Through a small telescope or even a pair of binoculars you can see that the dark areas are smooth while the bright areas are covered with numerous larger circular pits called craters. Craters usually have a raised rim and range in size from tiny holes less than a centimeter across to gaping scars in the Moon's crust that are over 200 km across. Some of the larger craters have mountain peaks at their center.
The large, smooth dark areas are called maria, from the Latin word for 'seas'. However these regions, like the rest of the Moon, are totally devoid of water. This usage comes from early observers who believed the maria looked like oceans.
The bright areas that surround the maria are called highlands. The highlands and maria differ in brightness because they are composed of different rock types. The maria are basalt; dark, congealed lava rich in iron, magnesium, and titanium silicates. The highlands, on the other hand, are mainly anorthosite, a rock type rich in calcium and aluminum silicates. This difference has been verified from rock samples obtained by astronauts, samples that also show that the highland material is generally less dense than mare rock and considerably older.
The highlands are not only brighter and their rocks less dense than the maria, they are also more rugged, being pitted with craters. In fact, highland craters are so abundant that they often overlap. This is in contrast with the mare region which usually contain only a few, small craters.
From many craters long, light streaks of pulverized rock called rays radiate outward. A small telescope reveals still other surface features. Lunar canyons known as rilles, perhaps carved by ancient lava flows, wind away from some craters. Elsewhere, straight rilles gouge the surface, probably the result of crustal cracking.
Nearly all the surface features we see on the Moon- craters, maria, and lunar rays- were made by the impact of solid bodies on its surface. When such an object hits a solid surface at high speed, it disintegrates in a cloud of vaporized rock and fragments. The resulting explosion blasts a hole whose diameter depends on the mass and velocity of the impacting object. The hole's shape is circular, however, unless the impact is grazing.
As the vaporized rock expands from the point of impact, it forces surrounding rock outward, piling it into a raised rim. Pulverized rock spatters in all directions, forming rays. Sometimes the shattered surface slumps partially back into the crater, pushing up a central peak.
As the Moon continued to cool, its crust thickened. But before the crust grew too thick, a small number of exceptionally large bodies- objects over 100 kilometers across- struck the surface, blasting huge craters and pushing up mountain chains along their edges. Subsequently, molten material from deep within the Moon gradually flooded the vast crater and congealed to form the smooth, dark lava plains that we see now. Because the denser material sunk to the Moon's interior during its molten stage, the erupted lava from those depths was denser than crustal rock into which it flooded. Moreover, because it was molten more recently, the mare material is therefore younger than the highlands. By the time the maria formed, most of the impacting bodies were gone- collected into the Earth and Moon by earlier collisions. Thus, too few bodies remained to crater the maria, which therefore remain smooth.
The Moon's small size relative to the Earth explains the differences between the two bodies. We have seen that radioactive elements in the Earth's interior hear it. The Moon is also heated by radioactive material, but because its volume compared to its surface area is small relative to the Earth's, heat escapes far more easily from the Moon. Thus the Moon has cooled far more than the Earth. Moreover, because its mass is much smaller than the Earth's, the Moon contains much less radioactive material and so cannot generate as much heat. Thus, without a strong heat source, the Moon lacks the convection currents that drive plate tectonic activity on the Earth.
The Moon's surface layer is shattered rock which forms a regolith- meaning 'blanket of rock'- tens of meters deep. The regolith consists of both rock chunks as well as fine powder, the result of successive impacts breaking rock into smaller and smaller pieces. This powdery nature is easily seen in the crispness of the astronaut's footprints. Samples of the regolith picked up by astronauts show that these surface rocks are typically the same type as the underlying rock. That is, the regolith on maria is generally basaltic, whereas that on the highland is anorthositic. In places the regolith may extend several hundred meters below the surface. Analysis of the regolith shows that over time its rocks have been broken up by high-velocity impacts, supporting the interpretation that the surface has been bombarded by meteoritic bodies.
Below this surface layer of rocky rubble is the Moon's crust, about 100 kilometers thick, on the average. The crust is much thinner (about 65 kilometers) on the side of the Moon that faces the Earth than on the far side, but the reason for this difference is not clear.
