Chapter 5: The Earth-Moon System

Earth's Moon.

We think of ourselves as living on a single planet, but in reality, we live in a system of two worlds. Our sister world, the Moon, is easily visible in our sky, and we can see its daily effects on ocean tides. The relationship between the two bodies was first appreciated in 1968 when humans started to explore the other half of our system.

The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the Sun. This diagram looks down on Earth from the North. Earth's rotation and the Moon's orbit are both counter-clockwise here. Sunlight is coming in from the right, as indicated by the yellow arrows. From this diagram, we can see, for example, that the Full Moon will always rise at sunset, but the waning crescent Moon is high overhead around 9:00 AM.

The Moon affects the Earth in several observable ways. Consider the monthly movement of the Moon around our planet - we see the phases of the Moon cycle daily, as different parts of the Earth-facing side of the Moon are illuminated by the Sun. Many people superstitiously believe that the Moon influences human behavior by some unknown force, causing people to act strangely during a full Moon. This is the origin of the word lunacy. However, this has never been convincingly proven. Occasionally, a study claims to find a correlation between Moon phase and murders or admissions to emergency rooms, but large-scale statistical studies show the effects are not significant. (Though before artificial lights there was probably some tendency for more people to be out on full Moon nights, and hence for more interesting events to happen!)

Schematic of the tidal forces of the Moon on the Earth's oceans.

The liquid ocean can move much more freely in response to tidal forces than the solid rocks inside the Earth. Water flows until it "piles up" in tidal bulges on each side of the Earth. You may wonder about the fact that tides occur on both sides of the Earth. Why doesn't the attraction of gravity just cause the water to pile up on the side closest to the Earth? Just as the force of gravity is strongest on the side of the Earth facing the Moon, the force is weakest on the side away from the Moon. Less gravitational force on the far side means the water is not attracted as strongly, and it moves away from the center of gravity. Think of a spring as an analogy. When the spring is stretched, the distance between all parts of the spring increases. In the same way, the tidal stretching force applies to the oceans on both sides of the Earth. Since there are tides on opposite sides of the Earth, and the Earth rotates once per day, a given spot on the rotating Earth passes through two high-tide zones in one day.

Scale model of the Earth and the Moon, with a beam of light traveling between them at the speed of light. It takes approximately 1.26 seconds.

The Sun is much more massive than the Moon, but it's also a lot farther away. When you calculate the resulting gravitational forces, the Sun's gravitational force on the Earth is much stronger than the Moon's gravitational force on the Earth. This is how the Sun keeps the Earth in orbit around it. However, because the Moon is so much closer, the differential force caused by the Moon is larger than the differential force caused by the Sun. So the Moon's tidal force is larger. The Sun has some tidal force on the Earth, too, but solar tides are only about half as high as lunar ones. When the Sun and Moon line up (new Moon and full Moon), the tides on Earth are especially high (spring tides). When they are 90º in opposition, the tides partially cancel each other out, and the resulting lower tides are called neap tides.

A illustration demonstrating simple synchronous rotation. As the Moon takes exactly one orbit to rotate once about its axis, the inhabitants of the planet will never be able to see the green side of the Moon.

You've probably noticed that the features of the Moon always appear the same. The Moon actually orbits the Earth in such a way that the same side always points towards the Earth. This is because the force of gravity is working both ways; the Moon is slightly elongated by its own tides, caused by the gravity of the Earth. Earth's gravity has forced the long axis of the Moon to face the planet so that the Moon is tidally locked in synchronous rotation.