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How to Describe the Relative Size & Distance Relationships Among the Sun, Earth & Moon

In the words of humorist Douglas Adams, space is big. Really, really big. Improbably big. Conveying the scope of space is a challenge to teachers and educators, and even journalists and science fiction writers, because the numbers are simply past the normal boundaries of what human beings deal with in everyday life. Couple this with how space --- and space travel --- are portrayed by Hollywood, and you're up against a half-century or more of misinformation.

Instructions

- 1
  Ask your audience if they've ever had to fly coast to coast in the United States, from New York to Los Angeles --- a trip of about 3,961 kilometers. Explain that Earth's circumference is roughly 39,900 kilometers. So, a New York--Los Angeles flight covers only about 10 percent of the Earth's circumference.
- 2
  Compare that distance to the one between the Earth and the Moon, which is roughly 385,000 kilometers (not factoring in extremes in orbit). Therefore, the long flight between New York and Los Angeles represents a little over 1 percent of the distance from the Earth to the Moon.
  
  For another perspective, explain that the distance light travels in one second --- 300,000 kilometers --- is known as a light second. The Moon is so far away that it takes its light 1.3 seconds to reach Earth.
- 3
  Use light seconds to compare the trip to the Moon to the distance between Earth and the Sun. The Sun is approximately 500 light seconds away from the Earth, or 150 million kilometers. This happens to be 400 times the distance between the Earth and the Moon, and going back to our cross-country flight example, roughly 400,000 times the distance from New York to Los Angeles.
- 4
  Explain that since the Sun is 400 times farther away from Earth than the Moon, but both are the same size in the sky --- as shown by solar eclipses --- we know the Sun's true diameter is 400 times the true diameter of the Moon. Since the Moon's diameter is roughly one-quarter of the diameter of the Earth, or roughly 3,474 kilometers, this makes the Sun about 3,500 times 400, or about 1.4 million kilometers in diameter.

Astronomy