Equinox and solstice are words you may have seen on your calendar or heard the local weather person talking about during a forecast. Each of these terms indicates a specific time in the annual revolution of the Earth around the sun. Two equinoxes and solstices happen every year. Each equinox and solstice also indicates the beginning of a new season.
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The word "equinox" is both a foreign and ancient term originating in medieval Latin; "aequus" and "nox," mean "equal" and "night," respectively. "Equal night" or "equinox" happens when day and night are nearly the same length at any latitude on the globe. The first equinox of the year, the spring or vernal equinox, occurs in March, generally March 20 or 21, depending on whether the year is a leap year. In the Southern hemisphere, this date marks the autumn equinox.
Summer solstice is the first of two solstices that occurs every year. The Earth is tipped at an angle as it makes its way around the sun; the north and south pole are not completely straight up and down, if viewing Earth from afar. Solstice indicates the points at which one of the poles is closest or farthest away from the sun. Summer solstice for the Northern hemisphere occurs when the north pole is tipped closest to the sun, generally on June 21. In the Southern hemisphere, on this same date, the opposite is true -- the south pole is tipped farthest from the sun, making it the winter solstice. Summer solstice is also the longest day of the year, in terms of daylight, and marks the first day of summer.
Autumn equinox, like the spring equinox, is a special day in which night and day are of nearly equal lengths, within minutes of one another. It also marks the beginning of autumn or fall. In the Northern hemisphere, this date happens on or around September 22. In the Southern hemisphere, this date is the spring equinox.
Winter solstice is the shortest day of the year, in terms of the amount of time between sunrise and sunset. In the Northern hemisphere, this occurs on December 21 or 22; it varies due to leap years. It is also the first day of winter. Summer solstice is on the same day in the Southern hemisphere, where this day is the longest of the year.