3/3/2012
LHSD
MOON?
[Type the document subtitle] | Sarah Broussard and Caitlin Owen
Table of contents. Page 1 Composition Page 2 Size- distance from Page 3 earth? Phases of the Page 4 moon Page 5 Orbit? Page 6 Page 7 ADEVERTISMENT Page 8 How was the moon formed?
Effects on the earth
Missions to the moon
How was the moon formed? Before astronauts went to the Moon there were several main theories about how the Moon formed. One of the goals of the Apollo missions was to try and choose which theory fit best, but in the end, a completely new theory was born. Before we look at all the different theories, we need to look at some of the differences and similarities between the Earth and Moon:
The Earth has a large iron core, but the Moon does not. Earth has an average density of 5.5 g/cm3 and the Moon has an average density of 3.3 g/cm3 (because it is depleted in iron). The Moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and
meteorites from other parts of the Solar System (e.g. the asteroid belt) have much different isotope compositions. Co-accretion This theory says that the Earth and Moon formed (accreted) in the same part of the Solar System from similar building blocks, and that the Moon is a sister planet to the Earth. This theory failed because it can’t explain why the Moon is depleted in iron compared to the Earth. Fission Fission describes the somewhat crazy idea that the young Earth was spinning so fast that it spun a lump of material off that went on to form the Moon. This theory fails because it defies the laws of physics! Giant Impact You’ve already heard about the Giant Impact theory, which is the current favored formation mechanism for the Moon. This theory accounts for both the similarities and the differences between the Earth and Moon’s composition. It assumes that the Earth’s iron had already drained into the core by the time the impact happened, and that the impactor also contained an iron core. The majority of debris thrown out by the giant impact came from their rocky mantles, and the core of the impactor melted on impact and merged with the iron core of the Earth. There is one thing this theory doesn’t explain, however, and that is why only one Moon formed, or why we don’t see evidence for this happening elsewhere in the Solar System!
Composition What is the Moon made of? Start by looking at its surface. This picture shows it very well. There are light areas and dark areas. The dark areas are called seas, but they do not contain water. The seas are flat regions, whereas the light areas are rugged and are higher on average. For this reason, they are also called highlands. Across the whole surface, craters caused by meteoric impacts are evident. The craters are much more enhanced and frequent within the light areas. In the years from 1969 to 1972 the Moon has been explored by man. Astronauts brought back to Earth some samples of lunar soil, so it has been possible to
analyze and date those rocks. It has been found that the younger lunar rocks are the dark ones, those of the seas, and that they are 3.2 billion years old. The older ones are the light rocks of the highlands, and they are 4.6 billion years old. Even if the mass of the Moon is just one hundredth of that of the Earth, the ratio of the mass of the Moon over that of the Earth is very large, if one compares it to the case of the other planets. Leaving aside the Pluto-Charon system, the Earth-Moon system is indeed a unique case in the Solar System. Generally speaking, satellites are much smaller than the planets which hold them into an orbit. A well known phenomenon is connected to this peculiarity. The tides.
Size- distance from earth? 1 pixel = 600 kilometers
The average distance between Earth and Moon is approximately 30 times Earth's
diameter. If you could fly to the Moon at a constant speed of 1000 kilometers per hour, which is the speed of a fast passenger jet, it would take sixteen days to get there. Apollo astronauts reached the Moon in less than four days even though they coasted "uphill" almost the entire distance. They got a fast start.The Sun happens to be 400 times the Moon's diameter, and 400 times as far away. That coincidence means the Sun and Moon appear to be the same size when viewed from Earth. A total solar eclipse, in which the Moon is between the Earth and Sun, blocks the bright light from the Sun's photosphere, allowing us to see the faint glow from the corona, the Sun's outer atmosphere. When the Moon is at apogee, it is 11% farther from Earth than it is at perigee. This is far enough that it cannot entirely block the bright light, so eclipses which occur near apogee are not total. Perigee 363,300 km Mean 384,400 km Apogee 405,500 km Gravitational interaction (tides on the Earth caused by the Moon) transfers kinetic energy from Earth to the Moon, slowing Earth's rotation and raising the Moon's orbit, currently at a rate of 3.8 centimeters per year.Earth and Moon Compared The Moon has approximately 1/4 Earth's diameter, 1/50 Earth's volume, and 1/80 Earth's mass. Earth is very dense overall (it is the densest planet in the Solar System), but the Moon is light for its size. The difference is partly because Earth has a large core of iron and other heavy metallic elements, while the Moon has only a small core, if it has a core at all. The Moon's surface gravity is 1/6 of Earth's, and escape velocity from the surface is about 1/5 of Earth's. The Moon's surface is covered with rock and grit that are mostly dark-gray minerals, so it reflects light poorly compared to Earth, which always has highly-reflective clouds. The Moon reflects visible light about 1/3 as well as Earth, and because of its
much smaller size, has a visual brightness less than 1/40 that of Earth, when both are fully illuminated and seen from the same distance -- a difference of four stellar magnitudes. Earth Moon Mean diameter 12,742 km 3,476 km 12 1.08321 x 10 2.199 x 1010 Volume 3 km km3 7.349 x 1022 Mass 5.9736 x 1024 kg kg Mean density 5.515 3.342 Surface gravity 9.78 m/s2 1.62 m/s2 Escape velocity 11.2 km/s 2.38 km/s Visual albedo 0.367 0.12 Visual -3.86 +0.21 magnitude At right: Earth and Moon to the scale of 1 pixel = 50k The Moon's surface area is a bit greater than Africa's.