Beneath the crust is a thick mantle of solid rock, extending down about 1000 kilometers. Unlike the Earth's mantle however, it appears that the Moon's mantle is too cold and rigid to be stirred by the Moon's feeble heat. The Moon's low density tells us its interior contains little iron. Some molten material may lie below the mantle, but the Moon's core is smaller and contains far less iron and nickel than the Earth's. These factors, plus the Moon's slow rotation, lead astronomers to think that the Moon's core is unable to generate a magnetic field as the Earth does. Measurements made by the Apollo astronauts confirm that the Moon has essentially no magnetic field.
The Moon's surface in never hidden by lunar clouds or haze, nor does the spectrum of sunlight reflected from it show any trace of gases. With no atmosphere to absorb and trap heat, temperatures on the Moon soar during the day and plummet at night. Likewise, no wind blows, and so the lunar surface lies dead.
The Moon has no atmosphere for two reasons. First, its interior is too cool to cause volcanic activity, which was probably the source of much of the Earth's early atmosphere. Second, even if volcanoes had created an atmosphere in its youth, the Moon's small mass creates too weak a gravitational force for it to retain the erupted gas. With no atmosphere and no plate tectonics, the Moon has essentially unchanged for billions of years.
As its orbits, the Moon keeps the same side facing the Earth. In order to do this, the Moon turns on its axis but with a rotation period exactly equal to its orbital period, a condition known as synchronous rotation. The Earth's gravity causes this locking of the Moon's spin to its orbital motion.
The Moon's orbit is tilted about 5
with respect to the Earth's orbit around the Sun. It is also tilted with
respect to the Earth's equator and is thus unlike most of the moons of
Jupiter, Saturn and Uranus, which lie nearly exactly in their planets'
equatorial plane. These oddities indicate that our Moon formed differently
from the moons of other planets, a conjecture supported by the odd mass
ratio of the Earth and the Moon.
Lunar rocks brought back to the Earth by Apollo astronauts have led astronomers to radically revise their ideas of how the Moon formed. Originally there have been three theories of the origin of the Moon, called the capture, the double planet, and the fission theories.
The double planet theory was suggested in the early 1800s and is the oldest. It holds that as the Earth formed from a spinning disk of material, not all of that material coalesced to form the Earth. A smaller part of it was left orbiting the Earth and formed into the Moon.
In 1878, astronomer Sir George Howard Darwin, son of the biologist Charles Darwin, proposed that the Moon was once part of the Earth and broke (or fissioned) from it due to forces caused by a fast rotation and by solar tides. The large basis of the Pacific Ocean was proposed as the place from which the Moon was ejected in this fission hypothesis.
Early in this century, another theory was proposed. It holds that the Moon was originally a separate astronomical object that happened to come near the Earth and that it was captured by the Earth's gravitational field so that it settled into orbit as the Moon. This is the capture theory.
How doe these theories fit the data? Based simply on comparing densities, the double planet theory seems to be ruling out, for if the Moon formed along with the Earth its density should match Earth's. In actuality, the density of the Moon is 3.3 grams per cubic centimeter. The Earth's density, on the other hand, is 5.5 grams per cubic centimeter. If the capture theory is correct, however, it would certainly be possible for the Moon to have a density less than the Earth's, for the two objects would have little in common. In fact, it would be a coincidence that the Moon's density matches that of the Earth's crust. The other theory- the fission theory- seems to fit the density data best, for if the Moon came from the Earth's outer layer it would have a density similar to that layer.
There is a problem with the fission theory, however. Astronomers have difficulty explaining how an object as massive as the Moon might have been pulled out of- or thrown off- the Earth. No satisfactory mechanism for this event has ever been proposed. In addition, the Moon does not orbit in the plane of the Earth's equator as it would if it were ejected from a spinning Earth.
There are also problems with the capture theory. If one astronomical object comes close to another, each of their paths will be changed by the gravitational force between them, but one will not capture the other unless there is contact between the two or unless a third object is involved, so that the interaction of the three objects results in one of them being slowed down to an orbital speed. Such a near-collision between three objects seems highly unlikely.
