On March 21, 1864, the Italian mathematician, astronomer, and engineer Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) discovered Saturn’s moons Tethys and Dione. Cassini used a refractor telescope with an aperture of 108 mm to make this observation.

Today’s (March 21) story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Giovanni Domenico Cassini

Giovanni Domenico Cassini
Giovanni Domenico Cassini (8 June 1625 – 14 September 1712)

Giovanni Domenico Cassini was born in Perinald, Imperia, Italy, on 8 June 1625. Because of his interest in astrology and the emerging discipline of science, he was employed by a rich amateur astronomer, Marquis Cornelio Malvasia (1603 – 1664), in Bologna.

In Bologna, Cassini indulged his passion for the skies using the Marquis’s instruments and was taught by Jesuit scientists. His work was exceptional in its quality and precision and formed the grounding for his later prestigious academic positions.

In his thirties, Cassini worked for the Bolognese government and simultaneously held the chair at the University of Bologna. His work included observations of the Sun but, as he obtained more powerful telescopes, he turned his attention to the planets.

Cassini needed bigger buildings to house these new instruments. He placed his ‘Meridiana’ instrument in the San Petronio Cathedral and used it to compute the exact date of Easter based on the apparent motion of the Sun across the sky.

In 1666, he used observations of Mars to calculate that the red planet rotated once every 24 hours 40 minutes. We now know it to be 24 hours 37 minutes 22.6 seconds, a “sol”.

In 1668 Cassini compiled tables showing the positions of Jupiter’s satellites and these were used by the Danish astronomer Ole Rømer to establish that the speed of light is extremely fast but not infinite.

On hearing of these great works in 1669, King Louis XIV of France invited him to Paris to join the recently formed Acadèmie Royale des Sciences. By 1671, Cassini was director of the Observatoire de Paris and two years later became a French citizen, changing his name to Jean-Dominique.

Known for his work on astronomy and engineering, he discovered four moons of Saturn (Iapetus in 1671, Rhea in 1672, and both Tethys and Dione in 1684) and the large gap in Saturn’s rings in 1675, now called the Cassini division.

He also was the first director of the Paris Observatory, measured the sun’s apparent motion through the sky, and even approximately calculated the distance from the Earth to the Sun. In addition, he used the shadows of Jupiter’s moons and shadows on Jupiter’s surface to determine the length of Jupiter’s day, and he is also credited with discovering Jupiter’s Great Red Spot.

Giovanni Domenico Cassini died in Paris on 14 September 1712, aged 87.

NASA, ESA, and the Italian Space Agency’s joint project, the Cassini spacecraft (see notes 1) was named after him.


Dione, Cassini Image (August 17, 2015)
Dione, the fourth-largest moon of Saturn (and the 15th largest moon in the Solar System) hangs in front of the gas giant and its icy rings. This photo was captured on August 17, 2015, during Cassini’s final close flyby of the icy moon. North on Dione is up. The image was obtained in visible light at a distance of approximately 45,000 miles (73,000 kilometers) from Dione. It has probably a huge ocean, under an icy shell, like Enceladus, which is buried about 60 miles (100 kilometers) beneath the shell.

Dione is a small moon of 349 miles (562 km) in mean radius orbiting Saturn every 2.7 days at a distance of 234,500 miles (377,400 km), which is roughly the same distance that the moon orbits around the Earth.


Saturn's rings and Tethys from Cassini (October 29, 2007)
Saturn’s rings and Tethys (October 29, 2007). The gas giant’s dark-side rings glow in shades of brown and gold, contrasting with the more neutral appearance of the icy moon Tethys, which is 1,062 kilometers, or 660 miles across. This view looks toward the anti-Saturn side of Tethys and the unilluminated side of the rings from about 2 degrees above the ring plane. North is up and rotated 35 degrees to the right. Images taken using red, green, and blue spectral filters were combined to create this natural color view. The view was acquired with the Cassini spacecraft narrow-angle camera on October 29, 2007, at a distance of approximately 2.1 million kilometers (1.3 million miles) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 21 degrees.

Tethys was discovered by Giovanni Domenico Cassini on March 21, 1684, together with Dione, another moon of Saturn, and is named after the titan Tethys of Greek mythology.

It is Saturn’s fifth largest moon. Its irregular shape is 331 miles (533 kilometers) in mean radius, with dimensions 669 x 657 x 654 miles (1076.8 x 1057.4 x 1052.6 kilometers).

The Italian astronomer named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea (“the stars of Louis”) to honor King Louis XIV (5 September 1638 – 1 September 1715), known as Louis the Great (Louis le Grand) or the Sun King (le Roi Soleil), a monarch of the House of Bourbon who reigned as King of France from 1643 until his death in 1715.

Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface. The largest impact crater on Tethys, which can be seen in the image above, Odysseus, is about 250 miles (400 km) in diameter.

March 21 in Science, Technology, Astronomy, and Space Exploration history


  1. The name of the mission was actually Cassini-Huygens. It was a joint project of NASA, ESA (the European Space Agency), and the Italian Space Agency (ASI). The spacecraft, named after astronomers Giovanni Cassini and Christiaan Huygens, comprised both NASA’s Cassini probe and ESA’s Huygens lander which would be landed on Saturn’s largest moon, Titan. Cassini released the Huygens probe on December 25, 2004, by means of a spring and spiral rails intended to rotate the probe for greater stability. It entered the atmosphere of Titan on January 14, 2005, and after a two-and-a-half-hour descent landed on solid ground. Cassini probe was destroyed by diving into Saturn’s atmosphere on September 15, 2017. This method of disposal was planned to avoid potential biological contamination of Saturn’s moons, since Titan, Enceladus, and other icy moons of Saturn may harbor oceans and alien life.


M. Özgür Nevres
Latest posts by M. Özgür Nevres (see all)

Leave a comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.