From all around the world, you can see Jupiter near the Moon tonight and over the next few nights. They are the two brightest objects in our night sky at the moment. Look to the West to find them.
Jupiter has a lot of moons, with 4 of the largest, Io, Europa, Ganymede and Callisto, being visible through a telescope, or even steady binoculars. Earlier this month however, it gained a new "moon", named Juno.
Juno has spent the last 5 years travelling through space to take up its orbit around our solar systems largest planet. It is the first spacecraft to journey to Jupiter since Galileo arrived there in 1995.
The four Galilean moons, named after the astronomer Galileo who first observed them in the early 1600s, gave him the idea that the planets may actually orbot the Sun rather than our Earth.
Using Jupiter's Moons to Determine Longitude
When Galileo made his discovery of the moons, he also noticed that their motions followed Kepler’s laws which explains scientifically the motion of planets around the Sun. This meant that the Galileon moons could be used to determine time, or the observers longitutde on Earth.
By observing when the moons entered or exited Jupiter's shadow as they passed behind the gas giant,and matching this against a table listing the predicted times of these eclipses as seen from a particular observatory on Earth - and knowing that our Earth rotates 360 degrees each day, you can then work out your longitude relative to that observatory.
Jupiter's Moons and the Speed of Light
In 1676 the Danish astronomer Ole Romer used Io, Jupiter's innermost moon, to come up with the first approximate measurement of the speed of light. In those times it was widely believed that light was instantaneous, rather than limited in speed.
What Else Can I See Tonight?
If you look to the South after sunset, you may also catch a glimpse of Mars and Saturn before they dip below the horizon in the early hours of the morning.
Take your opportunity to see Jupiter near the Moon tonight along with it's 4 visible moons, as by late August or early September it will be gone from our sky - lost in the twilight glare.