On May 9, Mercury moved across the face of the sun. This process, called a transit, took seven and a half hours. From Earth, Mercury looks just 1/158 as wide as the sun, so you can’t see its transit with your naked eye. When magnified by a telescope, it looks like a tiny black dot. Astronomers use transit events like this to search for planets around distant stars. As the planets move across their host stars, telescopes pick up changes in the stars’ brightness. This data helps astronomers calculate the size and orbit of the planet that caused the change. NASA’s Kepler mission has used this method to detect 1,041 planets.
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