The vernal and autumnal equinoxes, meanwhile, occur when the center of the sun's disk crosses what's known as the celestial equator, an imaginary line that projects outward from Earth's Equator, Chester noted. What's more, Earth's atmosphere bends sunlight when it's close to the horizon, making the sun appear to rise a few minutes earlier than it actually does. Most people will never see the full 12 hours of sunup and sundown on the autumnal equinox, the University of Massachusetts' Young added.
That's because most people have hills or trees blocking their views of a flat horizon. Thus, they see the sun rise later and set earlier than it does for a horizon without obstruction, she said. What's more, for people who don't live on the Equator, the sun still rises and sets at an angle to the horizon, noted Young, who built a Stonehenge-like solar calendar and observatory on the University of Massachusetts campus.
Even though the sun rises due east and sets due west on the autumnal equinox, "you'll only see an east sun rising and west sun setting with an obstruction-free horizon," she said. In Pope Gregory XIII established the Gregorian calendar, which most of the world now observes, to account for an equinox inconvenience.
If the pope hadn't established the new calendar, every years the spring equinox would have come a full calendar day earlier, eventually putting Easter in chilly midwinter. Before the pope's intervention, the Romans and much of the European world marked time on the Julian calendar. Instituted by Julius Caesar, the old calendar counted exactly Every four years a leap day helped keep things on track. It turns out, however, that there are Using the Julian calendar, the seasons were arriving 11 minutes earlier each year.
By the spring equinox had fallen back to March To fix the problem, the pope decreed that most century years such as , , and would not be leap years. But century years divisible by , like , would be leap years. Under the Gregorian calendar, the year is With an average duration of Nowadays, according to the U.
Naval Observatory's Chester, equinoxes migrate through a period that occurs about six hours later from calendar year to calendar year, due to the leap-year cycle. The system resets every leap year, slipping a little bit backward until a non-leap century year nudges the equinoxes forward in time once again. All rights reserved. Equinoxes as Season Starters The march of the seasons—winter, spring, summer, and fall—stems from the "clearly definable" position of the sun on the summer and winter solstices, according to Judith Young , a professor of astronomy at the University of Massachusetts in Amherst.
Watch video: Equinoxes Explained. The Lag of the Seasons The defined start of the seasons based on the sun's positions may seem counterintuitive. Autumnal Equinox Illusions But don't be fooled by the notion that on the autumnal equinox the length of day is exactly equal to the length of night. The difference is a matter of geometry, atmosphere, and language.
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