The Planets

Every month the "Stars and Planets" pages of Sky & Telescope tell where to locate the moving planets.

Mercury can sometimes be located during twilight with the naked eye, but binoculars make it much easier to pick up. Once found, however, this little planet only appears starlike. As with many astronomical objects, the accomplishment lies in finding it at all.

Venus, on the other hand, will show its crescent phase in high-quality, firmly mounted binoculars. In the summer and fall of 1610, Galileo watched Venus change phases in the evening sky. If the traditional Earth-centered idea of the solar system had been correct, with Venus always staying between us and the Sun, Venus would always appear as a crescent. Instead Galileo saw it become gibbous — proving that it circles behind the Sun, thus providing crucial evidence for the Copernican system that would shake European beliefs for the next century. Can you repeat Galileo's observation?

Mars just looks like a bright orange star. Jupiter, on the other hand, is one of the binocular showpieces of the sky. Its four bright Galilean moons (so named for their discoverer) are lined up on either side of the planet in patterns that change every night. The outer two moons of Jupiter, Ganymede and Callisto, are the most easily seen in binoculars. Europa and Io remain hidden in Jupiter's bright glare until you catch them near their greatest elongation (distance) from the planet. The monthly Jupiter's Satellites diagram in Sky & Telescope can be used to identify the ones you spot. All four are very roughly the size of our own Moon. Comparing these tiny pinpoints with the full Moon itself in binoculars dramatizes how much farther away they are.

A much more difficult achievement is finding Titan, the lone binocular moon of Saturn. This 8th-magnitude speck only gets about as far from Saturn as 6th-magnitude Europa does from Jupiter. It needs large, high-power binoculars on a steady mount (I have seen it this way in 10x50s). Saturn's rings, unfortunately, cannot be seen very definitely with magnifications less than about 20x or 30x.

Uranus, Neptune, and the half dozen or so asteroids that reach 8th magnitude or brighter look like faint stars. Uranus and Neptune can be found in binoculars with the aid of the charts printed in Sky & Telescope each year. Charts for bright asteroids are also printed in the magazine from time to time. To be sure which "star" is the object you're looking for, draw a map of the stars you see near the correct location and watch for the one that moves from night to night. This is the method by which all the major asteroids were originally discovered.