Russell Eberst's Sky View

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December 2017

The month of December sees the Sun reaching its southernmost point in its annual journey round the sky. This circuit of the Ecliptic is a reflection of the Earth's yearly path round our Sun. At the time of the winter Solstice, which this year occurs on December 21, the Sun is located in the constellation of Sagittarius the archer, very close to the point where the ecliptic and plane of our galaxy (the Milky Way) cross. This is also close to the positions of two Messier objects, M8 and M20, also known as the Lagoon and Trifid nebulae. The Sun then follows its six-month path northwards, and the length of daylight gradually increases. The Earth continues to get closer to the Sun, reaching its nearest point, perihelion, during the first few days of January. Despite being closer to the Sun than usual, the United Kingdom is likely to experience some of the coldest temperatures of the year, due largely to having the Sun above the horizon for only 7 of the 24 hours of each day and it never reaches more than about 12° above the horizon in Scotland. The duration of daylight is at its shortest, but (as happens every year), this does not mean that this date has the earliest sunset. Due to a number of effects, including the Earth's slightly eccentric orbit, the earliest sunset occurs about a week earlier at the latitudes of Scotland. So December 15 sees the earliest sunset of the year, but the latest sunrise occurs about December 29. The increase in the length of the night means longer opportunities to observe the dark sky. The winter nights are dominated by famous constellations such as Orion, Gemini and Taurus, with other bright stars on view such as Sirius, Capella and Procyon. These luminaries are all rivaled by the International Space Station. Orbiting at a height of 400 km. (250 miles), the Station can be easily spotted even without optical aid, as it crosses the sky taking about 5 minutes to go from horizon to horizon.

During the first fortnight of December it will be making passes in the evening sky for U.K. observers. Times and directions can be obtained from the website. Enter your location and your time zone, select ISS passes and they will be listed with dates and times for your observing site. From Scotland it will be seen moving fairly low across the southern sky. The stars of the square of Pegasus act as an useful guide, with the Station transiting well below the square, going right to left (west to east). The I.S.S. will far outshine any stars in this region. On some evenings it may be accompanied by a fainter companion when a cargo or ferry spacecraft is arriving or departing from the Station. These early evening passes are an ideal time to show children that large satellites can easily be spotted even without optical aid. Don't forget to wave to the six astronauts/cosmonauts that are onboard and don't be surprised if the Station fades rapidly from sight as it enters the Earth's shadow. By the time of the December New Moon on the 18th, U.K. passes of the International Space Station will have moved into daylight, leaving the Moon to hold sway in the evening sky. The following few nights will give opportunities to observe 'Earthshine'. This phenomenon is also known as the 'Old moon in the young moon's arms'. Looking at the narrow crescent of the illuminated Moon, it is possible to see the rest of the lunar surface as a dim but reasonably obvious disc. This fainter illumination results from the fact that the moonscape is lit by a tremendously bright "Full Earth". During December, the evening crescent is slightly smaller than usual, since the early hours of December 19 sees the Moon at a distance of 406,603 km (252,651 miles). This is the furthest distance at any time in 2017.

The middle of December will also see the predicted date of the maximum of the meteor shower known as the Geminids. The meteors radiate from a point in the sky not far from Castor in Gemini. Around 50-80 meteors per hour may be expected, so with the absence of a bright Moon in the sky, there should enough bright meteors to make watching worthwhile. Observations should also be made on the nights of December 12 and 14. These meteors move at around 35 km/sec, and seem to be associated with an asteroid (3200 Phaeton) and not a comet, as most shower meteors are. Observers may like to watch the sky well away from the radiant in Gemini, and instead look north towards Ursa Major/Minor, or higher in Cepheus or Cassiopeia. Orion or Taurus also provide suitable regions, so observers can choose their best target area depending on local circumstances. Later in the month, around December 23, the Ursid meteor shower may provide about 10 meteors per hour, with the meteors radiating from a point close to Kochab (beta Ursae Minoris). All nights between the Solstice and Christmas should show some activity.

So what about the planets? Most of the bright planets are confined to the morning sky. Venus is pulling in towards the far side of the Sun, and will be lost from view prior to its superior conjunction early in the New Year. Jupiter and Mars are becoming better placed, but still have a long way to go to reach their best at opposition next year. The pair are visited by the Moon on the morning of December 14 and they can give a guide to Mercury very low down in the south-east just before dawn in the second half of the month.

Moon phases:

  • Full Moon: December 3
  • Last Quarter: December 10
  • New Moon: December 18
  • First Quarter: December 26

Sky View Image

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 31st.