According to Alan MacRobert, a senior editor with Sky & Telescope magazine, although the length of the day and night are not equal on the equinox, the fall and spring equinoxes are the only two times during the year when the sun rises due east and sets due west. This happens everywhere on Earth.
The equinoxes are also the only days of the year when a person standing on the Equator will see the sun passing directly overhead.
On the Northern Hemisphere's vernal equinox, or spring equinox, a person standing on the North Pole would see the sun skimming across the horizon, beginning six months of daylight. A person at the South Pole would also see the sun skim the horizon, but it would signal the start of six months of darkness.
As people in the Northern Hemisphere celebrate the arrival of spring this Saturday, MacRobert says it's worth taking a moment to ponder a quirky rule in the Gregorian calendar that keeps spring almost always arriving on March 20 or 21 but sometimes on the 19th.
The calendar that most of the world now observes was established by Pope Gregory XIII in 1582 to keep the equinox from slipping backward one full calendar day every 128 years and eventually rendering the celebration of Easter in the middle of winter.
"It begins with the fact that there is not an exact number of days in a year," said MacRobert.
Before the pope's intervention, the Romans and much of the European world marked time on the Julian calendar, instituted by Julius Caesar, which counted exactly 365.25 days per year, averaged over a four year cycle. Every four years there was a leap year to keep things on track.
It turns out, however, that there are 365.24219 days in an astronomical "tropical" year, which is defined as the time it takes the sun to make one complete circuit of the sky as seen from Earth.
Using the Julian calendar, the spring and fall equinoxes and the seasons were falling 11 minutes earlier each year. By 1500 the vernal equinox fell on March 11.
To fix the problem, the pope instituted a new calendar wherein century years (such as 1700, 1800, and 1900) are not leap years, but those divisible by 400, like 2000, are. Under the Gregorian calendar, the year is 365.2425 days long.
"That gets close enough to the true fraction [so] that the seasons don't drift," MacRobert said.
With an average duration of 365.2425 days, Gregorian years are now only 27 seconds longer than the length of the tropical year, an error which will allow the gain of one day over a period of about 3,200 years.
Nowadays, according to 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 corrected by a century where no leap year is celebrated.
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