What colour is the Moon?

By Fred Watson 18 December 2009
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The Moon changes colour depending on several factors. It can appear blue, red or silvery.

What colour is the Moon?

According to that 99-year-old popular song, it’s “silvery” (and you might recall that its light makes you want to swoon, and to your honey you’ll croon love’s tune – and so on… Gosh, they don’t write lyrics like that anymore). Although silvery is an evocative description of the colour, to my eyes, I’m afraid it usually looks plainly and simply white.

There are exceptions, of course. When the Moon is low in the sky, close to the horizon, it often looks yellowish – and sometimes even orange or red. The reason for this is that light travelling through air undergoes a process of scattering, in which particles (photons) of blue light are catapulted off in random directions more frequently than their red counterparts. When the Moon is low in the sky, its light passes through a thicker layer of atmosphere than when it’s overhead, so the scattering effect is very pronounced. The blue light having been removed, our lunar satellite can look decidedly yellow.

A red Moon can occur during a total lunar eclipse, like the one on 28 August last year. Once again, the reason for the colour is the scattering of light by the Earth’s atmosphere. Even though the Moon is deep within the Earth’s shadow during the eclipse, scattered red light finds its way onto the lunar surface to give the appearance of a ‘blood Moon’.

The white moon

Under normal circumstances, when it’s high in the sky, the Moon looks white, with a few pale grey markings. Focus binoculars on it, though, and the grey areas reveal themselves as smooth, circular blotches, quite different in texture from the cratered highland regions between them. For centuries, the grey patches have been known as “maria” (seas), but we now know they are low-lying volcanic plains – the result of lava flows. On the airless Moon, they are as dry as dust, but they are geologically much younger than the white mountainous regions.

Colour enhancement techniques can be used to exaggerate slight differences in grey tones, providing a powerful tool for mapping the geology of the lunar surface. With this method, the greyish maria transform into delicate shades of blue or brown, corresponding to varying levels of iron and titanium in the basalt that fills them. The highland areas are lighter in colour, and show shades of yellow, pink and pale blue. Once again the differences are due to geological composition, but there may also be ageing effects due to bombardment by sub-atomic particles from the Sun.

The whitest parts of all are streaks of material ejected by asteroid impacts in the relatively recent past, providing pearly highlights to our silvery Moon.

Fred Watson is Astronomer-in-Charge of the Australian Astronomical Observatory at Coonabarabran in north-western NSW, and well-known to ABC radio listeners. Visit his website at fredwatson.com.au