Description

Northern Hemisphere

Ian Morison tells us what we can see in the Northern Hemisphere night sky during December 2020.

The Planets

• Jupiter, along with Saturn, still remains visible, low in the sky, west of south when darkness falls as December begins and sets around 19:00 GMT. Towards the end of the month it will be seen towards the southwest after sunset and sets by ~17:30 GMT. Its magnitude remains at -2.0 during the month whilst its angular diameter falls from 34.4 to 32.9 arc seconds. Sadly, even when first seen after sunset, it will only have an elevation of ~12 degrees above the horizon so the atmosphere will limit our views. Due its position in the most southerly part of the ecliptic this has been a very poor apparition for those of us in the northern hemisphere.
  
• SaturnClosely follows Jupiter into the sky, some 2 degrees behind at the start of the month but reducing to just 6 arc minutes on the evening of the 21st! [See highlight above.] Saturn is best seen in the south just after sunset on the 1st. Its magnitude remains steady at +0.6 whilst its angular size decreases from 15.7 to 15.3 arc seconds. The rings span some 35 arc seconds across and, at ~22 degrees to the line of sight, show up well. Saturn starts the month in Sagittarius and moves into Capricornus on the 15th. Sadly again, its low elevation of ~12 degrees when first visible in the evening will limit our views of this most beautiful planet.
  
• Mercury will be visible using binoculars very low in the southeast at dawn for the first few days of the month. On the first, it rises only 45 minutes before the Sun shining at magnitude -0.8. It will pass through superior conjunction (closest to the Earth) on the 20th of the month. Please do not use binoculars after the Sun has risen.
  
• Venus, rises in the southeast some 2 hours before the Sun at the star of December but by half an hour less by month's end. Its magnitude remains at -3.9 throughout the month whilst its angular size reduces from 11.7 to 10,7 arc seconds. At the same time its phase, the percentage illuminated disk, increases from 89% to 94% which explains why its magnitude remains constant.
  

Highlights of the Month

• Early December - still a good time to view Mars.This is still a good month to observe Mars which had its closest approach to Earth back on October the 6th when it will lay 39 million miles from Earth and reached opposition on the 13th. Wonderfully, at this opposition, Mars has been far higher in the sky than at recent oppositions. In Pisces, Mars, shining at a magnitude of -1.1 at the start of the month, can be seen crossing the meridian at 20:30 GMT. By month's end it magnitude will have dropped to -0.3 whilst being due south at ~19:15 GMT. Its angular size is just over 14 arc seconds at the start of the month dropping to 10.5 arc seconds by month's end. Reaching an elevation of ~45 degrees when due south as seen from the UK, amateur telescopes will enable one to see features, such as Syrtis Major, on its surface when the seeing conditions are good.
  
During this apparition, Mar's southern hemisphere is tipped towards the Earth and so the South Polar Cap should be visible, though much of its frozen carbon dioxide will have vaporised during the Martian summer. Though the North Polar Cap is beyond our view, one should be able to spot the haze of the North Polar Hood lying above the northern limb of the planet. At 24.6 hours long, the Martian day is similar to ours, so the surface details remain similar at the same time each night. Mars takes 41 days to make an apparent rotation as seen from Earth.

• 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 new so, pleasingly, its light will not hinder our view. The Geminids can often produce near-fireballs and so the shower is well worth observing if its 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 21st - after sunset - The closest visible conjunction of Jupiter and Saturn since 1226!After sunset on the 21st of December, let us hope for clear skies as Saturn and Jupiter will be at their closest in the sky since the middle ages at just 6 arc minutes apart! This means that with a telescope at moderate power one would be able to encompass both planets and their brighter satellites; Io Europa, Ganymede and Callisto with Jupiter and Titan with Saturn.
  
• 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. Pleasingly, this year the first quarter Moon Moon will set around midnight so its light will not 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 its worth having a look should it be clear.
  
• December - evening: the Double Cluster and the 'Demon Star', Algol.This month, in the evening, is a good time to look high in the south 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 as seen in the diagram below. Normally the pair has a steady magnitude of 2.2 but every 2.86 days this briefly drops to magnitude 3.4.
  
• December: find M31 - The Andromeda Galaxy - and perhaps M33 in TriangulumAround new Moon (14th 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 12th - 1 hour before sunrise - Venus and a thin crescent Moon.Before dawn on the 12th of December, Venus will be seen, if clear, down to the lower left of a very thin crescent Moon. Look out for the 'dark' side of the Moon illuminated with light reflected from the Earth - 'Earthshine'.
  
• December 17th - after sunset - Jupiter, Saturn and a thin crescent MoonAfter sunset on the 17th of December, Saturn and Jupiter will be seen, if clear, to the right of a very thin crescent Moon. Look out for the 'dark' side of the Moon illuminated with light reflected from the Earth - 'Earthshine'.
  
