The Dark Side of the Moon
“I'll see you on the dark side of the moon.” Folks with a little
astronomy knowledge cringe when they hear these lyrics. They will
patiently explain there is no dark side of the moon. The moon turns
a revolution over about 4 weeks. The far side as well
as the near side see two weeks of darkness as well as two weeks of
But one side is darker. Since the moon is tide
locked, the far side never sees earthlight. On the other hand,
someone standing on the moon’s nearside will always see earth
hovering in the same region of the sky.
Viewed from the earth’s surface, both the sun and the moon subtend
about half a degree. The moon’s albedo is .12, meaning it reflects
about 12% of the sunlight that hits it. The moon is nearly as dark
as charcoal, it only looks bright against the black void of space
when our eyes have adjusted to the night’s darkness. Even the above
graphic exaggerates the moon's brightness -- the sun is about
100,000 times brighter than the moon.
Viewed from the moon’s surface, the sun subtends half a degree (just
as when seen from earth). But earth subtends about 2 degrees.
Moreover the earth reflects about 2.5 times more light than the
moon, having an albedo of around .3.
The larger apparent diameter and higher albedo means the earth seen
from the moon is about 34 times brighter than the moon seen from
The Bright Side
Let's imagine an astronaut standing at the moon's closest point to
the earth. Not far from Mösting A Crater, 0 degrees latitude, 0
degree longitude. From this location, the Earth always hovers
Here is our astronaut pointing his iPhone straight up to snap a
picture of the earth. In the foreground an iPad displays the picture
As the sun climbs towards high noon, earth is a waning crescent.
As the sun sinks towards the horizon, earth is a waxing crescent.
As the sun sinks deeper behind the horizon, earth is waxing
From midnight to sunrise, the astronaut sees a waning gibbous earth.
At sunrise we're back to where we started.
To The Dark Side
The astronaut hops in his buggy and starts driving east. As he drives closer to the far side, the earth sinks
toward the horizon. When the earth is near the horizon it's possible
for the sun to be below the horizon when the earth is a dim thin
crescent. Even so, the astronaut enjoys strong earthlight for most
When the astronaut drives over into the far side, there's no
earthlight. On the far side, it's a deep stygian blackness during
the two weeks from sunset to sunrise.
The far side is dark in another sense. Earth is a bright radio
source. The far side of the moon is always shadowed from earth's
radio noise. Radio astronomers salivate at the thought of a radio
telescope under the far side's dark skies.
So you see, the Pink Floyd lyrics make some sense even if you're not
under the influence.