Solstice vs. Equinox

Main Difference

The main difference between Solstice and Equinox is that the Solstice is a annual event and Equinox is a astronomical event where the Sun is directly above the Earth’s equator.

• Solstice

  A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes.

  The term solstice can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are “June solstice” and “December solstice”, referring to the months in which they take place every year. The word solstice is derived from the Latin sol (“sun”) and sistere (“to stand still”), because at the solstices, the Sun’s declination appears to “stand still”; that is, the seasonal movement of the Sun’s daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

• Equinox

  An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth’s equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator.

  The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality primitive cultures noted the day when the Sun rises due East and sets due West and indeed this happens on the day closest to the astronomically defined event.

  In the northern hemisphere, the equinox in March is called the Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox. The dates are slightly variable, dependent as they are on the leap year cycle.Because the Moon (and to a lesser extent the planets) cause the motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun’s more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0° and 180°.