Transit of Mercury

On Monday, November 11, 2019, one of my favorite celestial events occurs – a transit! Think of a transit as an “eclipse lite” – a planet will pass between Earth and the Sun, and we’ll be able to observe the planet’s shadow moving across the Sun. Here are six questions to ponder during this month’s transit of Mercury:

1. What exactly happens when Mercury transits the Sun?

For a transit of Mercury to be visible from Earth, the orbit of Mercury needs to line up just right, such that from our perspective, the planet goes directly between us and the Sun. So what does that look like? During a Mercury transit, a small dot will cross the face of the Sun over a matter of hours – this upcoming transit will take about 5 hours. This tiny spot covers a lot less of the Sun than what during a solar eclipse, when the Moon (our closest neighbor) covers most or all of the Sun. Mercury is only a little bit bigger than our Moon, and is about 30 million miles away from the Sun on average and more than 50 million miles from us. That means it will appear small and won’t cover up much of the Sun at all – but we’ll still be able to tell that it is, in fact, a planet.

2. When do transits of Mercury happen?

Mercury transits the Sun from our perspective 13 to 14 times per century! This is due to its fast orbit around the Sun, which is just 88 Earth days. We actually observed the last transit here at the Museum in 2016 (until it got cloudy in Washington, DC). The next transit of Mercury isn’t until 2032, and the next one visible from Washington, DC, won’t happen until 2049! How do we know this? Scientists can calculate the orbits of planets so that we can predict transits for centuries into the future, and accurately date historic transits as well. All modern transits of Mercury occur within days of May 8 and November 10, based on the inclination of Mercury’s path through space. These dates are slowly shifting – before 1600, transits of Mercury occurred in April and October.

3. Does a transit cause things to change here on Earth?

Do transits cause things to happen here on Earth? Do they cause dark shadows like an eclipse, or create light shows in our atmosphere like meteor showers? The answer is that transits have no effect on the natural goings-on here on Earth, but they can teach us a ton about physics, our solar system, and other planetary systems! For example, transits of planets like Mercury can give us an accurate measurement of the distance between the Sun and the Earth. By observing the change in the planet’s position during the transit from two different sites on Earth, a person can measure the absolute distance between the Sun and Earth.

4. Mercury is in retrograde! Does that mean this transit is special?

When Mercury is in retrograde, it appears to have changed its direction of motion in the sky from our perspective here on Earth. However, Mercury hasn’t changed direction at all – instead, it has just lapped us in its path around the Sun! This happens during three to four periods throughout the year. It also means that Mercury is always in retrograde when it transits the Sun – when Mercury goes between us and the Sun, it is in the part of its orbit that makes it appear as it is moving from East to West!

5. Can things other than Mercury transit the Sun?

Depends on where you are! For a transit to occur, it has to be between us and the Sun. So from Earth, the only planets that transit the Sun are Venus and Mercury, because they are closer to the Sun than we are. That also means that if you were on another planet, like Mars, the Earth and our Moon would transit the Sun!

6. Can I view the transit myself?

Definitely! You will need specific tools to safely view the Transit of Mercury. Here are some options for you to try on Monday:

• Build your own projection rig.
• Watch it online!
• Visit your local observatory to use their safe solar telescopes.

We’ll be open from 7:30 am-1:00 pm at the Phoebe Waterman Haas Public Observatory, so stop by!