December 30 – Launch of RXTE

December 30th seems to be a thin day astronomically, but I did manage to find, in 1995, the launch of the Rossi X-Ray Timing Explorer from Cape Canaveral.  Bruno Rossi, after whom the satellite was named, was the discoverer of the first source of x-rays beyond the Sun.

Artist's Impression of RXTE

Artist’s Impression of RXTE

Over a sixteen year lifespan, RXTE did exactly what its name suggests: it timed variations in emissions from x-ray sources using three experiments (a proportional counter array, theHigh Energy X-ray Timing Experiment, and an all-sky monitor).

Also today, in 1924, Edwin Hubble announced to the world that the Milky Way is not the only galaxy.  Using the 100-inch Hooker Telescope at Mount Wilson, Hubble was able to calculate the distance to Cepheid variable stars in the Andromeda Galaxy using a technique devised by Harvard astronomer Henrietta Swan Leavitt.  At the time, “spiral nebulae” were assumed to be patches of dust or gas within the Milky Way, but calculating a distance to Andromeda of approximately 860,000 light years changed the scale of the Universe for ever.


December 27 – Johannes Kepler

Today is Johannes Kepler’s birthday.

Kepler was born in Weil der Stadt, a small town near Stuttgart, on December 27th 1571, and was introduced to astronomy from an early age, whether he liked it or not, by being taken outside to witness the Great Comet of 1577, C/1577 V1, at age 6. The comet was also seen, incidentally, by Tycho Brahe, with whom Kepler would later spend some time studying at the site of Brahe’s new observatory near Prague.

Johannes Kepler

Johannes Kepler

Kepler’s works included many revolutionary (and I mean that in several ways) publications on the behaviour of planets. His Astronomia Nova, published in 1609, contained arguments in favour of a heliocentric ‘universe’, and Harmonia Mundi (“The Harmony of the World”, 1619) was the setting for his third law of planetary motion.

As well as being an influential astronomer, Kepler was also a major influence in the field of optics (possibly because his own eyesight wasn’t the best). He was the first person to explain how a telescope works; worked out how our eyes perceive depth; investigated how a pin hole camera might be used to produce pictures, and discovered total internal reflection.

Also today, globular cluster Messier 92 was discovered in 1777 by Johann Elert Bode.

M92 (Photo courtesy of the Isaac Newton Group of Telescopes, La Palma)

M92 (Photo credit: the Isaac Newton Group of Telescopes, La Palma)

M92 is located in the constellation of Hercules. It is one of the oldest and brightest globular clusters, and is about 27,000 light years from Earth.

1968 – Splashdown, south of Hawaii, of Apollo 8, following a 6 day flight that included the first Earthrise seen by humans, and the first Christmas broadcast from a craft orbiting the Moon.


December 24 – Rhea

Saturn’s second-largest moon, Rhea, was discovered on this day in 1672 by Giovanni Domenico Cassini, who had presumably finished his Christmas shopping early.

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Like our own Moon, Rhea always keeps one side, the one you can’t see in this photo, facing towards the parent planet, and from the above view of Rhea’s cratered anti-Saturnian surface, this does look to be a place very much like the Moon we are familiar with down here.  But appearances can be deceptive.  For a start, it’s a lot smaller than the moon, and if it soft-landed on the Earth it would fit quite comfortably inside the borders of Zaïre (in this country people usually use Wales as the benchmark for large areas, but I felt like a change).  Secondly, although it looks nice and rocky, Rhea is thought to be comprised of as much as three-quarters ice, and only one quarter rock (studies of Rhea’s inertia suggest that it doesn’t even posses a rocky core).

Rhea is named after a Titaness of some standing, the daughter of Uranus and Gaia.  As mother of Zeus and Hera, she was grandmother or great-grandmother to almost all the Olympian gods and goddesses worth mentioning.



There are several suggestions doing the rounds concerning the origin of the name Rhea.  It may derive from the word for “flow”, or it could have a root in a much older word for “powerful”.  But the guy who named the large, flightless, South American bird after her was almost certainly thinking of a derivation of the Greek έρα, meaning “ground”.

