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The Equinox Explained

May 14, 2009 by  


ibn tufail 5-11-09

An equinox occurs twice a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens.

The name “equinox” is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. It may be better understood to mean that latitudes +L and -Lnorth and south of the equator experience nights of equal length.

At an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (not two whole days), when there is a location on the Earth’s Equator where the centre of the Sun can be seen vertically overhead, occurring around March 20/21 and September 22/23 each year.

On a day of an equinox, the centre of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly the same length. The word equinox comes from the Latin words aequus (equal) and nox (night); in reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period when sunlight reaches the ground in the absence of local obstacles.

The cumulative effects of light bending through the atmosphere make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.

The date at which the time between sunset and sunrise crosses 12 hours, is known as the equilux. Because sunset and sunrise times vary with an observer’s geographic location (longitude and latitude), the equilux also depends on location and does not exist for locations sufficiently close to the equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.

The Earth’s seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth’s axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honour, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North and South Poles of the Earth are just on the terminator and day and night are divided equally between the hemispheres.

11-21

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