Moon & Planets

Crescent Moon and Venus at sunset

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...

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...

Rising Moon

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...

Uranus's rings feature in this 2005 shot from Hubble Space Telescope. NASA / ESA / and M. Showalter

What defines a planet’s north pole?

Uranus is often said to have a retrograde rotation with its axis tilted 98°. Why don’t we say it has a direct spin with the axis tilted 82°? Since 1982, the International Astronomical Union has defined the north pole of a planet to be the pole that lies north of the ecliptic plane (the...

Spokes in Saturn's rings

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...