Spring feels good. I like the sound of it. I wrung as much cold-night observing, shoveling and cross-country skiing out of winter as I could. There were days I thought my hands would never warm up. So now I’m ready for a change, and you probably are too. Good thing spring’s on the horizon. That’s not all. Only hours after the season begins on Wednesday March 20 at 4:58 p.m. Central Time, the Full Worm Moon supermoon rises in the eastern sky. A beautiful coincidence and all the more reason to celebrate the equinox.
You’ve already noticed that the sun is a lot higher than it was back in December. It’s been creeping north ever since the winter solstice, when it reached its lowest point in the sky. At the equinoxes, the sun arrives at the midpoint between the southern and northern extremes of its path across the sky. Wednesday at 4:58 p.m. the center of the sun crosses the celestial equator.
Picture the Earth’s equator and then expand it way, way up into space above the planet like an imaginary hula-hoop and it becomes the celestial equivalent. If you live somewhere along the equator, the celestial equator begins at the eastern point of the horizon, passes directly overhead and drops down to touch the western horizon point. The other half of the equatorial circle lies below your horizon. If you’re north of the equator, say if you’re in Chicago, the celestial equator cuts a half-circle midway up the southern sky. If you live south of the equator, it’s in the northern sky.
The first moment of spring marks a transition, when the sun arrives at the midpoint between its lowest point in the sky in December and its highest in June. It crosses the celestial equator, an imaginary circle in the sky, headed north. For northern hemisphere folk, the sun continues to climb upward in the sky toward its summertime heights. Observers in the southern hemisphere watch the sun move north and drop lower in the sky toward its winter low.
At the spring equinox, sunlight shines equally on both hemispheres on the spring equinox, so day and night across the planet are very close to equal — 12 hours each. “Equinox” is a mashup from the Latin words for equal + night. And since the sun lies for a short time directly on the celestial equator, it rises due east and sets due west, making it an excellent natural compass. Face the setting sun, and stick out your arms. Your left arm will point due south, and your right arm due north.
Shadows shorten as the sun gets higher in the sky, so the shortest possible shadows happen when the sun is directly overhead. That happens at local noon Wednesday for anyone living on the equator. There, your shadow will follow in your footsteps like a permanent puddle of shade you can’t run away from. Here’s a photo so you can see how weird it looks. The picture was taken on the fall equinox, the second time each year the sun crosses the celestial equator but moving south this time.
And we’ve got that big full moon. Because it’s relatively closer than most of the other full moons in 2019, the Worm Moon will be the year’s third and last supermoon. Supermoons appear a little larger and brighter than a normal moon. If you’re blessed with a good forecast, try to catch the moon at or near moonrise when you can see it at its most colorful and distorted (due to refraction by the atmosphere). Click here to find when that happens for your city. If you’re faced with dreary overcast, you can still watch the vernal moon rise live — over the ancient city of Rome no less — at Gianluca Masi’s Virtual Telescope site. Streaming starts at 11:45 a.m. CST.
A full moon rises around sunset because it then sits directly opposite the sun with the Earth in between. We look west to see the sun set then turn a full 180 and face east to watch the moon rise. It’s an alignment that never fails to please.