January 27, 2010
The night sky is a good place to test your powers of observation. Are you clear which ‘stars’ are actually planets? Do you, as our ancestors did, notice their wanderings around the constellations? Have you observed how they stay within a relatively narrow band of the stars – the ecliptic or Zodiac?
Well if haven’t, don’t feel too bad. There are wheels in motion within wheels and keeping track can be challenging to non-astronomers. In addition to the daily rotation of the Earth causing everything to slide out of view over a period of hours there is the Earth’s passage around the Sun, the Moon’s orbit of the Earth, the periodic disappearance of those planets behind the Sun, and the tilt of Earth’s axis to contend with, for example. If you were up all night every night under clear skies, you would soon pick up on some of the more local activity – that of two inner and three outer planets. Busy lives, cloudy skies and our own diurnal (opposite to nocturnal) nature make it very difficult.
Now is a good time to look up and catch an observable event. Mars, the red planet, is at opposition. Opposition is when the Earth and an outer planet line up on the same side of the sun. In the same way that the Moon is full when it is at opposition, the face of Mars visible from Earth is completely illuminated. It is also visible pretty well all night, rising around sunset, riding high at midnight and setting around sunrise. It is at its closest to the Earth since 2008, making it appear bigger and brighter than at any time from then until 2012!
Looking East January 26 2010 at 21:00 – 22:00 10 second exposure at f 5.6, ISO 400
It not only looks striking but something else is going on, too. During November 2009 Mars was slowly trekking through the constellation of Cancer. As it continued its journey East, it moved into the Constellation of Leo in December but early in January 2010 it appeared to turn around and head back towards Cancer, travelling Eastwards again. This is known as retrograde motion and is really an illusion caused by the fact that Earth (being nearer the Sun) moves in a faster orbit – Mars takes almost two Earth years to complete a round trip. For a time Earth overtakes Mars causing it to seem as if going backwards (we’ve all seen this effect whilst in cars and trains), but because the motion of both planets is a circle not a straight line, we swing back to our relative positions again, albeit much further apart. If that’s not clear, then watch this lovely little video on You Tube; not one of mine I’m sorry to say. Retrograde Motion and the Opposition of Mars
Mars is back in Cancer again. Cancer is a faint constellation and the bright moonlight bleaches it out at the moment. On January 29th the Moon, which moves much more quickly West to East than Mars, will be full in Cancer, also, probably expunging all the local stars. However Mars, very close by, will continue to shine brightly. As the moon wanes it will start to appear later and later in the night sky and not interfere with seeing Mars, which will remain prominent for a couple of weeks. (Leave that to the clouds). On March 11, 2010 the planet will turn around and start heading back towards Leo again. As it does so it will become progressively fainter, a far cry from the red eye presently burning a hole in the night sky.
One way to follow this is by taking photographs at intervals of a couple of days. You will need a tripod. For the image I took a 10 second exposure at f 5.6, ISO 400. Use the self timer or shutter release to avoid shaking the camera initially.