Although we have known the density of the Moon for a long time, its chemical composition was not well known until the Apollo program brought back lunar samples. This new data brought new problems for the three theories. In many ways, the chemical composition of the Moon is similar to that of the Earth's crust, for both have about the same proportions of some of the major elements: silicon, magnesium, iron and manganese. The Moon, however, has far smaller proportions of easily vaporized (volatile) substances than does the Earth. The Moon has much higher proportions of nonvolatiles (such as aluminum and titanium), which require a very high temperature to vaporize, than does the Earth's crust. The differences in chemical composition, along with the Moon's lack of an iron core, seem to rule out both the fission theory and the double planet theory.
Thus, none of the theories for the origin of the Moon seemed to fit all the data. A new hypothesis was proposed within the last twenty years which differs considerably from the previous theories. According to the new hypothesis, the Moon formed from debris blasted out of the Earth by the impact of a Mars-sized body. The great age of lunar rocks and the absence of any impact feature on the Earth indicate that this event must have occurred during the Earth's own formation, at least 4.5 billion years ago. The colliding body melted and vaporized millions of cubic kilometers of the Earth's surface rock and hurled it into space in an incandescent plume. As the debris cooled, its gravity gradually drew it together into what we now see as the Moon.
This violent birth hypothesis explains many of the oddities of the Moon. The impact would vaporize low-melting-point materials and disperse them. Computer models of such an event also show that only surface rock would be blasted out of the Earth, leaving our planet's iron core intact, thereby also explaining the low iron content of lunar rocks. The splashed-out rock would condense in an orbit whose shape and orientation were determined by the collision rather than by the orientation of the Earth's equator. Furthermore, we would expect both similarities and differences in composition between the Earth and the Moon because the Moon was made partly from Earth rock and partly from rock of the impacting body. A bonus of the hypothesis is that it explains why the Earth's rotation axis is tipped so much more than the rotation axis of Mercury, Venus, or the giant planets, Jupiter, and Saturn; the impact knocked the Earth part way over.
After the Moon's birth, stray fragments of the ejected rock pelted its surface creating the craters that blanket the highlands. A few huge fragments plummeting onto the Moon later in its formation process blasted enormous holes that later flooded with molten interior rock to become the maria. That rock was probably melted in the Moon's interior by radioactive decay, as happened in the Earth. During the time it took the rock to melt, about half a billion years, most of the debris remaining in the Moon's vicinity fell onto its surface. Thus, by the time the maria flooded, little material was left to fall on them and so they are only lightly cratered. Since that time, the Moon has experienced no major changes. It has been a virtually dead world for all but the earliest times in its history.
Eclipses are rare because the Moon's orbit around the Earth is tilted with respect to the Earth's orbit around the Sun. Without that tilt we would have lunar and solar eclipses every month. With the tilt, the Moon lies above the Earth's orbit for half of the month and below it for the other half. The result is that at full moon the Earth's shadow generally falls either above or below the Moon, and at new moon the Moon's shadow falls below or above the Earth. Thus eclipses do not generally occur very often.
Another reason that eclipses don't occur very often has to do with the size of the respective shadows. The Earth's shadow (and the Moon's shadow) extend behind the Earth in the shape of a cone. This shadow is smaller than the size of the Earth. This occurs because the source of light- the Sun- is so much larger than the Earth.
Because of the way the shadows form, there are two distinct regions created by the Earth's shadow. The full shadow (region A) is called the umbra. As we get farther and farther from the Earth, we see that it tapers down to a point. At the distance of the Moon, the width of the umbra is only three-fourths the diameter of the Earth. So the Moon is less likely to pass through the shadow of the Earth than we think. When the Moon does enter the umbra, we have a total lunar eclipse.
If the Moon is in region B, it is only in partial shadow, for light from the lower (or upper) part of the Sun is hitting it. When the Moon is here, in the penumbra, it will not receive the full light from the Sun and will appear dim to Moon-watchers on the Earth. The penumbral shadow increases in size at greater distances from the Earth but it is not equally dark across its width. Right next to the umbra it is very dark and it gets brighter and brighter out towards its edge. When the Moon passes through the outer penumbra, we don't even notice the darkening. When it passes partially through the umbra, we see the Moon dark over part of its surface. This is a partial lunar eclipse.
