Observing Project 4B Joining an Astronomy Club

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Picking a good club provides several very important benefits to you as an amateur astronomer. To list just a few, first among them would be the companionship of those who enjoy our hobby and science as you do. Among those people you will find some surprising talents. In my own club, I have already benefited from the friendship of one amateur who is an electrician who helped repair my telescope when its internal inverter failed. He got me back up and running with a few passes of his soldering iron. Another helped locate us a reasonable dark sky observing site with clear horizons and dry ground to observe from. Others have lent their time to teach others and us about the areas of astronomy they are most passionate about. Others have allowed us to share in their innovations, their large telescopes and their hot beverage supplies.

Most important to remember is one simple fact that can make the club experience a joy or a frustration. Astronomy clubs come in two distinct types, those that stargaze and those that talk. A club that goes out and observes on a regular basis will encourage you to go out and do it yourself. The more you do it the better you will get at it! Clubs that do nothing but talk will eventually bore you to death. The thrill of the first night out will bring you back for seconds if you read the first four chapters and came out with reasonable expectations. But without new challenges, and someone to teach you how to accomplish them, you will quickly become bored. Find a club that goes out and observes, with members who own a wide variety of telescopes and even better, one that sponsors at least one annual weekend star party. Sky & Telescope maintains an excellent listing of clubs on its website. The database can be sorted by state and will allow you to easily find a club near you. Go to some meetings and find out what that club is about.

Now we have trained out eyes, purchased our equipment, learned to set up and care for it and demonstrated that it will perform as it should. Now it's time to take that equipment and begin to unravel the mysteries of deep space. But before pressing into the deepest parts of the cosmos, it would be perhaps better to sharpen our observing skills by starting a bit closer to home.

CHAPTER FIVE

- Mysteries of

the Moon

Figure 5.1. Ten-day old Moon. Photograph by author using 35-mm SLR and Celestron 8 at prime focus.

Figure 5.1. Ten-day old Moon. Photograph by author using 35-mm SLR and Celestron 8 at prime focus.

Professional astronomers hate it as much as romantic poets love it. It floods the sky with horrific quantities of unwanted light that washes out the grandeur of the deep sky for two weeks out of every month. It forever presents the same face to us, never allowing us to see its other hidden half. It is bland and it is boring and it never changes over billions of years, never mind in the span of a human lifetime.

"It" is our Moon.

The Moon is Earth's stalwart companion, loyally following it through space completing one circle around Earth each 27 days. It begins to appear in the west shortly after sunset each month, gradually growing fatter and brighter and higher in the sky each night until its full brilliance washes out the night sky of any FFTs (faint, fuzzy things) for days at a time. Then it begins to slim and fade as it transitions into the morning sky waning to half lit, then to a crescent and finally disappears in the east leaving behind a few precious nights of dark sky before starting the cycle all over again.

But the Moon is actually anything but a boring place and not quite so static as one might believe. The Moon is a geographical record, frozen in time, of the earliest days of the solar system and of what Earth might have been like. It will fill your eyepieces with amazing stories of meteorite impacts of astonishing power and lava flows that filled entire "seas." Parts of it have been battered to a point where there is no flat spot to be found anywhere and other parts of the lunar surface are as flat as prairie country. Different areas of the Moon have been exposed to very different environments. In this chapter, we will explore the various different elements of the lunar surface and use it as a practice ground for honing our observing skills under reasonably bright conditions. After all when the Moon is in the sky, no one will be spending much time looking into deep space, so it makes an excellent time to see the stories the Moon has to tell.

Before setting out to explore the mysteries of the Moon, we should first dispel some myths about the Moon, the first being the statement that the Moon is a moon. In fact it is not. The Moon is not in any way a true satellite of Earth. The Moon in fact fails the most basic test of whether or not an object is a satellite or a planet. As an object orbits a parent body, it "falls" towards that body and only that body. In a stable orbit, the falling motion is counterbalanced by forward speed. An object in a perfectly circular orbit moves forward at such a speed that the curve of the attracting body causes the surface to fall away from the satellite as fast as the satellite is falling, thus the object remains at a constant altitude as it circles its parent. When in orbit of a body, the orbiting moon falls only towards its parent planet. Jupiter's moons for example fall only towards Jupiter, never towards anything else. The Moon on the other hand, never falls only towards Earth, but rather falls towards the Sun. The Moon and Earth revolve together around a common center of gravity (called a barycenter) that resides approximately 1,600 kilometers below Earth's surface. Earth and the Moon revolve together around the barycenter once each 27.1 days and both fall together towards the Sun. So the Moon is not a satellite of Earth, but rather the two comprise a dual planetary system moving together around the Sun. Except for Pluto and Charon, there is no other pair of large bodies in the solar system so closely matched in size and mass as Earth and the Moon and none are so large. Both bodies have important effects upon the other.

