Mercury begins its May 9 transit across the face of the Sun in this NASA image. The planet is the tiny black dot at the lower left of the Sun's disk. A large sunspot group is visible to the upper left of the center of the disk, with smaller spots below it. This was the first Mercury transit since 2006. [Bill Ingalls/NASA]
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Mercury Transit II
The bodies of the solar system are like the hands on a complicated clock, returning to the same point at regular intervals. The Sun returns to the same position against the stars, for example, in exactly one year. The Moon crosses between Sun and Earth about once a month. And Mercury crosses in front of the Sun in May every 13 or 33 years.
In fact, early tomorrow the Sun’s closest and smallest planet will stage just such a crossing, known as a transit. It begins when Mercury’s outline first makes contact with the solar disk at 6:12 a.m. Central Daylight Time, and ends five-and-a-half hours later.
Because of the orbits of Mercury and Earth, Mercury transits can happen only in May and November. The May transits take place when Mercury is near its farthest point from the Sun, and its closest point to Earth. As a result, Mercury covers a slightly larger fraction of the solar disk during May transits.
Because of that geometry, though, Mercury is more likely to miss the Sun, so May transits are only half as common as November ones. They happen at intervals of 13 years, followed by 33 years.
All of tomorrow’s transit will be visible from east of a line from Louisiana to North Dakota. From west of that line, the transit will be underway as the Sun rises. Don’t look at the Sun directly because it’s dangerously bright. Instead, find the transit online or at a local museum or planetarium — and enjoy the clockwork precision of the solar system.
Script by Damond Benningfield