The Moon, though, follows an eccentric elliptical orbit, getting as close as 356,300 kilometers to the Earth. So it does get close enough that its umbra can reach the Earth. When this occurs, we can experience a total solar eclipse. Even when the Moon is at its closest, the width of the umbral shadow at the Earth's distance is only about 270 kilometers. If the shadow hits the Earth fairly straight on, its width will be 270 kilometers. If the shadow happens to strike the Earth at a slant, the show on the surface may measure wider than 270 kilometers but it seldom exceeds 400 kilometers.
When the Moon's distance is farther than 377,000 kilometers, the shadow of the Earth is not large enough to completely block out the Sun's surface. In this case, we see a dark disk where the Moon is surrounded by a bright ring. This is known as an annular solar eclipse. Finally , when the Moon's umbral shadow does not pass in front of the Earth, but instead above or below it, it is still possible for the Earth to pass through the penumbra of the Moon. When this happens, we experience a partial solar eclipse. So, if total eclipses are so unlikely, how do they happen at all?
As the Earth orbits the Sun, the Moon's orbit keeps nearly the same direction of tilt. This orbit tilt is kept fixed- like that of the spinning Earth- by a gyroscopic effect, or more technically by the conservation of angular momentum. The result is that twice each year the Moon's orbital plane points at the Sun. At those times- eclipse seasons- eclipses will happen when the Moon crosses the Earth's orbital plane. Only during these seasons are eclipses possible; at other times the shadows of the Earth and Moon always fall on to empty space.
When a solar eclipse does occur, it occurs at new moon. At this time, conditions are right for a lunar eclipse to happen at either the previous or the following full moon. Thus eclipses generally occur in pairs, with a solar eclipse followed approximately 14 days later by a lunar eclipse, or vice versa.
This simple pattern does not always work because the tilt of the Moon's orbit is not exactly fixed. That is, the Moon's orbit precesses, swinging once around about every 18.6 years. This orbital precession make the dates of the eclipse season shift by 1/18.6 year (about 20 days) each year.
If one of the eclipse seasons occurs in early January with the next in June, a third season may sometimes happen in late December. As a result, as many as five solar and five lunar eclipses can occur each year. No matter when the eclipse season falls, at least two solar and two lunar eclipses must happen each year, but that does not mean they will be visible to an observer at a given location, since the eclipse may visible only from another part of the Earth. Because the Moon is so small compared with the Earth, its shadow is small, and therefore you can see a solar eclipse only from within a narrow band. Lunar eclipses, however, are visible from anywhere the Moon is above the horizon at the time of the eclipse.
A solar eclipse begins with a black "bite" taken out if the Sun's edge as the Moon cuts across its disk. However, unless a large part of the Sun is covered by the Moon, you may not even notice an eclipse is happening. On the other hand, if you are fortunate enough to be at a location where the eclipse is total, you will see one of the most amazing sights in nature.
As the moment of totality approaches, the landscape takes on an eerie light. Shadows become incredibly sharp and black; even individual hairs on your head cast crisp shadows. Sunlight filtering through leaves creates tiny bright crescents on the ground. Seconds before totality, pale ripples of light sweep across the ground and to the west the deep purple shadow of the Moon hurtles down on you at more than 1000 miles per hour. In one heartbeat you are plunged into darkness. Overhead the sky is black, and stars may appear. The corona of the Sun- its outer atmosphere- gleams with a steely light around the Moon's black disk. Perhaps a solar prominence, a tiny glowing red flame like cloud in the Sun's atmosphere, may protrude beyond the Moon's edge. Birds call as if it were evening. A deep chill descends for a few minutes because the Sun's warmth is blocked by the Moon. The horizon takes on sunset colors: the deep blue of twilight with perhaps a distant cloud in our atmosphere glowing orange. As the Moon continues in its orbit, it uncovers the Sun and instantly it is daylight again. Now the cycle continues in reverse.
Just as the Earth exerts a gravitational pull on the Moon, so too does the Moon exert a gravitational pull on the Earth. Thus pull happens to the whole Earth and draws material towards the Moon. The attraction is stronger on the side of the Earth near the Moon and weaker on the far side, because the force of gravity weakens with distance. The difference between the strong force on one side and the weaker force on the other is called a differential gravitational force.
The differential gravity draws water in the oceans into a tidal bulge on the side of the Earth facing the Moon. But curiously, it creates an identical tidal bulge on the Earth's far side. This second tidal bulge can be viewed as a result of the Moon's gravity pulling the Earth "out from under" the water on the far side.
