Driftless Dark Skies: Vernal Equinox

March 12th, 2019 by John Heasley

Spring returns to the Driftless (and all over the northern hemisphere) at 4:58 pm on March 20. We’ve now traveled a quarter of the way around our sun since the winter solstice on December 20. Earth’s axis is pointed neither towards nor away from the sun as it is on the solstices but is balanced wonderfully between the two extremes. On the equinox, we have roughly equal day and night with twelve hours of each. That’s three more hours of daylight than we had back on the winter solstice. And the amount of daylight is quickening. Every day we have about three more minutes of it–almost 80 more minutes of daylight from the start of the month to the end. The amount of daylight continues to increase until it reaches fifteen hours on the summer solstice on June 21.

The path of the sun across the sky has also shifted. The sun is not rising in the southeast or setting in the southwest as it did in December or rising in the northeast and setting in the northwest as it will in June. Watch through the month as the places of sunrise and sunset move steadily northward. On the vernal equinox, the sun rises exactly in the east and sets exactly in the west. And the sun tracks higher in the southern sky. At solar noon (around 1pm after we move our clocks ahead one hour for daylight saving time on March 10), the sun is 47° above the horizon. That’s 23 ½ ° (the amount of axial tilt) higher than winter solstice when the sun only got 23° above the horizon but 23 ½ ° lower than summer solstice when the sun will be 70° above the horizon.

You can imagine yourself experiencing the vernal equinox all over our planet. Subtract your latitude from 90° to get the maximum altitude of the sun. If you stood at the north or south pole (90°) on this day, you would be amazed to see the sun neither rising nor setting but skimming along the horizon (0°)  in a full circle all day long. Our fellow travelers along the equator (0°) in Ecuador, Kenya, Indonesia, and Kiribati will see the sun directly overhead (90°) at solar noon.

This year’s vernal equinox is all the more special because it coincides with a full moon. This the closest coincidence of the March equinox and full moon since March 20, 2000. This one is called the Full Sap or Full Worm Moon.  I like how the name celebrates a seemingly dead world returning to life as the sap starts flowing in the trees as the worms return to composting the soil. On the vernal equinox, the full moon is directly opposite the sun and follows the same path across the sky twelve hours later.  That morning, you’ll see the moon setting in the west at 7:13 as the sun is rising in the east at 7:05. That evening, the moon rises in the east at 6:57 just as the sun sets in the west at 7:12.  Experience the syzygistic alignment for yourself as you watch your long shadow from the rising and setting sun point the way to the rising and setting moon! The moon will be 100% full at 8:44pm. Moon and Earth are at their closest, so this full moon might appear a little bigger and brighter. This day is even weirder at the poles.  At the north pole, the moon neither rises nor sets but stays opposite the sun as it chases it around the horizon all day long.  A most amazing sight!  For the  folks at Amundsen–Scott South Pole Station, the full moon never rises above the horizon. Wherever you are on March 20th, I hope you are awed by the vernal equinox.

John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. He volunteers with NASA/JPL as a Solar System Ambassador. For more information about stargazing in southwest WI, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies. Driftless Dark Skies appears monthly in the Voice of the River Valley.

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