NEOWISe & Noctilucent clouds

A bright comet is becoming more readily visible from the northern hemisphere. It is named NEOWISE after the satellite that discovered it and is given the designation of 2020 F3 as it was the third comet to be discovered in the second half of March. It will be very easy to see with binoculars, which may help to find it at first, but you should be able to see it with no equipment at all. It has a tail several degrees long, which will make it obvious when you have spotted it. In the evening twilight (roughly 10:30 pm in Cambridge) it is low in in the north-north-west about 7 degrees up. It will remain visible all night until swamped in the dawn sky around 2:40 am when it will have moved round to the north-north-east. Each evening it will be a little higher in the sky and become visible a little earlier as the nights draw in, however it will also steadily get fainter. Because it is quite low down you will need a clear northern horizon to spot it.

Noctilucent Clouds, also known as Polar Mesospheric Clouds form at about 80 km up in the atmosphere. This part of the atmosphere is coldest during the summer months, hence allowing clouds to form. Because they are so high up, the clouds remain illuminated long after sunset (or before sunrise) and glow with a beautiful silvery-blue colour. The colour comes from scattering in the ozone layer (at about 20 km altitude) during the long path of the sunlight through the atmosphere. The clouds are most frequently seen low towards the northern horizon, though occasionally much higher up.

The clouds often can’t be distinguished from the bright sky background until an hour or more after sunset, so for Cambridge that means 22:00 – 22:30 in the evening at the moment. The chances of seeing them are even higher before sunrise, but you would have to be up at 2 in the morning. Another factor that improves the chances of seeing them is solar activity (or lack of it) and we are currently near sunspot minimum in the 11-year cycle which means that the atmosphere is a bit colder at 80 km altitude than it is at solar maximum. The season for seeing the clouds lasts until early August.