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The Question

(Submitted April 20, 1998)

I am studying to become an elementary teacher, and I am interested in how the distances of the planets and stars were discovered. I read the question asked about stars, but I am interested in how these methods were discovered. In other words, what is the history? It seems difficult for me to imagine how we can estimate these distances when they are so far away. It is especially interesting how scientists first judged these distances before we sent any space craft or satellites into space. Could you please tell me how I might begin to research this question?

The Answer

The people we call Ancient Greeks (some of whom lived in e.g., Egypt) did a lot of this stuff a few centuries B.C., to the limits of naked eye observation. Eratosthenes measured the size of Earth, to good accuracy. Hipparchus and Aristarchus measured the distances to the Moon and to the Sun respectively (the Sun distance was off by a factor of ~20).

The ancient Greeks knew about parallax, and from the fact that the stars didn't move over the course of the year, they determined that the Earth did not move--unless of course the stars were so ridiculously far away that the movement would not be seen (Aristotle, I believe).

Once people started flitting around the world in boats, navigation became important--which made it useful to determine the size of the Solar system to make calculations more accurate. Measuring the transit of a planet across the Sun's disk from multiple places on Earth was an accurate way of measuring the parallax distance. Some of Captain Cook's voyages were for this purpose.

A good college-level text on astronomy will give you a simplified history of the development of 'the cosmological distance ladder' which allows us to use a chain of different techniques, each calibrated against the previous one, to determine the distance to distant galaxies and quasars.

David Palmer
for Ask an Astrophysicist

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