A rare transit of Mercury across the Sun occurs 11 November 2019. Here's a guide to when and how to observe the planet's small disk crossing in front of the Sun. Check it out if you can—the next Mercury transit isn't until 2032!
What is a transit?
A transit in this context is when one celestial body (e.g., Mercury) passes in front of another body (e.g., the Sun) and blocks a small portion of it as seen from a third celestial body (e.g., Earth).
What is a Mercury transit?
A Mercury transit of the Sun as seen from Earth occurs when the Sun, Mercury, and Earth line up so that Mercury appears in front of the Sun as a small black disk moving across the Sun for several hours.
How often and when do Mercury transits occur?
Mercury transits occur about 13 to 14 times per century. The last one was in 2016, but the next isn't until 2032. The timing is tied to not only the interplay of the orbital periods of the two planets, but also the relative tilt of the two orbits. Mercury's orbital plane is tilted a few degrees relative to Earth's orbital plane. The Sun-Mercury-Earth line-up can only occur when Mercury is passing through the plane of Earth's orbit, and only if that occurs when Earth is in the right part of its orbit. Random Space Fact: currently Mercury transits can only occur during May or November. Those are the times in Earth's orbit when Mercury can line up with the Sun IF Mercury is at the right point in its orbit.
From where is the 11 November 2019 Mercury transit visible?
This transit will be visible from South America and Africa, and most of North America and Europe. All that is required for you to see it is that it's daytime at your location. For Europe and most of Africa, the Sun will set while the transit is still occurring. For the east coast of North America and all of Central and South America, the entire transit will be visible. For central and western North America, the Sun will rise with Mercury already in transit.
What time is the 11 November 2019 Mercury transit?
Here's a handy table to help you out. The start time is when Mercury just broaches the edge of the Sun, and the end is when it moves off the Sun on the opposite side.
Start of transit
Middle of transit
End of transit
How can I observe the transit safely?
Because of Mercury's small size and substantial distance from Earth, it will require a telescope to see. When using a telescope, it is crucial to use proper safety filters over the front of the telescope or you risk serious eye damage. Do NOT look through a telescope, a telescope's finder scope, or binoculars (or with just your eyes) at the Sun without filters designed for solar viewing, such as those that can be obtained from reputable telescope makers. If you are unsure whether you have the right equipment, try to find a public viewing in your area. You can also watch live streams from several telescopes around the world, right from the comfort of your own home! These include the following:
With a small telescope, Mercury will be a tiny dot on the Sun moving across the whole face of the Sun over the several hours of the transit. A larger telescope will distinguish Mercury as a black disk, but it will still be tiny compared to the Sun. Frankly, the view of the transit is not visually spectacular. There's a little black spot on the Sun. What is spectacular is that you are viewing another planet tens of millions of kilometers away passing directly in front of the Sun (ponder that)! You are witnessing a rare celestial event that won't occur again until 2032.
Do any other planets transit the Sun as seen from Earth?
Finally, one of the ways we learn about exoplanets is by watching them cross in front of their host stars! We can't see exoplanets as a single disc like Mercury, but we know they are there due to temporary dips in their stars' light.