The Moon Guide
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U. S. N. O.
The phase of the Moon right now

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< Observing the Moon introduction

Earthshine shows as a faint glow
on the dark portion of the Moon
Earthshine is the faint illumination of the Moon’s dark side, caused by sunlight being reflected onto the Moon by the Earth. Earthshine is most obvious when the Moon is a thin waxing crescent in a reasonably dark sky, a spectacle sometimes referred to as ‘the old Moon cradled in the young Moon’s arms’. If you were standing near the centre of the Moon’s near-side at this time, the Earth would appear directly above your head, a big, nearly fully illuminated blue sphere 60 times brighter than full Moon.

Earthshine can be followed for several days until around first quarter phase and picked up again in the last quarter of the lunar month. It is best seen if the ecliptic between the Sun and crescent Moon makes a steep angle with the horizon, when the Sun is positioned far beneath the horizon, either in the evening skies of spring or the morning skies of autumn.

For all its apparent brilliance, the Moon is actually a rather dark object. It’s one of the least reflective worlds in the entire Solar System, with an average reflectivity of just seven percent. In other words, only 7 out of every 100 photons which hit the lunar surface manage to bounce back into space – the rest are absorbed by the dark surface and re-radiated in other wavelengths.

Atmospheric effects
Lunar rainbows are created under the same circumstances as those produced by the Sun. However, lunar rainbows have a maximum brilliance of a mere 1/500,000 of their day time counterparts and as such they are a rarer phenomenon, appearing very dim and lacking in vivid colours.

A bright pearly white region called the lunar corona can often be seen around the Moon.
A lunar halo. Credit Robin Scagell/Galaxy
A lunar halo
It is caused by the reflection and diffraction of moonlight by water droplets in the lower clouds. In addition to the corona, the Moon is sometimes encircled by one or two (on rare occasions even three) rainbow coloured lunar haloes caused when moonlight is diffracted by countless water droplets or ice crystals in the upper atmosphere. Lunar haloes can be particularly vivid when moonlight passes through a thin homogeneous cloud layer composed of very minute water droplets. Sometimes the lunar halo may be host to ‘Moon dogs’ which are diffuse images of the Moon located 22° either side of it, caused by the light’s refraction among ice crystals in Earth’s upper atmosphere.

‘Seas’ and mountains >

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Crescent Moon

Observing the Moon

Moon lighting

'Seas' and mountains

How much can you see?

Using binoculars and telescopes

Drawing the Moon

Getting to know the Moon

Three-day crescent Moon

Six-day crescent Moon

First-quarter Moon

Gibbous Moon

Interactive Moon map

spacerMaintained by SPA Webmaster: Last modified 5 November 2008
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