As the Earth travels around the Sun in its orbit, the north-south position (declination) of the Sun changes over the course of the year due to the changing orientation of the Earth's tilted rotation axes with respect to the Sun. It is this change in the position of the sun that is responsible for seasons. Solstices occur when Sun reaches maximum offsets of from the equator projected on the sky (i.e., the celestial equator). This offset corresponds to the tilt angle of Earth's rotational axis with respect to its orbital plane, called the Earth's obliquity.

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. Here is a QuickTime movie illustrating the tilt of the Earth's equatorial plane relative to the Sun.

The northern hemisphere summer solstice (approximately June 22) occurs when the sun is farthest north. The northern hemisphere winter solstice (approximately December 22) occurs when the sun is farthest south. (In the southern hemisphere, winter and summer solstices are exchanged since when one hemisphere is pointed towards the Sun, the other is pointed away.) The declinations of the Sun on the summer solstice and winter solstice are known as the tropic of cancer and tropic of capricorn, respectively ( ).

The summer and winter solstices are the longest and shortest days of the year, respectively, in the sense that the length of time elapsed between sunrise and sunset on these days is a maximum and minimum, respectively for the year. Of course, daylight saving time means that the first Sunday in April has 23 hours and the last Sunday in October has 25 hours, but these human meddlings with the calendar and do not correspond to the actual number of daylight hours.

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!

Equinox, Obliquity, Season, Summer Solstice, Winter Solstice