The Sky Show Never Sleeps

This Thursday, July 31, the moon will be "new" (utterly invisible — positioned on our "day side" so that all the light reflected from the sun bounces straight back to the sun rather than toward us here on Earth). When this lunar location also happens to fall in the plane containing sun and Earth, the moon can actually block the sun’s rays and produce a solar eclipse.

The Earth is large enough, and arrangements have to be exact enough, however, that a solar eclipse visible to one part of Earth is not necessarily visible to all parts of Earth. This involves one such example. Thursday will bring a total solar eclipse, but unless you happen to have plane tickets to Northern Canada or, better, Russia and Asia, you will be aware of it only intellectually.

So let us instead discuss what we can see this week from the greater Berkshire area — preferably without optical aids. After sunset, by far the most luminous object in the western sky will be the evening star, which is in fact not a star. It is our neighboring and rocky planet Venus. The sooner after sunset time and the closer to sunset place that you look, the greater the chance that the prominent disc you see is Venus. Like the sun, moon, all the other planets and the zodiac constellations, Venus rises in the east and sets in the west. So when you spot Venus in the west, you are catching her on the way down. It’s the first "star you see tonight" due to a brightness with which nothing but the moon can compete. But she will not hang around much past the full darkening of the sky and the emergence of the thousand-odd other, less radiant, objects. If you do have the pleasure of basking in Venus’s unique shine, then use her as a guide and turn your head a bit to the left (east). That next evident object is Saturn. Venus will be crawling toward Saturn over the next two weeks until the former passes underneath the latter on Aug. 13.

If you miss Venus, then instead look high and due south. There, holding court from the constellation Sagittarius, Jupiter’s gleam dominates. It rises in the southeast right after sunset and is at that time probably too low to see. But as the night progresses, as Venus exits stage-right and as the absence of a moon reduces the possibility of optical distraction, Jupiter takes over. It deserves to: Its mass is approximately two-and-a-half times greater than that of all the other solar system planets combined!

The longer you wait, the higher and more centrally Jupiter will climb. If you are fortunate (or devoted) enough to possess even a cheap pair of binoculars, look for four of Jupiter’s 63 moons: Io, Callisto, Europa and Ganymede. They will look like stars (and would otherwise be visible to the naked eye, were it not for Jupiter’s singular blaze), but will be arranged along a line extending from Jupiter’s midriff.

Galileo’s first observation of this little world within the world served as a nail in the coffin of his rejection of an Earth-centered universe. Jupiter’s frequent eclipses of these fast-moving moons also functioned as one of the earliest reliable clocks that could be used from a ship sailing rocky seas. Finally, unexpected irregularities in such eclipses suggested to the late 17th-century Danish astronomer Ole Roemer that light, as instantaneously as it may appear to travel, actually takes time to get from one place to another —186,000 miles per second. So take an evening stroll and appreciate the sublimity of Venus. Or contemplate Jupiter’s mini-universe.

Daniel Yaverbaum teaches science at Berkshire School in Sheffield, MA. He is director of the observatory there, as well. Send star questions to him via compass@lakevillejournal.com.