December Sky Notes
Written by John Stapleton
Image Credit: Sky at Night Magazine
Last Chance to see…?
Recent poor journalism suggested that right now would be your last chance to see Saturn’s rings as they were going to disappear, apparently forever, according to the article. This is of course not, at all true.
Because of the relative tilts of Saturn’s and the Earth’s orbits, our view of the planet changes over a period of years. When our view passes across the plane of Saturn’s rings they seem to disappear altogether. This is because the rings, although extensive around the planet, are only between 10 metres and 1 Kilometre thick, that is not large enough to be seen through most earthbound telescopes. The next ring-plane crossing takes place in March 2025, hence the (incorrect) panic in the press.
This event takes place once every 13 – 15 years and can occur up to three times in each cycle (depending on the relative positions and motions of the two planets). Before the ring-plane crossing we see the rings from above or below and more open, and after the crossing we see them from the opposite angle. The rings do not disappear, but they simply tilt through their smallest dimension rendering them invisible to earthbound observers.
This month also provides an opportunity to see one of the minor planets, or asteroids, that orbit the Sun between the orbits of Mars and Jupiter. A German astronomer named Johannes Elert Bode noted an approximate (but ultimately flawed) mathematical sequence between the distances of the known planets and suggested that there was a planet “missing” and that it should lie in the gap between Mars and Jupiter. He theorised that this planet would be small and that this is why it had not been found before. He formed an observing group with several other distinguished German astronomers of the time to scour the sky between them in search for the “missing” planet. This group styled themselves the “Celestial Police” because of the hunt they were undertaking. In the event, the planet was found by an Italian astronomer, named Guisseppe Piazzi, on January 1st 1801. Piazzi, who worked at the observatory in Palermo, Sicily, named the planet Ceres, after the patron goddess of the island. Olbers then discovered the second asteroid which he named “Pallas” and the other members of the “Celestial Police” (following the clues given to them by the first two discoveries) went on to discover several more.
There are now estimated to be between 1 and 2 million objects in the so-called asteroid belt. When William Herschel discovered Uranus in 1781 it did not fit with Bode’s sequence and the gaps between the planets were seen to be entirely random.
Sky Notes
Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.
Sun: Throughout December the Sun sets before 4.30 and there are just 8 hours of daylight. The Winter Solstice (shortest day) occurs on December 21. On this date the Earth is closer to the Sun than at any other time in its orbit. We experience Winter in the Northern Hemisphere because the North Pole is tilted away from the Sun and its warming energy is spread over a greater area on the surface of the Earth.
Mercury: Mercury is visible in the morning sky, rising about an hour before the Sun. It reaches its greatest elongation (distance East) from the Sun on 4th. If you observe before the Sun rises above your horizon it will be safe to scan the sky with binoculars to try and locate this elusive little planet.
Venus: Venus is also a morning object throughout the month, It is very bright and easily found near the star Spica in Virgo. On December 9, Venus will appear very close to the Moon.
Mars: Mars is a morning object but lies too close to the Sun to be readily seen, this month.
Jupiter: Jupiter is a brilliant object throughout the night and is well placed for observation. Jupiter is currently seen against the background stars of Aries and can be seen in the south-eastern sky. The Galilean moons can be seen with a good pair of binoculars or a small (bird-spotting) telescope. DSLR images of the planet will also pick up the moons. The cold winter skies (if they are clear) give a much clearer and steadier image in a telescope or binoculars making observations much more satisfying. The Moon will be very close to Jupiter on 22nd.
Saturn: The ringed planet can be found against the stars of Aquarius in the Southern sky. This is the last opportunity to view the ringed planet for a while. Its rings are now closing (from our point of view) and this is why it is becoming fainter, although still a naked-eye object.
Uranus and Neptune: Uranus is visible all night long throughout December and is seen against the background stars of Aries and close to the Pleiades star cluster. At magnitude 5.8 it is visible in binoculars. Look for a tiny greenish disc compared to the pinpoints which are the stars. Uranus will also be occulted by the Moon on 5th. Neptune is seen against the background stars of Pisces and close to the asterism called “The Circlet” which represents one of the fishes. At magnitude 7.8 it will require large binoculars or a larger telescope to find it and a larger telescope to discern its disc. Neptune displays a smaller and truly blue disc compared to that of Uranus although it will appear only as a bluish star to most small instruments.
Meteor Shower: There are two meteor showers due in December. The first, which peaks on the night of 13th, is the Geminids. The Geminid shower is the only shower associated with an asteroid rather than a comet. This asteroid is called Phaethon. The Geminid shower can be intense, and a New Moon the night before makes this ideal for observations this year. The second shower this month is the Ursids (maximum on 21st) that seem to emanate from a point in the constellation of Ursa Minor, which also contains the Pole Star.
Comet: There are no predicted bright comets this month. However, the minor planet (or Asteroid) 4 Vesta will be visible, definitely in binoculars and possibly to the naked eye. Vesta is the brightest of the asteroids and was discovered by one of the German “Celestial Police” named Heinreich Olbers. The minor planet can be found above Orion and just below Gemini, heading in the direction of the horns of Taurus. This passage will continue throughout the whole month, so you need to find the speck of light that changes position over the month.
The Last Quarter Moon occurs on December 5th with New Moon on 12th, First Quarter then follows on 19th December and Full Moon on 27th, so there will be plenty of light for Santa to see by.