Saturday, 19 March 2011

The March Equinox Explained

The March Equinox Explained

The March equinox will occur on March 20 in 2011, marking the beginning of spring in the northern hemisphere and fall (autumn) in the southern hemisphere from an astronomical viewpoint. The March equinox will occur at 23:21 (or 11:21pm) at Coordinated Universal Time (UTC) on this date.


This illustration, which shows an example only of the March equinox, is not to scale.

Twice a year, around March 20 or 21 and September 22 or 23, the sun shines directly on the equator and the length of day and night are nearly equal in all parts of the world. These two days are known as the March(vernal or spring in the northern hemisphere) equinox and the September equinox.

To find the March equinox date in other time zones or other years, please use the Seasons Calculator.

What does equinox mean?

The word “equinox” derives from the Latin words meaning “equal night” and refers to the time when the sun crosses the equator. At such times, day and night are everywhere of nearly equal length everywhere in the world.

It is important to note that while the March equinox marks the beginning of spring in the northern hemisphere, it is the start of autumn in many parts of the southern hemisphere.

March Equinox Explained

The March equinox is the movement when the sun crosses the true celestial equator – or the line in the sky above the earth’s equator – from south to north, around March 20 (or March 21) of each year. At that time, day and night are balanced to nearly 12 hours each all over the world and the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun.

In gyroscopic motion, the earth’s rotational axis migrates in a slow circle based as a consequence of the moon’s pull on a nonspherical earth. This nearly uniform motion causes the position of the equinoxes to move backwards along the ecliptic in a period of about 25,725 years.

Nearly Equal?

During the equinox, the length of night and day across the world is nearly, but not entirely, equal. This is because the day is slightly longer in places that are further away from the equator, and because the sun takes longer to rise and set in these locations. Furthermore, the sun takes longer to rise and set farther from the equator because it does not set straight down - it moves in a horizontal direction.

Moreover, there is an atmospheric refraction that causes the sun's disk to appear higher in the sky than it would if earth had no atmosphere. timeanddate.com has a more detailed explanation on this topic. timeanddate.com has more information on why day and night are not exactly of equal length during the equinoxes.

During the March equinox, the length of daylight is about 12 hours and eight to nine minutes in areas that are about 30 degrees north or south of the equator, while areas that are 60 degrees north or south of the equator observe daylight for about 12 hours and 16 minutes. Many regions around the equator have a daylight length about 12 hours and six-and-a-half minutes during the March equinox.

Moreover, one day does not last for the exact same 24 hours across the world and due to time zone differences, there could be a small difference in the daylight length between a far-eastern and far-western location on the same latitude, as the sun moves further north during 24 hours. For more information, find out the length of day in a particular city. Select a location in the drop-down menu below to find out the length of day around the time of the March equinox.

Vernal Equinox vs. Autumnal Equinox

The vernal equinox occurs in the spring while the autumnal equinox occurs during fall (autumn). These terms are derivatives of Latin. It is important to note that the northern hemisphere’s vernal equinox is in March while its autumnal equinox is in September. In contrast, the southern hemisphere’s vernal equinox is in September and its autumnal equinox is in March.

This distinction reflects the seasonal differences when comparing the two hemispheres. timeanddate.com refers to the two equinoxes simply as the March and September equinoxes to avoid false assumptions that spring is in March and fall (autumn) is in September worldwide. This is simply not the case.

Historical Fact

A Greek astronomer and mathematician named Hipparchus (ca. 190-ca.120 BCE) was attributed by various sources to have discovered the precession of the equinoxes, the slow movement among the stars of the two opposite places where the sun crosses the celestial equator. Hipparchus made observations of the equinox and solstice. However, the difference between the sidereal and tropical years (the precession equivalent) was known to Aristarchus of Samos (around 280 BCE) prior to this.

Astronomers use the spring equinoctial point to define their frame of reference, and the movement of this point implies that the measured position of a star varies with the date of measurement. Hipparchus also compiled a star catalogue, but this has been lost.

March Equinox across Cultures

In the northern hemisphere the March equinox marks the start of spring and has long been celebrated as a time of rebirth. Many cultures and religions celebrate or observe holidays and festivals around the time of the March equinox, such as the Easter holiday period.

The astronomical Persian calendar begins its New Year on the day when the March equinox occurs before apparent noon (the midpoint of the day, sundial time, not clock time) in Tehran. The start of the New Year is postponed to the next day if the equinox is after noon.

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