The Double Planet

Perhaps no other object in the night sky is as compelling as the Earth’s Moon.

Full moon. Photo by Eric Fitzgerald

Our moon is bright: second only to the Sun. Our Moon is large: its disk’s diameter can be exactly the same size as the Sun’s. While the sun is much larger, it is also much further away. This lucky coincidence is what makes solar eclipses so dramatic here on Earth. At over a quarter the size of Earth, the Moon is so large in relation to the planet it orbits that we have sometimes been called “The Double Planet.”

The Moon is only a little younger than the Earth. Most likely our Moon had a violent birth when an object the size of Mars slammed into the Earth about four and a half billion years ago. This ancient, doomed proto-planet is sometimes called Theia, the Greek titaness that was the mother of Selene—goddess of the moon.

It was the Moon that first aroused my curiosity about telescopes and the idea of getting a closer look at our nearest neighbor in space. When I finally managed to point my small telescope at the Moon I was shocked to see the vivid craters and jagged mountains in such stark relief. Since the Moon is so bright and close, it is a good object for a modest telescope. The photo below was taken with a 400mm telephoto lens— that’s a magnification factor of 8x—or the power of an average pair of binoculars. More important than magnification power is a steady mount on a sturdy tripod.

Besides being unusually large for a moon and strangely the same apparent size as the Sun, the Moon has another unusual feature. As she orbits Earth only one side faces us. That means a lunar day is just about the same as her orbital period around Earth. This synchronous rotation is not a coincidence but rather the result of a phenomena called tidal locking. As the Earth and the Moon formed, the Moon developed a bulge and slowly became locked by gravity to the Earth. This is why we always see “The Man in the Moon” side regardless of her phase and why there is a so-called dark side that we never see from Earth. So much myth has been dreamed up by the Man in the Moon and the mysterious dark side.

So, let’s follow our Moon through her phases as seen here on Earth. As this issue of the Messenger Mountain News comes out, the Moon will be in a waxing gibbous phase. Gibbous is the phase between half illumination and the full Moon. Waxing means that each consecutive night the globe of the moon becomes fuller and more illuminated. The term waxing is evoked by candle-making whereby a candle gets bigger each time it is dipped in wax. Gibbous comes from words meaning hunched, humped, or humpbacked.

This coming Wednesday, October 24, the Moon will be full. This is the Hunter’s Moon for 2018.  A full moon always rises when the sun sets. If you think about it, this explains all the phases of the Moon. For the illumination of the Moon’s globe to be 100 percent, the light source (the Sun) must be directly behind the observer on Earth. That means the Sun must be 180 degrees from an observer between the Sun and Moon. Half-illuminations occur twice in the monthly orbit when the Moon is 90 degrees to the observer and the Sun. These half-illuminations are actually called quarters because they occur at the one quarter and at the three quarter points in the Moon’s monthly orbit. The first quarter Moon always crosses the midpoint in the sky (also called the meridian) at sunset and sets at midnight. The third quarter Moon always rises at midnight and crosses the meridian at dawn.

To look up into the sky and visualize where we are on a grand scale and how we all are hurtling through the vastness of the cosmos is a thrilling pursuit.

Okay, so now that we see how we are positioned in space, let’s take a closer look. It is perhaps counter-intuitive, but the full Moon is not the best time to train a telescope on the Moon. While the full Moon is an attention-getter for the naked eye, it is too bright and, more importantly, too flatly lit to reveal much detail. This dividing line between night and day on the Moon is called the terminator.

Tonight (October 19th), the Moon is in the waxing gibbous phase. The most dramatic area on the terminator is the Jura Mountains, Sinus Iridum, and Mare Imbrium. The large craters Tycho and Copernicus deeper into the lunar day also catch our eye. These two craters have extensive ray systems or scars radiating out from the craters caused by ejecta blasted out from the cataclysmic impact of the meteors that caused the craters. This is a sign of a relatively recent crater. Sinus Iridum is actually a very large ancient crater. The floor of this crater was flooded with lava long ago and its ray system eroded away over the eons by countless other meteors.

The Man in the Moon is caused by the light mountains and highlands of the Moon contrasted with the darker maria, or “seas” in Latin. The mare (singular of maria) were formed when low-lying areas of the Moon were flooded with lava long ago. Surface volcanism has long since subsided to deep within her core. As the Moon waxes to a thin crescent out of the new phase, the first mare that becomes visible is too flatly lit to reveal much detail. Like here on Earth, when the shadows get long at sunset and sunrise, is when most of the dramatic contrast takes place.


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