![]() ![]() In other words, the geometric center of the sun is actually more than eight-tenths of a degree below a flat and unobstructed horizon at the moment of sunrise. Naval Observatory routinely uses 34 arc minutes for the angle of refraction and 16 arc minutes for the semi/half diameter of the sun's disc. In their calculations of sunrise and sunset times, the U.S. The sun's apparent diameter is roughly equal to half a degree.īut the main reason that this happens is due to Earth's atmosphere it acts like a lens and refracts (bends) its light above the edge of the horizon. This fact alone would make the time of sunrise and sunset a little more than 12 hours apart on the equinox days. One factor to consider is that when we refer to sunrise and sunset, it refers to when the very top edge of the sun appears on the horizon. As the table below shows, days and nights are equal not on the equinox, but on Saint Patrick's Day: In fact, on the equinox dates in both March and September, the length of daylight is actually longer than darkness by several minutes.Ĭheck out the situation for New York City. Perhaps someone skimming through the weather page of their newspaper on the day of the equinox, looked at the almanac box which provides the local time of sunrise and sunset and noticed that the length of day and night is not equal at all. Not "equal" on the equinox!Īnother complexity involving the vernal equinox concerns the axiom, "equal days and equal nights on the equinox." Yet each year I always get at least one or two inquiries asking why that isn't so. Had the year 2000 not been a leap year, the equinox would be occurring this year on Tuesday (March 21), not Monday. The vagaries of our Gregorian calendar, such as the inclusion of a leap day in century years divisible by 400 also help contribute to the seasonal date shift. This shift in dates happens because the Earth's elliptical orbit changes the orientation of its axis, and because our year does not contain an even number of days. the last Monday of each month.Next year in fact, spring will officially begin on March 19. Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. Central Daylight Time, a good time to go outside and celebrate the event. The sun crosses the equator today at 4:24 p.m. ![]() In Chaco Canyon, the ancestral Puebloan people carved spiral designs into rock that track the seasons. At Machu Picchu, stones mark the four cardinal directions so that exactly at noon on the equinox, no shadow is cast. Stonehenge was constructed in a way that marks its relation to the Sun’s position in the sky and in different seasons. During the vernal equinox, the sun is moving from south to north as it crosses above the equator.įor a long time, people have marked the changing of seasons and the sun’s track across our skies. The vernal equinox happens when the Earths axis reaches a particular. During the spring and fall equinoxes, the sun is above the horizon for all locations on Earth for 12 hours. S pring 2022 has finally arrived, and will be marked by the vernal equinox, which takes place on this Sunday, March 20. The equinoxes occur when the sun’s rays strike the equator at noon at an angle of 90 degrees. Satellite view of Earth on March 20th 2023, Spring Equinox in the Northern Hemisphere. ![]()
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