The Blue Moon is a dazzling astronomical spectacle, tied to the Moon’s irregular orbit of the Earth. The phenomenon only appears once in a Blue Moon – roughly once every two years – but when it does, stargazers get excited about its arrival. Tomorrow (May 18), the Blue Moon will peak in the late night hours during the Moon’s full phase. When this happens, around 10.11pm BST (9.11pm UTC), the Moon will face the Sun head-on.