A blue moon usually occurs every two or three years and tonight (August 19), Brits can look out for it in the night sky.

Royal Museums Greenwich (RMG) said: “Which of the 13 full moons is the blue moon is up for debate. Traditionally the definition of a blue moon is the third full Moon in an astronomical season containing four full moons.

“This is the most complicated definition for people using the standard calendar, as the astronomical seasons begin and end at the equinoxes and solstices (e.g. the winter season begins at the winter solstice and ends at the spring equinox, the spring season begins at the spring equinox and ends at the summer solstice and so on).”

How to see the blue moon in the UK tonight

The type of blue moon that will be visible tonight in the UK is a seasonal one – the third full Moon in an astronomical season containing four full moons.

TimeOut explains when and how you can see the blue moon tonight: “The sun is going to set just after 8pm, but it’s currently due to be a tad overcast. Clear skies always make it easier to see the moon, so if there are thick, dark clouds it could be more difficult.

“Your best bet is getting out of city centres and heading somewhere with less light pollution away from buildings that might obstruct your view.

“You don’t need any equipment to see the moon; all you need to do is look up. Feel free to take a telescope or a pair of binoculars to a hill for an even better view.”

Stonehenge UK on X, formerly known as Twitter, posted a picture of a full moon above the famous landmark and explained that the blue moon will be visible from 8.45pm on Monday, August 19.

What colour is a blue moon?

While the blue moon would make sense to be blue, it’s actually a different colour.

RMG explained: “Blue moons remain the same colour as any other full moon except in two rare cases:

  • “During a lunar eclipse, the Moon can turn blood red, lit only by the light bent around the Earth by its atmosphere onto the face of the Moon. As lunar eclipses occur only during full moons, and blue moons are one type of full moon, it's very rare that a blue moon will be red!
  • “In very rare circumstances, the Moon can appear blue, but in this case the colour would be caused by viewing the Moon through a haze of dust particles in our atmosphere, perhaps from a recent volcanic eruption. From space, however, the Moon will look just as grey as it always has!”

The next blue moon after this one will be on May 31, 2026 when a monthly one (second full Moon in a month) will be visible.