A Transit and an Eclipse

(This column was first published in the May 5, 2003 issue of The Buffalo News.)

Within the next ten days two interesting astronomical events will occur: one quite rare, the other common; one during daylight, the other at night.

At dawn on Wednesday morning, May 7, the planet Mercury will pass between the Earth and the Sun. Astronomers call this a transit of Mercury.

Only two planets, Mercury and Venus, cross between us and our star and neither event is common. Transits of Mercury usually occur on average about every eight years while transits of Venus happen in pairs still less often: over a century apart. The next Mercury transit will be November 8, 2006 and the next pair of Venusian transits will be June 8, 2004 and June 6, 2012. The most recent previous transit of Venus occurred in 1882.

These events are of particular historical interest. Observations of transits of Venus in 1761 and 1769 gave scientists the first good estimates of the distance from the Earth to the Sun. (Interestingly, one of the expeditions undertaken to observe those transits also provided a test of one of the clocks John Harrison constructed to solve the problem of calculating longitude.)

For several reasons, however, western New York will not be a good place to see the transit of Mercury Wednesday morning. Most of the passage will take place while the sun is still below the horizon and, even if the east is not cloud covered on that day, less than four minutes of the transit will be observable.

Not only that, but the size of Mercury is so small compared to the size of the Sun ã its diameter only 1/158 of the Sun's ã that a 50 to 100 power telescope will be required even to see it.

Finally, and most important, looking at the Sun at such magnification can blind an observer unless appropriate filters are used. An alternative is to project the telescope image onto a flat surface, but even then constant attention is called for as the surface may catch fire.

The other astronomical event, a total lunar eclipse, will happen on the night of May 15-16. In an odd way this event is also a transit. If you were standing on the Moon that night you would be able to observe a transit of Earth for a lunar eclipse occurs when the Earth passes between the Moon and the Sun.

It is not by chance that the Moon will be full that night, because lunar eclipses only occur when the Sun is shining past the earth on the full sphere.

If you are interested in observing this eclipse, here are the times provided by the U. S. Naval Observatory:

Moonrise 8:15 p.m.

Moon enters penumbra 9:05

Moon enters umbra 10:03

Moon enters totality 11:14

Moon leaves totality 12:06 a.m.

Moon leaves umbra 1:17

Moon leaves penumbra 2:15

Moonset 6:08

All of this will take place in the southeastern to southwestern sky.

For those of us who have forgotten those rarely-used terms umbra and penumbra from our middle school science lessons, I'll review what they mean. The Sun is a very large light source so it casts two kinds of shadows. The penumbra is the region where only part of the sunlight is blocked by the earth. The umbra is the smaller region where all of the sun is hidden. For a total eclipse, the Moon must move completely within that umbra.

Viewing a lunar eclipse is free of the problems of watching transits across the solar disk. And they are rather common: the next ones will be November 9 this year and October 28, 2004.-- Gerry Rising