What’s Mercury doing to the Sun?
Get ready for an amazing astronomical alignment! On May 9th, 2016, we’ll be able to see the planet Mercury during the daytime as its orbit takes it right across the face of the Sun! Astronomers call these events transits.
Both Mercury and Venus orbit closer to the Sun than the Earth does, so occasionally they’ll pass between the Earth and Sun, appearing as a tiny black dot against the Sun from our perspective. But it doesn’t happen every time they go around the Sun, because their orbits are tilted at different angles than the Earth’s orbit. The orbits have to line up just right for a transit to happen. Transits of Venus occur in pairs 8 years apart, with roughly 120 years between each pair of transits. The next one won’t be until 2117! Transits of Mercury occur more frequently, about 13 or 14 times in a century, because Mercury is closer to the Sun than Venus and orbits faster. The other planets (Mars, Jupiter, Saturn, Uranus and Neptune) never appear to transit the Sun from our viewpoint, because they orbit further away from the Sun than the Earth. If you were visiting any of those planets, though, it would actually be possible see the Earth transiting the Sun!
Transits aren’t just interesting because of their rarity. They were also useful to astronomers in the 18th century who were trying to determine the size of our solar system. By timing how long it takes each planet to orbit the Sun, astronomers already knew their relative distances from the Sun, so if they could make even one actual distance measurement then they would be able to work out all the others. The transits of Venus in 1761 and 1769 provided that opportunity. Astronomers observed Venus’ path across the Sun from different locations on the Earth and used geometry to calculate the distance from the Earth to the Sun, getting impressively close to the actual value (149,598,000 km). The Exploratorium has a really great in-depth discussion of the math. Check it out here.
Transits are also a way for us to hunt for planets that orbit distant stars. With a powerful enough telescope, astronomers can detect tiny changes in a star’s brightness if it has any planets whose orbits bring them between the star and the Earth. By carefully observing how Venus’ atmosphere affects our Sun’s brightness during a transit, we’re refining our techniques for discovering whether these faraway planets also have atmospheres, and what they might be made of.
So how can you observe one of these rare events for yourself? The first thing to know is that observing the Sun is dangerous. You risk permanent damage to your eyes if you aren’t being careful and using the right equipment.
Since Mercury appears very tiny next to the Sun, some kind of magnification will be necessary. You won’t be able to see Mercury with just your eyes, and you shouldn’t be looking directly at the Sun anyway! A telescope will do the job nicely, but it needs to have a solar filter in place to reduce the amount of incoming light to a safe level for both your eyes and the optics of the telescope to handle. A proper filter is made of either glass or plastic, with a metallic coating that reflects 99.99% of the light and heat from the Sun. Be wary of cheap Sun filters that come with inexpensive telescopes. They could crack in the heat, potentially blinding anyone looking through the eyepiece.
Since it will take some special equipment to safely observe the transit, your best bet might be to make friends with someone who already has the right gear, like the Sudbury Astronomy Club, who will be camped out in front of Science North on the morning of the 9th along with some of our Bluecoats (weather permitting). They’ll be ready to share their expertise and equipment with students and curious visitors.
For observers in and around Sudbury, Mercury’s upcoming transit starts at 7:13 am EDT and ends at 2:41 pm EDT on the 9th. For more information on the timing of this and other astronomical events, this website is a great resource. Here’s hoping for clear skies! Don’t fret if you miss out this time, though. Our next chance to see a transit of Mercury will be November 11th, 2019.
Happy (and safe) observing!