On March 9, 2006, the Cassini Space probe took an astounding photograph that confirmed the theory that the moon Enceladus was contributing to the ring system of Saturn. The photo here was taken by NASA after engineers used Saturn itself to obscure the Sun but illuminate the rings and satellite, here clearly shown with a cryovolcano erupting water and gas into the E-ring.
Three years later, the Cassini space probe was positioned again to use the light from the sun to illuminate the rings and surface of the giant planet, with a shadow thrown across the ring system. Mathematical models of the ring system show that they are unstable beyond 10,000 years, meaning that they are constantly replenished from a source such as Enceladus.
Launched on October 15, 1997, the Cassini-Huygens spacecraft left Earth for flybys of Earth, Venus and Jupiter, before arriving at Saturn on Christmas Day, 2004 and separating the Huygens probe which reached Titan a few weeks later. Cassini-Huygens was named for Italo-French Astronomer Giovanni Domenico Cassini (later known as Jean Dominque Cassini), known for his discovery of four moons of Saturn, and Dutch astronomer Christiaan Huygens, who discovered Saturn’s moon Titan. The Cassini spacecraft is still in service today, currently located at the south Pole of Saturn, taking images of the famous hexagon. The spacecraft is scheduled to remain in service until September 2017.
Images courtesy NASA/ESA, in the public domain.
Although scientists as early as Aristarchus of Samos knew the relationship between the sun and earth around 270 BCE, it wasn’t until 1543 that Nicholas Copernicus published his masterwork De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), published the same year that he died that the heliocentric model received wide distribution. Perhaps his death and inability to defend his thesis led to the very slow spread and adoption of his idea, so that by the year 1616 a group of cardinals and bishops under the direction of the Vatican met to denounce Galileo Galilei, who was using the results of his observations made with the new technology of the telescope to re-introduce the heliocentric model of the solar system.
A decade and half passed before Galileo was dangerous enough to draw a trial, which commenced in 1633. Galileo was furious with the philosophers, theologians and scientists who denounced his idea, complaining to his friend and fellow astronomer Johannes Kepler,
My dear Kepler, I wish that we might laugh at the remarkable stupidity of the common herd. What do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an asp and do not want to look at either the planets, the moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times? Truly, just as the asp stops its ears, so do these philosophers shut their eyes to the light of truth.
He lost the trial and spent his last eight years under house arrest, working on his theories from his home in Pisa.
The word heliocentric comes from Ancient Greek, a combination of the words ἥλιος (helios) meaning sun and κέντρον (kentron) meaning center. It would take another three centuries for scientists to understand that not only is the Earth not the center of the Universe, neither is the Sun.
Happy Birthday Galileo Galilei, born February 15, 1564.
Painting of Galileo Facing the Roman Inquisition by Cristiano Banti, 1857, in the public domain.
Image from Copernicus in the public domain.
When is a planet not a planet? This question has troubled scientists and students for decades, and the recent demotion of Pluto from planet to plutoid marked a significant change in how scientists classify planets. In 2006 the International Astromical Union identified three key criteria to classify a body as a planet:
By this definition Pluto was excluded, but the discovery of a transneptunian object on January 5, 2005 added very briefly a planet to the solar system.
Many teams were racing to map the outer solar system, specifically the belt of asteroids and debris in the scattered disc known as transneptunian objects, and the discovery of Eris and Dysnomia took several years of searching, two years of analysis with both computers and good old fashioned legwork. The team of Mike Brown, Chad Trujillo, and David Rabinowitz had programmed their computers to rule out objects moving too slowly to count and missed seeing Eris when it was first photographed. Manually pouring over the photos almost a year and a half later, they found the largest (to date) plutoid or dwarf planet.
Wanting to name more sky objects after women, internally the team referred to the dwarf planet as Xena, after the popular television series. After a period of several years (and much consensus building) the name Eris was settled on. Eris was the goddess of strife in Greek mythology, the meddling troublemaker who doomed the marriage of Peleus and Thetis, the parents of Achilles, which led to the disastrous war that destroyed both Troy and much of the invading Greek armies. Eris’s own son was named Dysnomia, meaning lawlessness, and when the Hubble Space Telescope discovered that Eris had a moon, well, the name was inevitable. Since the discovery of Eris and Dysnomia, several more plutoids have been discovered, but none yet as large. Eris was considered a planet for less than a year, before both Eris and Pluto were reclassified as plutoids.
Images of Eris and Dysnomia courtesy NASA/Hubble.