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May 7, 2016

A couple of the small worlds of the solar system will cross in front of two different stars on Sunday and Monday. One passage will be visible only from the other side of the world, while the other is well placed for viewers in North America.

Early tomorrow, the Moon will pass in front of Aldebaran, the leading light of Taurus, the bull, blocking the star from view. This “occultation” will be visible from parts of Asia, Africa, and Europe, but just barely — the Moon and Aldebaran stand so close to the Sun that they’re difficult to spot through the evening twilight.

And early Monday, Mercury will cross in front of the Sun. The Sun’s closest planet is so tiny, though, that it won’t block much of the sunlight. Instead, Mercury will look like a tiny dot tracking across the Sun’s disk. At least part of the transit will be visible across all of North America, although you don’t want to look at it directly — the Sun is just too bright. Instead, follow it online or at a local museum or planetarium.

In centuries past, astronomers used transits of Mercury, and especially of Venus, to measure the scale of the solar system. Watching transits from different points on the globe gave them a 3-D view, allowing them to plot the distances to the Sun and the transiting object. That showed that the Sun is about 93 million miles away — providing a good “yardstick” for measuring the scale of the solar system.

More about the transit of Mercury tomorrow.


Script by Damond Benningfield

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