Welcome to the December night sky. This is the month of the winter solstice in the northern hemisphere. The Earth is now close to the point in its orbit where the north pole is tilted away from the Sun to its maximum extent. This implies that the Sun rays reach the surface of the Earth at shallow angle, so they do not heat the ground as much.
We are lucky in the northern hemisphere as this reduced heating effect happens when the Earth is slightly closer to the Sun, so our northern winters are not quite as cold as they could be. This is not always the case as the relationship between the tilt of the Earth’s axis and the position of the Earth in its orbit changes slowly over periods of tens of thousands of years and this is one of the factors that can cause ice-ages.
The month of December has no bright planets easily visible, but we do have a reliable meteor shower called the Geminids. This is usually one of the best shows of the year and its peak is on the night of the 13th on to the morning of the 14th.
Meteor showers normally happen when the Earth crosses a dust trail from a comet, but, in the case of the Geminids, the dust trail is from an asteroid called Phaethon. This asteroid is made of rock and comets are made mostly of ice and rock particles mixed.
The orbit of Phaethon is very much like a comet as it is highly elliptical. When it is at its maximum distance from the Sun, its orbit takes it beyond Mars and, when it returns in its orbit, it is closer to the Sun than Mercury. When this happens, the surface temperature of Phaethon rises to around 700Cº on the side facing the Sun and, because this asteroid rotates in a little over three hours, the rapid cycling of temperature is thought to crack the surface and allow small particles of rock to leave the asteroid and create a dust trail that our Earth encounters every year in December.
This asteroid will pass the Earth at a distance of 10 million kilometres on December 10 of this year. There is no chance of a collision though.
The Moon will be full on the 3rd, last quarter on the 10th, new on the 18th and first quarter on December 26.
By Clive Jackson
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Clive Jackson is the Director of the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.
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To see the December Sky Map click on the pdf link below