This is Dudley Observatory’s Skywatch Line for Friday, March 11, through Sunday, March 13, 2016, written by Alan French.
This weekend marks the beginning of Daylight Saving Time, so set your clocks ahead by one hour before retiring on Saturday night.
The Moon was new last Wednesday and is moving toward first quarter. A waxing crescent Moon will be visible in the early evening sky after sunset.
Look for the lunar crescent toward the west as darkness falls Friday night. The Moon will set at 9:21 pm. The Moon will be fatter and higher on Saturday night and will set at 10:33 pm. Because the clocks spring ahead Sunday morning, moonset is after midnight on Sunday, at 12:41 am Monday morning.
We recently wrote about the four brightest moons of Jupiter – Io, Europa, Ganymede, and Callisto, and how they are visible through even a very modest telescope. The four were discovered by Galileo in 1610 and were the first moons discovered orbiting another planet. They are known as the Galilean moons.
Io is the Galilean moon closest to Jupiter, taking less than two days (1.77 days) to orbit the planet. Hence its positions changes markedly from night to night. Io is very active and has hundreds of volcanoes, spewing material high above its surface. The material continues to resurface the moon’s landscape, which is covered with lava flows and liquid sulfur. The volcanoes are powered by the immense tidal forces stretching Io as it orbits close to Jupiter. These strong tidal forces can raise surface features by as much as 100 meters.
Europa is the next farthest moon away from Jupiter, taking just over three-and-a-half days (3.55 days) to orbit the planet. It is the smallest of the Galilean moons and its surface is largely covered with water ice. Tidal forces are smaller on Europa because of its greater distance from Jupiter, but the heat they generate may produce a liquid ocean under its icy crust. Some astronomers believe strongly that we should explore Europa’s oceans and look for life there.
Third outward from Jupiter is Ganymede, the largest moon orbiting the planet and the largest moon in our solar system. It travels around the planet in just over 7 days (7.15 days). Its surface is dominated by ancient cratered regions, bright linear features, mountain ranges, and flows of water ice.
The Galilean moons are tidally locked – one side is always toward Jupiter. The inner three moons are in resonant orbits. Europa makes one orbit for every two orbits of Io, and Ganymede makes one trip around Jupiter for every four of Io.
Farthest from Jupiter, of the four Galilean satellites, is Callisto, which makes a leisurely seventeen day (16.69 days) journey around the planet. It is not in a resonant orbit nor is it affected by tidal forces.