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Mercury Transit

May 6, 2016

A tiny black dot will cross the face of the Sun early Monday: Mercury, the Sun’s closest planet. The entire event — known as a transit — will be visible across the eastern half of the United States, with the rest of the country seeing most of it. Don’t look at it directly, though — the Sun is too bright. But many museums and planetariums will display it, and you can also watch it online.

During the latter half of the 19th century, astronomers hoped to use transits to find another solar system body: Vulcan.

Oddities in the orbit of Mercury suggested that the little planet was being pulled by the gravity of something even closer to the Sun. At first, astronomers thought it was a planet. Later, they decided it might be several large asteroids. Such bodies would be too close to the Sun to see directly, but they should occasionally transit the Sun. Observing a transit would yield the object’s size and its distance from the Sun.

In 1859, an amateur astronomer in France reported seeing something transiting the Sun, which the pros interpreted as Vulcan. Unfortunately, though, no one else saw it. Repeated attempts led to a few other sightings — but none was ever confirmed.

In 1915, it became clear why. Albert Einstein’s theory of gravity showed that the Sun warps the space around it. That accounts for the shift in Mercury’s orbit — without the need for another planet.

You can find out more about Vulcan in “The Hunt for Vulcan,” by Thomas Levenson. And we have an excerpt in the current issue of StarDate magazine. Subscription information at


Script by Damond Benningfield

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