Transit logo
Why Do
Transits
Happen?

Conjunctions

A transit occurs whenever a planet comes between the the earth and the sun. At that time we see a small black disk crawl across the face of our star. Obviously, only the planets Mercury and Venus can ever do this because those are the only planets with orbits smaller than the earth's, and hence the only planets that can get between us and the sun. Although the outer planets (Mars through Pluto) can be seen in the same direction as the sun, they are always on the far side as seen by us. Both Mercury and Venus can appear on the far side too, but no transit will occur in that case. When Mercury or Venus passes between us and the sun, we call that event an inferior conjunction . If they are on the far side of the sun (but seen in the same area of the sky), we call that a superior conjunction . Mercury moves one complete revolution around the sun every 88 days. Since we view this from the moving earth, it takes a little extra time (28 days!) for Mercury to catch up with us so it can be at inferior conjunction again. The time from one inferior conjunction to the next inferior conjunction is therefore 116 days.

Extra Credit
Just to give you a little vocabulary, the time it takes the planet to go once around the sun (360°) is called its siderial period . The time from one alignment to the next (inferior conjunction to the next inferior conjunction, for instance) is called the synodic period .

Orbits

We mentioned in " Timing, Timing " that the last transit of Mercury visible in the United States happened more than 25 years ago. You might wonder why this is so if we find Mercury at inferior conjunction about every 116 days. The explanation is not too difficult to visualize. As the earth orbits the sun it defines a plane we call the ecliptic . Most of the planets orbit in pretty much the same plane, but not exactly. Mercury orbits in a plane that is tilted 7 degrees to our orbit. From our point of view therefore, the great majority of the time Mercury passes above or below the sun when it is at inferior conjunction. To make matters even worse, at inferior conjunction, Mercury is closer to us and the perceived angle above and below the ecliptic is even greater.

Mercury's Orbit
Click the picture above for a QuickTime animation (328k) showing Mercury's movements as seen by us on our moving earth. Here you can see Mercury and its orbit from our viewpoint on earth as we go around the sun. Note that most of the time (except for Nov 15th, 1999) Mercury would pass above or below the sun.
Produced with Starry Night by Sienna Software.

Sizes

The sun is only 1/2 degree across in the sky. This is like viewing a quarter coin (about 1 inch diameter) placed 9-1/2 feet from your eyes. Mercury is much smaller still, only 1/200 the diameter of the sun as seen during the transit, only 10 arc-seconds across. This is like viewing that same quarter from 3-1/2 miles away! This lead to some serious difficulties for our webcast. You can easily see the tiny disk of Mercury through a good quality consumer telescope (see " How To View Safely " section), but we greatly reduce the resolution when we put a TV camera on the telescope. A video signal has only 640x480 pixels of resolution, much less than your eyes. We then take that lower resolution signal and further muddy the waters by making an even lower resolution (320x240) webcast picture. The JPEG webcast compression of the video further degrades the image. We hoped that there would be enough left for you to see! Fortunately, since we were using the second largest refractor in the world, the giant 36" Lick Refractor at Lick Observatory on Mount Hamilton, we had a monstrous telephoto lens that helped magnify Mercury's tiny disk. It worked beautifully! Thank you Lick Observatory!