Another Rhea

Another Rhea



December 22 – Asteroid 264 Libussa

Discovered on December 22nd 1886 by C H F Peters, asteroid 264 Libussa is an asymmetrical S-type main belt asteroid of somewhere between 50 and 60 km diameter.

It was named after Libussa (Lubossa or Libuše), a half-elf  daughter of the mythical Czech king Krok.  She eventually became the founder of the city of Prague, and ancestor of the whole Czech nation.

Libuse (photo credit: Palic Kap)

Libuse (photo credit: Palic Kap)

Today’s photo shows a detail from the Octárna Hotel, Kroměříž, Czech Republic.  This is alunette (a half moon shaped space) by local artist Max Švabinský. It shows Princess Libuse prophesying the glory of Prague.

There’s not a great deal more to say about Libussa, so it’s back to orbital characteristics.  To date we have come across aphelion (3.18 AU for this particular rock), perihelion (2.42 AU), semi-major axis (2.8 AU), longitude of ascending node (49.8°), and inclination (10.4°).  Today we add another important element of an asteroid’s orbit: the argument of perihelion.  For Libussa, this is 339.27°, and in orbital calculations it is depicted by the letter  ω.  It’s not a “w”, by the way, but a lower-case omega (the word means “great o”) the last letter of the Greek alphabet.  Remember that, because one of these days I’m going to work out how to put some of these calculations in this blog in the form of a table.

The argument of perihelion is the angle between the perihelion and the ascending node.  So if, for example, the value was 90°, that just means that the asteroid would be at perihelion when it reached the northerly point in its orbit.

1981  ⇒  Asteroid 323 Brucia becomes the first asteroid to be discovered by astrophotography, by Max Wolfe.


December 19 – Asteroid 397 Vienna

Asteroid 397 Vienna was discovered by Auguste Charlois on December 19th, 1894. It is an S-type main belt asteroid of about 43 km diameter. There are no prizes for guessing the origin of the name.

In the 1960’s, the only way to spy from above on your decadent imperialist western enemies was to send collections of cameras into orbit, shoot a few rolls of film, bring the whole thing back down, and hope you could make it land in a suitable place for retrieval.
Kosmos 24 (also known officially, but less publically, as Zenit 2, Number 15) was one such Soviet reconnaissance satellite, launched on December 19th, 1963, and recovered by the military nine days later. The launch took place at the Baikonur Cosmodrome via a Vostok 2 rocket, and as far as anyone is aware it was a success.

Kosmos 24

There were over 500 Zenit 2 launches, mostly carrying four cameras, with each camera capable of shooting 1500 frames. And what I didn’t know until very recently was that the Zenit camera I owned in the late 1970’s was made by the same company who manufactured the equipment for the Kosmos satellites. I Wish I’d kept it.

2013 – Launch of the Gaia Space Observatory.


December 19 – Tau Ceti ‘e’ and ‘f’

Tau Ceti e and Tau Ceti f are as yet unconfirmed exoplanets orbiting, fairly obviously, the star Tau Ceti, a G-type main sequence star in the constellation Cetus (usually referred to as “the whale”, but actually a sea monster from Greek mythology). They are the fourth and fifth planets out from the star, and were discovered on December 19th, 2012, by the “radial velocity” method, the oldest known means of detecting planets outside our system.

Location of Tau Ceti

Location of Tau Ceti

Both planets are likely to be a fair bit bigger than the Earth (“f” could be up to seven times larger), but they have excited astronomers by being in the habitable zone of Tau Ceti. It is thought they could have temperatures of up to 50 or 60°C, which is plenty warm enough for the existence of life. Unfortunately for any Tau Cetians though, the parent star is known to have an extensive “debris disk”, meaning that any planets nearby would face a regular pounding by rocks of varying shapes and sizes, almost certainly including some big enough to cause serious problems for any fledgling species trying to evolve.

