BY AMY SAYLE
Join us at the Morehead Sundial on Monday, May 9, 2016 to see something no one has seen in nearly a decade: a transit of Mercury.
On May 9, Mercury will pass directly between Earth and the Sun and will look like a tiny dark spot crossing the Sun from 7:12 a.m. to 2:42 p.m. Eastern time.
The Morehead Sundial skywatching session is set for lunchtime hours, from 11 a.m. to 1 p.m., providing an opportunity to have a safe look at the transit through a solar telescope. This event is weather permitting. (Warning: Never view the Sun with the unaided eye, binoculars, or an ordinary telescope without a proper solar filter.)
You can also see a near-live feed of transit images from NASA’s Solar Dynamics Observatory at www.nasa.gov/transit.
Mercury transits happen only about 13 or 14 times each century. Although Mercury passes between Earth and the Sun much more frequently than that, it usually passes above or below the Sun from our point of view, rather than directly across. That’s because the plane of Mercury’s orbit around the Sun is tilted with respect to the plane of Earth’s orbit.
The transit is like a teeny-tiny eclipse, as the little disk of Mercury blocks out a tiny portion of the Sun’s light. These kinds of transits can happen with other solar systems as well. The Kepler Space Telescope has used the transit method to discover hundreds of “exoplanets” that orbit stars other than the Sun in such a way that, from our point of view, the planet periodically passes directly in front of its parent star, blocking out a tiny portion of the starlight that can be measured and used to infer the planet’s existence.
A transit of Mercury provides an opportunity to study Mercury’s extremely thin atmosphere and to test spacecraft and instruments. Historically, transits of Mercury have been used to measure the apparent size of Mercury and help estimate distance between Earth and the Sun.
Prepare to be underwhelmed visually by the transit of Mercury. If you viewed one of the most recent transits of Venus, in 2004 and 2012, you know how small that planet looked against the Sun. Mercury will look even smaller. In the solar telescopes at the Morehead skywatching session, you’ll be looking for a tiny black dot on the Sun, sort of like a sunspot, but very dark and very circular.
If you miss this transit of Mercury, your next chance is November 11, 2019. If that seems like a long time from now, it’s nothing compared to the next opportunity for a transit of Venus, the only other planet that can appear in front of the Sun from Earth’s viewpoint. That won’t happen again for another century, in 2117.
Image caption: The path of the November 2006 transit of Mercury, as seen from SOHO (Solar and Heliospheric Observatory)