Cassini was born in Perinaldo, Italy, near Nice, France. He studied with the Jesuit priests and astronomers Giovanni Riccioli (1598-1671) and Francesco Grimaldi (1618-1663) before becoming an astronomy professor at the University of Bologna at the age of twenty-five.

While at Bologna Cassini observed the moons of Jupiter carefully enough for an extended time to publish an ephemeris, a table which tracked the moons in their orbits. Other astronomers using the ephemeris noticed a discrepancy: when the Earth and Jupiter were far apart, the moons appeared to take longer to pass in front of Jupiter than Cassini had listed. Some scientists postulated that the light reflected from the moons needed extra time to travel the increased distance between the Earth and Jupiter. Working from this postulation in 1676, Olaus Roemer used Cassini 's ephemeris to calculate the speed of light.

In 1669 King Louis XIV of France invited Cassini to...