Monday, February 9, 2015

The Moon is the only natural satellite of Earth

The Moon is the only natural satellite of Earth:
        orbit:    384,400 km from Earth
        diameter: 3476 km
        mass:     7.35e22 kg

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.
Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.
The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.
The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day. (This is a greatly simplified model; actual tides, especially near the coasts, are much more complicated.)
But the Earth is not completely fluid, either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimetres per year. (The opposite effect happens to satellites with unusual orbits such as Phobos and Triton).
The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.
Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side (left) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term "dark side" in the past may have referred to the far side as "dark" in the sense of "unknown" (eg "darkest Africa") but even that meaning is no longer valid today!)
The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well.
The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the Moon's interior is no longer active. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.
There are two primary types of terrain on the Moon: the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon's surface) are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.
Most of the craters on the near side are named for famous figures in the history of science such as Tycho, Copernicus, and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by Luna 3). In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the the largest impact basin in the solar system and Orientale on the western limb (as seen from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater.
A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these precious samples.
Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.
Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impacttheory: that the Earth collided with a very large object (as big as Mars or more) and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.
The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moon's history.
With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Over its 4 billion year lifetime many ions from the solar wind have become embedded in the Moon's regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.
More about the Moon
more pictures of the moon
Moon calendar showing past & future moon phases
from NSSDC
a Teacher's Guide to the Moon (concentrating on lunar geology)
a Socratic Dialog about the Moon's motion
Lunar rocks and regolith
Io and Luna, a comparison from LANL
Life Under the Moon, from Phil Plait's excellent Bitsize Astronomy site
Why the Moon looks bigger near the horizon
The Lunar size illusion, why the Moon looks bigger near the horizon
a longer explanation of the tides and their causes and effects
leap seconds and the slowing of the Earth's rotation
historical info about observations of the Moon
Lunar Meteorites
Moon Phases explained
The impact theory of lunar formation
The Origin of the Moon, with nice paintings by Dr. William K. Hartmann
The Origin of the Earth and Moon, written by G. Jeffrey Taylor, Hawaii Institute of Geophysics and Planetology
Big Bang, New Moon (from Southwest Research Institute)
New theory links moon's current orbit to its formation via a giant impact (from Southwest Research Institute)
lunar exploration
Exploring the Moon from LPI, nice images from Lunar Orbiter and others plus lots of explanatory text
Apollo info and images html
Apollo Missions
Scientific Discoveries Made During Apollo
Clementine - DSPSE (including some nice images)
Lunar Prospector
SMART-1, the first European lunar mission
The Artemis Project, a private venture to establish a permanent, self-supporting community on the Moon!
LunarSat
Yes, we really did go to the Moon, debunking the Moon Hoax theories
Fraud - Books Moon Hoax
another article on lunar ice
Lunar Nomenclature Table
(Earth and) Moon Viewer
Transient Lunar Phenomena
Inconstant Moon by John Walker, info about the Moon's changing appearance
Inconstant Moon by Kevin Clarke, for amateur astronomers
Lunar Info for the Amateur Astronomer
Names of the Full Moons
The Triple Triumph of the Moon by Isaac Asimov
The Face of the Moon, an exhibit of rare books and maps from the Linda Hall Library
The Moon in landscape photography by Joe Decker
fiction
The Moon in Science Fiction, a huge bibliography
From the Earth to the Moon, fiction by Jules Verne
Chinese Moon Legends

Eugene M. Shoemaker: Obituaries
Open Issues
Why are the maria concentrated on the near side?
Why is the Moon's center of mass off center? Because of the tidal lock with the Earth?
Now that we've found water on the Moon, what are we going do to with it?
Only twelve men have ever walked on the surface of the Moon. Who will be the 13th? Who will be the first woman?

7 reasons the world will end in 2012

Don’t know whether its true or not, but just thout of sharing it to you.
7 reasons the world will end in 2012
Scientific experts from around the world are genuinely predicting that five years from now, all life on Earth could well finish. Some are saying it’ll be humans that set it off. Others believe that a natural phenomenon will be the cause. And the religious folks are saying it’ll be God himself who presses the stop button…
1. Mayan Calendar
The first mob to predict 2012 as the end of the world were the Mayans, a bloodthirsty race that were good at two things:
Building highly accurate astrological equipment out of stone and Sacrificing Virgins.
Thousands of years ago they managed to calculate the length of the lunar moon as 329.53020 days, only 34 seconds out. The Mayan calendar predicts that the Earth will end on December 21, 2012. Given that they were pretty close to the mark with the lunar cycle, it’s likely they’ve got the end of the world right as well.
2. Sun Storms
Solar experts from around the world monitoring the sun have made a startling discovery: our sun is in a bit of strife. The energy output of the sun is, like most things in nature, cyclic, and it’s supposed to be in the middle of a period of relative stability. However, recent solar storms have been bombarding the Earth with so much radiation energy, it’s been knocking out power grids and destroying satellites. This activity is predicted to get worse, and calculations suggest it’ll reach its deadly peak sometime in 2012
3. The Atom Smasher
Scientists in Europe have been building the world’s largest particle accelerator. Basically its a 27km tunnel designed to smash atoms together to find out what makes the Universe tick. However, the mega-gadget has caused serious concern, with some scientists suggesting that it’s properly even a bad idea to turn it on in the first place. They’re predicting all manner of deadly results, including mini black holes. So when this machine is fired up for its first serious experiment in 2012, the world could be crushed into a super-dense blob the size of a basketball.
4. The Bible says…
If having scientists warning us about the end of the world isn’t bad enough,religious folks are getting in on the act aswell. Interpretations of the Christian Bible reveal that the date for Armageddon, the final battle between Good an Evil, has been set down for 2012. The I Ching, also known as the Chinese book of Changes, says the same thing, as do various sections of the Hindu teachings.
5. Super Volcano
Yellowstone National Park in the United States is famous for its thermal springs and Old Faithful geyser. The reason for this is simple – it’s sitting on top of the world’s biggest volcano, and geological experts are beginning to get nervous sweats. The Yellowstone volcano has a pattern of erupting every 650,000 years or so, and we’re many years overdue for an explosion that will fill the atmosphere with ash, blocking the sun and plunging the Earth into a frozen winter that could last up to 15,000 years. The pressure under the Yellowstone is building steadily, and geologists have set 2012 as a likely date for the big bang.
6. The Physicists
This one’s case of bog-simple maths mathematics. Physicists at Berekely Uni have been crunching the numbers. and they’ve determined that the Earth is well overdue for a major catastrophic event. Even worse, they’re claiming their calculations prove, that we’re all going to die, very soon – while also saying their prediction comes with a certainty of 99 percent- and 2012 just happens to be the best guess as to when it occurs.
7. Slip-Slop-Slap- BANG!
We all know the Earth is surrounded by a magnetic field that sheilds us from most of the sun’s radiation. What you might not know is that the magnetic poles we call north and south have a nasty habit of swapping places every 750,000 years or so – and right now we’re about 30,000 years overdue. Scientists have noted that the poles are drifting apart roughly 20-30kms each year, much faster than ever before, which points to a pole-shift being right around the corner. While the pole shift is underway, the magnetic field is disrupted and will eventually disappear, sometimes for up to 100 years. The result is enough UV outdoors to crisp your skin in seconds, killing everything it touches.