Why does Mercury sometimes appear as a tiny dot crossing the Sun?

On May 9, 2016, the planet Mercury crossed directly between Earth and the Sun. Scientists call this event a transit. During the transit, Mercury appeared as a small black dot slowly moving across the face of the Sun. The whole crossing took about seven and a half hours. Transits of Mercury are rare because Mercury's orbit is tilted compared to Earth's orbit. The two orbits only line up perfectly about thirteen times every century. The next transit of Mercury will not occur until November 2032. Scientists use transits to study Mercury's thin atmosphere and to measure distances in the solar system. In the 1700s, astronomers used transits of Venus and Mercury to calculate how far Earth is from the Sun. Modern scientists use transits to study planets around other stars too. When a distant planet crosses in front of its star, the star's light dims slightly. By measuring that tiny decrease in brightness, scientists can figure out the size of the planet. This method, called the transit method, has helped discover thousands of exoplanets -- planets orbiting stars other than our Sun.