Sky Notes for May 2012, with James Abbott #stargazing
May brings the onset of summer twilight conditions. Here in Essex, from May 21st onwards, the Sun does not pass far enough below the Northern horizon for full night conditions to occur. The residual twilight around midnight is faint, but can be seen away from streetlit areas. Under the clearest skies this twilight arch can be a beautiful deep blue colour.
May also sees the start of the noctilucent cloud season. Because the Sun is not that far below the Northern horizon during the night, its rays still illuminate the top of our atmosphere. At an altitude of 50 miles, noctilucent clouds can form in summer, despite the very low gas pressure and humidity at that height. The best time to try to see these rare clouds is from around 10.30pm onwards. Its best to have a good Northern horizon looking over fields so that trees and buildings do not block the view. The clouds can be seen as silvery blue streaks or wave-like forms low down above the twilight arch. As the Sun becomes more active with increased ultraviolet flux, the NLC may be seen less often this season.
There is an eclipse of the Sun this month, on the 20th, but unfortunately it cannot be seen from the UK. Full Moon is on 6th May and now down at summer altitudes so that even at midnight it is barely 20 degrees above the horizon from Essex.
Venus starts the month still very bright and prominent in the West as it gets dark, but will be closing towards the Sun and more difficult to see in the second half of the month. From the 19th onwards the phase is less than 10%.
Mars continues to fade as the distance from the Earth increases. Nevertheless, it remains fairly bright and is visible until after midnight in the constellation of Leo. Diameter mid-month is around 9 arc seconds.
Saturn is well placed and in mid-month is in the South at about 11pm BST, just 5 degrees above Spica. The two objects will be of similar brightness, but different colours. Spica is blue/white, but Saturn is obviously yellowish.
With the Moon full early in the month, the best time for darker skies at a reasonable hour will be after about the 9th. Looking East at around midnight, the main summer constellations of Lyra and Cygnus are well up and under good skies the summer Milky Way should be visible as a hazy band, roughly parallel with the horizon.
James Abbott is an astronomer, NEAS member and CfDS Regional Information Officer.