December 18 – Epimetheus

Saturn’s moon Epimetheus, discovered today in 1966 by Richard Walker, is a non-identical twin.  It, and its larger brother, Janus, which had been discovered three days previously by Audouin Dollfus, fly round Saturn in almost identical orbits (their mean orbital radii differ by a mere 50km or so).  Obviously, at the time of discovery, with only  the evidence of Earth-based telescopes to go by, this caused a certain amount of confusion among astronomers trying to work out where the supposed new single moon was.  At the time it was thought, also fairly obviously,  highly unlikely that two moons would share the same orbit without hitting each other.

Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)

Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)

It is possible that at some time in the past Epimetheus and Janus were part of a single larger body, but if that is the case it is likely to have been a very long time ago.  Both are heavily cratered, and their surface features display softening of their edges by the accumulation of dust, suggesting nothing much has changed there for some time.

Epimetheus was, in Greek mythology, a Titan (nowadays the name of a rather larger Saturnian moon), brother of Prometheus (also a moon of Saturn).  The pair were sons of Iapetus (another Saturnian satellite).  Epimethius was, according to some myths, married to Pandora (and guess what she gave her name to?).

Epithemius was given the job, shortly after the creation of the Earth, of finding a positive trait to give to each of the newly designed animals.  Unfortunately he ran out of good characteristics with one animal remaining, which you’ve probably already guessed was man.  That’s when Prometheus came along and gave  man the gift of fire, after which it all went downhill.

Epimetheus (3rd left) meets Pandora for the first time.

Epimetheus (3rd left) meets Pandora for the first time.

As it happens, though, they do share the orbit, and they don’t hit each other.  One moon does catch the other up every four years, but interactions between their gravities causes the inner moon to move faster (which causes it to move to a higher orbit) and the outer moon to slow down, causing its own orbit to drop a little.  So they actually exchange places, which is why I’m not going to call them the fifth and sixth moons, because they are each neither and both.

Launch, in 1958, of SCORE, the world’s first communications satellite (Signal Communications by Orbiting Relay Equipment).  The launch, from Cape Canaveral, was via an Atlas-B 10B rocket.

December 16 – Asteroid 351 Yrsa

Asteroid 351 Yrsa was discovered today in 1892 by Max Wolf.  There isn’t a great deal to say about it:  it’s in the main belt, and is about 40 km across.  It is thought to have been named after the wife of King Eadgils of Swedish legend.  her father, Helghe, who had only had a brief fling with her mother, visited the region where she lived years later and, not knowing she was his daughter, got her in the family way, as they say.   Unlike the average Greek god, who would have thought nothing of it, he did the decent thing and killed himself upon discovering the truth.

Lithograph depicting Yrsa

Lithograph depicting Yrsa

Max Wolf was a great friend of today’s birthday boy, Edward Emerson (E.E.Barnard, namesake of Barnard’s Star, one of the most closely observed objects in the Galaxy.  Barnard was mainly a prolific comet finder, discovering 17 in all (15 solo, and 2 co-discoveries), but he’s always going to be associated with the one star in my mind, because it was, and indeed may still be for all I know, the intended target of Project Daedelus, the mind-bogglingly ambitious unmanned interstellar mission proposed by the British Interplanetary Society.  When I was an impressionable teenager in the 1970’s, Daedelus seemed likely to happen “any year now”.  Unfortunately, 30 years later, they still haven’t quite got around to working out how to get the necessary helium-3 fuel back from Jupiter in order to get it to work, and I suspect that the cost in today’s money of a nuclear-powered spaceship bigger than a Saturn 5 and built in orbit would be rather more expensive than gold-plating the Isle of Wight or  buying Australia and Canada back.

E E Barnard

E E Barnard



December 15 – Janus

J R Hind discovered today’s main belt asteroid, 23 Thalia, from Hyde Park, London, on December 15th, 1852 (I’d like to see him try that nowadays). Thalia is an S-type asteroid of about 107 km diameter, located between the 3:1 and 5:2 Kirkwood gaps.

In Greek mythology, Thalia, daughter of Zeus and Mnemosyne, with a name derived from the verb “to flourish”, was the muse of comedy and pastoral poetry. She may or may not (depending on which source you believe) have been the mother of the Corybantes, attendants to the Great Mother of the Gods, and associated with particularly orgiastic rites.

