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Determine Why the Sun and the Moon Appear to Be the Same Size in the Sky

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Author: Janice VanCleave

What You Need to Know

Apparent size is how large an object appears to be from a specific distance. An object's actual size is its true measurement.

How Does Apparent Size Work?

The actual size of an object, such as a car, is its true measurement. When you stand next to the car, the car is much bigger than you are. When the car moves away from you, it looks smaller than it really is. An object's apparent size decreases as the object's distance from you increases.

How Does Apparent Size Work?

What Does This Have to Do with the Size of the Sun and the Moon?

Viewed from Earth, celestial bodies, such as the Sun and the Moon, appear to be much smaller than their actual size. The greater their distance from Earth, the smaller their apparent size compared to their actual size. In order for two celestial bodies of different sizes to have the same apparent size, the ratio between their diameter and their distance from Earth must be equal. The ratio for their distances can be written as: Dsun/Dmoon. The ratio for their diameters can be written as: dsun/dmoon. A formula representing the comparison for the two ratios is:

  • Dsun/Dmoon = dsun/dmoon

Real-Life Science Challenge

Because the Sun and the Moon, as seen from Earth, have the same apparent size, the German scientist Albert Einstein (1879–1955) proved that gravity (the force of attraction between all objects in the universe) bends light rays. He predicted that a star positioned behind the Sun would be visible during a total eclipse. This is because the gravity of the Sun would cause the light rays coming from the star to change direction and make the star appear to be next to the Sun. Einstein's prediction was proven correct during a total solar eclipse in 1919.


Now, start experimenting with the relationship between the Sun and the Moon's size and their distance from Earth.


  • Find the diameters of Earth and the Sun.
  • Paper disks can be used to model the diameters of the Moon and the Sun.
  • Find the actual distances of the Moon and the Sun from the Earth.
  • Create a scale model (see chapter 1) to place Moon and Sun models at appropriate distances from an observer on Earth.
  • Use the formulas above to compare the ratios of the Sun's and the Moon's diameters and their distances from Earth.

Fun Fact

Since the Moon orbits Earth, sometimes it comes between Earth and the Sun. Because the Sun and the Moon appear to be the same size when viewed from Earth, the Moon can block the Sun completely. When this happens, it is called a total solar eclipse. The region from which a total solar eclipse is visible, called the path of totality, is very narrow. Few people have seen a total solar eclipse.

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