Tonight Jupiter will have no moons. At least that will appear to be the case for a short while, as a rare phenomenon takes place in which all of the four bright moons of Jupiter will not be visible for viewers on Earth.
Any night of the year that Jupiter is visible, you can usually spot several of the four Galilean moons. Io, Europa, Ganymede and Callisto are big enough that they are easy to find in a telescope or even binoculars. The inner three moons, Io, Europa and Ganymede orbit Jupiter very rapidly, in 2, 4 and 8 days respectively. So on any given night the relative position of the four moons changes and the pattern of the four moons around Jupiter - from our point of view - is always in flux. The above image from Sky and Telescope Magazine helps to illustrate this.
To make things more interesting, as a moon passes directly in front of Jupiter or behind it (or in the shadow of Jupiter) it becomes invisible for a few hours. This evening from 7:00 to 12:00 pacific time, the moons will be invisible for some period of time, with all four moons being out of view from 9:43 until 11:29 pacific. If you have a telescope or binoculars, use them to catch the changing pattern of the moons and witness this unusual alignment.
Jupiter, by the way, can be easily spotted tonight as the bright "star" in the east near the full Moon.
A Weekend at Conway Observatory
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