What is a blue supermoon and when will it appear in UK skies?

The UK has enjoyed a weekend of vivid sunsets and sunrises, with the moon taking on a distinctly orange hue. Monday night also promises the relatively rare lunar phenomenon of a blue supermoon, as a blue moon and supermoon coincide.

Why have the skies been red and the moon orange?

The spectacular shades over the weekend in the UK are due to smoke from wildfires across the Atlantic in North America. According to the Met Office, a red sky appears when dust and small particles are trapped in the atmosphere by high pressure. This scatters blue light, leaving only red light to give the sky its notable appearance.

When the moon is low, it appears more orange as its light must pass through a more substantial thickness of the Earth’s atmosphere, according to the BBC’s Sky at Night magazine. In summer, the moon is below the celestial equator when it is full or nearly full, so from the UK it does not rise high enough above the horizon to lose its orange hue.

What is a supermoon?

According to Nasa, a supermoon occurs when a full moon coincides with the moon’s closest approach to Earth in its elliptical orbit, a point known as perigee. This occurs about three to four times a year.

At its closest point, the full moon can appear up to 14% bigger and 30% brighter than the faintest moon of the year, which occurs when it is farthest from Earth in its orbit.

It might be hard to detect a supermoon visually, but it does have an effect on Earth. Because the moon is in its closest approach, it can cause higher tides than usual.

This supermoon will appear full in the sky for three days.

What is a blue moon?

Confusingly, there are two different definitions of a blue moon.

Normally there are 12 full moons in a year, one in each calendar month. But the lunar cycle does not always match our calendar.

A blue moon happens every two to three years when there are two full moons within a single calendar month. But a blue moon is also defined as the third full moon in an astronomical season, with the astronomical seasons beginning and ending at the equinoxes and solstices.

Monday night’s blue supermoon is under the seasonal definition. There was also a blue supermoon under the monthly definition last August.

The moon will not appear blue. In fact, it will appear more red or yellow at dusk thanks to the light refracting around the atmosphere at the horizon.

How rare is a blue supermoon?

According to Nasa, Monday night’s combination of a full moon, a supermoon and a blue moon is quite “irregular”. The terms blue moon and supermoon are not scientific, meaning definitions for terms differ, adding to the variability of estimates for when the next blue supermoon could be.

Can you see the blue supermoon in the UK?

The full blue supermoon will become 100% full at 7.25pm on Monday evening. The moon rises at about 8.50pm in the roughly east sky and will then set on Tuesday morning at about 6.35am in the western sky.

According to the BBC weather presenter Simon King, the weather forecast is not looking favourable, with cloud and patchy rain spreading eastward across the UK. There will be some limited times when the skies should clear, at different stages of the night.

Though the blue supermoon will not technically be full on Tuesday night, it will still be near-full and there will be a greater chance of clear skies, says King.