More information about the Equinoxes

 The name "equinox" is derived from the Latin aequus (equal) and nox (night). Find out why below.

An equinox occurs  when the tilt of the Earth's axis makes a right angle with the Sun's rays.

 Because of this,  equal latitudes to the North and South of the Equator (eg. 10 degrees and -10 degrees) experience equal spread of solar energy and consequently, nights of equal length. About the equator, the length of night and day will be about 12 hrs each. However, this is not true throughout the globe due to difference in latitude. Also, the word 'equinox' denotes a moment in time and not a whole day or two.  A day in which day and night are closest to being 12hrs each is referred to as equilux.

There are 2 equinoxes a year, one in March and one  in September. They are respectively referred to as the Vernal or Spring Equinox and Autumnal Equinox in the Northern hemisphere. The words can be reversed on the other hemisphere as spring and autumn occur at opposite times in the different hemispheres. 

One important thing to remember is that the Equinox denotes the first day of that season. The Autumnal Equinox, for example, is the first day of autumn. Same goes for spring. 

The above plots show how the date of the autumnal equinox shifts through the Gregorian calendar due to the fact that one year is not exactly 365 days. It is roughtly 365.25 days long. The .25 is made up after four years by the Leap Year, which has an extra day. As a result, the specific date and time of the equinoxes migrate through a period that occurs about six hours later from year to year.

The system resets every leap year, slipping a little bit backward until a non-leap century year nudges the equinoxes forward in time once again.


(a century year is a year that is exactly divisible by 400. Eg, 2000. A non-leap century year is a century year that is not a leap year)

When the Sun passes the autumnal equinox, nights begin to grow longer than days, and continue to do so until the Winter Solstice. Conversely, when the sun passes the vernal equinox, days continue to grow longer until the Summer Solstice.


The table below gives the universal time of the autumnal equinox.  (In the southern hemisphere, this situation is reversed).

year	Equinox Mar		Solstice June		Equinox Sept		Solstice Dec
	day	time	day	time	day	time	day	time
2004	20	06:49	21	00:57	22	16:30	21	12:42
2005	20	12:33	21	06:46	22	22:23	21	18:35
2006	20	18:26	21	12:26	23	04:03	22	00:22
2007	21	00:07	21	18:06	23	09:51	22	06:08
2008	20	05:48	20	23:59	22	15:44	21	12:04
2009	20	11:44	21	05:45	22	21:18	21	17:47
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

The autumnal equinox for year 2011 has been calculated to be on September 23rd, 9:04 AM.


Interesting facts on the perception of the Equinox:

• Day and night would each be exactly 12 hours long on a spring or fall equinox only if the sun were a single point of light and Earth had no atmosphere.
• The true days of day-night equality always fall after the autumnal equinox and before the vernal, or spring, equinox, according to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C.
• The equinoxes are also the only days of the year when a person standing on the Equator can see the sun passing directly overhead.
• On the Northern Hemisphere's autumnal equinox day, a person at the North Pole would see the sun skimming across the horizon, signaling the start of six months of darkness. 
• On the same day, a person at the South Pole would also see the sun skim the horizon, beginning six months of uninterrupted daylight.
• These two effects are caused by the sun's heat only hitting the earth's surface that faces the sun and not the poles (see below)

• If the earth had no tilt, we would technically be experiencing a perennial equinox.
• Equinoxes are celebrated in many cultures, for example Japan, where Vernal Equinox day is a national holiday where ancestral graves are worshipped. In Arab countries, Mother's Day is celebrated on the March equinox. In India, Tamil and Bengali New Years are celebrated according to the Vernal Equinox. In Korea, Chuseok is a major harvest festival that occurs during the Autumn Equinox where three days of public celebrations are held.
• All these examples show us that the equinoxes do, in fact, affect the lives of people and are not just purely astronomical happenings that pass by without people noticing them. 

PS. Ever had one of those annoying 'we cannot show any TV due to satellite interference caused by solar outage' on Starhub Cable TV?

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 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. 

Posted by Shu En, Sat Feb 26