Bath Astronomers

February Night Sky

Mars is technically visible in the night sky for much of the year, but for reasons I have explained before, its distance from the earth varies by a huge factor, and much of the time it presents only a very small disc where your chances of seeing anything more than a bright red blob are small. That is true this month, but there is added interest, since at the end of the month Mars will pass very close – less than a degree, so in the same field of view at low power in some telescopes – from Uranus. The closest approach is on February 27. If you have never seen Uranus, or have difficulty in finding it, this is a good time.

The map below shows the lower part of the western sky at 6.30 pm on 27 February. Venus shines very brightly quite low down, and Mars should be easy to pick up with the naked eye about 12 degrees to the northeast. It is magnitude 1, so brighter than anything else in the vicinity, and its red colour will be distinctive.

Western Sky 26 February 2017 - Copy

 

 

Uranus is so close to Mars that their names are jumbled up on the star map. I have therefore added another map below, with a closer in view, that shows you how to find Uranus from Mars.

Mars and Uranus 26 February 2017

 

Mars and Uranus should make an interesting comparison. Mars is very much brighter – 5 magnitudes more than Uranus at Magnitude 6. Magnitudes work on a geometric scale, with each magnitude representing a change factor of 2.5, so 5 magnitudes is a difference of 2.5 to the 5th power, which is about 100. And you will see this if you look at the two in turn. But their difference in apparent size is quite small – Uranus has a disc of 3.4″, and Mars is not much bigger at 4.7″.  So why is Mars so much brighter than Uranus? There are 3 main factors in play. The biggest is distance from the sun. Uranus is 13 x further away from the sun than Mars. This means that the amount of sunlight reaching as far as Uranus is less than to Mars  by a factor of 13 squared or 169. However, the cloud tops of Uranus are about 3 times better at reflecting sunlight than the deserts of Mars. And the third factor is that the slightly bigger apparent size of Mars amounts to about 2 times the apparent angular surface area in our night sky. Putting these factors together suggests that the brightness difference should be 169×2/3, which is a factor of 113. That is a little more than we find in practice, but we can take account of other factors. The image of Mars we see now is only about 95% illuminated. That brings the expected difference down to 107, which is near enough what we find.

Happy viewing. The colour contrast – the red of Mars compared to the blue/green of Uranus – should be interesting.