Transit of Mercury

Transit of Mercury

What is the Transit of Mercury?

The planet Mercury will provide one of the most arresting celestial sights of 2019 when, on Monday 11 November, it will pass across the face of the Sun. It’s an event that’s among the rarest of predictable astronomical phenomena. The transit will be visible across almost all of North America, South America, Europe, Africa and western Asia.

Similar to a solar eclipse, a transit occurs when a planet passes directly between the Earth and the sun. Transits are rare and only occur about 13 times every 100 years, with the next one not happening until Nov. 13, 2032.

However, to see the transit of Mercury next month, casual spectators will need the proper safe solar glasses (our eclipse glasses) to safely look at the sun, as well as cloud-free weather. Order now (by clicking below box) so that the glasses arrive in time for the November transit.

What can be seen during a Transit of Mercury?

The tiny black disc of Mercury, just 10 arc seconds across, will take about five and a half hours to pass across the Sun. It will cross from east to west.

Xavier Jubier, an eclipse-chaser from Paris, France and a member of the IAU Working Group on Solar Eclipses found another way to view Mercury’s last solar transit.

“It’s actually visible to the naked eye with solar eclipse glasses,” he says. “Although the apparent diameter of Mercury is much smaller than Venus, which means that without enough magnification it’s more difficult to observe.”

He recommends using a telescope with a focal length of 1,000mm, though any smaller telescope will get you a view of Mercury as a tiny black dot.

“Simple and inexpensive solar scopes can also be used,” he says.

The transit starts at 4:35 a.m. PST, but viewers in certain areas, such as the West Coast, won't be able to see it until the Sun is visible in the sky. Thankfully, this transit will last about 5.5 hours, so there will be plenty of time to catch the show. At approximately 8:20 a.m. PST, Mercury will be as close as it is going to get to the center of the Sun. 

Safely watching The Transit of Mercury

A word of caution to all prospective viewers: Mercury will appear as a black dot, only about 0.5% the diameter of the sun, so a telescope magnifying at least 50 power will be needed to see it well. Yes, as mentioned above by Xavier, it will still be possible to see with the naked eye (and our eclipse glasses).

If there are any sunspots on the sun's disk, take note of how much darker the silhouette of Mercury appears to be compared to a sunspot. In addition, special precautions must be taken when viewing the dazzling solar disk. Be very careful to never look directly at the sun with a telescope. The visual requirements are identical to those for observing sunspots and partial solar eclipses — you need to use safe solar filters to protect your eyes. 

If you are using a telescope with a large aperture, say 8 inches (20 centimeters) or more, you should place a circular mask in front of the objective lens or mirror to "stop down" the image, thereby reducing the amount of light and heat striking the lens or mirror.