Earth’s moon averages 238,000 miles or 380,000 kilometers distance from the Earth. It has an average diameter of 3,474 kilometers. While the Earth’s Moon appears to be the same relative size as the Sun, it has one quarter of the Earth’s diameter.
Dark side of the Moon. © NASA
Gravity on the Moon’s surface is one sixth that of Earth. Earth’s moon is the second largest in the solar system.
The light we see at night is a reflection on the Sun’s light from the Moon. The waxing and waning of the Moon is actually the Earth’s own shadow onto the Moon. We see the same face or side of the Moon all of the time. This is because our Moon rotates synchronously. Each “day” for the moon is as long as its rotational period with Earth. If you were on the Moon, sunrise would occur every 28 days. The side of the Moon we never see is called the dark side of the Moon. The cycles of the moon are considered the basis of the 12 “months” most cultures on Earth use to break up the year. The Moon orbits the Earth every 27.3 days, resulting in about 13 lunar months versus the 12 months recognized in English influenced cultures. The moon has no seasons of its own and almost no axial tilt. The moon moderates the Earth’s axial tilt.
It has no active magnetic field, and the surface is essentially a vacuum. There is more erosion on the Moon from the solar wind than the miniscule atmosphere. The foot prints left by human explorers like Neil Armstrong are estimated to last up to a million years before weathered away.
The Moon does not orbit relative to Earth’s equator as the Earth orbits the Sun. It actually orbits near an ecliptic plane. The Moon’s orbit is elliptical, not circular.
Hidden history of the Moon
Formation of the Moon. © NASA
When a moon forms along with the parent planet, it has the same chemical composition as the parent planet. Earth’s moon seems to have been formed by the collision of a Mars sized body with the Earth about 4.5 billion years ago. The debris of its impact coalesced into the Moon that we now see. The material ejected into orbit was mostly surface material, primarily light elements like aluminum and silica.
Denser materials like iron had settled in Earth’s core by the time the Mars-sized body hit the Earth and while less was lost in the impact. This is why the Moon has little iron and no molten core of its own. If the Moon had formed from the same material as the Earth, it would have more iron. A lot of water also boiled away in the impact. Much of the water vapor would freeze in space and coalesce with the new moon. When the new moon cooled and solidified, ice would boil on exposure to sunlight. The molecular bonds of the water would be broken. Hydrogen is lost to the solar wind while the oxygen remains and combined to create the aluminum oxide abundant on the Moon today. There is still water ice on the Moon today, buried underground.
The disproportionate material loss on the side of the planet where the impact occurred is also a possible explanation for the formation of the super-continent Pangaea later in Earth’s history.
Effects of the Moon on Earth
Effects of the Moon on Earth(tide). © NASA
The pock marks of craters on the Moon are visible from the naked eye. The light areas we see are the low lands while the darker areas are the high points. The largest mountains on the Moon are the result of molten material thrown up in a matter of minutes after asteroid strikes, not volcanic activity. And every meteor that hit the Moon is one that did not hit the Earth. We know how disruptive asteroid strikes are. An asteroid strike in the Yucatan peninsula 65 million years ago is blamed for the extinction of the dinosaurs. The Moon has shielded Earth repeatedly from asteroids, helping life on Earth to evolve with fewer catastrophic events to survive. The existence of one large moon early in our history may be why Earth has never captured smaller asteroids as moons like Mars or Jupiter.
The gravitational tug of war between the Earth and the Moon is most evident in our tides. The pull on Earth by the Moon has slowed down Earth’s rotation from a few hours to the 24 hour day we enjoy now. The water of Earth is pulled upward toward the moon and tries to rotate in sync with it, despite the Earth‘s own rotation. The result is tides that are highest when the Moon is closest. The high tides help wash continental material into the oceans. This gave early life more nutrients to use while increasing the salt content of the oceans. The Earth’s oceans are also pulled toward the Sun, but the solar tides are perhaps a third of the tides caused by the much closer Moon.
Orbit of the Moon. © NASA
The Moon continues to affect the Earth today. The tidal effects slow down Earth’s rotation roughly two milliseconds per hundred years. The interaction between the Moon and Earth also increases the Moon’s orbit about 4 centimeters per year.
Structure of the Moon. © NASA
Because the Moon is so close to Earth relative to any other bodies, the Moon is the only body that has received human visits. There have been six manned lunar landings, all between 1969 and 1972 by the United States. There have been dozens of rovers and craft sent to the Moon. Over 500 man-made satellites orbit the Earth. The Japanese satellite Kaguya was sent into orbit around the Moon in 2007. The Chinese satellite Chang’e-1 entered lunar orbit shortly after the Japanese satellite. The Indian orbiter Chandrayaan reached lunar orbit in 2008.
Several “impactors” were sent to the Moon to crash into it, sending up debris for analysis by telescopes on Earth.
One of the most famous impactors carried the ashes of Eugene Shoemaker, one of the scientists who discovered the Shoemaker-Levy comet that crashed into Jupiter. While there are hundreds of space burials each year with human ashes sent into Earth orbit before crashing back to Earth, Shoemaker is the only person to date given the honor of a lunar burial to date. NASA made this decision, and private lunar burials are not an option at this time.