Formation of the Moon

From the Earth the Moon has looked the same throughout human history. Looking up on clear nights, the Moon's familiar face is reassuring in its constancy. Comparing the Moon to the Earth strengthens that image: The Earth has hurricanes that change shorelines, volcanoes that make new islands, clouds that move and change, and plants whose colors change with the seasons. Above us, the Moon has no atmosphere to make weather, and therefore no oxygen to support life, and it has no volcanoes. Aside from its waxing and waning each month, regularly and smoothly, the Moon never seems to change.

This was not always the case. The origins of the Moon are hugely violent and hot, and for billions of years it had active volcanoes dotting its surface.The large dark areas filling the Moon's craters are seeing the largest volcanic flows in the Earth-Moon system, more volcanic rock than can be seen at a glance anywhere else in the solar system. The huge dark basalt flows exist almost exclusively on the near side of the Moon (see the photo below), while the far side has fewer giant craters and almost no dark basalt flows (see the photo on page 120). The

The near side of the Moon shows the familiar dark mare basalt flows filling ancient impact craters. (NASA)

The Moons far side, never visible from Earth, has fewer distinctive features than does the familiar near side. (NASA)

The Moons far side, never visible from Earth, has fewer distinctive features than does the familiar near side. (NASA)

reasons for the differences between the two sides are not agreed upon in the scientific community. Basic physical parameters for the Moon are given in the table on page 121.

Each planet and some other bodies in the solar system (the Sun and certain asteroids) have been given its own symbol as a shorthand in scientific writing.The symbol for the Moon is shown on page 118.

The orbital relationships of the Earth, Sun, and Moon create the gorgeous phenomenon of the eclipse, as shown in the figure on page 122.There are lunar eclipses, in which the shadow of the Earth is cast over the Moon, making it dark when seen from the Earth. More spectacularly, there are solar eclipses, in which the Moon moves between the Sun and the Earth, covering the Sun from the vantage point of people on parts of the Earth. Because the shadow of the Moon (the umbra) is relatively small on the surface of the Earth, solar eclipses are seen only in narrow paths that curve across the Earth's surface. The path over the Earth's surface where the Sun completely disappears behind the Moon is called the path of totality, and the regions adjacent to it where a partial eclipse is seen are known as the penumbra.

As implied by the terms near side and far side, the same side of the Moon faces toward the Earth at all times. This relationship is called synchronous rotation, and is created by the period of the moon's rotation on its axis being the same as the period of the moon's orbit around its planet. Information about the Moon's orbit is given in the table on page 123.

Tidal locking causes synchronous rotation. Gravitational attraction between the Moon and the Earth produces a tidal force on each of them, stretching each very slightly along the axis oriented toward its partner. The tidal force causes each to become slightly egg-shaped; the extra stretch is called a tidal bulge. If either of the two bodies is rotating relative to the other, this tidal bulge is not stable. The rotation of the body will cause the long axis of the bulge to move out of alignment with the other object, and gravitational force will work to reshape the rotating body. Because of the relative rotation

Was this article helpful?

0 0

Post a comment