feathered serpent god Kukulkan. Besides incorporating elements of the Mayan calendar, the pyramid is known for the “snake” that seems to undulate down the stairs of the pyramid as the Sun sets on the days around the equinoxes. The design of the building creates this effect of light and shadow.
There is an idea that makes the rounds each year that on the vernal equinox you can get an egg to balance on its end on a flat surface. Balancing an egg seems to be an old custom in some parts of China. If done on the first day of spring, it brings good luck. But someone in America decided to apply it to the vernal equinox, and people claim that the egg only balances at that time because the day and night are equal, and gravitational forces are in balance. Others say that it works at both equinoxes, but it's usually associated with the spring.
The equinox doesn't affect the gravitational forces on the Earth. In fact, even the day and night are only approximately equal. When day and night are of equal length it's an equilux, and various local factors determine when that happens in a given area.
If you have the patience – and nothing better to do – you could get an egg to balance on the vernal equinox . . . or the autumnal equinox . . . or any other day of the year.
KE Eduljee, “Zoroastrian Inheritance” http://www.heritageinstitute.com/zoroastrianism/nowruz/
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