We also have a moon today.  The discovery of Janus, one of the inner Saturnian satellites, is attributed to Audouin Dollfus, who first observed it on December 15th 1966. Three days later, Richard Walker also observed an object in the right place but at the wrong time, which caused confusion for a while, but was eventually found to be another moon, Epimetheus, which shares an orbit with Janus.

Janus, photographed by the Cassini probe (image credit: NASA / JPL / SSI)

Janus, photographed by the Cassini probe (image credit: NASA / JPL / SSI)

Janus is the two-faced Roman god of beginnings, entrances, gates, doors, etc. You should thank him the next time an automatic door opens for you. Janus is also one of the select group of deities after whom a month (January) is named, and strangely he has no Greek counterpart.

1965  –  launch of San Marco 1 by Italy. Being their first satellite, the Italians wisely did not fill it with lots of expensive equipment. It did, though, contain a couple of experiments to study the ionosphere, the layer of the atmosphere stretching from about 60 km to 1,000 km, a region you need to know about if you’re planning to become a space-faring nation, needing to send radio messages over great distances.

2000  –  Death of George Alcock, aged 88, hunter of novae and comets. I believe he found five of each (remarkable for south-eastern England), some of them from indoors using binoculars, and even occasionally through double glazing1!  His eyesight must have been unbelievable.

2014  –  Voyager 1 crossed the termination shock.


2015  –  Launch of Expedition 46 to the International Space Station.  This caused considerable press interest in my homeland (in fact I’m going to call it a frenzy) because in addition to Russian cosmonaut Yuri Malenchenko and American astronaut Tim Kopra, the three-man crew contained Tim Peake, the first Briton to float into the ISS (I was going to say “set foot aboard” the ISS, but I’ve seen the footage, and feet don’t feature much).  Because of the  numbering system they use at the ISS when crews overlap, these three also formed part of Expedition 47.

ISS Expedition 46 Patch

ISS Expedition 46 Patch

As a supporter of Port Vale FC, I was distraught to discover that one of Tim Peake’s tasks whilst on this mission was to unveil a flag featuring the name of our local rivals, Stoke City.  I’ve gone off him a little.


1   Journal of the British Astronomical Association, vol.111, no.2, p.64-66

December 14 – Tycho Brahe

Tycho Brahe was born today in 1546 in Denmark. As the child of a wealthy family (he was born in a castle) he was well placed to receive a good education, and took up astronomy while studying law.

Tycho Brahe wearing the Order of the Elephant (and the Moustache of the Walrus)

Tycho Brahe wearing the Order of the Elephant (and the Moustache of the Walrus)

Tycho made several leaps of the imagination, including the development of his own model of the solar system, which he nearly got right. He correctly deduced that the Moon orbits the Earth, and that the planets orbit the Sun. unfortunately he also decided that the Sun must orbit the earth; an easy mistake to make I suppose at that time, if you spend your entire life seeing it cross the sky thousands of times in front of your very eyes.

Tycho was also the first person to decide that novae (they weren’t called that at the time) originated further away than the Moon. The popular view was that the stars were fixed and unchanging, and anything that appeared in the sky had  to be closer than them. But Tycho noticed that a bright new star in Cassiopeia (now called SN1572) did not move against the background stars, and so must be farther away than all the objects that did. It sounds fairly reasonable to us, but in the 16th century the night sky was anything but obvious.

2009 – Launch of the Wide-field Infrared Survey Explorer (WISE) by NASA, on a mission  to map 99% of the sky. So far, it has found more than 33,000 asteroids and comets, over 200 Near-Earth Asteroids (including 43 potentially hazardous ones), the most luminous galaxy in the known universe, and numerous brown dwarfs.

2013  –  the Chinese Chang’E 3 mission lands on the Moon.

The Chang'E Lunar Lander, imaged by the Yutu rover (image: China National Space Administration)

The Chang’E Lunar Lander, imaged by the Yutu rover (image: China National Space Administration)