Remembering Gian Domenico Cassini, the man who discovered the Cassini Division


Gian Domenico Cassini astronomer
Gian Domenico Cassini astronomer

Italy-born French astronomer Gian Domenico Cassini discovered the Cassini Division-the gap present between Ring A and Ring B of Saturn. Born on June 8, 1625, he also discovered four moons of Saturn and was the first person to record zodiacal light.

So groundbreaking was his work that in 1669, French King Louis XIV invited the astronomer to Paris in order to join the Acedemie des Sciences which has been formed only recently. In 1671, when the Observatorie de Paris was completed, Cassini assumed directorship of the same. In fact, he was the first of four generations of his Cassini family who went on to be directors of the Paris Observatory.


He also had a big hand in the direction of the activities carried out in the French school of astronomy until the French Revolution. Cassini passed away on September 14, 1712.

Read: 13 years of Cassini's trip around Saturn: A timeline of the journey of the NASA spacecraft

Here is a list of Cassini's contributions to astronomy and space science:

  • Cassini was the first to observe Jupiter and its satellites as the moons passed between Sun and Jupiter
  • Cassini used the observation of the spots on Jupiter to measure the time the planet requires to make one rotation
  • The French astronomer also observed Mars the same way and in 1666, calculated its rotational period as 24 hours and 40 minutes. Now, the rotational period of the Red Planet is considered at 24 hours 37 minutes 22.66 seconds
  • He discovered four of Saturn's satellites - Iapetus in 1671, Rhea in 1672, and Tethys and Dione in 1684
  • In 1668, Cassini recorded the positions Jupiter's moons in a table. In 1675, Ole Romer, a Danish astronomer, used the data to establish that the speed of light was actually finite
  • The astronomer discovered that Jupiter was flattened at the poles as a result of its axial rotation
  • Cassini also worked with applied hydraulics and wrote a number of memoirs on flood control
  • Cassini carried out intensive work to record the size of our solar system as accurately as possible. To this end, he sent his colleague Jean Richer to South America so that simultaneous recordings of Mars' position could be taken from Paris and from Cayenne in French Guiana. This could make up for the parallax error somewhat, thus making the final recording more accurate both for the position of Mars and for the distance of the sun
  • From 1671 to 1679, Cassini observed Moon and compiled a large map from his data, which he gave to the Academie
  • It was in 1675 that he discovered the Cassini Division. He was also the first to say that Saturn's rings were not solid objects as Huygens had noted earlier but were actually formed with thousands of tiny moonlets revolving around Saturn
  • Cassini made careful observations of zodiacal light and concluded that the light was not a meteorological phenomenon but was cosmic in nature. Zodiacal light is a diffused cone of light seen at times in the night sky which extends from the sun's vicinity

Read: Songs of Saturn: Astrophysicists produce music from Saturn's moons and rings before Cassini's last dive

Though Cassini had trouble accepting new ideas and theories, his contributions were significant enough to term him one of the most important astronomers in the 17th and 18th centuries.

With the 13-year-old Cassini spacecraft making its final crash landing on Saturn tomorrow after 'bidding goodbye' to Saturn's moon, Titan, let's pay our respects to Gian Domenico Cassini, the man who started our fascination with the ringed planet.



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