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I noticed in today's newspaper (Boston Globe, September 26) that sunrise and sunset were both at 6:25. That's the twelve hour day I'd have expected at the equinox.

(Please retag if there's a better one)

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A day would be twelve hours long at the equinoxes if the Earth had no atmosphere and if the Sun was a point rather than a sphere. Sunrise and sunset are defined as the time at which the upper limb of the Sun appears to rise above or set below an ideal oblate spheroid Earth and assuming average atmospheric conditions. While this calculation ignores terrain and variations in atmospheric condition, it does not ignore that the basic facts that the Earth does have an atmosphere and that Sun is a sphere. The standard atmosphere conditions means that sunrise (sunset) occur when the upper limb of the Sun first reaches (first falls below) 34 arc minutes below the horizon. That the Sun is not a point adds another 16 arc minutes to this, making sunrise/sunset occur when the center of the Sun is 50 arc minutes below the horizon.

This means that at the equinoxes, a "day" is at least six minutes longer than the twelve hours we are naively taught (and incorrectly reported twice a year by naive weathermen). As an aside, days are always longer than nights at the equator.

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The day and night are not equal on the equinox, though they are about equal. Equinox is when the Earth's axis is not tilted towards the sun, or equivalently the path of the sun passes from the Northern to the Southern hemisphere.

Sunrise and sunset are measured from the "top" of the sun, not the centre, and the refraction of the atmosphere means that the sun is visible from the surface, even after it would have set were there no air to bend its light. Finally the Earth's elliptical orbit also varies the length of the day.

The day on which the day and night are equal varies with latitude. In Boston, at 42 degrees North (it's as far south as the French Riviera - always surprises me) the day is 12 hours long on Sept. 26.

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