How do scientists know the distance between the planets?

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Two cartoon girls look at the night sky.

Our friends at the Museum of Aviation in Robbins, Georgia, asked how astronomers know the distances between the planets.

The planets look like pinpoints of light to our unaided eyes, and we can't simply look up at the sky and tell how far they are from Earth or from each other, or even see that Earth and the other planets actually revolve around the Sun. But astronomers have come up with some brilliant ways to discover their distances.

Johannes Kepler

Until less than 500 years ago, most people thought Earth was the center of the solar system. By the time Johannes Kepler was born in 1571, people were starting to get the idea that the planets revolve around the Sun. Kepler was the first to explain the puzzling movements of the planets by realizing that their orbits around the Sun are not perfect circles, but instead are ellipses, like elongated circles. Kepler discovered that the motions of the planets could be described very accurately by some simple mathematical formulas. The closer a planet is to the Sun, the faster it travels, and Kepler found a method to connect the average distance of a planet from the Sun to the time it takes the planet to make a complete orbit around the Sun.

Planetary orbits.

Kepler's discoveries allowed him to figure out how much closer or farther all the planets are to the Sun than Earth is, even though he could not figure out the actual distances. For example, he knew Mars is closer than Saturn, because one Mars orbit takes less than 2 years, while one Saturn orbit takes about 29 years. Although he couldn't use that to calculate their real distances, he did figure out that Mars is about 1.5 times farther from the Sun than Earth is, and Saturn is 10 times farther away than the Sun.

If only astronomers could determine the distance from Earth to any other planet or the Sun, they could use that to find the distances to all the planets, and their understanding of the arrangement of the solar system would be greatly improved.

Gian Domenico Cassini

One of the first people to make a good measurement of the distance to a planet was the great astronomer Gian Domenico Cassini. In 1672, Cassini used a technique called parallax to measure the distance to Mars.

You can understand parallax by holding your thumb up at arm's length and looking at it first with one eye, and then your other. Notice how your thumb seems to shift back and forth against the objects that are farther away. Because your two eyes are separated by a few inches, each views your thumb from a different position. The amount that your thumb appears to move is its parallax. When astronomers measure the parallax of an object and know the separation between the two positions from which it is observed, they can calculate the distance to the object. Using observations on Earth separated by thousands of miles -- like looking through two eyes that are very far apart -- parallax measurements can reveal the great distances to planets.

Although he didn't get quite the right answers, Cassini's results were very close to the correct values. The Sun is about 93 million miles from Earth. As Earth and Mars move in their separate orbits, they never come closer than 35 million miles to each other. Saturn, the most distant planet known when Cassini was alive, is around 900 million miles away. Imagine how exciting it must have been for him to discover that the solar system is so fantastically big!

Parallax used to measure distance to stars.

Astronomers can use parallax to find distances to objects much farther even than planets. To calculate the distance to a star, astronomers observe it from different places along Earth's orbit around the Sun. If they measure the object's position several months apart, their "two eyes" will have a separation of well over 100 million miles!

Now astronomers have technologies to measure distances to other planets more directly. When we have a spacecraft at another planet, we know the time it takes a radio signal to travel between Earth and the spacecraft. We can also send a powerful radar signal toward a planet and time how long it takes for the echo to return. Astronomers know how fast these signals travel (the speed of light), so measuring how long they take makes it easy to calculate the distance very accurately.