I hope you enjoyed the recent great pictures from comet Churyumov–Gerasimenko (or 67P for short) as much as I did. Launched in 2004 by the European Space Agency, the Rosetta spacecraft finally rendezvoused with the comet last summer. On Nov. 12, the small Philae lander made contact and settled on the surface after a couple of big bounces.
If you missed either the lunar or solar eclipses in October, you don’t have to wait very long to see another. As a matter of fact, eclipses are visible every few days. At least they are visible if you count eclipses of the Galilean moons of Jupiter.
From closest to farthest away from Jupiter, the four moons discovered by Galileo in 1610 are Io, Europa, Ganymede and Calypso. They regularly pass either in front of or behind Jupiter. When passing in front, that is called a transit, and when behind, it is called an eclipse.
With a small telescope, it is easy to see both phenomena, although transits are most noticeable by the dark shadow they cast on the planet’s surface.
Galileo’s discovery of these moons was important in establishing that the Earth was not the center of the universe. Among his many other contributions to science, Galileo suggested that the timing of the observed eclipses could be used as an accurate clock to solve the navigational problem of longitude. Galileo lacked the technology (and I lack the space to explain it) but within a century of the discovery of these moons, their eclipses became an important part of mapmaking.
Io, the closest moon, has a period of only 42½ hours, so that is the most convenient moon to observe.
Another important 17th-century contribution using these regular eclipses was the first estimate of the speed of light. Ole Roemer realized that if light didn’t travel instantaneously, then the finite propagation speed would slightly change the observed period of Io depending on if Earth were moving toward or away from Jupiter.
Venus and Saturn are well within the glare of the sun, and Mars will be joining them shortly.
Jupiter is rising after 11 p.m., so will be well-positioned for viewing during the next few months. Right now, it can be seen just to the right of the backward question mark in the constellation Leo. Every 21¼ hours, Io will either make a transit or be in an eclipse, so you have your chance to make your own observations.
The Geminid Meteor shower peaks on the morning of Dec. 14.
Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory. firstname.lastname@example.org.