35 years ago this week Jupiter's thirteenth satellite (in order of discovery), Leda, was discovered by Charles T. Kowal.
Kowal used Palomar's 48-inch Schmidt (now called the Samuel Oschin Telescope) to make the discovery. For the 2-hour exposure (seen above) he tracked the telescope to move through the sky at the same rate as Jupiter, which is subtlety different from the motion of the stars. The result is that anything moving across the sky at the same rate as Jupiter appears as a dot and the stars as streaks. Naturally, the arrow was added later.
With a diameter of just 10 kilometers (~ 6miles) Leda is a pretty small moon.