At the equinoxes, the astronomers tell us, the sun can be seen to be directly above the Earth's equator. If, like most people, you are not an astronomer who lives on the Earth's equator, you probably know the equinoxes best as the first days of spring and fall.

In 2011, the March equinox — marking the first day of spring in the Northern Hemisphere and of autumn in the southern — occurred at 7:20 p.m. ET on March 20.

The September equinox in 2011 — marking the first day of autumn in the Northern Hemisphere and first day of spring in the Southern Hemisphere — occurs at 5:04 a.m. ET on Sept. 23.

'Equal' night? Not quite

The word equinox is derived from the Latin words for "equal" and "night," and a common description of the equinox is the day when there are equal hours of daylight and night. This isn't quite correct. On the equinox, the centre of the solar disk spends an equal amount of time above the horizon and below.

But since the upper half of the sun above the horizon provides sunlight at dawn and dusk, daylight on the equinox is actually several minutes longer than the night. The dates when there are 12 hours of day and 12 hours of night, called the equiluxes, are a few days closer to winter than the equinoxes and fall on different days at different latitudes.

Equinox in holidays and calendars

Two girls walk around the Sun Pyramid at Teotihuacan where Mexicans come to receive the first sunlight in Mexico City, March 21, 2004. The date marks the spring equinox. (Jaime Puebla/Associated Press)The spring equinox is the first day of the year in both the Baha'i calendar and the Iranian calendar, where the day is called Norouz. It is a holiday in many countries in the Middle East and Central Asia, and for followers of Zoroastrianism, Sufism and the Baha'i faith.

Conversely, the French Republican Calendar (used in France for just 12 years in the late 18th and early 19th centuries, before Napoleon Bonaparte came to power and abolished it) used the autumn equinox as New Year's Day, the first day of the month called Vendémiaire, or "wine harvest."

The March equinox also marks the beginning of the astrological sign of Aries, the first sign of the Zodiac. However, because of the "wobble" in the Earth's axis and the shift over hundreds of years of the sun's apparent position against the background of stars, the March equinox no longer corresponds with the sun's position in the constellation Aries.

In the Christian church, Easter is calculated as the first Sunday after the first full moon on or after the March equinox, although for this purpose, the date of the equinox is always considered to be March 21. So, the earliest Easter can be is March 22, and it hasn't fallen on that date since 1818, and won't again until 2285.

In Japan, both the spring and autumn equinox are national holidays, commonly spent holding reunions and visiting family graves.

In neopaganism, the spring and autumn equinoxes are called Ostara and Mabon, respectively, although these names are modern in origin and don't correspond to any ancient festivals.

The egg thing

Incidentally, there's no truth to the myth that perennially clogs e-mail in-boxes that an egg can be made to stand on end only on the spring equinox. Any success you have with the balancing of eggs on any given day has more to do with the geometry of the egg — and your own patience — than with the geometry of the solar system.