WELCOME TO the December night sky and to the cold clear nights of winter. On December 21, the Sun is at its lowest point in the heavens and this gives us the shortest day and the longest night of the year. The further north you travel the more extreme this effect becomes, until you reach the artic circle, where the Sun fails to rise on this day and there is zero hours of daylight and 24 hours of night.
On December 5, the Moon is Full and will be in the constellation of Taurus. A Full Moon happens simply because it is directly opposite the Sun in the sky. This implies that if the Sun is low the Full Moon must be high and indeed it is in December, when it will be more than 80 degrees above the southern horizon at midnight on December 5.
Around Christmastime, I often get calls from people that have seen a bright sparkling object, low on the southeastern horizon, asking if this is perhaps the Christmas star? The fact is, that rising in the early evening at this time of year is Sirius, the brightest of the nighttime stars. Sirius is nine light years away in the constellation of Canis Major. When this star is low in the sky, its light has to travel through many miles of the Earth’s atmosphere and this causes a prismatic effect that breaks the white light of the star up into various colours of the rainbow. Therefore, Sirius will be seen to flash red, green and blue, maybe giving an impression that it is rotating and moving from side to side. This is purely an optical illusion.
By 9pm mid month, the ringed planet Saturn will rise in the east and it is presently in the constellation of Leo. Through a small telescope, the rings are visible with a power of 30 times or more.
The Moon is Full on December 5, Last Quarter on December 12, New on December 20, and First Quarter on December 27.
Clive Jackson is the director of the Astronomical Observatory of Tavira (Sitio do Malhão) and the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach. Tel 281 321 754, Fax 281 324 688, e-mail: [email protected] or visit http://www.cdepa.pt
