The Moon - Frequently asked questions

How was the moon created ?

A depiction of the impact that could have caused the earth-moon system. Picture courtesy of Southwest Research Institute / NASA Up until the Apollo landings there were a number of competing theories about the origins of the moon. Study of moon rock samples brought back by the astronauts has led to the development of the impact theory. This suggests that the young earth collided with an object about the size of Mars. The debris thrown up from this collision went into orbit around the earth and formed into the moon.

Why does the moon sometimes appear to be very large in the sky ?

When you see the moon near the horizon it looks a lot bigger than when it is high up in the sky. This is due to the way the brain perceives size and distance. When the moon is close to the horizon the trees, houses etc indicate to the brain that the moon is a very long way away. When the moon is high in the sky there are no clues to the brain so it concludes that the moon is closer. Since an object's apparent distance determines its perceived size, the moon near the horizon looks twice the size compared to when it is high in the sky.

What causes the different phases of the moon ?

The moon is illuminated because it reflects the light from the sun. However only the half of the moon facing the sun is lit up. Th side facing away from the sun is in darkness. How much of the moon we see from earth depends on the angle between the earth, moon and sun. As the moon orbits around the earth we see it grow from a thin crescent to a full disk (or full moon) and then shrink back to a thin crescent again before vanishing for a few days.

How does the moon cause tides on earth ?

The gravitational pull of the moon causes the earth's oceans to bulge. This bulge is the high tide. In fact the moon causes the oceans to bulge in two places, the oceans facing the moon and the oceans facing away from the moon. The moon's gravity has a greater effect on the oceans facing the moon than on the earth itself. This causes the water directly under the moon to bulge. The moon pulls the rocky part of the earth more strongly than the water on the far side. This causes the oceans on the far side of the earth to get left behind and the water bulges there as well.

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