This takes place around the same time every year, where the Sun shines directly over the equator and distributes the same amount of light on either side of the planet.įor the Northern Hemisphere, this marks the beginning of shorter days and colder weather. Eastern.ĭuring that time, the Sun will be exactly above the Earth’s equator and will appear overhead at noon as seen from the equator. The 2021 fall equinox will take place on Wednesday, Septemat 3:21 p.m. What exact time is the September 2021 fall equinox? Inverse breaks down everything you need to know about this annual event.
The seasonal change is actually due to our planet’s tilt on its axis, and a time when the Northern and Southern Hemispheres essentially trade places. On September 22, the Sun will cross the celestial equator and mark the beginning of a new season and the September 2021 fall equinox.įor that brief moment during the autumn equinox, night and day are almost exactly equal before the day begins to get shorter. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. Thus, thanks to atmospheric refraction, the length of daylight on any given day is increased by approximately six or seven minutes. Or, put in another way, when you watch the sun either coming up above the horizon at sunrise or going down below the horizon at sunset, you are actually looking at an illusion – the sun is not really there, but actually is below the horizon.Īs a result, we actually end up seeing the sun for a few minutes before its disk actually rises and for a few minutes after it has actually set. In other words, the geometric center of the sun is actually 0.83º below a flat and unobstructed horizon at the moment of sunrise. Naval Observatory routinely uses 34 minutes of arc for the angle of refraction and 16 minutes of arc for the semi diameter of the Sun's disc. In their calculations of sunrise and sunset times, the U.S. It's an illusionīut the main reason that this happens can be attributed to our atmosphere it acts like a lens and refracts (bends) its light above the edge of the horizon. The sun's apparent diameter is roughly equal to half a degree. This alone would make the time of sunrise and sunset a little more than 12 hours apart on these days. One factor is that the moments of sunrise and sunset are considered when the top of the sun, and not its center, is on the horizon. As the table below shows, days and nights are equal not on the equinox, but actually, a few days earlier, on Saint Patrick's Day (March 17): The difference in the number of hours separating sunrise and sunset on the day of equinox are not equal at all.Ĭheck out New York City. In fact, on the days of the spring and fall equinox the length of time that the sun is above the horizon is actually longer than the time it spends out of sight below the horizon by several minutes.Įvery year around this time, almost like clockwork, I will get an email from someone who was studying the weather page of his or her newspaper, looking at the section listing the times of sunrise and sunset and noticing that something appears to be wrong. Naval Observatory or the sunrise/sunset tables in any reputable almanac, you will find that this is not so. Yet, if you check the calculations made by the U.S.
Since grammar school we all have been taught that on the first days of spring and autumn, that day and night are equal to exactly 12 hours all over the world. Inequal day and nightĪ complication revolving around the vernal equinox concerns the length of day versus night. The March and September equinoxes occur when both the Northern and Southern Hemispheres equally face the sun and all parts of the world have the sun above the horizon for exactly 12 hours, and below the horizon for exactly 12 hours.Įqual days and equal nights: The equinox. The December solstice marks the beginning of winter, when the direct rays of the sun are shining down on that part of the globe 23.5 degrees south of the equator - called the Tropic of Capricorn. For the Northern Hemisphere, the June solstice marks the start of the summer season and occurs when the direct rays of the sun shine down on that part of the globe 23.5 degrees north of the equator - the so-called Tropic of Cancer. This tilt causes different latitudes on Earth to receive varying amounts of heat and light from the sun throughout the course of the year. Our seasons take place because as our planet revolves around the sun, its axis is tilted at a 23.5-degree angle.
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