ransits of Mercury have long helped astronomers since viewing the event for the first time in 1631. In addition to providing a better understanding of the planet’s disk size and the distance between the Earth and sun, transits now allow NASA to test their spacecraft and calibrate their instruments.

A transit of Mercury only occurs when it is passing between the Earth and sun, and passing the Earth’s orbital plane, which occurs in early May and November each year. The next occurrence will not be until November of 2019, and then not again until 2032.