Phases of the
moon
Orbit?
just 0.05 today. (A circle, in contrast, has an eccentricity of zero.)
The Moon's orbit is fiendishly difficult to explain, moving as it does around a rotating Earth, which together form a "doubleplanet" system that orbits around the Sun. It is a classic example of a three-dimensional, gravitational three-body problem. The Moon's peculiar bulge -- to which the French mathematician PierreSimon Laplace first drew attention in 1799 -- makes the problem even more complex.
Moreover, they believe that the Moon may have been spinning much faster than it is today and behaved a bit like Mercury does today -- rotating three times about its own axis for every two revolutions about the Sun (a socalled 3:2 resonance). Now, of course, the Moon spins just once for every revolution around the Earth, which is why we can never see its far side. The team also showed that the bulge can be explained by a 1:1 resonance with an eccentricity of 0.49 and semimajor axis of 22.9 Earth radii.
Garrick-Bethell and colleagues now think that the strange bulge can be accounted for if the Moon moved along a very different orbit than it does now (see figure). Based on simple classical mechanics, rather than computer simulations, the MIT team suggest that when the Moon was just 100-200 million years old it was less than 30 Earth radii away, compared to about 60 Earth radii now. As well as being much closer to Earth, they also believe the Moon had a much more elliptical orbit at that time. They calculate that its "eccentricity" -- a measure of how much an ellipse differs from a circle -- was 0.61, compared to
According to Garrick-Bethell's team, the proximity of the Moon to the Earth, together with its elliptical orbit, were ideal conditions for the bulge -- which was still forming as the Moon cooled -- to "freeze" into its present form. This explanation also ties in with the most widely accepted theory of the Moon's origins, which says it was created when a massive Mars-like object crashed into the Earth. In this theory, the Moon formed at about 4 Earth radii and it has been gradually moving away from us at a rate of about 3.8 cm a year ever since.
Effects on the earth
occur. Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon.
The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. The tidal flow period is synchronized to the Moon's orbit around Earth, but the phase isn't. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year.
The Moon is most clear at night, but can sometimes be seen during the day. The Moon (and also the Sun) appear larger when close to the horizon. This is a purely psychological effect. The angular diameter of the Moon from Earth is about one half of one degree. During the brightest full moons, the Moon can have an apparent magnitude (the amount of light received from the object) of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8.For any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon comes highest when it is full, and the full moon comes highest in winter. Like the Sun, the Moon can also give rise to an optical effect known as a halo (an optical phenomenon that appears near or around a light source).The event known as a blue moon is related to the western calendar system. A blue moon is the second of two full moons to occur in the same calendar month. Blue moons occur infrequently (thus the saying once in a blue moon to denote a rare event), because the length of the calendar month in this system is close to the length of the period of the moon's phases. They are not impossible, because every month except February is longer than this period by 1 or 2 days. Blue moons occur every 2.72 years. The next blue moons will be on June 30, 2007; and December 31, 2009.*
The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no total eclipses will
Missions to the moon
The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation
from Earth. The invention of the optical telescope brought about the first leap in the quality of lunar observations. Galileo Galilei is generally credited as the first person to use a telescope for astronomical purposes; having made his own telescope in 1609, the mountains and craters on the lunar surface were among his first observations using it. In 1969, NASA's Project Apollo first successfully landed humans on the Moon. They placed scientific instruments there and returned rocks to Earth.
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