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Earth's Moon


Earth's Moon - WholeThe moon is Earth's closest celestial neighbor. It has a huge influence upon our lives, governs the tides (the sun plays a small part, but not nearly as much as the Moon), and is responsible for eclipses. Some biologists say that the moon is even responsible - in part - for life on Earth, for without the tides causing currents and mixing of chemicals, life might never have formed.

The moon lies, on average, 384,400 km (238,900 miles) away and takes 27.3 days to orbit the Earth. Strange as it may seem, its rotation period is exactly the same as its orbital period, something that scientists call tidally locked. Therefore, one half is always facing the Earth while the other half is always facing away. This means that there isn't really a "Dark Side" of the moon. Also, contrary to popular opinion, humans have landed on the far side, and it has been extensively photographed and mapped.

Surface and AtmosphereEarth's Moon - Hemisphere

The moon has a very heavily cratered surface, which tells the tale of the solar system. Earth would be the same way if it had no atmosphere nor plate tectonics. Without an atmosphere or water to erode the craters, the lunar landscape has remained relatively constant for billions of years.

It is through these craters that we can actually tell some of the solar system's history. Through dating the craters, planetary scientists have been able to determine that most of the craters were made in the early days of the solar system, approximately 3.8-4.5 billion years ago. This means that early in the solar system, there was a period of heavy asteroid bombardment. This is most likely due to all of the extra material left over from the solar system's formation. After approximately 1.2 billion years, most of the material had collected into the Asteroid Belt, been ejected from the solar system, or hit the planets and moons.

If you look at the moon closely, you can see dark patches. After the period of heavy asteroid bombardment, the moon's surface cracked in many places. The lowlands of the moon filled with volcanic material, which then cooled. This lasted for approximately 750 million years. They are called maria (singular is mare) due to their resemblance to dark oceans.

The moon has remained relatively static (unchanging) for the last 2.5 billion years, with the occasional asteroid impact.

The moon is considered to have no atmosphere. It does, however, have a very light one, with a total mass of only 25,000 kg. At night, this layer of gas exerts a surface pressure of only 3x10-15 bars at about 2x105 particles per cubic cm. The estimated composition is shown in the pie chart at left, and can be summarized as:

  • Helium 4: 4He - 40,000 particles per cm3
  • Neon 20: 20Ne - 40,000 particles per cm3
  • Hydrogen: H2- 35,000 particles per cm3
  • Argon 40: 40Ar - 30,000 particles per cm3
  • Neon 22: 22Ne - 5000 particles per cm3
  • Argon 36: 36Ar - 2000 particles per cm3
  • Methane: CH4 - 1000 particles per cm3
  • Ammonia: - 1000 particles per cm3
  • Carbon Dioxide: CO2 - 1000 particles per cm3

Besides these, there are also trace amounts of Oxygen (O+), Aluminum (Al+), Silicon (Si+), and possibly Phosphorus (P+), Sodium (Na+), and Magnesium (Mg+). The true composition is poorly known and variable. The numbers presented are estimates of the upper limits of the night ambient atmosphere composition. Day levels were difficult to measure due to heating and outgassing of Apollo surface experiments.


There are four main theories about the creation of the moon, although only one is generally considered to give an accurate description of what actually occurred.

The first theory states that the moon was created the same way the planets were - through the coalescing of gas and dust during the solar system's formation. The nice part of this is that there was a lot of material around in the early solar system for this to happen. However, this does not account for why Earth and Luna have different compositions, such as the moon lacking a significant amount of iron. Another con of this is that the Earth-Moon system has too much angular momentum compared to other planets for this to be a likely scenario.

The second theory says that the moon is a captured asteroid. The only "pro" of this is that it's not impossible. It is, however, extremely improbable. Besides the miniscule chance for this to happen, the energy that the moon would have had would need to have been carried off by a third body, otherwise the moon never would have parked itself in Earth orbit.

