dark side of the moon

Watching the lunar eclipse (from the cambie bridge) this morning left me motivated to go on a rant about one of my least favourite terms.

The “dark side of the moon.” I am referring to the celestial object not to the gnarly album by pink floyd.

The moon orbits the earth once every 28 days and spins on its axis once every 28 days. This means it transits the sky with exactly the same part of its surface facing the earth each and every time we see it. The only humans to actually have that other side of the moon face them, flew around the other side. The moon is lit up by the sun. We see most clearly the portion of the moon that is lit in this way, but no matter which parts of the moon are being lit by the sun, the same parts of the moon face the earth. In a typical 28 day cycle, all parts of the moon receive equal sunshine; the parts that face us and the parts that don’t face us each spend equal amounts of time facing the sun.

So what the hell is the “dark side of the moon”?!? When people refer to the dark side of the moon, do they simply mean the far side of the moon? Tragically, yes.

But wait, there’s more…

A lunar eclipse occurs when the shadow of the earth passes over the moon. By definition this can only happen during a full moon. This reduces the amount of sunlight that hits the side of the moon facing us, but an eclipse will never occur to the part of the moon facing away from us – the eclipse will always take place during a part of the month where the far side of the moon was due to be dark anyway. This means the near side of the moon actually get LESS total light than the far side.

Therefore: dark side of the moon = the side that faces us.


p.s. the moon is still eclipsed and I am listening to wish you were here.

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6 Responses to dark side of the moon

  1. gai says:

    I am jealous you could see it. We set the alarm to watch it, but cloud covered that beautiful clear moon and the stars that we so clearly saw last night.

  2. Rafferty says:

    Also, Gai, you told me the wrong time. You said 4:30, wasn’t it just after 6:00?

  3. emilystewart says:

    I missed it! Neat blog :)

  4. Eric says:

    Due to small relative motions between the Earth and the Moon about 59% of the Moons surface is actually visible from the Earth, considering that you look at it long enough.

    • kaan says:

      I definitely felt at the time I wrote that post that I was using rather strong language: “exactly the same part of its surface facing the earth”. Apparently in my pre-sunrise rage I was quite content to not think that through. Thank you for setting the record straight.

      Also, I am impressed you were able to produce such a precise figure during the wikipedia black out. My world fell apart.

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