Thursday, October 30, 2008

Tidal Forces and Moon

Newton proposed that every object with mass exerts a gravitational pull or force on every other object with mass in the universe. Well, the earth is much more (80more) massive than the moon, which is why the moon orbits us, and not we it. (If you want to get technical, we both actually orbit an imaginary point called the center of mass.) However, the moon is sufficiently massive to make the effects of its gravitational field felt on the earth.
Anyone who lives near the ocean is familiar with tides. Coastal areas experience 2 high and 2 low tides within any 24-hour period. The difference between high and low tides is variable, but, out in the open ocean, the difference is somewhat more than 3 feet. If you’ve ever lifted a large bucket of water, you know how heavy water is. Imagine the forces required to raise the level of an entire ocean 3 or more feet! What force can accomplish this?
The tidal force of gravity exerted by the moon on the earth and its oceans. The moon and the earth mutually pull on each other; the earth’s gravity keeping the moon in its orbit, the moon’s gravity causing a small deformity in the earth’s shape.
This deformity results because the moon does not pull equally on all parts of the earth. It exerts more force on parts of the earth that are closer, and less force on parts of the earth that are farther away. Just as Newton told us: Gravitational forces depend on distance. These differential or tidal forces are the cause of the earth’s slightly distorted shape—it’s ovoid rather than a perfect sphere— and they also make the oceans flow to two locations on the earth: directly below the moon, and on the opposite side. This flow causes the oceans to be deeper at these two locations, which are known as the tidal bulges. The entire Earth is pulled by the moon into a somewhat elongated—football—shape, but the oceans, being less rigid than the earth, undergo the greatest degree of deformity.
Interestingly, the side of the earth farthest from the moon at any given time also exhibits a tidal bulge. This is because the Earth experiences a stronger gravitational pull than the ocean on top of it, and the Earth is “pulled away” from the ocean on that side. As the Earth rotates beneath the slower-moving moon, the forces exerted on the water cause high and low tides to move across the face of the earth.
The tides of largest range are the spring tides, which occur at new moon, when the moon and the sun are in the same direction, and at full moon, when they are in opposite directions. The tides of smallest range are the neap tides, which occur when the sun and the moon are at 90 degrees to one another in the sky. Tides affect us every day, of course, especially if you happen to be a sailor or a fisherman. But even if you live high and dry in Kansas or Nebraska, say, tides (and the moon) still affect you. Every day, the earth is spinning a little slower on its axis because of the moon.

Impact theory of moon

The favored theory today combines elements of the daughter theory and the capture theory in something called an impact theory. Most astronomers now believe that a very large object, roughly the size of Mars, collided with the earth when it was still molten and forming. Assuming the impact was a glancing one, it is suggested that shrapnel from the earth and the remnant of the other planetesimal (a planet in an early stage of formation) were ejected and then slowly coalesced into a stable orbit that formed the moon.
This model is also popular because it can explain some unusual aspects of the earth (the “tip” of its rotational axis, perhaps) and the moon. In the impact model, it is further theorized that most of the iron core of the Mars-sized object would have been left behind on the earth, eventually to become part of the earth’s core, while the material that would coalesce into the moon acquired little of this metallic core. This model can explain why the earth and moon share similar mantles (outer layers), but apparently differ in core composition.

Captive theory of moon

A third theory suggests that the moon was formed independently and far from the earth, but was later captured by the earth’s gravitational pull when it came too close.
This theory can account for the differences in composition between the earth and the moon, but it does not explain how the earth could have gravitationally captured such a large moon. Indeed, attempts to model this scenario with computer simulations have failed. Moreover, while the theory accounts for some of the chemical differences between the earth and moon, it does not explain the many chemical similarities that also exist.

A sister theory of moon

Another theory holds that the moon formed separately near the earth from the same material that formed the earth. In effect, the earth and the moon formed as a double-planet system.
This theory seemed quite plausible until lunar rock samples were recovered, revealing that the moon differs from Earth not only in density, but in composition. If the two bodies had formed out of essentially the same stuff, why would their compositions be so different?

Daughter theory of moon

The oldest of the four theories speculates that the moon was originally part of the earth, and was somehow spun off a rapidly spinning, partially molten, newly forming planet.
Once prevalent, this theory (sometimes referred to as the fission theory) has largely been rejected, because it does not explain how the proto-Earth could have been spinning with sufficient velocity to eject the material that became the moon. Moreover, it is highly unlikely that such an ejection would have put the moon into a stable Earth orbit.

