Up in the Air: Mercury comes between us and the sun next month

In a few weeks, we will be treated to a celestial event that only occurs every several years – a transit of Mercury. This is when the closest planet to the sun comes directly between us and the sun, and Mercury is seen in silhouette.

The May 9 event will start just after sunrise and end mid-afternoon, but you need to have the proper equipment to look at the sun safely. You should never look at the sun without using the correct equipment and techniques.

This event dims the sun as seen by us, but only by a very small fraction. Yet, it is exactly this technique that astronomers use to find planets, so-called “exoplanets,” around other stars. Some of those exoplanet transits of their host stars have been found from ground-based telescopes, but the majority – over a thousand – have been found by the Kepler satellite. High above Earth’s atmosphere, the Kepler satellite could detect incredibly small variations in the brightness of stars due to exoplanets.

The details of the drop in the star’s brightness are used to measure the size of the planet – its diameter in miles, say. Astronomers had already been using this technique for a century to measure the size of stars before modern equipment allowed us to do it for exoplanets. For orbiting binary stars, those that eclipse each other, the measurements of the changes in light during eclipses yield the sizes of the stars.

We don’t get a transit of Mercury every time it comes between us and the sun (about every 116 days) due to the relative tilt of Mercury’s orbit with respect to Earth’s orbit. The 7-degree angle puts Mercury “above” or “below” our view of the sun for most of those alignments. This is similar to why we do not get a lunar eclipse at every full moon, or a solar eclipse (a transit of the moon) at every new moon: the tilt of the moon’s orbit being the culprit in that case.

We get a transit of Mercury about a dozen times per century, the last one being in 2006. The next one will be in 2019, when the eastern U.S. will only get a glimpse of the end of the event (South America has a favored view).

So, find a public-viewing event near you (check and give it a shot! Even in the age of space telescopes and the Internet, nothing beats seeing celestial objects with your own eyes. And, for your kids, it could spark an interest that might lead to a future in science, technology engineering, or mathematics: Astronomy is a gateway science to STEM careers!

Daniel B. Caton is a physics and astronomy professor, and director of observatories at Appalachian State University. Email: More on this month's column: