June 08 – Giovanni Domenico Cassini

On this day in 1625 the Italian mathematician and astronomer Giovanni Domenico Cassini was born in the tiny municipality of Perinaldo.

Cassini (Leopold Durangel, 1879).

Cassini (Leopold Durangel, 1879).

Cassini held several important astronomical positions during his career, including professor of astronomy at Bologna University, and director of the Paris Observatory, and was responsible for the discovery of four saturnian moons: Iapetus, Rhea, Tethys and Dione.  He also discovered the Great Red Spot on Jupiter (at the same time as Robert Hooke, so he only gets half a credit for that one), and the “gap” in Saturn’s rings which now bears his name, the Cassini Division. I’ve put the word “gap” in inverted commas because recent visits to the planet have found it to be actually quite busy (see below).

The Cassini Division (image credit: NASA / JPL / Space Science Institute)

The Cassini Division (image credit: NASA / JPL / Space Science Institute)

Cassini wasn’t just a gas giant geek, though, and made observations of our nearest planetary neighbour, Mars, from Paris (simultaneously with a colleague a long way away in French Guiana to make the angle as big as possible) to make the first calculation of the size of the solar system using parallax.  The relative positions of all the known planets had already been calculated, so only the distance to one was needed in order to have a stab at working out how far away they all were.  Mars was the obvious choice because it’s the closest, so the apparent shift would be greatest.  Cassini’s measurements turned out to be not too far from the values we have now; he used his observations to calculate the Earth-Sun distance as 21,700 “Earth radii”.  Today we use the accepted value of 23,455.


1873  –  Main belt asteroid 146 Lucina discovered by Alphonse Borrellyand named after the Roman goddess of childbirth.


1887  –  Themistian asteroid 268 Adorea discovered by Alphonse Borrelly.


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