Solstices and Equinoxes

Solstices and equinoxes mark the beginning and end of each season. The vernal(spring) equinox denotes the end of winter and the beginning of spring while the summer solstice marks the end of spring and the beginning of summer. The end of summer and beginning of autumn is demarcated by the autumnal equinox, followed by the winter solstice, marking the end of autumn and beginning of winter.

The vernal equinox typically falls on the 20th of March, while the autumnal equinox usually falls on the 22nd of September. The instances of the equinoxes are not fixed but fall about six hours later every year, amounting to one full day in four years, but then they are reset by the occurrence of a leap year, as shown in the table below. Technically speaking, an equinox occurs when the center of the sun is directly above the earth's equator (referred to as zenith, the point directly overhead), resulting is a 0-degree sun angle at the equator. For Cape Cod, the sun is located at about 42 degrees above the horizon at noon on both the vernal and autumnal equinoxes.

It is a common misconception that the day of the equinox has an equal amount of daylight and darkness in a 24 hour-period. In actuality, the day of equal daylight and darkness occurs about 4 days before the equinox.This is because the sun is a disk and not a single point. At sunrise, the very top edge of the sun appears over the horizon first, and the time at which this occurs is the time of sunrise. At sunset, the very top edge of the sun is the last to disappear over the horizon, and this is the time of sunset. This effect, combined with atmospheric refraction, combine to make the day about 10 minutes longer than the night here on Cape Cod on the day of the equinox.