This is almost a full moon. The photo was taken 16 hours and 19 minutes before the exact moment of full. The photo was shot at 1:25 AM, EDT on June 24th 2002. With a good imagination, you might be able to see the "Rabbit in the Moon", or maybe the "Man in the Moon". Some people can see the "Lady in the Moon". For centuries, people have looked at the light and dark areas of the moon and imagined all sorts of things in the moon. What do you see? Near the west limb of the moon are two very small craters, 10 and 8 miles in diameters. These little dots are the craters Galileo and Galileo A, two of the smallest named craters on the moon. These were named for Galileo Galilei, the Italian astronomer & physicist who lived from 1564 to 1642. Galileo was the first astronomer to aim a telescope at the moon and the first to see any of its craters and mountains. Moon's age: 13 days 5 hours 37 minutes.


Neil Armstrong during his first EVA on Apollo 11.


Charles M. Duke on the rim of Plum crater, Apollo 16


James B. Irwin next to the LRV in front of Mount Hadley, Apollo 15


Harrison H. Schmitt at Lunar Roving Vehicle (LRV), Apollo 17


David R. Scott and the LRV during Apollo 15 moonwalk.


LRV tire tracks leading to the Lunar Module, Apollo 14.


James B. Irwin loads equipment from the Lunar Module into the LRV; Apollo 15.


Trampled lunar soil trimmed from the bottom of the previous image. This makes for a backdrop thats less like a photo and more like a texture.

What are the three major theories for the origin of the Moon?

  • The Fission Theory: This theory proposes that the Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. This theory was thought possible since the Moon's composition resembles that of the Earth's mantle and a rapidly spinning Earth could have cast off the Moon from its outer layers. However, the present-day Earth-Moon system should contain "fossil evidence" of this rapid spin and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.
  • The Capture Theory: This theory proposes that the Moon was formed somewhere else in the solar system, and was later captured by the gravitational field of the Earth. The Moon's different chemical composition could be explained if it formed elsewhere in the solar system, however, capture into the Moon's present orbit is very improbable. Something would have to slow it down by just the right amount at just the right time, and scientists are reluctant to believe is such "fine tuning". Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.
  • The Condensation Theory: This theory proposes that the Moon and the Earth condensed individually from the nebula that formed the solar system, with the Moon formed in orbit around the Earth. However, if the Moon formed in the vicinity of the Earth it should have nearly the same composition. Specifically, it should possess a significant iron core, and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

There is one theory which remains to be discussed, and it is widely accepted today.

The Giant Impactor Theory (sometimes called The Ejected Ring Theory): This theory proposes that a planetesimal (or small planet) the size of Mars struck the Earth just after the formation of the solar system, ejecting large volumes of heated material from the outer layers of both objects. A disk of orbiting material was formed, and this matter eventually stuck together to form the Moon in orbit around the Earth. This theory can explain why the Moon is made mostly of rock and how the rock was excessively heated. Furthermore, we see evidence in many places in the solar system that such collisions were common late in the formative stages of the solar system. This theory is discussed further below.

More About The Giant Impactor Theory

In the mid-1970s, scientists proposed the giant impact scenario for the formation of the Moon. The idea was that an off-center impact of a roughly Mars-sized body with a young Earth could provide Earth with its fast initial spin, and eject enough debris into orbit to form the Moon. If the ejected material came primarily from the mantles of the Earth and the impactor, the lack of a sizeable lunar core was easily understood, and the energy of the impact could account for the extra heating of lunar material required by analysis of lunar rock samples obtained by the Apollo astronauts.

For nearly a decade, the giant impact theory was not believed by most scientists. However, in 1984, a conference devoted to lunar origin prompted a critical comparison of the existing theories. The giant impact theory emerged from this conference with nearly consensus support by scientists, enhanced by new models of planet formation that suggested large impacts were actually quite common events in the late stages of terrestrial planet formation.

The basic idea is this: about 4.45 billion years ago, a young planet Earth -- a mere 50 million years old at the time and not the solid object we know today-- experienced the largest impact event of its history. Another planetary body with roughly the mass of Mars had formed nearby with an orbit that placed it on a collision course with Earth. When young Earth and this rogue body collided, the energy involved was 100 million times larger than the much later event believed to have wiped out the dinosaurs. The early giant collision destroyed the rogue body, likely vaporized the upper layers of Earth's mantle, and ejected large amounts of debris into Earth orbit. Our Moon formed from this debris.