Moon & Planets

Four Galilean moons shadows

Can all four Galilean moons cast their shadows on Jupiter ’s disk at the same time? No. A triple shadow transit is as good as it gets (S&T: July 2004, page 94). The motions of the three inner moons are gravitationally locked so that, as English astronomer George B. Airy wrote more than a...

How is the time of new (or full) Moon defined?

How is the time of new (or full) Moon defined? Astronomically, the Moon is new when it and the Sun have the same celestial longitude. The Moon is at first-quarter phase when its longitude is 90° greater than that of the Sun. The Moon is full when its longitude is 180° greater, and at...

Is it possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear when Ganymede is at greatest elongation from Jupiter?

I’ve heard it might be possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear together when Ganymede is at greatest elongation from Jupiter. Will this happen anytime soon? Jupiter is now setting soon after sunset. But three times in 2008, Texans (and North Americans generally) will have a chance...

What does “true color” mean in a deepspace photograph?

What does “true color” mean in a deep-space photograph? How would a galaxy or nebula actually look to the naked eye from close up? “True color” is how an object would look if you were nearby or were viewing it in a telescope under a dark sky, and if its surface brightness (the amount...

What happened to the “spokes” in Saturn’s rings?

In 1979 the Voyager spacecraft revealed “spokes” in Saturn’s rings. Yet recent images from Cassini have shown no spokes. What happened to them? Those spokes have kept planetary scientists scratching their heads for decades. It’s thought that the spokes are radial fingers of dark, extremely fine dust particles that become electrostatically charged by pulses...

When the Moon misses the Sun by the greatest amount north (or south), could the lunar crescent be seen in a telescope at new Moon?

Is the Moon’s orbit inclined sufficiently that, when it misses the Sun by the greatest amount north (or south), the lunar crescent could be seen in a telescope at new Moon? Probably not. The inclination of the Moon’s orbit to the ecliptic varies from 5.0° to 5.3°. French astronomer André Danjon (1890–1967) showed that...

Why are Kuiper Belt discoveries made at aphelion?

Why is it that so many of the recent Kuiper Belt discoveries — including the “10th planet,” 2003 UB313 — have been made while the objects are at aphelion? Timing and luck. Objects that orbit the Sun spend most of their time near the farthest point of their orbits, where they move most slowly....