Equinoxes occur when the Sun passes over the equator, marking a change of season. Day and night are approximately equal in length, but not exactly. The event is celebrated in many cultures and is associated with routine maintenance and seasonal festivals.
An equinox is a period in a planet’s orbit when the planet’s orbit and position cause the Sun to pass directly over the equator. Besides being a noteworthy astronomical event, it also marks a change of season. On Earth, the equinoxes appear around September 22 and March 20 each year, although the days can change slightly, especially since the date is calculated using Coordinated Universal Time (UTC), which means that the equinoxes in the Far East they can fall a day or more later than the equinoxes in Europe and the Americas, due to time zone differences.
Every planet has a centerline or equator, along with a celestial equator, an imaginary plane created by moving outward from the equator. When the equinox occurs, the Sun is directly above the celestial equator, meaning that someone standing at the equator would be able to see the Sun immediately overhead. It also means that half of the planet is fully illuminated and that day and night are approximately the same length.
Given the name, which implies the root of “same,” one might think that day and night are of equal length at this point in the year; however, this is not actually the case. A number of factors can affect the length of daylight experienced in any given area, and exactly the same days and nights actually fall slightly before the spring equinox and just after the fall equinox. However, two people the same distance from the equator would experience the same length of days and nights.
For people at the North Pole, the vernal equinox marks the beginning of six months of nearly continuous light, while people at the South Pole have six months of night to look forward to, as fall and winter have just begun. On this calendar day, however, they both see the same amount of sunlight, even though it marks the start of different seasons.
Another interesting event occurs at this time: the sun rises in the east and sets in the west. Many cultures have associated specific meanings with these times of the year. These dates traditionally mark the changes of the seasons, much like the winter and summer solstices, which mark the shortest and longest days of the year, respectively. Festivals have often fallen on or around these dates historically, and people may use the event as a helpful reminder to do routine maintenance on homes or cars.
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