This is Dudley Observatory’s Skywatch Line for Friday, March 18, through Sunday, March 20, 2016, written by Alan French.
The Moon was at first quarter last Tuesday and is moving toward full, so a waxing gibbous Moon dominates the night sky this weekend. The Moon will be full next Wednesday.
We’ve been writing about Jupiter’s Galilean moons and how they are easily visible in any telescope. This weekend features some close pairings of moons.
Look for Jupiter toward the east southeast at 9:00 pm. On Friday night all four moons will be on one side of the planet. Io and Europa will be nearest Jupiter and very close together – in some telescopes it may be hard to split the pair and they may look like a single, perhaps, elongated star, especially if the air is unsteady and the planet itself appear to shimmer. Callisto is next farthest away with Ganymede most distant.
On Saturday at 9:00 pm three moons will be on one side of the planet and the fourth, Io, will be on the opposite side. Ganymede will be closest to the planet with Europa a little farther away, with Callisto well away from the planet.
By Sunday Ganymede and Europa, close together, will be on one side and Io, closer to the planet, will be on the other side. Considerably farther away on that side you’ll find Callisto.
Even when we don’t write about the moons they are fun to follow from night to night and hour to hour. Apps are available that show the positions of the moons and various Galilean satellite events.
From our perspective, the Galilean moons shuttle back and forth. When they are moving from east to west they cross in front of Jupiter. As they move from west to east the go behind the planet and vanish from sight. Both Jupiter and its moons cast shadows, so the moon’s shadows can appear on the cloud tops of the planet, and the moons can be seen moving into or out of Jupiter’s shadow as they pass behind the planet.
A moon moving in front of the planet is called a transit. When a shadow moves across the planet it’s a shadow transit. Apps give times for moon and shadow transit beginnings, known as ingress, and moon and shadow transit ends, known as egress.
A moon can disappear behind the western limb of Jupiter, being hidden or occulted by Jupiter. It can also emerge from behind the planet, an occultation end. Right now Jupiter’s shadow lies to its east, so a moon emerging from behind the planet’s eastern limb is in the shadow – eclipsed – and invisible at first. Then it moves out of Jupiter’s shadow into sunlight and becomes visible.