I celebrated the transition from winter to spring by taking a walk with my dog in the rain. We stepped out shortly after midnight this morning, me with umbrella in hand and Sammy nosing the ground. At 12:14 a.m. CDT at the stroke of spring I strained to listen for the first frogs. None were heard. They don’t usually begin calling until mid-April, but with temperatures in the 70s the past few days and snow vaporizing faster than a comet’s nucleus, my expectations were high.
Spring is when day and night are nearly equal across the entire planet. That’s because Earth’s axis is oriented neither toward nor away from the sun. If the southern hemisphere is the planet’s feet and northern hemisphere its head, today we’re showing the sun our belly or profile if you like. In winter, the northern hemisphere is tipped away from the sun with short days and a low sun to pay for it. In summer, we’re tipped toward the sun with long days, a high sun and heat to spare.
Spring and fall are the ‘tween times when temperatures moderate and the sun rests for a brief moment between extremes. And don’t forget the bonus alignment: at the equinoxes the sun rises due east and sets due west. If you’ve never been sure of your directions, this week is the time to get reacquainted. Face the sunset and stick out your arms. Your right arm points due north, your left south and your back faces east. Couldn’t be easier.
Spring is that astronomical moment when the sun’s path crosses an imaginary circle in the sky called the celestial equator. The celestial version is an extension of the Earth’s equator into the sky. That’s why the sun is exactly overhead at the real equator. An observant equatorian might notice that flagpoles or power poles cast no shadows at noon, because the poles literally stand right on top of them.
From here on out, the sun continues moving northward in the sky, which for those living in the northern hemisphere, means the sun gets higher and higher and daylight hours longer and longer until maxing out on June 20, the solstice. Not so for those living south of the equator, where the seasons run in exactly the opposite direction. It’s the autumnal equinox down under. The sun’s headed lower in the sky, bringing with it shorter days and longer nights.
The sun’s apparent movement north or south in the sky is a result of the Earth’s axial tilt of 23.5 degrees, which in turn is amount the Sun moves north or south of the equator during a year.
If you’re trekking to the North Pole today, situated at latitude 90 degrees north, the sun will make its first appearance of the year on your horizon at local noon … and it won’t set for the next six months! It doesn’t matter what direction you look either since it’s up all night and day.
At 90 degrees north, the celestial equator rings the horizon. Not until the sun reaches this point – which happens on the first day of spring – does it finally return for observers at the pole. Conversely, today is the last day the sun is up for the next six months for an observer at the South Pole. I hope your spirits rise today like the sun in the new season. Happy equinox!
The new supernova in M95 in Leo, shining at magnitude 13.1, now has a name – 2012 aw. It doesn’t sound like much but if you add an “e” you’ve got an eyeful of “awe”.