Description

Northern Hemisphere

Ian Morison tells us what we can see in the Northern Hemisphere night sky during December 2019.

The Planets

• Jupiter, shining on the 1st at magnitude -1.8 and with an angular size of 32 arc seconds, can be seen very low in the southwest as darkness falls at the start of December but, soon after, will be lost in the Sun's glare. Jupiter lies in the southeastern part of Ophiuchus and is heading towards the southernmost part of the ecliptic so, as it appears in the twilight, will only have an elevation of ~6 degrees. With its low elevation, atmospheric dispersion will take its toll and an atmospheric dispersion corrector would greatly help to improve our views of the giant planet and it four Gallilean moons.
  
• Saturn will be seen west of south as darkness falls at the start of the month. Then, its disk is ~16 arc seconds across and its rings - which are still, at ~24 degrees, nicely tilted from the line of sight - spanning some 36 arc seconds across. During the month its brightness remains +0.6 with an angular size of 15.4 arc seconds. Sadly, now in Sagittarius and lying on the south-eastern side of the milky way, it is at the lowest point of the ecliptic and will only have an elevation of ~12 degrees after sunset. As with Jupiter, an atmospheric dispersion corrector will help improve our view.
  
• Mercury,. Following its transit of the Sun and reaching greatest elongation west on the 28th of November, can be seen in the pre-dawn sky low in the southeast at the start of December. On the 1st it will have a magnitude +0.29 and will rise around an hour before the Sun. It will then have an elevation of some 9 degrees before being lost in the Sun's glare. With an angular size of ~5 arc seconds, it will then fall back towards the Sun and be lost from view by the middle of the month.
  
• Mars, Mars can be seen towards the southeast in the pre-dawn sky at the start of its new apparition. It rises some two and a half hours before the Sun at the start of the month and will have an elevation of ~15 degrees before it is lost in the Sun's glare. It then has a magnitude of +1.7 and a 3.9 arc second, salmon-pink, disk. By month's end it will be seen further round towards the south before dawn and its magnitude will have increased slightly to +1.6.
  
• Venus, Venus may just be glimpsed in the south-west at the start of the month, but will be difficult to see due to the fact that the ecliptic is at a shallow angle to the horizon and so Venus will have a very low elevation. As the month progresses, it will rise higher in the sky and on the 31st will have reached an elevation of 14 degrees as darkness falls. During December, its magnitude remains at about -4 and its disk increases from 11.6 to 13 arc seconds across. A low horizon and possibly binoculars will be needed to spot Venus, but please do not use them until after the Sun has set.
  

Highlights

• November - still a chance to observe Saturn. . Saturn is now low (at an elevation of ~13 degrees) just west of south as darkness falls lying above the 'teapot' of Sagittarius. Held steady, binoculars should enable you to see Saturn's brightest moon, Titan, at magnitude 8.2. A small telescope will show the rings with magnifications of x25 or more and one of 6-8 inches aperture with a magnification of ~x200 coupled with a night of good "seeing" (when the atmosphere is calm) will show Saturn and its beautiful ring system in its full glory.The thing that makes Saturn stand out is, of course, its ring system. The two outermost rings, A and B, are separated by a gap called Cassini's Division which should be visible in a telescope of 4 or more inches aperture if seeing conditions are good. Lying within the B ring, but far less bright and difficult to spot, is the C or Crepe Ring.
  
• December, late evening: the Double Cluster and the 'Demon Star', Algol. December is a good time to look high in the Southeast after dark towards the constellations of Cassiopea and Perseus. Perseus contains two interesting objects; the Double Cluster between the two constellations and Algol the 'Demon Star'. Algol in an eclipsing binary system. Normally the pair has a steady magnitude of 2.2 but every 2.86 days this briefly drops to magnitude 3.4.
  
• December: find Uranus. This month is a still good time to find the planet Uranus in the late evening as it reached opposition on October 28th. With a magnitude of 5.7, binoculars will easily spot it and, from a really dark site, it might even be visible to the unaided eye. A medium aperture telescope will reveal Uranus's 3.7 arc second wide disk showing its turquoise colour. It lies in Aries, close to the boarders of Pisces and Cetus as shown on the chart.
  
• December: find M31 - The Andromeda Galaxy - and perhaps M33 in Triangulum. Around new Moon (26th December) - and away from towns and cities - you may also be able to spot M33, the third largest galaxy after M31 and our own galaxy in our Local Group of galaxies. It is a face on spiral and its surface brightness is pretty low so a dark, transparent sky will be needed to spot it using binoculars (8x40 or, preferably, 10x50). Follow the two stars back from M31 and continue in the same direction sweeping slowly as you go. It looks like a piece of tissue paper stuck on the sky just a bit brighter than the sky background. Good Hunting!
  
• December 1st If clear before dawn and looking southeast, one could see a nice lineup of Mercury, Mars and Spica. Arcturus will be seen high up to their left.
  
• December 10th In the evening one could see the Moon, close to full, lying between the Pleiades and Hyades Clusters. Aldeberan is a red giant star far closer to us.
  
• December 12th - before dawn: Mars and the double star Alpha Librae.If clear before dawn on the 12th, one will see Mars (magnitude +1.67) just above the double star Alpha Librae (Magnitudes +2.74 and +5.15) or Zubwnelgenubi. Despite its name it is the second brightest star in Libra. This would make a nice image using a small telescope.
  
• December 14th and 15th after midnight: the Geminid Meteor Shower.The early mornings of December 14th and 15th will give us the chance, if clear, of observing the peak of the Geminid meteor shower. The Moon is at First Quarter and will set around 11 pm so, when Gemini is highest in the sky, its light will not hinder our view. The Geminids can often produce near-fireballs and so the shower is well worth observing if it is clear. An observing location well away from towns or cities will pay dividends. The relatively slow moving meteors arise from debris released from the asteroid 3200 Phaethon. This is unusual, as most meteor showers come from comets. The radiant - where the meteors appear to come from - is close to the bright star Castor in the constellation Gemini as shown on the chart. If it is clear it will be cold - so wrap up well, wear a woolly hat and have some hot drinks with you.
  
• December 22nd/23rd - late evenings: the Ursid Meteor ShowerThe late evenings of the 22nd and 23rd of December are when the Ursid meteor shower will be at its best - though the peak rate of ~10-15 meteors per hour is not that great. Sadly, this year Full Moon is on the 21st, so its light will greatly hinder our view. The radiant lies close to the star Kochab in Ursa Minor (hence their name), so look northwards at a high elevation. Occasionally, there can be a far higher rate so it is worth having a look should it be clear.
  
• December - Evenings of the 5th and 18th: The Straight WallThe Straight Wall, or Rupes Recta, is best observed either 1 or 2 days after First Quarter or a day or so before Third Quarter. To be honest, it is not really a wall but a gentle scarp - as Sir Patrick has said "Neither is it a wall nor is it straight!"
  

Southern Hemisphere

Haritina Mogosanu and Samuel Leske from the Carter Science Centre in New Zealand speaks about the Southern Hemisphere night sky during December 2019.


• Where we are This time we went to Stonehenge Aotearoa, but don't worry this is still in the Southern Hemisphere. Here is what's in the store for December.
• Overview of the night sky
• Orion
• The Planets Venus and Saturn will have a close encounter on the 11th of December, look west to see them just after sunset. Have a good look as the next day it will be full Moon, which will light pollute the skies.
  

• Meteor Showers
• The Moon
• Clear skies from Haritina and Sam from New Zealand, and see you next year!