June 8, 1625, is the birthday of Giovanni Domenico Cassini, the Italian and French astronomer.

The 17th century was rich in scientific discoveries. Physics and astronomy were of special interest to the scientific community of the century. Giovanni Cassini cast his lot with the latter. His mentors were Giovanni Riccioli and Francesco Grimaldi, Italian science popularizers. Their influence on Cassini and the rapid development of astronomy at that time formed a stable obsession with the cosmic space, which remained with him throughout his life.

Cassini started his astronomical studies with observation of the Sun and astronomical calculations. In 1662, he published a paper with cataloged data on the Sun – sun tables. Following the sine law, the astronomer developed an atmospheric refraction theory, whereby the cause of position changes of celestial objects was refraction of the rays from sources of light, which the bodies reflected.

The observations of space bodies continued. The scientist went even further – in 1664, he started observing planets. Telescopes with good optical characteristics helped him with that. Cassini went down in history as the first scientist who was able to determine the rotation period of Jupiter and explained the system of bands on the planet. Two years later, he determined the axial rotation period of Mars. In 1668, he created a theory and developed tables of movements of Jupiter’s moons. The Italian astronomer did not leave his optical observations at that. On December 23, 1672, he discovered the fifth moon of Saturn – Rhea. The name was borrowed from the ancient Greek goddess, the daughter of Uranus and Gaea. Cassini loved to look at the planet. To him, Saturn was a mysterious celestial object enthralled by some kind of a ring…

Cassini Division, a snapshot made from the Cassini orbiter

Cassini Division, a snapshot made from the Cassini orbiter

Source: Wikipedia

Now, Saturn is known to have dozens of moons. Cassini discovered and named four of those: Iapetus, Rhea, Tethys, Dione. Collectively, he named them Sidera Lodoicea, meaning the Stars of Louis in Latin. Thus Cassini thanked King Luis XIV of France for the invitation to work in France, in the Royal Academy of Sciences created not long before that. Cassini supervised the construction of the Paris Observatory and then became its director. It was there that the astronomer performed his observations and discovered the moons of Saturn.

In 1675, Cassini noticed a strange black band in the disk of the ring, embracing his favorite planet. The phenomenon would be later called the Cassini Division. In fact, it is not a void but a ring. This was the conclusion drawn thanks to snapshots from the space probes Voyager and Cassini. The material is similar to that of the C Ring in color and thickness. The Cassini Division has real gaps inside it that are virtually empty: the Huygens Gap (on the inner edge) and the Laplace Gap, with small thin rings.

Years of observations of the mysterious celestial bodies brought results. In 1679, Cassini published the most accurate map of the Moon of his time. His hypothesis regarding the existence of the zodiacal light that he put forward in 1683 was based on the assumption that the sunlight dispersed on dust clouds in the ecliptic plane. Today, this is no longer an assumption, but a complete scientific theory.

Cassini can be rightfully called a space pioneer. His name is associated with the mysterious Great Red Spot of Jupiter – a gigantic vortex that has been raging in Jupiter’s atmosphere for centuries. It is so huge, that there is no bigger vortex in the Solar System. In terms of its linear dimensions, three planets the size of the Earth could fit into it.




The Great Red Spot

The Great Red Spot

Source: Galspace

The first observation of the Great Red Spot is often attributed to Robert Hooke. He described a spot he saw on Jupiter in 1664. It became clear afterwards that Hooke’s spot was probably in another belt altogether (the North Equatorial Belt, as opposed to the current Great Red Spot's location in the South Equatorial Belt). Giovanni Cassini gave a far more convincing description. The official discovery of the Great Red Spot is believed to have happened in 1665.

Nothing was impossible for Giovanni Cassini. It would seem that the dimensions of the Solar System could not be even imagined, let alone measured. Especially, without modern technology. But the Italian astronomer proved the opposite. In 1672, he calculated the distance between the Earth and Mars using the parallax method. The scientist knew that Mars was close to the Earth, so he used his improved telescopes to measure the distance. If he were to measure the angle to a point on Mars from two different points on the Earth at the same time, he could use those angles and the geometry of the triangles to calculate the distance to Mars. For the calculation work, he sent the French astronomer Jean Richer to Cayenne in French Guiana on the north coast of South America, while remaining in Paris himself. One night in 1672, the scientists measured the angle to Mars and placed it precisely against distant stars. Upon Richer’s return to Paris, Cassini was able to calculate the distance to Mars. Then he applied Kepler equations to discover that the distance to the Sun had to be 146 million km. Cassini's calculation was very close to the truth (according to the modern data, the distance to the Sun is 149.6 million km).

Photo on the page: Spacegid

Photo on the main page: Cassini Saturn Images