December Sky Notes
Considered the most prolific annual meteor shower, the Geminids are active from 7 to 16 December, reaching a peak on 14 December. When conditions are favourable observed rates can approach 50-60 per hour, however this year a waning gibbous moon will severely restrict numbers and only the brightest Geminids will be noticeable.
Geminids originate from debris shed by a small asteroid called Phaethon, which may even be the nucleus of a ‘dead’ comet. Phaethon passes within ten million miles of the Sun causing the object to deposit material over great swathes of the inner solar system. However it is only within the last eighty years that Geminid activity has increased to the levels now recorded, a result of Earth passing through a much denser debris strand, a situation that will only continue for another hundred years or so.
A typical Geminid meteoroid is about the size of a large coffee granule, Brighter Geminids regularly produce long luminous trails, sometimes green or orange in hue. The shower radiant lies close to Castor, visible in the east by 9pm.
Conditions for the less prolific Ursid meteor shower are much more favourable. The shower is active from 17 to 25 December, peaking on 22/23 December. Hourly rates of around 10 are the norm, but occasionally and erratically, Ursids can produce strong outbursts, so it may be worth keeping an eye open that night. The radiant lies close to the Great Bear – Ursa Major.
The Winter Solstice
The Sun reaches its lowest position in the sky on 22 December, the date of the winter solstice and officially the start of winter in the northern hemisphere. From our latitude, useful daylight amounts to just 7 ½ hrs ’the shortest day’. Latest sunrise and earliest sunset do not however occur on the winter solstice date. The Sun sets earliest mid-month (15th or 16th) and rises latest near the end of December (27th or 28th).
The Early Winter Sky
Seasonal winter constellations begin to dominate the sky in much of the south and east. But our celestial ramble begins directly overhead: a position known as the Zenith, which often induces a crick in the neck. Throughout the early part of the evening the zenith is occupied in turn by the distinctive ‘W’ pattern of Queen Cassiopeia, then by Perseus, a group of stars that resemble a distorted figure Pi symbol. Later in the evening the stars of Auriga the charioteer, highlighted by brilliant Capella, occupy the station. The autumnal groups of Andromeda and Pegasus occupy much of SW aspect high up, with the square of Pegasus resembling huge a diamond shape. Dropping down in the WNW, two bright stars normally associated with summer nights catch the eye, Vega and Deneb, both members of the ‘summer triangle’. These stars remain visible all year from our latitude, the third member, Altair, sets earlier in the evening - due west. To the north lie the two celestial bears, Ursa Major and Minor, with Draco the dragon winding between them. The ‘pointer stars’ in the bowl of the Plough (part of Ursa Major) point toward the pole star located some three hand spans above the N horizon in Ursa Minor, the little bear.
The glittering array of winter constellations begins to dominate in the east and southeast. Taurus leads the way, readily identified by the “V” outline of the Hyades star cluster highlighted by fiery hued Aldeberan. Above and to the west look for the beautiful Pleiades star cluster – commonly known as the seven sisters. Following Taurus across the southern aspect majestic Orion stands proud dominated by brilliant Rigel, ruddy hued Betelgeuse, and the three belt stars. The belt stars point down to the location of the brightest night time star, sparkling Sirius, the Dog Star in Canis Major. Some distance above and left, solitary Procyon, in the lesser dog of Canis Minor, is yet another brilliant winter jewel. Above Orion’s left shoulder stands Gemini, marked by bright Castor and Pollux,. Extending westwards from these two stars, two lines of less conspicuous stars mark the ‘stick like’ outline of the twins. Late in the evening the first celestial shoots of spring sprout from the eastern horizon. The faint stars of Cancer lead the way, which scuttle after Gemini. Leo the lion follows, best identified by the ‘Sickle’ asterism at the foot of which shines bright Regulus. It may be of some comfort that even though we are in the dark days of winter, by midnight ‘spring’ is already making a show. Happy Solstice.
Whitby & District Astronomical Society
For more information about Whitby & District Astronomical Society see: www.whitby-astronomers.com