The long, cold, snow-laden winter of 2004-05 officially comes to its much-anticipated end at 7:33 a.m. Eastern Standard Time on Sunday, March 20th (12:33 p.m. Greenwich Mean Time that same day).
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 (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 23.5 degrees 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 by more direct solar rays, 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 north 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 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:
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.)
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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