Watch Mercury make a rare transit across the sun on Monday

Starting at 7 a.m. ET Monday morning, Mercury will be visible as a small, dark circle going across the sun.

Mercury passing in front of the sun happens just 13 times a century — the last time was in 2006 — and the event will last in the United States for at least seven hours. NASA says it will be visible in the Americas, Europe, Africa, and large parts of Asia, and has a warning — in order to see Mercury, don't stare at the sun, but instead use a "telescope or high-powered binoculars fitted with solar filters made of specially-coated glass or Mylar."

The first transit was observed in 1631, NPR reports, and previous ones have been used to measure the distance between the Earth and the sun. Today, scientists wait for Mercury to pass in front of the sun in order to study its exosphere. "Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there," NASA scientist Rosemary Killen of the Jet Propulsion Laboratory said in a statement. For those who don't have a special telescope or pair of binoculars handy, NASA will be livestreaming the event. Catherine Garcia

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