Unless you never look at the sky, you’ve probably noticed the Sun and Moon are the same size in the sky, which is 0.5 degrees of arc. The reason they have the same apparent size is because the Sun has a diameter about 400 times greater than the Moon, yet is also 400 times further away.
- The Sun and Moon are roughly the same size in the sky or 0.5 degrees of arc.
- The reason is that the diameter of the Sun is 400 times greater than the diameter of the Moon, but the Sun is 400 times further away.
- But, sometimes one is a bit larger than the other because the Moon has an elliptical orbit around the Earth and the Earth has an elliptical orbit around the Sun. The Moon reaches its largest size when it is closest to Earth, while the Sun reaches its largest size when the Earth is closest to the Sun.
Comparing the Size and Distance of the Moon and Sun
The diameter of the Sun is 1,390,000 kilometers (864,000 miles) and its average distance from the Earth is 1.496×108 km (93 million miles). The diameter of the Moon is 3475 kilometers (2159 miles) and it averages 384,400 kilometers (238,900 miles) from Earth. So, the Sun is about 400x larger than the Moon and 400x further away.
Are the Sun and Moon Always the Same Size?
But, the Sun and Moon don’t usually appear exactly the same size in the sky. Two factors cause this:
First, the Moon does not have a constant distance to the Earth. At its furthest distance (apogee) it averages about 363,400 kilometers away from Earth. At its closest approach (perigee) it is 405,500 kilometers away. A full moon at perigee is a “supermoon” and it appears larger than a full moon at apogee.
Second, the distance between the Earth and Sun changes as well. The Sun appears slightly larger than in the sky when the Earth is at its closest distance (perihelion) and slightly smaller in the sky when the Earth is at is greatest distance (aphelion). The difference in distance between perihelion and aphelion varies depending on the year, but it is approximately 4.8 million kilometers (3 million miles).
The Moon is moving further away from the Earth, so millions of years in the future, it will appear smaller than the Sun. Meanwhile, in the distant past, the Moon always looked larger than the Sun. We’re just in the right place at the right time to experience the two bodies having the same apparent size.
How This Relates to Solar Eclipses
When the two elliptical orbits place the Moon and the Sun at just the right distance and orientation relative to the Earth, the two bodies appear the same size and a total eclipse of the Sun occurs. During a total solar eclipse, the Moon completely obscures the disc of the Sun. If the Sun and Moon were always the same relative size, all solar eclipses would be total solar eclipses. Instead, sometimes the Moon is a bit smaller than the Sun and annular eclipses occur. In an annular eclipse, the Moon doesn’t fill the whole disc of the Sun, resulting in a bright ring.
Are the Sun and the Moon the Same Size on Other Planets?
Moons form via multiple processes, but they aren’t typically the same apparent size as a planet’s star. For example, consider Mars and its moons. Phobos and Deimos both appear quite a lot smaller than the Sun in the Martian sky. While the Sun is farther from Mars than it is from Earth, the big difference is that the moons are tiny. None of the other inner planets have moons, so Earth is special, at least in our solar system.
Each of the gas giants has multiple moons. A small moon could potentially appear the same size as the distant Sun.
References
- Harrington, Philip S. (1997). Eclipse! The What, Where, When, Why and How Guide to Watching Solar and Lunar Eclipses. New York: John Wiley and Sons. ISBN 0-471-12795-7.
- Karttunen, Hannu (2007). Fundamental Astronomy. Springer. ISBN 9783540341444.
- Link, F. (1969). “Lunar Eclipses”. Eclipse Phenomena in Astronomy. Springer-Verlag Berlin Heidelberg. ISBN 978-3-642-86475-9. doi:10.1007/978-3-642-86475-9
- Littmann, Mark; Espenak, Fred; Willcox, Ken (2008). Totality: Eclipses of the Sun. Oxford University Press. ISBN 978-0-19-953209-4.