A lunar halo

Bob Marriott

Immediately following the total eclipse of the Moon during the late evening of 3 March 2007 the sky became slightly hazy, and a lunar halo appeared. Lunar and solar halos are caused by refraction and reflection of moonlight or sunlight by ice crystals on cirrus clouds. Lunar halos typically occur within five days either side of full Moon. The commonest have an angular diameter of about 40° and are usually white, but the smaller halos – the lunar corona – can show a prismatic effect, with blue light refracted to the outer edge and red light to the inner edge.