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Sizes and Distances: The Earth-Moon-Sun System

Just how big, and how far, are the Sun and Moon? Compare their sizes and distances, and discover the 400x coincidence behind total solar eclipses.

⏱️ 16 min 🎯 13 activities
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What you'll cover

How big, how far?

The Earth, Moon and Sun are wildly different in size and distance, yet the Sun and Moon look almost the same size in our sky. This module measures the system and reveals the coincidence that makes total solar eclipses possible.

The diameters

Learn the approximate diameters (note the powers of ten):

Match each body to its approximate diameter

  • The Sun
  • The Earth
  • The Moon
  • About 1,400,000 km
  • About 12,700 km
  • About 3,500 km

Smallest to largest

Put the three bodies in order of diameter, from smallest to largest.

  • The Moon (about 3,500 km)
  • The Earth (about 12,700 km)
  • The Sun (about 1,400,000 km)

The distances

Now the distances: - Earth to Moon: about 384,000 km. - Earth to Sun: about 150,000,000 km (1.5 x 10^8 km), called 1 astronomical unit (AU). The Sun is vastly further away than the Moon.

Earth to Sun

The mean Earth-Sun distance is 1.5 x 10^8 km, which is 150,000,000 km. How many MILLION km is that?

A 400x coincidence

A remarkable coincidence: the Sun's diameter is about 400 times the Moon's, and the Sun is also about 400 times further away. Because both ratios are about 400, the Sun and Moon appear almost the same size in our sky, which is why total solar eclipses are possible.

How many times bigger?

The Sun's diameter is about 1,400,000 km and the Moon's is about 3,500 km. How many times larger is the Sun's diameter than the Moon's?

Same size in the sky?

Why do the Sun and Moon look almost the same size from Earth?

  • The Sun is about 400 times bigger but also about 400 times further away
  • They are actually exactly the same size
  • The Moon is really larger than the Sun
  • The Sun is very close to the Earth

True or false

Select the TWO true statements about the Earth-Moon-Sun system.

  • The Sun's diameter is about 1.4 x 10^6 km
  • The Earth is larger than the Moon
  • The Moon is larger than the Sun
  • The Earth-Sun distance is smaller than the Earth-Moon distance

Match each quantity to its value

  • Earth-Moon distance
  • Earth-Sun distance
  • 1 astronomical unit (AU)
  • The Sun's diameter
  • About 384,000 km
  • About 150,000,000 km
  • The mean Earth-Sun distance
  • About 1.4 x 10^6 km

Sizes-and-distances frame

The Sun's diameter is about 1.4 x 10^6 km, far larger than the Earth (_____ km) and the Moon (_____ km). The mean Earth-Sun distance is 1 astronomical _____, about 150 million km. The Sun looks the same size as the Moon because it is about _____ times bigger and 400 times further away.

12,700 3,500 unit 400 1,400,000 384,000 mile 100

Explain the eclipse

Explain why the Sun and Moon appear almost the same size in the sky, and why this makes a total solar eclipse possible. Use the sizes and distances you have learned.

  • State that the Sun is about 400 times bigger than the Moon
  • State that the Sun is also about 400 times further away
  • Explain that the two ratios cancel, so they look the same size and the Moon can cover the Sun