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On Spring Equinox, Day and Night Equal in Name Only

John Roach
for National Geographic News
Updated March 20, 2006
 
In the Northern Hemisphere spring officially begins today, the vernal equinox, at 1:26 p.m. ET. Soon trees will bud, snows will melt, and for the next six months daylight will tick more minutes off the clock than darkness.

The reverse is true in the Southern Hemisphere. But whether you are entering the season of light or darkness, don't be fooled into thinking that on the equinox the length of the day is exactly equal to the length of the night. It's not.

The day of light and dark equality always happens before the spring and after the fall equinoxes, according to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C.

"Exactly when it happens depends on where you are located on the surface of the Earth," he said.

By the time the center of the sun passes over Earth's Equator—the official definition of equinox—the day will be slightly longer than the night everywhere on Earth. The difference is a matter of geometry, atmosphere, and language.

Geometry, Atmosphere, Language

Chester explains that if the sun was just a tiny point of light and the Earth had no atmosphere, then the day and night of the equinox would each be exactly 12 hours long.

But, to begin with, the sun is bigger than a point—it appears nearly as large as a little fingertip held at arm's length, or half a degree wide, as seen from Earth.

As such, sunrise is defined as the moment the top edge of the sun appears to peek over the horizon, and sunset is when the very last bit of the sun appears to dip below the horizon. The equinox, however, is when the center of the sun crosses the Equator.

Additionally, the Earth has an atmosphere that bends the light cast by the sun when that light is close to the horizon. The golden orb appears a little higher in the sky than it really is.

As a result, the sun appears to be above the horizon a few minutes longer than it really is. Therefore, on the equinox, the daylight hours are actually longer than the length of time between when the sun crosses the horizon at dawn and when the sun crosses the horizon at sunset.

"Those factors all combine to make the day of the equinox not the day when we have 12 hours of light and darkness," Chester said.

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.

Marking Time

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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