Why Do the Sun and Moon Look the Same Size in the Sky?

The Sun is about 400 times wider than the Moon, yet both look almost exactly the same size in our sky. That strange coincidence is what makes total solar eclipses possible.

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The Sun and Moon look almost exactly the same size in our sky, but in reality they are enormously different.

The Moon is about 2,159 miles [3,475 km] in diameter and averages about 238,855 miles [384,400 km] from Earth. The Sun is about 864,938 miles [1,392,700 km] in diameter and about 93 million miles [149.6 million km] away.

That means the Sun is roughly 400 times wider than the Moon, but also about 400 times farther away. Those two facts nearly cancel out from our point of view, so both appear to span about 0.5° of sky, or about 30 arcminutes. That remarkable coincidence is why the Moon can completely cover the Sun during a total solar eclipse.

To make the scale easier to picture, if Earth were the size of a basketball, the Sun would be about 87 feet [26.5 m] across. The Moon, by comparison, would be only about 2.6 inches [6.7 cm] across.

The episode also explores why the Moon can LOOK huge near the horizon even though its actual size does not change as it rises in the sky. That effect is largely the Moon illusion. And because moonlight is just reflected sunlight, the Moon can also appear yellow, orange, or red when its light passes through more of Earth’s atmosphere.

This is one of the most elegant and surprising facts in the sky: a nearby rocky Moon and a gigantic distant Sun just happen to look almost perfectly matched from Earth.

Download the companion infographic here:
go.dirtfarmerjay.com/sun-moon