Vernal equinox

In astronomy, the vernal equinox (spring equinox, March equinox, or northward equinox) is the moment when the Sun appears to cross the celestial equator, heading northward. It is the precise moment that spring begins in the Northern Hemisphere and autumn in the Southern Hemisphere. In Chinese culture the vernal equinox marks the middle of spring. The term can also be used to refer to the point on the sky defined as the first point of Aries.

Related Topics:
Astronomy - Sun - Celestial equator - Spring - Northern Hemisphere - Autumn - Southern Hemisphere - Chinese culture - First point of Aries

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The equinox can be as early as March 19 or as late as March 21, the precise time being about 5 hours 49 minutes later in a common year, and about 18 hours 11 minutes earlier in a leap year, than in the previous year. It is the balance of common years and leap years that keeps the calendar date of the vernal equinox from drifting more than a day from 20 March each year.

Related Topics:
March 19 - March 21 - Precise time - 20 March

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If the day is defined as the period during which the true position (ignoring atmopheric effects) of the middle of the Sun is above the horizon, the length of the day (and the night) is almost exactly 12 hours at an equinox. The Sun would rise due East and set due West all over the Earth except at the poles, where it would lie on the horizon. At the equator at noon the Sun is straight overhead.

Related Topics:
East - West

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More commonly the day is defined as the period that sunlight may reach the ground in the absence of local obstacles. Because the Sun is not a point but appears as a disc, and because sunlight is refracted downwards by the atmosphere, at the equinox daylight is increased by almost 7 minutes at the equator, decreasing nighttime by the same amount. This causes the day to be longer than the night by almost 14 minutes, making the practical equality of day and night happen a few days towards the winter side. The disparity is increased toward the poles because there the Sun sets at an angle to the vertical, increasing the time it takes to set.

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