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In this month: December


15th December 1966

Janus, a moon of Saturn, is discovered by French astronomer Audouin Dollfus. Astronomers were first mistaken that another moon, Epimetheus, was the same as Janus. In 1980 Voyager 1 confirmed that these two moons share the same orbit. Janus is potato-shaped with a diameter of 180 kilometres and orbits 151,000 kilometres from Saturn in the gap between the F and G rings, taking 17 hours to complete one orbit. It is extensively cratered with several craters larger than 30 kilometres but has few linear features.

Janus is named after the two-faced Roman god Janus, god of gates, doors, beginnings, and endings. He is usually represented as having one face to look forward and another to look back. Although the name was informally proposed soon after the initial 1966 discovery it was not officially adopted until 1983.

Photograph of Janus by the Cassini mission during its flyby of this Saturnian moon on March 27, 2012. Image Credit: NASA/JPL-Caltech/Space Science Institute

21st December 1968

Launch of Apollo 8 the first mission to take humans to the Moon and back. It was the first crewed spacecraft to leave low Earth orbit, and also the first human spaceflight to reach another astronomical object - the Moon - which the crew orbited without landing. The three astronauts, Frank Borman, James Lovell, and William Anders, were the first humans to witness and photograph an Earthrise. They orbited the Moon ten times over the course of twenty hours, during which they made a television broadcast on Christmas Eve where they read the first ten verses from the Book of Genesis.

The historic mission happened because of a last-minute call by NASA. At the time the United States and Russia were engaged in a "space race" and NASA, being mindful of crew safety, wanted to bring Americans to the moon as soon as possible. Sending Apollo 8 into lunar orbit after only one previous Apollo mission, which had remained in Earth orbit, was a bold decision.

3rd December 1973

The first flyby of Jupiter by the Pioneer 10 spacecraft. After becoming the first spacecraft to pass through the asteroid belt, it passed by the planet at a distance of 130,000 kilometres above the cloud tops. The rendezvous provided the first close-up images of the planet and data on the planet’s magnetic field which proved Jupiter to be a primarily liquid planet. After Pioneer 10 flew by Jupiter it began its journey into interstellar space heading towards the star Aldebaran which it will encounter in 2 million years. Pioneer 10 carries a plaque with a message to any intelligent life it might meet on its journey. The plaque includes diagrams of Earth's location and drawings of a man and a woman.

After operating for more than 30 years, the successful Pioneer 10 spacecraft sent its last, very weak, signal to Earth on the 23rd January 2003.


12th December 1803

James Challis. English physicist and astronomer.

He is best remembered for his missed opportunity to discover the planet Neptune in 1846. As early as 1844 John Couch Adams had predicted the location of an eighth planet in the

Solar System but failed to promote his prediction successfully. In 1846, the Astronomer Royal, George Airy, persuaded a skeptical Challis to join in the search for the planet. Challis began his, somewhat reluctant, search in July 1846, unaware that French astronomer Urbain Le Verrier had independently made an identical prediction. The planet was found and named "Neptune". It soon became apparent from Challis's notebooks that he had observed Neptune twice, a month earlier, failing to make the identification through lack of diligence and a current star chart. Despite the embarrassment over Neptune, Challis did make genuine contributions to astronomy and he succeeded Airy as Director of Cambridge Observatory (image above courtesy Wiki Commons).

He published 225 papers in mathematics, physics, and astronomy, and published, in 12 volumes, Astronomical Observations Made at the Observatory of Cambridge (1832–64).

The Lunar crater Challis is named after him

16th December 1857

Edward Emerson Barnard, American astronomer.

He was recognized as a gifted observational astronomer and is best known for his discovery of the high proper motion of Barnard's Star in 1916, which is named in his honour. This is the second nearest star system to the Sun, second only to the Alpha Centauri system. In 1892 he discovered Amalthea, a moon of Jupiter, using the 91 cm refractor telescope at Lick Observatory. It was the last natural satellite to be discovered by direct visual observation, all later moons were discovered by photographic or digital imaging.

He was also a pioneering astro-photographer. His Barnard Catalogue lists a series of dark nebulae, known as Barnard objects, with numerical designations similar to the Messier catalogue.

He published his initial list together with a paper, "On the Dark Markings of the Sky with a Catalogue of 182 such Objects" in the Astrophysical Journal

Barnard has many astronomical objects named after him including a lunar crater and a crater on Mars and the Asteroid 819 Barnardiana. Image above by EDWARD EMERSON [Astrophotograph of the Andromeda Nebula], 1889 Albumen print Courtesy Wiki Commons.

28th December 1944

Sandra Moore Faber. American astrophysicist .

Faber is known for her research on the evolution of galaxies. She is the University Professor of Astronomy and Astrophysics at the University of California and works at the Lick Observatory, California. She has made discoveries linking the brightness of galaxies to the speed of stars within them and was the co-discoverer of the Faber–Jackson relation (one of its main uses is as a tool for determining distances to external galaxies.) She was also instrumental in designing the Keck telescopes in Hawaii.

The minor planet #283277 Faber is named for her.

Photograph courtesy University College Santa Cruz.


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