The brilliant point of light near the waxing gibbous moon tonight – Monday, February 10, 2014 – is Jupiter, 5th planet outward from the sun. These two worlds – our companion moon and the solar system’s largest planet – are the brightest objects in the evening sky now. Look for them in the east, shortly after sunset. They will be in the sky together for most of the night, until shortly before dawn on Tuesday.
In fact, Jupiter and the moon have appeared near each other for several nights now, as seen from around the world. Because they are both so bright, they’re noticeable near each other for several days. They are closest tonight. But if you miss them tonight, try tomorrow.
Jupiter – the king of planets in our sun’s system – has over 60 known moons of its own. Only four of these moons are large enough to be seen through a small telescope. In their order outward from Jupiter, these four moons are Io, Europa, Ganymede, and Callisto. You might even glimpse them through binoculars. Click here to know the positions of Jupiter’s moons tonight – or any night.
Two of Jupiter’s moons, Io and Europa, are about the size of our moon. The other two, Ganymede and Callisto, have diameters that are approximately 1.5 times the moon’s diameter. Ganymede, the largest moon in the solar system, is actually larger than the planet Mercury.
These four major moons are called the Galilean moons because Galileo first observed them in the early 1600’s. Galileo believed that the eclipses of Jupiter’s moons would enable sailors to find longitude at sea, if an accurate almanac giving the eclipse times for the moons could be provided.
The Danish astronomer Ole Romer studied the moon Io in the late 1600s for the purpose of creating a reliable almanac. But Romer ran into a snag. He found that when Jupiter was farther away from Earth, the eclipses came later than expected. And when Jupiter was closer to earth, the eclipses happened sooner than expected.
Ole Romer correctly surmised that the finite speed of light must be responsible for the observed phenomena. Light takes less time to travel when Jupiter is closer to Earth, and more time to travel when Jupiter lies farther distant. This explanation proved shocking to people in Romer’s day because many took for granted that the speed of light was instantaneous.
Bottom line: Learn the surprising story about Jupiter’s moons and the finite speed of light. Then look eastward after sunset on February 10 for Earth’s moon near Jupiter. Jupiter and the moon will be up nearly all night tonight.