The third theory says that when the Earth was first formed it was spinning so rapidly that it split in two; this is often referred to the "fission" theory. This theory can account for why the density of the moon is similar to that of Earth's outer layers. Unfortunately for this theory, the moon would then be orbiting along Earth's equator, which it is not. Second, the composition of Moon rocks are dissimilar to that of Earth's surface. Finally, Earth would have had to have been spinning at a rate of about one rotation every hour and 25 minutes, as found by balancing the centripetal and gravitational forces:

5040 sec

The fourth theory is the one that most scientists currently believe is correct. It states that when the Earth was quite young, a Mars -sized planet crashed into it. The planet crashed with such speed that it was completely destroyed, and almost destroyed the Earth. The planet was coming in with such force that when it was destroyed, the molten iron in its core continued to travel through Earth, to eventually be included it its core. The crash, comically dubbed the "Big Splash," sent trillions of tons of rock and debris into orbit. These fragments eventually coalesced to form the Moon.

This theory has many pros, the first of which is that simulations confirm that this is what would happen in this situation, and that collisions in the early solar system did happen. This would explain why the moon has very little iron, and it also explains why there are very few volatiles in the moon. The tidal and rotational forces in play also would account for why the moon's day is exactly the same as it's "year." As a bonus, it also explains the tip in Earth's axis. Two cons are that in order to get material past the Roche limit, the impactor body would need more angular momentum than is now in the Earth-Moon system. Also, there is no reason in this theory why there should just be one moon, yet there is.


Eclipses are caused by one celestial body passing in front of another - in our case the moon passing between the sun and Earth or Earth passing between the sun and moon. However, eclipses do not occur every new nor full moon because the moon's orbit is tilted approximately 5° relative to the Sun-Earth plane. Because of this, the moon passes through this plane only twice in its orbit, and it is rare that these crossings correspond with a full or new moon.Solar Eclipse Diagram

A solar eclipse occurs during a new moon, when the moon is directly between Earth and the sun, as per the illustration on the left, which is not to scale. When this happens, the moon blocks the sun's light from reaching Earth in two main ways.

The first is represented by the inner, darker triangle, and is where a total eclipse will appear on Earth. If the moon is just the right size and at just the right distance for it to block all of the sun's disk during a total solar eclipse. Generally, a total solar eclipse lasts about 5 minutes. If the moon is not at quite the right distance - because its orbit is an ellipse and not a perfect circle - then it will be too small to cover the entire solar disk, and an annular eclipse will result.

The second manner that the moon blocks the sun's light is represented by the two outer triangles in yellow. In these areas on Earth, there will be a partial solar eclipse where the sun's disk is not fully covered by the moon.

A fourth type of solar eclipse is a fairly rare event, and is called a hybrid. It is when an annular eclipse is seen on one part of Earth and a total is seen by another.

Lunar eclipses generally occur more often than solar eclipses, if for no other reason than Earth's shadow is much larger at the moon's distance than the moon's shadow on Earth. Other than it's the Earth casting a shadow on the moon, lunar eclipses are exactly the same as solar eclipses.

The Earth lies between the sun and moon, and if the moon crosses the Sun-Earth plane when it is full, then a lunar eclipse will result. The first part is a partial lunar eclipse, as the moon begins to move through Earth's penumbra. Then, if it is aligned just right, the moon will then pas through the umbra, and go into totality. Totality during a lunar eclipse lasts much longer - generally about 40 minutes - than totality during a total solar eclipse.

Partial Lunar EclipseTotal Lunar EclipseDuring a total lunar eclipse, the Earth's atmosphere creates two very noticeable effects. The first is that it causes the moon to appear red. This is because the atmosphere scatters sunlight around Earth; shorter wavelengths are scattered more easily, and so do not pass all the way around Earth with nearly as much intensity as the shorter, redder light. This effect hints at the second, which is that the atmosphere acts as a lens to amplify the light that would reach the moon.

Therefore, without a terrestrial atmosphere, a lunar eclipse would appear darker and not red. Also, the atmosphere has a blurring effect, so without an atmosphere, the distinction between the penumbral and umbral shadows would be much sharper.

The picture on the left is a partial lunar eclipse, taken at 22:38 EDT on May 15, 2003. The picture on the right is a total lunar eclipse, taken at 23:35 EDT on May 15, 2003.

TidesTides from Moon

Tides are a very complex phenomena, yet they can be boiled down to a few key concepts. The first to develop an accurate theory of the tides was Sir Isaac Newton over 300 years ago. His theory, "The Equilibrium Theory of Tides," is what will be discussed here.

First examined will be the relationship between just the Earth and Moon. In the diagram at right is presented a top cartoon view of the Earth-Moon system. This is not to scale, and the water bulges have been greatly exaggerated.

