Sun dog over Canterbury, NZ
© Ross Day
It looks like something out of a science fiction novel but it's really just a quirk of the clouds that gives the appearance Earth is now orbiting two suns.

Ice prisms in a veil of high cirrus, about 8 kilometres up, refract sunlight and can create many different optical phenomena, including the "mock sun" or "sun dog" photographed by Ross Day near Sugar Loaf on Christchurch's Port Hills close to sunset on Tuesday.

The phenomenon - more properly known as a parhelion - is not especially rare and is one of several tricks of the light generated by hexagonal ice crystals in clouds at such high altitudes.

In the best displays, a whole range of intersecting haloes and arcs may be visible around the Sun.

Sun dogs appear as patches of light, sometimes subtle, sometimes bright, at the same elevation in the sky as the Sun and about 22 degrees either side of it.

And yes, moon dogs are a thing too.

The origin of the term sun dog is unclear, although some believe it relates to how the mock sun follows the real Sun around.