Astronomy: There are lessons to be learned from transit of Mercury across sun – The Columbus Dispatch

Observing the transit of an inner planet requires a telescope with a solar filter that allows viewing the sun without eye damage. The transit appears as a small black dot that slowly moves across the face of the sun. It isnt visible without magnifying the suns image.

The transit of Mercury is a bit like what happens in a solar eclipse. Instead of the moon coming between Earth and the sun, which can entirely block the suns light, Mercury is much farther away, so it blocks only a small fraction (less than half a percent) of the sun. But even though its only a small black dot, its clearly visible with the right telescope.

The historical importance of the transit of Mercury goes back to the 1600s, when Johannes Kepler, famous for his planetary orbital laws, was the first astronomer to predict it. Back then, there was still public controversy about the model of Copernicus, where planets go around the sun, and the older view that the sun orbited Earth. The transit of Mercury was first observed in 1631 by French astronomer Pierre Gassendi, and was irrefutable evidence that Copernicus was right.

In 1677, Edmund Halley, for whom Halleys comet is named, realized that he could use the transit of Mercury to find the distance to the sun. He did that by measuring the time of the start and finish of Mercurys shadow as it went across the sun, and then used the mathematical technique of parallax to calculate the distance. This was a tour-de-force calculation for its time.

Today, the transit of Mercury is more of a curiosity than a groundbreaking scientific event. However, an interesting application of this technique has been applied to look for planets around other stars. The NASA space telescope Kepler made very careful measurements of the brightness of stars, and found periodic times when the brightness dipped by about 1%. Thats expected to happen when an exoplanet orbits the star and comes between that star and the view from Earth. Thousands of exoplanets have been discovered using the method.

The transit of planets can be used as a great teaching moment for amateur astronomers, either young or old. Its one thing to learn about the planetary orbits in a textbook, but another entirely to see a planets shadow move across the sun in real time.

Kenneth Hicks is a professor of physics and astronomy at Ohio University in Athens.

hicks@ohio.edu

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Astronomy: There are lessons to be learned from transit of Mercury across sun - The Columbus Dispatch