As theorized by Newton, every object exerts a pull on every other object (Newton's Third Law). As the moon pulls on Earth, Earth also pulls on the moon. Earth is so much larger than the moon that it hardly moves at all, and the rock is so rigid that it hardly deforms at all; however, the water flows much more easily. Because of the pull from the moon on Earth's water, the water forms a bulge on the moon-ward side. This is high tide.

Earth has a certain velocity as it orbits the sun, and this causes an outward force. Just as the moon's gravity pulls the water and creates a bulge towards it, the outward force also creates a bulge, but on the side opposite the moon. Thus there are two high tides on the planet on opposite sides at once.

There is another basic complication to this theory, and that is represented by the arrows. The gray arrows pointing towards the moon represent the relative force felt by different sections of Earth at different distances from the moon. The close side of Earth is about 50 times Earth's radius from the moon, while the far side is about 52 times. This creates a difference in the force, so the far side does not feel as much of a pull from the moon. The green arrows pointing away from the moon represent the force outward. Their strength is opposite to that of the moon in direction. They are stronger on the far side of Earth and weaker on the near side.

These act to create a net - total - force where on the side closer to the moon the pull is to the moon and on the side farther from the moon the pull is away from the moon. Conversely, there is a resultant inward force in between these extremes, and this causes the low tides. As the moon orbits the Earth, the tidal bulges move with it, making one revolution every 24 hours and 50 minutes. The equation below shows the effective tidal force due to the moon (see any advanced classical mechanics textbook for its derivation):

Tidal Force Equation

In this equation, D is the closest distance between Earth's surface and the moon's surface, r is the radius of Earth, and R is the distance between the moon and the point on Earth where the tide is being calculated. Due to the vectors and trigonometry, the previous qualitative explanations can now be shown quantitatively.

Tidal Bulges from the Moon and SunNow we add the sun. The tides caused by the sun follow the exact same methods as those by the moon. In the diagram at the left, the light blue bulge of water is caused by the sun's influence (purple-blue is from the moon). The yellow arrows indicate the pull of the sun. Even though the sun is so much larger than Earth, it is so much farther away that the difference in the force between opposite sides of Earth is approximately 45% (derived below) that of the Moon's. Thus, the tides that would be produced by the sun are approximately 45% as strong as those from the moon.

One final bit will finish the Equilibrium Theory of Tides. Moon and Sun do not stay at fixed points relative to Earth. The moon orbits around Earth, and Earth orbits around the sun. Therefore, the total effect of the sun and moon act to create two high and two low tides per day, but they vary in intensity. When the moon and sun are at right angles, as shown in the picture at the left, the moon's tidal effects cancel out that of the sun's, but as a result its pull is diminished, causing what are called neap tides. When the sun and moon line up relative to Earth, the two act together to create much greater high tides and much lower low tides, causing what are called spring tides.

Tidal Force (Relative)In order to calculate the force of the sun's tide relative to the moon's, the above formulation is use, dividing the values for the sun by those of the moon. The equation boils down to that seen on the right. Using this, and then inputting the appropriate mass for the moon and the sun and the distances to both, the result comes out to approximately 45%.

Data for Earth's Moon

Distance from Earth (103 km)
Mass (1020 kg)
Perigee (103 km)
Volume (1010 km3)
Apogee (103 km)
Equatorial Radius (km)
Revolution Period* (days)
Polar Radius (km)
Synodic Period** (days)
Ellipticity (Flattening)
Average Orbital Velocity (km/s)
Average Density (kg/m3)
Maximum Orbital Veloocity (km/s)
Bond Albedo
Minimum Orbital Velocity (km/s)
Geometric Albedo
Inclination to the Ecliptic
Visual Magnitude
Inclination to the Equator
18.28 - 28.58
Apparent Magnitude
Orbital Eccentricity
Solar Irradiance (W/m2)
Recession Rate from Earth (cm/year)
Black-Body Temperature (K)
Surface Gravity (m/s2)
Discovery Date
Escape Velocity (km/s)

* The revolution period is a measure of the average time for the moon to make one revolution about Earth from one point in its orbit to the equivalent point (e.g. equinox to equinox).

** The synodic period is the time interval between similar configurations in the orbit (e.g. opposition).

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