The May 9th Transit of Mercury

Dr. Mel Blake

The solar system is a flat disk shape for the most part, with the Sun at the center, and the planets moving around it on elliptical orbits.   The planets’ orbits all line up very close to the plane of the Sun’s equator.   It might be wondered if a viewer on the Earth can therefore see a planet pass directly between the Sun and Earth.   This indeed can happen for Mercury and Venus, which lie closer to the Sun and the Earth.  Such an occurrence is called a transit.  Mercury lies very close to the Sun, and so there are about 13 such transits every century.  On May 9^th of this year viewers will be treated to such an event.   From here in Alabama sky watchers should be able to see the entire event provided skies are clear.  However, you will need access to a telescope in order to see the tiny disk of Mercury pass between us and the Sun.   Never look directly at the Sun with a telescope that does not have proper screening.  The transit of Mercury will start at about 6:12 am on May 9^th, and end at about 1:42 pm in the afternoon.  So if you are watching the event, even if it is cloudy in the morning you might get some clearing later to see it.   

The transits of Mercury and Venus were crucial in the history of astronomy for setting the scale of the solar system.   Johannes Kepler used detailed positions of the planets to devise three laws governing the movement of the planets, and Newton built on these to come up with the law of gravity that laid out the physical cause of the planetary motions.  However, astronomers could only determine the positions of the planets relative to the size of the Earth’s orbit.  To get the actual distances, they needed to determine the distance to one planet and then use that to get the distances to all the others.   When a transit occurs, observers from two different locations on the Earth will see the path of the planet across the Sun at slightly different locations on the Sun’s disk (see the figure below).   This offset, or parallax, can be used to solve for the distance to the planet if the distance between the two observers is known.  Transits of Venus and Mercury were therefore important and great efforts went into observing these events from places on earth that were as distant as possible.

UNA Planetarium will have solar telescopes available for people to view the transit.