Satellites of Jupiter Jupiter has a large number of satellites.
Of these, four are comparable to the Earth's Moon in size; the
rest are orders of magnitude smaller. When Jupiter is at
opposition and closest to the Earth, the stellar magnitude of
its four large moons is between 5 and 6.  This means that, were
it not for the shielding brightness of Jupiter, these bodies
would be visible with the naked eye. The aperture of the
telescope used by Galileo in 1610 and its magnification thus
brought these four "Galilean" satellites within his grasp. But
first Galileo had to make adjustments to the instruments. When
viewing bodies that are very bright and very small, the optical
defects of the telescope can be crippling. By trial and error
Galileo learned to stop down the aperture of his instrument
until he could begin to make useful observations. At the end of
1609, as he was finishing his series of observations of the
Moon, Jupiter was at opposition and the brightest object in the
evening sky (not counting the Moon)