March 12, 2003
Alan MacRobert, Senior Editor
617-864-7360 x151, amacrobert@SkyandTelescope.com
|Note to Editors/Producers: This release is accompanied by a high-quality animation and illustration; see details below.|
The long, cold, snow-laden winter of 2002-03 officially comes to its much-deserved end at exactly 8:00 p.m. Eastern Standard Time on Thursday, March 20th (1 a.m. Greenwich Mean Time on Friday, March 21st). At least that's what astronomers say regardless of whether anything springlike is happening at that moment.
Why is spring said to begin at such a precise time, regardless of day or night, snow or warmth?
Because that's the moment of the March equinox.
The seasons' official starting times are determined by the Earth's motion around the Sun — or equivalently, from our point of view, the Sun's annual motion in Earth's sky. The start of spring (for the Northern Hemisphere) is defined as the moment in March when the Sun passes over Earth's equator heading north — an event called the vernal equinox. This moment can come at any time of day or night.
The Sun appears to move north and south in our sky during the year because Earth's axis is tilted with respect to our orbit around the Sun (see the diagram below). For skywatchers at mid-northern latitudes, the effect is to make the Sun appear highest in the sky in June. At that time the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. Six months later, when we're on the opposite side of our orbit, the Northern Hemisphere is tipped away from the Sun; the solar rays come slanting in to our part of the world and heat the air and ground less, making winter.
An equinox happens when the Sun is halfway through its journey from one of these solstices to the other. Several other noteworthy things happen on the equinox date:
- Day and night are approximately the same length; the word "equinox" comes from the Latin for "equal night." (A look in your almanac will reveal that day and night are not exactly 12 hours long at the equinox, for two reasons. First, sunrise and sunset are defined as when the Sun's upper edge — not its center — crosses the horizon. Second, whenever the Sun is very near the horizon, refraction by Earth's atmosphere shifts its position upward slightly.)
- The Sun rises due east and sets due west, everywhere on Earth. The spring and fall equinoxes are the only times of the year when this happens.
- If you were standing on the equator, the Sun would pass exactly overhead in the middle of the day. If you were at the North Pole, the Sun would be skimming the horizon just beginning the six-month polar day.
- In the Southern Hemisphere, the March equinox marks the start of autumn, and the September equinox marks the start of spring. Summer for kangaroos begins in December, winter in June.
- Eggs do not balance on end more easily at the equinox than at other times! Actual tests have demolished this bit of New Age goofiness; the ability of eggs to balance depends on tiny irregularities on their shells (and the persistence of the would-be balancer!), not on what day it is. "This perennial silly-season story has nothing to do with how eggs balance," says Sky & Telescope senior editor Alan MacRobert, "and everything to do with how some media can't say no to a wacky story even if it's wrong."
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