The sky has been just beautiful the past few days. In the evening, the crescent moon has been making its way across the western sky after sunset, weaving a path passing near Venus and Jupiter. In the morning, before sunrise, a cast of characters romps high in the sky to the south: Taurus the bull with the seven sisters of Pleiades riding on his back, chased by Orion the hunter followed his faithful hunting dog Canis Major.
And while the forecast doesn’t look too promising for the next few days, as this splendid indian summer comes to an end, I’ve got a little observing project for you.
These days, everything is about change. Change is interesting too in astronomy. Our common perception is that most things in the sky are mostly the same night after night. Indeed, citizens of civilizations around the world far back in time have been comforted by the steady stars and the unchanging constellation patterns they form.
Not everything about the sky is unchanging, however. The moon, for example, obviously changes from day to day as it moves through it phases. The planets also change, more subtly than the moon, as they drift slowly against the background stars. While the changing moon and planets are in some ways not as nice as the unchanging stars and constellations, at least the moon and planets follow predictable patterns. The moon does what it does over and over again month after month, and the planets do what they do year after year. This is comforting.
Unexpected change in astronomy has historically been perceived as a harbinger of doom. Comets and eclipses, before they could be predicted, were significant and scary events. Even today, we are not immune to change-induced irrational hysteria: remember Y2K?
Fortunately, change in the heavens is infrequent, in human-life-span terms, or it is subtle. Most commonly, it is subtle.
To see the subtle ways the heavens change you have to look carefully, not casually. For example, if you were to have a look at Jupiter over the period of a couple clear nights with a small telescope you would plainly see that the brightest four of its moons change positions with respect to one another and Jupiter.
The project I have for you today is to spot a star that changes noticeably in brightness from time to time. The star’s name is Algol; it is the second brightest star in the constellation Perseus. Perseus is presently just about straight up about an hour after sunset.
Most of the time, Algol is in the top 50 list of brightest stars visible from West Michigan. But periodically, the star suddenly dims to barely remain in the top 200 list. You can see this happen if you look carefully.
Algol dims every 2 days 20 hours and 49 minutes. It stays dim for only about two hours. To see this you need only to know when and where to look.
Saturday night at 11 pm, Algol will be dim. If you were to seek it out and look at it about 9 pm, again at 11 pm, and perhaps again at 1 am you would surely see it change. Next Tuesday at 8 pm Algol will also be dim.
For other days and times, and for a sky map showing where to look to find Algol, follow this URL to an article at Sky & Telescope magazine.
I write about this now because Perseus and Algol are best observed in the fall and winter. Put this on your list of things to do when the sky clears up again.
This column originally appeared in the Grand Haven Tribune on 25 October 2008.
