The pull of gravity from the other planets also affects the location of Earth in its orbit.Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time Earth reaches each 90-degree location in its orbit around the sun. Another reason is that Earth's elliptical orbit is changing its orientation relative to the sun (it skews), which causes Earth's axis to constantly point in a different direction, called precession.It eliminates leap days in century years not evenly divisible by 400, such 1700, 18, and millennium years that are divisible by 4000, such as 800. To try to achieve a value as close as possible to the exact length of the year, our Gregorian Calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for Earth to complete one orbit around the sun. A year is not an even number of days, and neither are the seasons.There are a few reasons why seasonal dates can vary from year to year. Still, for many March 21 is ingrained in our culture as the traditional first day of spring, even though it's been more than 30 years since that last happened. Yet from 1981 to 2102, Americans celebrate the beginning of spring no later than March 20. In fact, did you know that during the 20th century, March 21 was actually the exception rather than the rule? The vernal equinox landed on March 21 only 36 out of 100 years - most of these occasions coming during the early and middle part of the 20th century. Is this unheard of? Not if you look at the statistics. After all, when some of us of a certain age were growing up, the first day of spring was always on March 21, not March 20, right? Now all of a sudden spring is coming on March 20, and as we just noted, even earlier - March 19 - for some. After the summer solstice, the days start getting shorter (receiving less daylight time), reaching an equal amount again on the autumnal equinox.Now maybe this rings false. After the winter solstice, the days start getting longer (receiving more daylight time), reaching an equal amount on the day of the spring equinox. During the winter solstice, Earth is tilted away from the sun and receives sunlight for the shortest time, resulting in the shortest day of the year. During the summer solstice, Earth is tilted toward the sun and receives sunlight for the longest time, resulting in the longest day of the year. While the day of an equinox has equal amounts of daylight and darkness, the days on which the solstices occur are known for being the longest and shortest days of the year in terms of daylight. On most calendars, these days are used to mark the beginning of spring and fall similar to how the days of the two solstices are used to mark the beginning of summer and winter. However, the word is most commonly used to refer to the day on which this happens.
Since Earth is in motion, the exact positioning considered an equinox only happens for a moment. It comes from the Latin aequinoctium, meaning “the time of equal days and nights,” from equi-, meaning “equal,” and nocti-, meaning “night.”Īn equinox is a moment, not an entire day. The first records of the word equinox come from the 1300s.
In the Southern Hemisphere, it’s the reverse.Įxample: Many ancient cultures recognized and observed the equinoxes as times that marked the change of the seasons. In the Northern Hemisphere, the summer solstice occurs in June and the winter solstice occurs in December.
There are two solstices during the year: one that occurs around June 20–22 (usually June 20 or 21) and one that occurs around December 20–23 (usually December 21 or 22). In contrast, a solstice is one of the two times of the year when the positioning and tilt of Earth relative to the sun results in the most amount of daylight time or the least amount of daylight time in a single day. In the Southern Hemisphere, it’s the reverse. In the Northern Hemisphere, the vernal equinox (or spring equinox) occurs in March and the autumnal equinox occurs in September. The equinoxes are traditionally considered to mark the start of spring and fall. This can also be called the equinoctial point. The word equinox can also refer to the position of the sun at this moment. The equinoxes occur on these days, but an equinox is not the whole day-it is the moment when the sun is directly above Earth’s equator. The two equinoxes occur around March 20–21 and September 22–23. An equinox is one of the two times of the year when the amount of daylight and nighttime hours are just about of equal length.