Editor’s note: The author, Shannon Schmoll, PhD, is Director of MSU’s Abrams Planetarium. The photograph above is of SunSpotter in use showing an image of the sun.
When people think of astronomy, they often think of the stars and planets in the darkness of night. However, we have a rare treat of a daytime astronomical event coming up. On Monday, May 9, 2016, the planet Mercury will transit in front of the sun. Weather permitting, East Lansing residents will be able to observe the phenomenon as explained below.
Mercury passes between the sun and the Earth several times a year, though its orbit is tilted out of the plane of the solar system slightly. As a result, most of the time Mercury is just a bit above or below the sun from our perspective when it passes. About thirteen times a century our position lines up just right for us to see Mercury cross directly in front of the sun.
Transits can only happen for planets inside Earth’s orbit, which means we only see this for Mercury and Venus. Venus transits are far rarer and happen in pairs eight years apart. The pairs then happen over a century apart each time. The last Venus transit was in 2012 and the next one will not happen until 2117.
On May 9th specifically, we will see Mercury start to pass in front of the sun as a tiny speck beginning at 7:13 a.m. for East Lansing. It will follow a straight line across the sun until 2:42 p.m. (Article continues after the following image, which shows the apparent path of Mercury across the sun on May 9, 2016. The times listed are in Universal time, four hours ahead of Eastern Daylight Time. This also gives a sense of size of Mercury compared to the sun.)
An observer must be on the day side of the Earth to see the transit. The entire transit will be visible from roughly the eastern half of the United States including East Lansing, most of South America, and the western most sections of Africa and Europe. The rest of the U.S. and most of Europe will see at least a part of the transit. It will not be visible from Australia and eastern most parts of Asia.
The last time Mercury transited was November 9, 2006, and next transit will be November 11, 2019. The next two transits after that will also be in November (2032 and 2039 respectively). This is not a coincidence as all Mercury transits happen in either November or May. The points in Mercury’s orbit where it crosses into the plane of the solar system are aligned with Earth’s position in November and May. The next May transit will not be until 2049. So, this may be the last time for a while we will be able to have a good chance for clear skies in Michigan to see this event.
If you want to safely observe the transit of Mercury, you will need to take precautions as it is not safe to observe the sun directly and can cause serious damage to your eyes. One way you can safely observe the sun is to purchase eclipse glasses that block out almost all light. However, Mercury will be so small, you will likely not see it cross the sun without some help from a telescope.
At the Abrams Planetarium we will have solar telescopes set up for viewing from 8:30 a.m. until the transit ends. We will be in front of the planetarium with two different viewing options. One is our SunSpotter, a small device that projects the image of the sun onto a piece of paper. (See photo at the top of this article.)
We will also have our Coronado solar telescope out. This is a special telescope with a Hydrogen Alpha filter on it. The filter only lets a very specific wavelength of light produced by hydrogen through the telescope. The amount of light that makes it to the eye piece is not enough to harm your eyes. In this telescope, the sun will appear in a distinctive reddish hue. (Article continues after this image, showing a view of the sun through a Hydrogen Alpha filter, the kind of filer in our Coronado solar telescope.)
In case of inclement weather, we will also have a drop-in session in the planetarium theater to watch a real-time simulation on the dome as well for people to just ask questions about Mercury. We will also attempt to show a live stream from other observatories of the transit in our lobby.
For more information on this particular transit, a map of where it will be visible, and the dates for transits through the year 2100, visit NASA’s page here: http://eclipsewise.com/oh/tm2016.html