Summer is out of business in the night sky

Published 9:25 am Thursday, September 17, 2015

As hard as it is to say goodbye to summer, the time has come. Autumn is great with longer nights for stargazing and the leaves put on their great show … and then there’s winter. Enough about that for now. It’s time for autumn of 2015 to commence at 1:20 a.m. this coming Wednesday, the exact moment of the Autumnal Equinox.

Astronomically the Autumnal Equinox is an annual rite of passage as the Sun crosses the celestial equator and starts moving farther south and lower in our sky. All this happens because Earth revolves around the sun with its north to south pole axis tilted by 23.5 degrees to its orbit around our home star. As a result the Sun’s path among the backdrop of stars, known as the ecliptic, is inclined to the celestial equator by that same 23.5 degrees. The celestial equator is just a projection in our sky of the Earth’s terrestrial equator and occupies the same mathematical plane. Because of where we live the celestial equator runs from the eastern to western horizon, and its highest point is due south just less than halfway from the horizon to the overhead zenith.

At 1:20 a.m. Wednesday morning, the sun in the sky will be smack dab on the celestial equator in our sky. That makes sense when you think that on the day of the Autumnal Equinox in September, and the Vernal Equinox in March, the sun is shining directly over Earth’s equator, and at noon anywhere along the equator the sun will be directly overhead. The entire surface of the Earth experiences equal amounts of sunlight and radiation.

On Winter Solstice day in late December the sun’s most direct rays shine over the Earth’s southern hemisphere and the sun is low in our sky at its maximum separation south of the celestial equator. That makes our nights much longer than our days. On Summer Solstice day in late June the sun’s most direct rays are shining over the Earth’s northern hemisphere and the sun is very high in our sky at its maximum separation north of the celestial equator. That makes our days much longer than nights.

Almost without fail, on either the autumnal or vernal equinox, you’ll see someone on a TV news show attempt to balance an egg on its side. When they finally stand it on its end they say, “Look, this really works. It’s because of the equinox day since the sun is shining right over the equator.” The crux of the myth is that the sun will not pull down the egg. This makes absolutely no sense. The truth of the matter is that if you try to balance an egg any other time of the year you would be just as successful. You can pull off that egg trick anytime with enough patience.

You have no doubt also been told that on the days of the both Vernal and Autumnal Equinox everyone worldwide has equal amounts of day and night. Guess what? That’s yet another myth. Certainly it’s true that both hemispheres receive equal sunlight, but the days are actually still longer than the nights. Just check the almanac data for Wednesday morning and you’ll see that we still have 12 hours and 10 minutes of daylight. Equal day and nights won’t be achieved until next weekend.

If we didn’t have an atmosphere night and day would be equal this Wednesday, but fortunately we do, as it makes life possible as we know it. The sun’s incoming light from 93 million miles away is bent by the shell of our atmosphere, something called astronomical refraction. The thicker the atmosphere, the more the sun’s light is bent. When the sun is rising or setting any time of the year, its light has to cut through a lot more of the atmospheric shell than it does when it’s overhead. The bending of the sunlight is so extreme at the horizon that the sun’s disk will appear above the horizon when it’s actually still below it, giving us extra daylight.

Next Sunday night we have an extra fantastic celestial happening. It’s a triple-header. We have a full harvest moon, a super moon, and a total lunar eclipse over Everett. Start saying your prayers for clear skies right now. I’ll have more next week in Starwatch.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications available at bookstores at www.adventurepublications.net