A lunar spectacular, which really does only happen once in a Blue Moon, is set to light up the night sky tomorrow.
The first Blue Moon since 2012 will appear over the UK. The next won't be until 2018.
But anyone hoping to see the dazzling spectacle of a glowing sapphire celestial body crossing the sky will be sadly disappointed. A blue moon (usually) is silvery grey, like any other.
The term "blue moon" refers to the second of two full moons appearing in the same calendar month.
It has more significance for astrologers than astronomers, marking a time of upset, change and possibilities.
Friday night's lunar phase on July 31 fits the calendar definition of "blue" because there was another full moon on July 2.
Just occasionally the moon really can be coloured blue - but you normally need to be near a volcanic eruption to see the phenomenon.