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The long, cold, snowy winter of 2006-07 officially comes to its much-anticipated end at 8:07 p.m. Eastern Daylight Time on Tuesday, March 20th (or 12:07 a.m. Greenwich Mean Time on the 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 at that moment, the Sun passes over Earth's equator heading north, an event called the vernal (or spring) equinox.
The Sun appears to roam north or south in our sky, depending on the time of year, because of what some might consider an awkward misalignment of our planet. Earth's axis is tilted about 23½° with respect to our orbit around the Sun. So when we're on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated more directly by sunlight, making summer. Six months later, when we're on the opposite side, the Northern Hemisphere is tipped away from the Sun, the slanting solar rays heat the ground less, and we get winter.
For a skywatcher at northern temperate latitudes, such as in the continental United States, the effect is to make the Sun appear to creep higher in the sky each day from late December to late June, and then slide back down again from late June to late December. An equinox comes when the Sun is halfway through each journey.
This celestial arrangement makes several other noteworthy things happen on the equinox date:
• Day and night are almost exactly 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 top edge — not its center — crosses the horizon. Second, Earth's atmosphere distorts the Sun's apparent position slightly when the Sun is very low. Have these facts on hand when you get the inevitable calls at the equinox from people saying your sunrise and sunset times must be wrong because they are not 12 hours apart.)
• The Sun rises due east and sets due west (as seen from any location on Earth). The fall and spring equinoxes are the only times of the year when this happens.
• If you were standing on the equator, the Sun would pass exactly overhead at midday. If you were at the North Pole, the Sun would skim completely around the horizon as the six-month-long polar day begins.
• In the Southern Hemisphere, March's equinox marks the start of autumn, and the September equinox marks the start of spring. (Summer for kangaroos begins in December, their winter in June). Not even the most diligent jet setter could manage to live in an endless summer, but by traveling between hemispheres at just the right time, you could live in an endless spring and summer.
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