When to watch the Moon
Keep an eye on our closest celestial neighbour to spot three ‘supermoons’, a blue Moon and a faint lunar eclipse
Our natural satellite is in for a busy 2021. It will eclipse the Sun twice and itself be eclipsed twice by Earth. But what’s sure to grab the headlines is the annual spate of ‘supermoons’.
“A supermoon is when the Moon is at its closest to Earth while at full phase,” says Nigel Henbest, about what astronomers call a perigee full Moon. They occur because the Moon’s monthly orbit of Earth is slightly elliptical. “The technical definition of a supermoon is a full Moon that’s within 10 per cent of its closest approach to Earth, which means we can usually declare three supermoons in a year.” adds Henbest, who thinks supermoons tend to get beginners interested in astronomy, but don’t tend to grab the attention of more experienced astronomers.
In 2021 that definition creates three ‘supermoons’ on 27 April, 26 May (the closest and therefore biggest perigee full Moon, so the ‘best’ of 2021) and 24 June, the latter the lowest-hanging full Moon of the year.
“When the Moon is viewed close to the horizon it always looks bigger,” says Henbest about the so-called ‘Moon illusion’. “An ordinary full
Moon close to the horizon will appear larger than a supermoon high up in the night sky.” This means it’s best to view the full Moon – supermoon or not – as it rises and sets.
Later in 2021 there’s a ‘seasonal blue Moon’, a calendar quirk that sees a full Moon on 22 August become the third of four full Moons in the astronomical season of summer. More interesting still is the full Moon of 19 November, which from the UK will just be visible as a slight penumbral lunar eclipse starting at 06:02 UT, during which our satellite will pass through Earth’s outer shadow just before moonset in the west-northwestern sky. From North America, the Pacific and East Asia it will be a partial lunar eclipse during which some of the lunar surface may redden slightly.