Planet Mercury to blaze across Sun today

November 8, 2006

A rare, but normal, astronomical event occurred today as the planet Mercury passed in front of the Sun from the perspective of the Earth. The event, called a was at least partially visible from most of the planet, except from Europe when the transit happened during local nighttime.

Visible as a perfectly round dot only 1/195th the diameter of the Sun, Mercury took about an hour and a half to move across the southern limb of the Sun, starting at 7:12PM UTC and ending at 9:41PM UTC.

From the point of view of the Earth, transits of this type can only occur with the two innermost planets - Mercury and Venus. Due to the orbital inclination of Mercury - 7° relative to the Earth - transits do not happen every time Mercury passes the Earth every 116 days. Only once every 23 times does a transit happen. The last time Mercury transited the Sun was May 7, 2003. The next transit will occur on May 9, 2016.

Such rare events have fascinated astronomers since the invention of the telescope first allowed them to view them. , for example, found Australia's east coast while on a voyage to track the transit of  in 1769. Recording these transits allowed astronomers to make the first accurate calculations of the distance between the Sun and the Earth.

While not scientifically used for this purpose anymore, transits provide a glimpse into one way astronomers can currently search for extra-solar planets. Astronomers using this method of searching will monitor other stars to see if the star dims ever so slightly as an extra-solar planet passes between the star it is orbiting and the Earth. Being able to detect such a small, temporary drop in brightness may indicate the presence of such a planet.

Viewing solar transits are fascinating, but only if done safely. People must never look directly at the Sun at any time! The safest way of viewing a transit is to project the image of the Sun as seen through a telescope onto a screen. It may also be viewed with properly made and placed solar filters.