The Moon and the Earth
It’s a clear night, and Earth’s beautiful full moon shines big and bright in the dark sky. Through myth, fact, and even first-hand exploration, we humans have a special fascination with the moon. Let’s explore our moon, and Earth’s relationship to it.
Where did the moon come from?
Although some planets have many moons, Earth has just one. Our moon is about one quarter the size of Earth. Scientists estimate that the moon is just a little younger than Earth, about 4.51 billion years old compared to 4.54 billion years for Earth.
In fact, what we know about the moon suggests that it is made of material that broke off from Earth when a huge object—some say a planet—crashed into our planet when it was very young. The impact destroyed the object and also knocked a huge amount of material from Earth into space. That material seems to have formed our moon.
In July, 1969, humans stepped foot on the moon for the first time. What an exciting moment that was! Those astronauts, and others who landed on the moon later, scooped up rocks to study back on Earth. By measuring the age of these rocks, scientists determined how old the moon is. Analysis of the moon rocks also confirmed that the moon is very similar in composition to the Earth.
By studying the moon, we’ve also learned more about other places in our solar system. For example, unlike Earth, the moon has no atmosphere to burn up meteorites, so they just smash into the surface. Each crash forms a crater. Comparing the age of moon rocks with the number of craters helps scientists understand how old features on other planets and moons might be.
Like all moons, Earth’s moon orbits its planet. The moon orbits around Earth every 27.3 days. Each rotation of the moon is called a lunar month. During each orbit, the moon goes through different cycles.
Sometimes we see a full moon. Other times only a small crescent. And for a night or two each month, we see no moon at all. How much moon we see from Earth depends on where the moon is in relation to the sun and the earth.
As the crescent of the moon grows bigger, we call that a waxing moon. After the full moon, the surface that we can see grows smaller. This is called a waning moon. When the moon is dark from Earth’s view it is called a new moon. At that time, the sunny side of the moon is facing away from Earth.
As the moon circles Earth, our planet is also rotating on its axis. That makes the moon seem to rise and set in the sky, but it’s really Earth that’s turning away from the moon. And even though the moon seems to glow in the sky, it doesn’t give off any light or heat of its own. Instead, that beautiful glow is actually the sun’s light reflected off the moon’s surface.
The earth looks to the moon
The moon varies from about 225,000 miles to 252,000 miles away from Earth. Even though that sounds pretty far, the moon has an important influence on our planet.
The main influence is ocean tides, caused by the tug of the moon’s gravity. In places, that gravitational tug, combined with the overall shape of the oceans and continents, can create a 40- foot difference between low and high tide! No wonder tides are such big factors in shoreline ecosystems, as well as a powerful influence on human civilization and history.
Moon cycles are also the basis for the months in our calendars. The months in our familiar 12-month year are not quite the same as lunar months. They are longer than moon cycles so that one year equals the time it takes the earth to go around the sun.
Some societies use a lunar calendar where each month is the length of one moon orbit of earth. However, 12 of those 27.3- day moon orbits don’t quite add to up a year. So lunar calendars need to add an extra month every few years, like the leap year in a 12-month calendar.
The moon gives us more than tides and our calendar. Without it, we wouldn’t have moon shadows or moonbeams, the man in the moon or the rabbit in the moon. Humans have always looked to the moon for inspiration.
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy