Hooray! Springtime has finally arrived — at least in Earth's Northern Hemisphere.

This year's official onset of spring occurs on March 19 at 9:30 p.m. PDT (12:30 a.m. EDT on March 20). Astronomers are fond of saying that the first day of spring is when the sun rises due east and sets due west and the length of our day and night are equal. As long as we don't concern ourselves with the fine details of our sun's position, those are both true statements.

So why, then, do we often cite the time for the beginning of spring?

That's because the beginning of spring — the vernal equinox — is not a day, but a moment in time. It's marked by the sun's passage from the Southern Hemisphere sky into the Northern Hemisphere sky. In fact, it's the exact moment when the sun crosses the celestial equator on its journey from south to north.

Confused? Well, try to picture what's going on in the heavens.

During wintertime, we in the Northern Hemisphere see the sun cross our daytime sky low in the south; during our summertime it crosses high overhead. It's all caused by the fact that the Earth's axis is tilted. As we orbit the sun, our wintertime sun shines more directly onto the Southern Hemisphere, while our summertime sun shines more directly onto the Northern Hemisphere.

At some point during the year, the sun has to cross the equator on its way north, and that occurs at the moment known as the vernal equinox. On that day the sun is positioned directly over Earth's equator.


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Try to imagine standing on the Earth's equator on the first day of spring. A few days or weeks earlier, the sun would have appeared in the southern sky, and a few days or weeks later it will appear in the northern sky. But on that day, you would see the sun pass directly overhead above the equator, and you'd discover that you have no shadow!

Another way of thinking about it is to imagine projecting the Earth's equator into the starry heavens around it. This would create a great circle in the sky that astronomers call the celestial equator. As follows, the moment when the sun crosses the celestial equator on its way north marks the vernal equinox.

The night sky also exhibits changes because of the season change. At this time of year, we begin to turn away from such constellations as Orion and Taurus — the so-called Northern Hemisphere winter stars — and begin to turn toward groupings like Ursa Major, the great bear, the Big Dipper (which makes up most of Ursa Major), Leo, the lion and Bootes, the herdsman. We know these as springtime stars, and within a month or so they will completely replace the winter stars.

South of the Earth's equator, of course, all is reversed at the start of spring: Southern stargazers will be watching their summer stars disappear in the west, to be replaced by autumn constellations in their east.

Visit Dennis Mammana at www.dennismammana.com.