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Can we use the Moon to tell time?

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Helen Czerski Helen Czerski | 15:01 UK time, Friday, 4 March 2011

d ~ 162'086'400 km: day 63

Earth's relationship with the Sun is of huge significance to the 23 Degrees team. Its 23.5 degree tilt is what makes our planet's annual journey around the Sun interesting and it's why we're encouraging everyone to help us record that journey. But maybe it's time we also looked up at the moon. Our planet's relationship with the Moon also plays a part in the Earth's cycle and the passing of time - the moon goes around the Earth and the Earth goes around the Sun which goes around the galaxy. It almost sounds like the solar system runs like clockwork right? But maybe it should be the other way around. Clocks run like the solar system. Our Sun dominates our concept of time but the moon is 400 times closer to us than the Sun is, and it moves around our planet steadily like the hand of a clock. Could we use the moon to tell the time if we wanted to?

Earthrise Apollo 8 Lunar mission 1968

It's tricky because the moon orbits Earth at a fixed rate but that rate has nothing to do with how fast the Earth spins or how fast the Earth goes around the sun. Inconveniently the numbers don't divide cleanly. However, during any single day the position of the moon doesn't change very much so it's quite a good reference point. Once the sun has set you're on the side of the Earth facing away from the sun slowly spinning back towards sunlight. If you know where you are on that journey you know the time. Here are some ways that the moon can help.

The ocean bulges out towards the moon attracted by its gravity. As the Earth spins we pass through the bulge and out the other side. We call that bulge a tide. If you're by the beach on a cloudy night you can get an idea of the time just by watching the tide level. In most places high tide is six hours after low tide, and you can use this even if you can't see the moon directly.

The moon's gravity doesn't only affect the water. It's also tugging on us, and if the moon is overhead pulling us upwards, it balances out a little bit of Earth's gravity pulling us downwards. If you stood on some very sensitive scales at a full moon, you'd notice that you weighed about a quarter of a gram less at midnight than you did at midday. So you could tell the time by monitoring how your weight changed during the night. It's not a very useful clock - you'd have to weigh anything you ate or drank and not put any extra clothes on. But in theory you could use your weight as a clock, because it changes systematically as you rotate closer to and further away from the moon. You have your own tide.

At full moon a normal sundial can be used as a moon dial. The moon is more or less exactly where the sun would be in the sky during daytime, except that the times on the sundial will be 12 hours too early. And of course, if you can see the moon, you can watch its progress across the sky and estimate the time in the same way that you would with the sun.

But why expend all this effort to help the moon tell solar time? The moon has its own time, and there's a tide-powered clock in London to tell you what that is.
The Aluna clock is at Trinity Buoy Wharf in London - a prototype for a larger one to be built next year. I love this idea because the progress of the sun going across the sky is only one way of keeping track of time. Everything in the universe with an orbit is like the hand of a different clock all measuring the same time in different units. The moon is just one clock of many!


  • Comment number 1.

    "and it moves around our planet steadily like the hand of a clock."

    No, it doesn't.
    It moves much faster along the zodiac when it is near perigee,
    and conversely slower when near apogee.


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