Moon Illusion

The full Wolf Moon of 25th February '13 rising over Fanad Lighthouse on the North coast of Donegal, Ireland.  Pictures by Peter O'Toole.

©Peter O'Toole, shown with permission

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The near horizon moon has an upper green rim (green because much blue light is scattered away) and a lower red rim.

A mirage is needed to magnify these into a green or red flash.

The moon really does appear larger when it is near the horizon. But its real angular size is no greater than when it is high in the sky.

Earth’s atmosphere near the horizon raises the moon slightly in the sky. This refraction becomes greater at the horizon and consequently the moon appears flattened. Also, compared to an airless world, the moon and stars rise early and set late.

Refraction has another effect. It is weakest for red light. The ‘red moon’ is raised least. Near the horizon, as here, the moon has a thin red lower rim and a green upper one.

A third near horizon atmospheric effect is to redden the moon.

All these are real, they can be measured or photographed. They do not alter the moon’s angular size. That is a perception by our eyes and brain. The reasons have long been debated.