Friday, October 17, 2008

Understanding Moon Phases

Take the time to observe the moon through all of its phases. When the moon is about three or four days “old,” Mare Crisium and other vivid features—including the prominent craters Burckhardt and Geminus—become dramatically visible, assuming it’s a clear night. You can also begin to see Mare Tranquilitatis, the Sea of Tranquility.
At day seven, the moon is at its first quarter. At this time, mountains and craters are most dramatically visible. Indeed, this is the optimum night for looking at lunar features in their most deeply shadowed relief.
As the moon enters its waxing gibbous beyond first quarter phase, its full, bright light is cheerful, but so bright that it actually becomes more difficult to make out sharp details on the lunar surface. An inexpensive “moon filter” or variable polarizing filter fitted to your telescope can help increase contrast on the bright lunar surface. As the moon verges on full, we do get great views of the eastern maria, the lunar plains. Past day 14, the moon begins to wane as the sunset terminator moves slowly across the lunar landscape. At about day 22, the Apennine Mountains are clearly visible. It was these mountains that Galileo studied most intensely, attempting to judge their height by the shadows they cast.
During the late waning phase of the moon, moonrise comes later and later at night, as the moon gradually catches up with the sun in the sky. By the time the moon passes day 26, it is nothing but a thin crescent of light present in the predawn sky. The new moon follows, and as the moon overtakes the sun, the crescent reappears (on the other side of the moon at sunset), and it begins to wax again. Here are some cold, hard facts about a cold, hard place. The moon is Earth’s only natural satellite, and in fact a very large satellite for a planet as small as the earth. The planet Mercury is only slightly larger than the moon. The mean distance between the earth and moon, as it orbits our planet from west to east, is 239,900 miles (386,239 km). The moon is less than one-third the size of the earth, with a diameter of about 2,160 miles (3,476 km) at its equator. Moreover, it is much less massive and less dense than the earth—1⁄80 as massive, with a density of 3.34 g/cm3, in contrast to 5.52 g/cm3 for the earth. If the earth were the size of your head, the orbiting moon would be a tennis ball 30 feet away.
Because the moon is so much less massive than the earth, and about a third as big, its surface gravity is about one-sixth that of our planet. That’s why the Apollo astronauts could skip and jump like they did, even wearing those heavy space suits. If you weigh 160 pounds on the earth’s surface, you would weigh only 27 pounds on the moon. This apparent change would give you the feeling of having great strength, since your body’s muscles are accustomed to lifting and carrying six times the load that burdens them on the moon. Of course, your mass—how much matter is in you—does not change. If your mass is 60 kilograms (kg) on the earth, it will still be 60 kg on the moon. the moon is in a synchronous orbit around the earth; that is, it rotates once on its axis every 27.3 days, which is the same time that it takes to complete one orbit around the earth. Thus synchronized, we see only one side of the moon (except for the tantalizing peek at the far side that libration affords).

What You Can See On The Moon?

Even if you don’t have a telescope, there are some very interesting lunar observations you can make. Have you ever thought that the moon looks bigger when it’s closer to the horizon? It’s just an optical illusion, but you can test it out. The angular size of the moon is surprisingly small. A circular piece of paper just about 0.2 inches in diameter held at arms’ length should cover the moon. At the next full moon, cut out a little disk of that size and prove to yourself that the moon stays the same size as it rises high into the sky.
The telescope through which Galileo Galilei made his remarkable lunar observations was a brand-new and very rare instrument in 1609; but you can easily surpass the quality of his observations with even a modest amateur instrument.
Why is it so exciting to point your telescope at the moon?
Because no other celestial object is so close to us. Being so close, the moon provides the most detailed images of an extraterrestrial geography that you will ever see through your own telescope.
When should you look at the moon?
The easy answer is: anytime the sky is reasonably clear. But if you’re thinking that you should always wait until the moon is full, think again. When is the best time to view a rugged Earthly landscape at its most dramatic? When the sun is low, early in the morning or late in the afternoon, and the light rakes across the earth, so that shadows are cast long and all stands in bold relief.
The same holds true for the moon. When you can see the sunrise or sunset line (the terminator), and the moon is not so full as to be blindingly bright, that is when the topography of the moon will leap out at you most vividly. This characteristic means that you’ll get some very satisfying viewing when the moon is at one of its crescent phases, and probably not at its full phase.

What Galileo Saw on the moon?

It is possible to observe many features of the moon without a telescope. One of the first things you should try is to track its daily motion against the background stars. Since the moon travels around the earth (360 degrees) in
27.3 days, it will travel through about 13 degrees in 24 hours, or about half a degree (its diameter) every hour.
Galileo was the first person to look at the moon through a telescope; indeed, its mottled gray face was one of the first celestial objects on which he trained his new instrument in 1609.
What he saw conflicted with existing theories that the surface was glassy smooth; it was instead rough and mountainous. He closely studied the terminator, or the boundary separating day and night, and noted the shining tops of mountains. Using simple geometry, he calculated the height of some of the mountains based on the angle of the sun and the estimated length of shadows cast. Galileo overestimated the height of the lunar mountains he observed; but he did conclude rightly that their altitudes were comparable to Earthly peaks.
Noticing mountains and craters on the moon was important, because it helped Galileo conclude that the moon was fundamentally not all that different from the earth. It had mountains, valleys, and it even had what were called seas—in Latin, maria though there is no indication that Galileo or anyone else maintained after telescopic bservations that the maria were water-filled oceans. Conten-ding that the moon resembled the earth in 1609 was not a small thing. This statement implied that there was nothing supernatural or special about the moon or perhaps the planets and the stars, either. Followed to its conclusion, the observation implied that there was perhaps nothing divine or extraordinary about the earth itself. The earth was a body in space, like the moon and the other planets.