You know the Sun is really big, but have you ever wondered whether the Sun fits between the Earth and the Moon? The quick answer is no. Here’s a closer look at the numbers.
- The Sun does not fit between the Earth and the Moon.
- The diameter or size of the Sun is 1,392,000 km (865,000 mi), while the average distance between the Earth and the Moon is 384,400 km (238,900 mi).
- The diameter of the Sun is about 3.6 times the distance between the Earth and the Moon.
How Big Is the Sun?
Measuring the size of the Sun is a bit tricky because it does not have a solid surface. Astronomers generally consider the photosphere as the edge of the Sun, where the photosphere is the visible solar surface. The Sun is a nearly perfect sphere with a diameter around 1,392,000 km (865,000 mi). To put this number into perspective, the diameter of the Sun is about 109 time greater than the diameter of Earth.
The Distance Between the Earth and the Moon
The average distance between the Earth and the Moon is 384,400 km (238,900 mi). This distance varies because the Moon’s orbit is elliptical. At its closest approach (perigee), the Moon is 356,400–37,0400 km from Earth. At it farthest point (apogee), the Moon is 404,000–406,700 km away.
Can the Sun Fit Between the Earth and the Moon?
To answer the question, you compare the diameter of the Sun (1,392,000 km) with the distance between the Earth and the Moon (384,400 km). It does not matter whether you use the average distance between the Earth and the Moon, its distance at perigee, or its distance at apogee. The answer is the same because the Sun is so much larger than the Moon’s orbit. In fact the diameter of the Sun is about 3.6 times larger than the distance between the Earth and the Moon.
Will the Sun Ever Fit Between the Earth and the Moon?
Nope. Even though the Moon is moving away from Earth, the Sun won’t ever fit inside its orbit around Earth. The Sun reaches its red giant phase and fries the Earth and Moon in about 5 billion years. The Moon is moving away from the Earth at the rate of 3.78 centimeters or a bit more than 1 inch per year. In 5 billion years, the Moon will be (very roughly) 190,000 kilometers further away than it is today. This comes from taking the average distance the Moon moves away from the Earth (which, incidentally, is not constant) times 5 billion years. The red giant phase happens long before the Moon gets far enough away and way before the Moon returns to the Earth and breaks apart. Technically, at this point, the Sun will be between the Earth and the Moon (just not the entire Sun).
What if the Sun did not enter the red giant phase? It still doesn’t matter because the Moon won’t keep moving away from the Earth.
The tidal attraction between the Earth and the Moon causes both bodies to bulge toward each other. This adds a slight forward component to Earth gravity that pulls the Moon in the direction of the Earth’s rotation and into a higher orbit. This also slightly slows down the Earth’s rotation. But, eventually the Earth’s rotation and the Moon’s orbital period will lock, tidal forces will lag the Moon’s orbit, and the Earth will slowly draw the Moon back. Then, the Moon moves closer until it reaches the Roche limit at 18,470 km (11,470 miles). When the Moon comes inside the Roche limit, it will break apart. This process takes an estimated 15 billion years.
References
- Dickinson, Terence (1993). From the Big Bang to Planet X. Camden East, Ontario: Camden House. ISBN 978-0-921820-71-0.
- Murray, C.D.; Dermott, Stanley F. (1999). Solar System Dynamics. Cambridge University Press. ISBN 978-0-521-57295-8.
- Williams, D.R. (1 July 2013). “Sun Fact Sheet“. NASA Goddard Space Flight Center.
- Zombeck, Martin V. (1990). Handbook of Space Astronomy and Astrophysics (2nd ed.). Cambridge University Press.