• December - Evenings of the 7th and 23th: 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 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!"
  

Haritina Mogosanu and Samuel Leske from the Carter Science Centre in New Zealand speaks about the Southern Hemisphere night sky during December 2020.

• The evening sky in December is commandeered by the edge of our galaxy, visually in the asterisms of Orion and Taurus. The region in question is so beautiful that it simply demands all attention. However, the entire December sky is shattered with bright planets and bright stars at dusk.
  
Jupiter is the brightest object after sunset, low in the west. Saturn is close to Jupiter, above and right of it at the beginning of the month. Mars is due north at dusk, still beautiful and bright. The three brightest stars in the sky are also visible at the same time: Sirius, the brightest true star is midway up the eastern sky, Canopus, the second brightest star, is high in the southeast and Alpha Centauri, the third brightest star is due south.

• In December, Jupiter and Saturn will be very close, as they near their once-in-20-years conjunction on December 21-22nd, 2020. At their closest, they will be only 0.1 degrees apart. That’s just 1/5 of a Full Moon diameter. They will be in close visual proximity from December 17th to the 26th. When two bright objects in the sky are in the same line of sight, we call the phenomenon a conjunction. Every twenty years, Jupiter, orbiting the Sun in 12 years, catches up with Saturn, which takes 30 years to do an orbit. Of course, a conjunction is a visual illusion. In reality, Jupiter is 879 million km away and Saturn 1,610 million km away, mid-month, almost twice further away from the Sun. From Wellington, it would be a bit tricky to photograph or see the two objects through a telescope on the 21st of December but if you observe them at 9PM when it is not yet night, they will be at about 15 degrees above the horizon. Try and find a place with a clear horizon. Otherwise, they will simply be two bright dots visible in the west after sunset.
  
• Another beautiful visual combination is the line that the brightest star Sirius makes with the Second brightest star, Canopus. Extend that line south and you will come across the Large and the Small Magellanic clouds. This is a very good trick to find our beautiful southern dwarf irregular galaxies in the night sky. From a dark sky, the Large Magellanic Cloud looks like a chunk of the Milky Way has been displaced nearby.
  
• The Southern Cross and the pointers are very low on the Southern horizon, making the asterism of the frying pan. The two pointers are the handle of the pan, and Epsilon Centauri (Birdun), Gamma Centauri (Muhlifain) and Delta Centauri (SAO 239689) are the frying pan. The Southern Cross is the fish frying in the pan. Close to the Southern Cross, the dark region of the Coalsack for Maori is the flounder also frying in the frying pan. Our gastronomical sky also contains the Pot, in Orion, with the bottom of the pot made by the three stars of Orion’s belt and the handle of the pot constructed from the metal of Orion’s sword. The pot is held in place by Eta Orionis.
  
• This is the best time of the year to observe our famous southern sky galaxies, the magellanic clouds. The first person to write about them, was the Persian Astronomer Al-Sufi around 964 AD. Explorer Amerigo Vespucci, in a letter about his third voyage around 1503 - 1504 was the next to write about the Magellanic Clouds and also about the coalsack, referring to them as the three “Canopes”, two bright and one obscure. And finally, Ferdinand Magellan wrote about it after his voyage in 1519. To spot the Magellanic clouds, you need a very dark sky and use your peripheral vision. With a telescope, the Large Magellanic Cloud is an amazing sight. One of my favourite deep sky objects, 30 Doradus or Tarantula nebula is a cloud of partially ionized gas in which star formation has recently taken place. Just like the horsehead nebula in Orion, the Tarantula Nebula in the Large Magellanic Cloud is a place where thousands of stars can form over a period of several million years. Supernova 1987a—the nearest supernova in recent years, co-discovered by New Zealand astronomer and Guinness book record holder Albert Jones, —was in the Large Magellanic Cloud. For regions like the Tarantula Nebula, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases, now easily visible in telescopes, leaving behind a cluster of stars which have formed. We can see what happened to such stars when we look at the starcluster the Pleiades.
  
• Close to Zenith is Achernar from Eridanus, all the beautiful stars of Grus and Fomalhaut. In Grus, the Grus Quartet is now visible. In Sculptor, the famous Sculptor galaxy is in a good position to observe. This galaxy has a visual magnitude of about 7 and it is visible with the naked eye, it looks like a blurred star. Sculptor galaxy is about 12 million light years away from us.
  
• A total solar eclipse will be visible from South America on the 15th of December as the New Moon disappears into the Sun’s glare. There will also be a few minor meteor showers, the geminids and leonids, nothing that compares with the northern Hemisphere’s perseids in August. Mercury will pass around the far side of the Sun on December 20th and on the 21st we will have the longest day of the year, the summer solstice.