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Planetary News: Mercury (2006)

Mercury Transit on November 8 -- Watching our Solar System at Work

by Melanie Melton Knocke
November 2, 2006

Click to enlarge > 1999 Mercury Transit The November 1999 Mercury transit was captured by the Transition Region and Coronal Explorer (TRACE) mission. TRACE is a mission of the Stanford-Lockheed Institute for Space Research and part of NASA's Small Explorer program. Credit: TRACE

It's one thing to understand intellectually the forces at work in our solar system. It's another thing to see immediate, physical proof of those forces. On November 8, 2006, you will get your chance to do just that when Mercury passes between Earth and the Sun.

Now, given the nature of this event -- known as a Mercury transit -- the forces at work are going to be a little difficult to see. In fact, while it is occurring, most people on our planet won't have a clue that anything special is going on. Even those who know about the transit will require specialized observing equipment to see it.

Even so, it will be worth the effort. Below are some examples of solar system forces you will be able to observe firsthand during this five-hour transit, as well as tips on when, where, and how to observe Mercury's passage across the Sun's disk.

Our Solar System at Work

1. During this transit, you will see that Mercury is actually moving.
While this may seem like an obvious point, when was the last time you actually saw a planet move across the sky? The movement of planets is usually subtle, requiring at least a couple of observations throughout an evening or even from night to night before you can register a slight change in its position with respect to the background stars.

During this transit, the Sun will provide an excellent (though bright) backdrop that will allow you to see Mercury's motion clearly. The planet will be visible as a tiny black dot moving slowly and steadily across the bright solar disk. Though the entire transit will take five hours to complete (see exact times below) it will still be moving fast enough for you to notice changes in its position in just a few minutes.

2. You will notice that Mercury is round.
You will see that Mercury definitely meets the "spherical shape" part of the International Astronomical Union's official definition of a planet. The added benefit is that seeing the sharply-defined disk of Mercury silhouetted against the fiery solar backdrop is really pretty awesome.

3. You will see just how small Mercury is.
Most of us know that Mercury is the smallest planet in the solar system (since Pluto was kicked out of the category in August). Here is your opportunity to see just how small it really is.

Keep in mind these numbers: the diameter of the Sun is 1,392,000 kilometers (864,400 miles) and the diameter of Mercury is 4,880 kilometers (3,030 miles). In other words, Mercury will cover a mere 0.06% of the Sun's surface.

4. As you watch the transit progress, you will notice that the orbit of Mercury is tilted with respect to the Sun.
One might expect Mercury to travel horizontally across the center of the Sun. Instead, the orbits of both Mercury and Earth are tilted with respect to the Sun's equator. As a result, Mercury will be cutting across the Sun at an angle and won't go through the center.

If the face of the Sun were a clock with its top (north) representing 12:00, Mercury would first cross in front of the Sun at the 4:30 position and exit the Sun at the 9:00 position. NOTE: These are not actual event times, merely position markers!

Transit Facts

1. When does the transit take place?
The transit will begin at 11:12 a.m. (PST) when Mercury first crosses the Sun's disk.

The halfway point of the transit will be at 1:41 p.m. (PST).

The transit will end when Mercury moves away from the Sun at 4:10 p.m. (PST).

2. Who can see it?
Viewers in North and South America, eastern Asia, Australia, and New Zealand will be able to see at least a portion of the transit.

Viewers in North and South America will get to see the beginning of the transit. The Sun will set before the transit is complete (although the west coast of the U.S will be able to see the transit in its entirety).

For viewers in eastern Asia, Australia, and New Zealand, the transit will already be underway when the Sun rises above your horizon on November 9. You will get to see the end of the transit.

The entire transit will be visible over most of the Pacific Ocean, southern Alaska and along the western coast of the United States.

People in Europe, Africa, western Asia, the Middle East, and India are out of luck this time around.

3. How to Safely Observe It.
The best way to observe this transit on your own is with a telescope and a solar filter. The telescope should have enough magnification so that the solar disk entirely fills its field of view. (Because the Full Moon and the Sun are approximately the same size in our skies, you can test your magnification on the Full Moon on November 5.) A solar filter should fit securely over the end of your telescope, in front of its optical system. With this arrangement, harmful solar radiation will be filtered out BEFORE it enters the telescope.

It is extremely important to use filters that were designed specifically for solar viewing. Though Mercury transits are fairly rare (only 13-14 occur each century), they are not worth losing your sight over. If you are not absolutely sure of the safety of your solar filter, DON'T RISK IT!

4. Viewing Without Equipment Hassles.
If you don't have a telescope and solar filter, don't fret. Many local planetariums, science centers, and astronomy clubs may set up observing opportunities for the general public. To find a planetarium in your area, visit the Astronomical Society of the Pacific's website. To find a hands-on science museum in your area, visit Carnegie Mellon University's website. Also, University of Hawaii astronomers will use special telescopes at the summits of Haleakala and Mauna Kea to transmit live images of the transit over the internet as a "Mercury Transit Hawaiian Style" webcast.

Whether or not you get a chance to view this month's Mercury transit, you've now had a little refresher course on the workings of our solar system, courtesy of its smallest planet..