The full moon is rising high

Even though our next official full moon is this coming this Thursday we more or less have a full moon every night this week in the Marshall sky, putting the kibosh on any serious stargazing or astrophotography. Even if you don’t have to put up with light pollution the moon kicks out so much reflected secondhand sunshine that all but the brighter stars get visually done in. The effect is amplified this time of the year because the full moon takes a much higher arc across the heavens from sunset to sunrise. The lunar whitewashing is also intensified if there’s fresh snow cover where you are.

The reason the full moon rides so high across the night sky this time of year is twofold. It has to do with the Earth’s tilted axis and the fact that all full moons lie almost directly opposite the sun in the celestial dome.

Because Earth’s axis is tilted 23.5 degrees with respect to its orbit around the sun the northern hemisphere is tilted away from the sun’s strongest rays this time of year, making it colder around here. This also causes the sun to take a much lower arc across the sky from sunrise to sunset. The sun hits the lowest point in its track on the winter solstice, or, if you will, the first day of winter. This year the winter solstice is on Dec. 22.

Meanwhile, because of the fact that any full moon is approximately 180 degrees from the sun or opposite of the sun in our celestial sphere, the full moon in the winter takes approximately the same high path across the sky as the sun does in the summer. We don’t get “moonburn” but we can sure get moonstruck!

Looking at the full moon through a telescope or binoculars is not as easy as you would think it would be. It’s just so bright. You can buy and use lunar filters that will help out, but even with those, it’s hard to see a lot of details in the lunar features with the sunlight blasting it. Nonetheless, one of the features that stands out on a full moon is the crater Tycho, found toward the lower limb of the moon as you can see in the picture. Tycho was named after the famous 16th-century Danish astronomer, Tycho Brahe. The crater Tycho is a little over 50 miles in diameter and nearly three miles deep. By comparison, the Grand Canyon is just over a mile deep. The Tycho crater was created when a large asteroid blasted into the lower limb of the moon a little over 100 million years ago. What really makes Tycho jump out at you is that even with the naked eye you can see the large system of rays that emerge in all directions from the crater. When the asteroid that created Tycho impacted the moon, a huge amount of material was thrown out in all directions, creating the rays we see today. Since Tycho is a relatively new crater on the moon, the ray system hasn’t had much of a chance to be eroded by a subsequent meteor or asteroid blasts. A similar ray system can be found around another young, large crater called Copernicus on the left-hand side of the full moon.

It’s a really good thing that we have the moon up there in the heavens because it provides a wonderful stabilizing effect on the Earth. It keeps our Earth’s axis from wobbling too much. As it is the Earth’s axis wobbles modestly in a 26,000-year cycle. However, if our moon wasn’t around with its stabilizing gravitational force, it’s thought that the gravitational force of Jupiter, as well as other planets, would cause the Earth’s axis to wobble more chaotically and quickly. That wobbling would really throw Earth’s seasons and our climate out of whack. Maybe without the moon, we wouldn’t be here! The moon is our friend!

The full moon this month is no friend to the annual Geminid meteor shower. The Geminids are one of the best meteor showers, but not so much this year. That’s because during the peak of the Geminids on the night of Dec. 13-14 the moon will still be pretty full. The moon’s light bombing will visually wipe out all but the brightest meteors or “shooting stars.”Without all the extra moonlight it would be possible to see over a hundred meteors an hour in the countryside and even quite a few in urban skies. The Geminids are produced by debris left in the wake of a passing asteroid known as 3200 Phaethon.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul.


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