Sky Diary

The long bright summer evenings begin in earnest in May. Twilight diminishes during April and by the 2nd May true darkness only occurs for 32 minutes after which, until the 10th August, true darkness does not return. The summer constellations are much less dramatic and bright than those in winter months, but the brightest stars which indicate the summer constellations can still be seen during darker times of the evening. Regulus, in Leo, can be seen towards the south-west while Arcturus of Boötes is high in the south by 10pm. By that time, the Summer Triangle of Vega, Deneb and Altair are rising in the east to north-eastern sky. Finally, looking north-west, the bright yellow star Capella will be seen in a northerly direction throughout the summer months.

The seven stars of the Plough are nearly overhead in late May evenings with the handle of the Plough pointing towards bright Arcturus, an easy way to identify this star. Have a look at the handle of the Plough and in particular the second star from the end of the handle. This star is called Mizar and, if your eyesight is better than mine, you may see another star close to Mizar. This is named Alcor and may be seen more easily using binoculars. These stars are moving together in the sky at a distance of 82 light years and it has been discovered that they are a pair of stars bound by gravity which orbit each other in about one million years. The pair Mizar and Alcor also have companions making a total of six stars in the multiple complex, a striking contrast to our own solitary Sun.

The end of May should see the start of the noctilucent cloud season. Because of warming in the lower layers of the atmosphere, tenuous ice clouds form at an extreme height of around 85km and these have regularly made their first appearances in the last week of May. They are so thin that they can only be seen after sunset, from about 10.30pm until 3am which explains their name which can be translated as night shining clouds. These pearly white clouds are generally seen towards the north but can be quite extensive. There is evidence to suggest that high ice clouds such as these are associated with global climate change but may also be influenced by solar activity. They are best seen during June and July and usually end around early August. I previously mentioned the star Capella of Auriga and this star swings round above the northern horizon as the evenings progress. It is one of the best markers for the position of noctilucent clouds and photographs taken often show Capella with bands and ripples of these ice clouds around the star.

None of the brightest planets is easily visible throughout May as they are all placed quite close to the Sun in the sky and rise or set at about the same time as the Sun.