Mercury makes rare journey across the sun

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Mercury's rare transit.

For seven-and-a-half hours Monday, the solar system's smallest planet was visible as it moved between the Earth and the Sun, blocking out a small amount of light.

Between 7:12 a.m. and 2:42 p.m. Mercury appeared as a small black dot moving across the Sun. The passage is known as transiting, and it was only visible through a telescope or binoculars.

The event is rare. On average, the transit happens just 13 times per century. Why is it rare? Well, Mercury's orbit is about 7 degrees off from that of our home planet, meaning that Mercury, the Sun and the Earth just don't line up all that often.

NASA scientist, Dr. Stephen Rinehart said despite the infrequency, the transits are an important part of the evolution of science. "If you go back about two centuries, people first used transit to build a yard stick for our solar system to understand just how big other planets are, and how far away they are," he said.

Astrophysicists such as Rinehart use this effect to look for planets around other stars, and they will use today's transit to continue learning and improving their understanding of the solar system.

"That data is going to be used to help calibrate the cameras from the mission and to understand better the data, which means that we are going to improve our understanding of what we are seeing," Rinehart said.

If you missed today's transit, don't worry. It will happen again in three years. After that, you'll have to wait until the year 2032 to see the next event.

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