Io and Europa in the same frame March 2, 2007, two days after New Horizon's flyby of Jupiter. As the spacecraft sped away it took this long-lens photo. The moons are in fact separated by 790,000 kilometres. Eruption of the volcano Tvashtar is still very visible. Io's night side is lit by Jupiter, a planet which gives off twice as much radiation as it receives from the Sun. Europa's night side is dark because it is on the other side of Jupiter.
The night of January 7, 1610 Galileo first noticed strange objects around Jupiter. By January 13 he had seen 4 moons. Discovery of new worlds was truly exciting and kept Galileo up every night. By March 22 he had finished his observations, and immediately started writing his observations in book form.
STARRY MESSENGER was published that same year and was immediately popular. The book was also controversial, because not everyone believed in the new moons. Someone named Horky, who had been a student of Johannes Kepler, wrote a book claiming that the moons were a fraud designed to gain fame and wealth. Kepler was very unhappy, but he counseled Galileo not to respond to such attacks.
Pictured to scale by New Horizons are Io, Europa, Ganymede and Callisto. The moons will be there long after the doubters are underground. Kepler's advice to Galileo was right on.