2005/11/14 | 昼夜平分点[Equinox]
类别(¤〖天文〗) | 评论(0) | 阅读(20) | 发表于 20:20
There are two equinoxes each year. Although, as the etymology of the word suggests, the term originally referred to the two times of year when night and day are the same length, the modern definition of the equinoxes is the instant at which the center of the Sun crosses declination 0 (i.e., the celestial equator, which is the projection of the Earth's equator onto the sky). With this definition, day and night are not quite the same length on the equinoxes due to (1) refraction of light from the sun as it passes through the Earth's atmosphere and (2) the fact that sunrise and sunset are calculated from the limb (not the center). Both of these effects slightly lengthen "day" relative to "night."

The vernal equinox (near March 21) occurs when the Sun crosses the celestial equator moving northward. The right ascension (i.e., the projection of lines of longitude onto the sky) at the vernal equinox was originally located in the constellation Aries, and this point of crossing was known as the first point in Aries. As a result of precession, the vernal equinox now actually occurs when the sun is in the constellation in Pisces. The autumnal equinox (near September 23) occurs when the sun crosses the celestial equator moving southward.

Here is a QuickTime movie illustrating the tilt of the Earth's equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.

There is a superstition that it is possible to stand an egg upright on its end on the date of the equinoxes (and/or solstices). While this is true, it is also possible to stand an egg on its end at any other time of the year. Although this feat is not trivial, a persistent person can usually succeed with enough practice and skill. However, the only connection between success in standing eggs upright and the equinox, to my knowledge, is that because of the superstition, many more people try and persist than would be the case on any other date. Of course, cheating by resting the egg on a thin layer of salt grains and then blowing all but a few invisible grains away is still far and away the easiest method!
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