September Night Sky

Neptune comes to opposition on September 2, and this is the best month to try to observe this elusive planet. It might seem strange that Neptune is so much harder to find than Uranus. The two planets are comparable in size and nature, and both are in the outer reaches of the solar system. But Neptune is much further out from the Sun than Uranus – an average of 4.5 Billion km to Uranus’ 2.9 Billion. This has a double whammy effect on Neptune’s apparent brightness from the Earth. The distance of Uranus to the Sun is only about 63% of Neptune’s, but that means that Neptune only gets about 40% of the amount of sunlight that reaches Uranus. (This is the effect of the inverse square law of radiation propagation. If you imagine a body the same size as the Earth, but twice our distance from the sun, that body will receive a fixed amount (by area) of the sun’s radiation at that distance which is spread by then over the surface of a  sphere of twice the radius that it was at the earth, and therefore  over a surface area 4 times as great. So it will only get 25% of the radiation the Earth receives.) The same effect  applies again with the light reflected back from these planets to the Earth, enabling us to see them. So you could say it is a double double whammy effect! The result is that Neptune’s apparent brightness from the Earth is about 40% x 40% = 16% that of Uranus. So, Neptune is about 6 times fainter than Uranus, which amounts to 2 Magnitudes (each Magnitude is a factor of 2.5, and 2.5 x 2.5 is about 6)

So Neptune is much harder to see than Uranus. The other problem is that it is still quite low in the sky, in a fairly obscure constellation, Aquarius. The better news, compared to last year, is that it has now moved closer to a bright star  (Lambda) in Aquarius, and should be reasonably easy to find from that starting point. The map below shows you how to find it – the blue dot near the “N” of Neptune. The other help the map should offer is the ring of stars in the upper left part which is the easy to spot Circlet in the constellation Pisces, itself south of the Square of Pegasus. Good luck!

Neptune Sept 2016