The most important effect of the Moon's influence on Earth is ocean tides. The Moon's gravity pulls on Earth's oceans, causing them to rise when the Moon is overhead and recede when the Moon is at the horizon. Earth's gravity also exerts tidal forces on the Moon. Earth's pull is strong enough that it holds the Moon with sufficient force that the Moon is tidally locked to Earth. As a result of this, the Moon rotates around its axis in the same amount of time it takes to complete on revolution around the common center of gravity between the two bodies. This means that the Moon presents the same face towards Earth theoretically at all times. There are three minor variants in this motion that allow observers on Earth to see more of the Moon than would ordinarily be possible. The first is the fact that the Moon does not always orbit Earth at the same speed. When it is at its closest to Earth, it is at a distance of about 356,400 kilometers and travels faster than normal. When it is at its farthest from Earth, it is at a distance of over 406,400 kilometers and travels a bit slower than normal. The rotation rate of the Moon however is constant. So the slight changes in velocity of the Moon along its orbit allow observers on Earth to catch a glimpse slightly around each limb. As the Moon accelerates and decelerates in its orbit, it appears to nod back and forth, showing us alternately a peak around its western limb, then a peak around its eastern limb. The Moon's axis is also tilted slightly with respect to its orbit, about five degrees. The Moon's orbit is also inclined slightly with respect to Earth's equator. These two facts allow us to alternately peer over the Moon's north pole and under the Moon's south pole. All together, an Earth bound observer can actually see up to 59% of the Moon's surface over time. So the Moon does not always show those of us on Earth exactly the same face all the time. The combined effects of these motions and changing viewpoints are collectively called libration. Those areas of the Moon that we only get to see part time holds some rather interesting terrain, which we'll get a chance to explore later in this chapter.

Another common misconception about the Moon is that it's pretty much the same all over, a homogenous ball of rock. That assumption falls apart upon cursory examination of the Moon's near face. Though the Moon is not nearly as diverse as is Earth, it is not uniform either by any means. Bright highlands areas around the circumference of the lunar disk alternate with the dark lowlands called maria or "seas" dominating the areas in the center of the lunar disk. A bright rocky material that is primarily anorthosite dominates the highland areas. This is a brightly colored mineral, which is high in aluminum content. These areas of the Moon are far more heavily cratered than are the seas because these areas have not had their geological past erased by volcanism. The low-lying maria are primarily made of miles deep accumulation of basalt. This type of rock forms in layers when volcanic lava cools in pre-existing depressions of the lunar surface. Basalts are very dark in color. Under pressure, when it cools, it creates a black colored glassy material called obsidian. By a simple observation we can determine that the low-lying maria are much younger than the heavily cratered areas nearer to the lunar limbs. The Moon was obviously very heavily bombarded throughout its early history, leaving the polar areas and the eastern and western limbs obliterated with craters. The same bombardment had to occur in the maria areas, but subsequent volcanic activity wiped out the craters and replaced it with a smooth surface, which is not so heavily cratered. The current surface formed at a time when the planets had swept the solar system relatively clean of the vast amounts of dust and rock that so decimated not only the Moon's surface but also that of all the inner planets.

A far more interesting question about the Moon is how did those maria come to be? Only a relatively small area of the Moon, about 17% of its total surface area, is of this type. The Moon's far side is known to be almost completely free of the volcanic seas. Are the low depressions where the seas formed an artifact of the Moon's creation? Could they be a "birth mark" of some kind? That in turn calls into question just how the Moon itself came into being. There are many theories that seek to explain how the Moon came into existence and came to orbit Earth in such an orderly manner. Some believe that the Moon may have formed independently and been captured after drifting too close to Earth. Others believe that the Moon formed as a part of Earth that broke away. We can learn a great deal about both theories by studying the surface features of the Moon. And by looking at many of the same decisions that were faced by those who were planning the Surveyor and Apollo flights of the 1960s and 1970s, we can learn for ourselves how these theories of lunar creation gained strength and encountered resistance.

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