Get Ready For The Spring Equinox Supermoon Mash-Up

Last month’s full moon supermoon rises over ice-covered Lake Superior in Duluth, Minn. Bob King

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.

Earth’s axis maintains a 23.5 tilt as it orbits the sun, but its changing position in orbit causes the axis to point toward, away and sideways to the sun during the year. On the first day of spring (March 20) it’s neither tipped toward nor away from the sun but evenly illuminated from the side, the reason day and night are equal across the planet. Sonoma University with additions by the author

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.

On the first day of spring the sun crosses the celestial equator, an imaginary extension of Earth’s equator onto the sky, moving north. As the sun moves north, it climbs higher and higher in the sky — with increasing daylight hours — until highest on the first day of summer. In the southern hemisphere, where the sun traverses the northern sky, March 20 marks the fall equinox (first day of fall). As the sun moves north, it gets lower in the sky en route to winter. Bob King

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.

This is what spring looks like in Duluth, Minn. — open water on the Lester River Sunday morning the 17th. Bob King

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.

Catkins in a poplar tree are silhouetted against the full moon. Bob King

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.

Clear skies!





10 Responses

  1. kevan hubbard

    Beautiful Moon tonight but watch it be cloudy tomorrow!20th at 2158 UTC ( or GMT as we use to call it but GMT seems to have joined Peking, Bombay, Madras,etc,in the history books presumably due to association with imperialism?). I must confess I am becoming more of a Moon person,I always found it a nuisance as it flushes out deep sky objects but tonight there it was casting a huge white line across the sea and the first bat I’ve seen of 2019 flew across it(in fact first time I’ve seen a bat on a beach ever!).

    1. astrobob

      Hi Kevan,
      I’ve turned into a moon guy over the years too. Just SO much to see. Happy to hear about that bat. They’re struggling here in the U.S. due to a fungus.

  2. caralex

    Yet Easter, which by normal ecclesiastical calculating, should fall next Sunday, is postponed for a month, as the church, (which only recognizes an equinox that falls on the 21st), hasn’t caught up with the 400 year leap day cycle, which sees the equinox fall on the 20th rather than the 21st right now. Do you know, Bob, if an adjustment leap day is anywhere on the horizon?

        1. astrobob

          Nothing out of the ordinary, no. The leap year schedule is a little bizarre, but it keeps the equinox and solstices dates from slipping. We don’t normally think about it, but leap year dates are more complex than a leap day every four years. I wrote about this in a blog a while back. Here’s the Wiki summary: Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.

          1. astrobob

            The reason the date bounces around from March 20 to the 22nd is because a year is not a whole number. It’s 365 1/4 days, or six hours longer. So one year, the date is March 20 at 8 p.m. Central Time, the next year, it’s about 6 hours later or 2 a.m. March 21. Then the next year, it’s 6 hours later again. Finally, one the leap year, we add a day and the equinox shifts back 18 hours (24 minus 6). When you add in the date changeover due to varying time zones (in Australia this year, the equinox happened on the 21st), then you get a small spread of equinox dates.

  3. kevan hubbard

    It wasn’t cloudy and I had an amazing view of the equniox Moon and it’s reflection across the sea.yes I have read about the fungal nose in the bats of the USA hopefully they’ll develop some kind of resistance to it?those bats down Austin, Texas,way look incredible,loads of them fly out at sunset. I think that they live under bridges? I have read that equinoxs and solstices can fall on the 20th/21st/22nd but never any other dates using our Solar calendar. I can’t remember any falling on a 22nd but some must have an aside tonights sunset reminded me of sunsets on Mars.obviously I have never been to Mars and I doubt that I ever shall!but I’ve seen pictures of Martian sunsets.thin clouds here on Earth acted as Martian dust they only thing to break up the illusion was the size of the Sun and the fact I could breathe!

Comments are closed.