On Christmas Day, nature will provide us with an interesting parting gift to the closing of the old year, and officially, the closing of the old millennium as well. The moon will come almost directly between the Earth and the Sun this coming Monday, and as result a good portion of the US will be able to witness a partial solar eclipse. By "partial solar eclipse," it's meant that only part of the Sun's disk is covered by the new moon - see Astronomy isn't hard. An eclipse of the Sun of any kind (partial, annular or total) occurs on the Earth's surface at most only five times a year, but it's extremely unusual to have such a celestial event coincide on the most special religious day of all for many people in the US.

Though partial eclipses can't match the grandeur of total eclipses, their nonetheless neat to observe. Since the eclipse will be visible over much of North America and all of the US except Hawaii and Alaska, lots of folks will have the opportunity to see it. If you're heading off to Central America or the Caribbean, it'll also be visible there (just barely), and if you're not happy with your tan, you'll have something to blame it on. Wherever you view the eclipse from, make sure you protect your eyes. Even if 99% of the Sun's disk is covered by the moon, the remaining 1% is still bright enough to damage your vision if you look at it directly. It's not that Sun is more dangerous or brighter during an eclipse - it's not. We're just more apt to look up it and stare. If Santa didn't come through with that special solar filter lens that you so much wanted, you'll have to observe the eclipse in an indirect way by using a pinhole projection. Punch a small hole through a piece of cardboard to let the Sun's light project onto a light colored piece of wood or paper. You can then safely watch the progress of the moon as it slips across the face of the Sun. Don't look at the eclipse through your brand new telescope. Otherwise, your only use for it will be to help you read the morning paper.

For the Washington DC area, greater than 53% of the Sun's diameter (this translates to about 42% of the Sun's area) will be obscured at the peak of the eclipse. It'll begin at about 11:00 in the morning when the edge of the new moon takes a small bite of the Sun's disk. The peak of the eclipse will be at 12:41 p.m., and it'll end at about 2:15 p.m. For all of you planning to spend your holidays in the North Atlantic and the Baffin Island area, 72% of the Sun's diameter will be eclipsed by the moon - more than anywhere else. In northern New England, about 60% of the Sun's diameter will be covered, close to 50% of the Sun's diameter will be covered in St. Louis, nearly 40% in Denver, and in Arizona, less than about 30% of the diameter will be obscured. If you're playing golf in Phoenix on Christmas Day, you may not even notice anything unusual, but you can use the eclipse as an excuse for that untimely triple bogey that ruined your round.

The last solar eclipse that occurred on Christmas Day was in 1954 - visible only over southern Africa, and the next one will also be visible in Africa, in 2307. According to Fred Espanek of the Goddard Space Flight Center, for the period from 2,000 BC through 3,000 AD, there are 31 solar eclipses which happen on Christmas Day. The last total eclipse of the Sun on Christmas Day occurred in 1666 over a portion of South America, and the next one won't happen until 2755, visible over Europe.

On Christmas Day in 1935, there was also a partial solar eclipse, but like the 1954 eclipse, this one was not visible in North America either. From the solar eclipse data sets assembled by Fred Espanek, I was unable to find a single solar eclipse that took place on Christmas Day and was centered over the US, or what became the US. In 1628, a partial solar eclipse, centered over the Atlantic Ocean (15 degrees north latitude and 44 degrees west longitude) may have been visible by Native Americans in what is now the southeastern US. Otherwise, it seems that on no other Christmas Day, in any year, during the last two millennia has an eclipse been visible over what is now the US! I didn't check the prior millennium, but even though there may have been an eclipse on December 25, Christmas wouldn't have been celebrated then. Thus the best answer to this week's question is D - within the last 500 years, if the 1628 eclipse was indeed visible over a portion of southeastern North America. Perhaps I should have included as a choice; E. none likely. Since a solar eclipse may never have occurred over the US on Christmas Day before, that makes this Christmas even more special, at least in a celestial way. Maybe we wish for snow on Christmas Eve and clear skies on Christmas Day.

For more about this see "Fifty year canon of solar eclipses: 1986-2035" by Fred Espanek, Sky Publishing Co., Cambridge MA and "Eclipses during 2,000, " Observer's handbook - 2000, Royal Astronomy Society of Canada, Toronto, Ontario.