So far, we have ignored the Earth's rotation. The tidal bulges are aligned approximately with the Moon, but the Earth spins. Its rotation therefore carries us first into one bulge and then the next. As we enter the bulge, the water level rises, and as we leave it, the level falls. Because there are two bulges, we are carried into high water twice a day, creating two high tides. Between the times of high water, as we move out of the bulge, the water level drops making two low tides each day.
This simple picture must be altered to account for the inability of the ocean to flow over land areas. Thus water tends to pile up at coast lines when the tidal bulge reaches shore. In most locations the tidal bulge has a depth of about 2 meters, but it may reach 10 meters or more is some long, narrow bays and may even rush upriver as a tidal bore- a cresting wave that flows upstream. The motion of the Moon in its orbit makes the tidal bulge shift slightly from day to day. Thus high tide comes about 50 minutes later each day, the same delay as in moonrise.
The Sun also creates tides on the Earth, but although the Sun is much more massive than the Moon, it is also much farther away. The result is that the Sun's tidal force on the Earth is only about one third the Moon's. Nevertheless, it is easy to see the effect of their tidal cooperation in spring tides, abnormally large tides that occur at new and full moons. At those times the lunar and solar tidal forces work together, adding their separate tidal bulges. On the other hand, at first and third quarters, the Sun's and Moon's tidal forces work at crosspurposes, creating tidal bulges at right angles to one another. The socalled neap tides that result are therefore not as extreme as normal high and low tides.
As the Earth's rotation slows, the Moon accelerates in its orbit, moving farther from the Earth, as required by the need to conserve angular momentum. The Moon accelerates because the tidal bulge it raises on the Earth exerts a gravitational force back on the Moon, which pulls the Moon ahead in its orbit. That acceleration makes the Moon move away from the Earth at about 3 centimeters per year. Thus the Moon was once much closer to the Earth, and the Earth spun much faster, perhaps as rapidly as once every 5 hours several billion years ago. Over that immense period of time, the Moon has receded to its present distance and the Earth's rotation has slowed to 24 hours. These processes occur even now; tidal braking lengthens the day by about 0.002 seconds each century.
Tidal braking is also the reason the Moon always keeps the same face to the Earth. Just as the Moon raises tides, which slows the Earth, the Earth raises tides on the Moon, which slow it. These lunar tides distort the Moon's crust and have braked the Moon severely, locking it into synchronous rotation. The Moon's braking of the Earth will eventually make the Earth rotate synchronously with the Moon's orbital motion. Billions of years from now the Earth and Moon will orbit so that each constantly presents the same face to the other: the Moon will then be visible only from one side of the Earth! Similar tidal effects have locked some of the moons of other planets into synchronous rotation, but the planets themselves have not been noticeably slowed. On the other hand, tidal braking by the Sun probably slowed the rotation of Mercury and Venus.
The Moon's gravitational pull on the Earth may also stabilize our climate. Astronomers have recently discovered with computer simulations that the tilt of the a planet's rotation axis may change erratically by many tens of degrees if the planet has no moon. Because the tilt causes seasons, changes in the tilt will alter the severity of the seasons. Our Moon is large enough that its gravitational attraction on the Earth's equatorial bulge helps hold the Earth's tilt relatively fixed, sparing us catastrophically large climate changes.
It might seem that the Earth would not precess because there is nothing below it trying to pull it over. There is, however, a force on the Earth tending to change the orientation of its axis of rotation. the Earth is not truly spherical, but us slightly pear shaped. It's diameter is about 26 miles greater across the equator than from pole to pole. This is caused simply by the fact that it is spinning. Now recall that the Moon exerts a gravitational force on each particle of the Earth. Since points near the Moon are pulled more than points farther away, the overall result is that the Moon attempts to change the tilt of the spinning Earth's axis. So we have what is needed to cause precession. The Earth's axis does indeed precess, although very slowly. The Earth completes one precession about every 26,000 years. This has the effect of shifting the apparent position of the north celestial pole. Right now, the pole is located near the star Polaris That pole will gradually change, until about 12,000 years from now the star Vega will be the new "North Star".
Last Updated: August 29, 1997
Comments to: D-Suson@tamuk.edu