The moon’s done it again. Ambled its way along the ecliptic to another full moon phase. Appropriately enough, this month’s full moon is called the Hunter’s Moon. It rises in the northeast in Aries the Ram and stands high in the south around midnight. While you’re out this evening, I’ve prepared a list of 10 things to see or reflect upon while you and the moon are beaming at one another.
Earth's atmosphere takes away the moon's blues, leaving it orange. Photo: Bob King
1. Dramatic colors – Moonrise for Duluth is 5:35 p.m. or about 35 minutes before sunset. If you can get to location with an open view to the east, try to catch the moon just as it’s clearing the horizon. Depending on the state of the atmosphere, the moon will rise in technicolor red or orange. Longer wavelength red light penetrates air and dust, while blues and greens are scattered or absorbed. Once the moon is a fist or so above the densest layer of air, it assumes its usual white color. You’ll recall that white light is a mix of all the colors of the rainbow spectrum. In the thinner air high above the horizon, all the colors – except a bit of the blue – make it through to give us back a pale moon.
The moon is very out-of-round (right) when rising compared to when it's higher up. Atmospheric refraction is the cause. Credit: Jim Schaff
2. Distorted shape – The atmosphere also acts like a lens and bends or refracts the moon’s light. There’s no time better to see this than during a full moon rise, when the moon’s lower half is closer to the horizon than the upper. Thicker air at the horizon “pushes” the lower half of the moon upward into the less-heavily refracted top half, causing the moon to appear flattened or squished. If you’re ever lucky enough to see the moon directly on the horizon, you’re only seeing an optical illusion. Refraction there is strong enough to lift the moon into view even before it’s truly risen!
The dusky band beneath the rising moon is the Earth's shadow. Photo: Bob King
3. Earth’s shadow – The moon rises along with Earth’s shadow tonight. You won’t see the shadow at first, because the sun won’t have set yet, but beginning at sunset and continuing for the next 20 minutes, you’ll notice a thick, blue-gray band in the east beneath the moon.Â That’s our planet’s shadow. It extends all the way around the eastern horizon, and rises until it gradually merges with the darkening sky.
4. Eminently quotable – You’ll find many mentions of the moon, particularly the full moon, in literature, TV and other media. Here’s a sampling:
“Promises are like the full moon, if they are not kept at once, they diminish day by day.” – German proverb
“Moonlight is sculpture; sunlight is painting.” – Nathaniel Hawthorne
” Everyone is a moon, and has a dark side which he never shows to anybody.” – Mark Twain
Can you see the moon rabbit or the lady in the moon? Tonight's the night.
5. Make a face – You can see almost anything you want in the face of the moon. The dark spots, called lunar seas, contrast with the white regions known as the lunar highlands, creating patterns that give us everything from the familiar “man in the moon” to the lady of the moon, the alligator of the moon and anything else your imagination can conceive. As you making faces up there, consider that the lunar seas are impact holes that later filled with lava from below, while the white areas are ancient crust carpeted with craters from asteroid and meteorite bombardment in the early days of the moon’s formation.
Craters are labeled in white in this annotated photo of the full moon
6. Binocular exploration – Bring your binoculars tonight and see how many lunar seas and craters you can spot. The seas are obvious, while the four most prominent craters labeled at right require only a little extra effort.Â Huge systems of bright rays surround each crater and make them easy to spot. The rays originated when boulders sent aloft from the crater impacts rained back down, striking the moon and creating thousands of mini-craters, each of which excavated fresh soil from the moon’s surface.
In this full moon photo, you can see the terminator scraping along the top or north edge of the moon. Nice shadow detail shows up in the craters on the moon's north edge, because the terminator there is the day-night border where shadows are long. Credit: NASA
7. Terminator madness – At full moon, the sun, Earth and moon are lined up in that order, and the sun shines squarely on the moon’s face, lighting it up completely. During other phases, we see only part of the moon; the terminator, or day-night line, separates the lit part of the moon from the unlit. At full moon, the terminator essentially disappears because the entire face of the moon is in sunlight. Truth be told, it doesn’t disappear completely. The lineup that I mentioned earlier is exact only in the case of a total lunar eclipse. The moon’s orbit is slightly inclined with respect to Earth’s orbit, so it’s usually a little north or south of the sun-Earth line. Tonight it’s far enough north of that line that telescopic observers will see the terminator running along the south edge of the disk below the prominent crater Tycho. To the naked eye, the moon will appear perfectly circular with no nibbling about the edges. By tomorrow, the terminator will have swung around to its usual post-full moon position on the west side of the disk. To see the “southern terminator” to best effect, look between 7-10 p.m. Central time. The exact moment of full moon is 8:36 p.m. CDT tonight.
The moon's black disk covers the sun during a total solar eclipse.Â In the distant future, this will be a thing of the past. Credit: Luc Viatour
8. Slip slidin’ away -The difference in the moon’s gravitational pull on the nearside versus the farside of Earth causes the two tidal bulges. Coastal residents are familiar with as the ocean tides. Some of the energy from this moon-Earth interaction causes our planet’s spin to gradually slow down. The spin energy is not wasted, but transferred out to the moon, causing it to move 1 1/2 inches farther from our planet each year. Over several billion years, the Earth’s spin will slow to eventually match the orbital period of the moon. On that far future day, we’ll be locked facing one another so only the inhabitants of the moon-facing side of the Earth will see the moon. Those living on the Earth’s “farside” will have to travel to the other hemisphere for a look-see at the moon. Weird to contemplate, but it’s inevitable. One other consequence of this tug of war is that a more distant moon will also appear smaller, and a smaller moon won’t be able to completely eclipse the sun. The days of the total solar eclipse will be over.
The Earth's tilted axis is stabilized by the moon's gravitational influence.
9. Thank you for being our friend – One of the wonderful things the moon does for our planet is keep its axial tilt stable. Without the moon, the Earth’s axis would wobble to more extreme tilts like Mars’ axis does, causing catastrophic climate changes. The moon stabilizes our axis and keep our planet’s alignment from going to extremes.
10. Moonlight on linen – After an evening with the full moon, leave the lights off for a few minutes when you return to your home or apartment. If you have an east or south-facing window, you’ll be treated to the sight of moonlight spilling on the floor, the bed and curtains. Though the moon only reflects 7% of the sunlight striking it, it seems strangely brilliant and alien. When I see that I’ve unwittingly allowed the moon into the house, it never fails to stoke my sense of wonder at how close the universe is.