Why does the moon get bigger when it's closer to the horizon?

Category: Space
Published: December 15, 2012

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

The moon stays approximately the same size, whether measured by apparent size or actual size. Actual size is the number you would measure if you went to the moon with a long ruler. It would take a cataclysmic event such as collision with a planet to change the actual size of the moon. Apparent size is the angular diameter that an object takes up in a camera's field of view and depends on both actual size and distance. The apparent size would change if the moon moved significantly away from the earth. But it doesn't. According to "The Moon Book" by Kim Long, the distance of the moon from the earth only changes by about 10%. This means that the observed diameter of the moon only changes by 10% throughout the month. This small variation of the moon's apparent size is easily verified with a camera. We only think the moon looks bigger on the horizon because of a psychological trick. When the moon is low, we can visually compare it to distant mountains and conclude that the moon is much bigger than a mountain. But when the moon is high, there is nothing to compare it to, so our minds perceive it to be smaller. The exact mechanism is still debated by psychologists, as set forth in the book "The Moon Illusion" by Maurice Hershenson.

The moon only changes its apparent size by about 10% throughout a cycle, as evident in this series of photos. This change is due to the moon coming closer to the earth during part of its orbit. Public Domain Image, source: Tom Ruen.

Topics: big moon, horizon, moon, perspective