This is the week of the summer solstice in the northern hemisphere. The longest day of the northern year falls on June 20 and according to astronomical traditions it is the beginning of summer.
In the southern hemisphere it is the winter solstice, marked by the longest night.
While such long days shorten the hours for traditional stargazing, other targets offer their own unique spectacles. Early summer is ideal for observing noctilucent clouds (NLCs). They can appear in the west about 30 minutes after sunset. This week, from London, the sun sets at around 21.20 BST.
NLCs are the highest known clouds to form in Earth’s atmosphere, existing at an altitude of about 50 miles (80 km). They are thought to be essentially natural phenomena made of ice crystals, although how the water molecules get so high in the atmosphere is a mystery.
The clouds appear as glowing white or pale blue streaks across a dark sky. They look a bit like cirrus clouds, but can be distinguished because cirrus clouds do not glow and are therefore only visible during the day. NLCs make a beautiful but ominous sight, shining brightly against the darkness of the night sky.
