Behind the scenes: Physicist in freefall

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    Helen Czerski Helen Czerski | 17:00 PM, Wednesday, 23 November 2011

    Distance travelled ~ 840'555'200 km

    helen czerski


    Being surrounded by sky is not a natural place for a human being. We have evolved to scoot about on the bottom of the atmosphere, stuck to the ground, and we don't often look up. Even when we do, we tend to see the sky as flat - clouds, the moon and aeroplanes move sideways across the sky. It's easy to forget that the sky has depth too, and that air in the atmosphere moves up and down as well as sideways.

    Your perspective changes quickly when you're in freefall, three thousand metres above the Earth's surface and travelling downwards at 120 mph.

    sky full of cumulus clouds


    Skydivers relish the sense of freedom that falling through the sky brings. There is nothing to get in the way, nothing touching you and a whole extra dimension to play in. For the air in our atmosphere, three-dimensional movement is normal. At the place where I jumped out of the plane, in Arizona, air that starts about 10 miles up is gradually sinking towards the ground. The air doesn't make the squeaking noises that I did, but then it isn't falling nearly as fast - it's a few millimeters per second on average. I was falling through a giant atmospheric waterfall, but a very slow one.

    It's not just in Arizona that this happens. Although weather maps tend to show sideways winds, the air making up those winds is all rising and falling as it travels around the Earth. The paths of air parcels weave in and out of each other, making the Tokyo subway map look simplistic by comparison.

    tokyo subway map

    Image courtesy of Tokyo Metro

    All this is very interesting, but not much comfort to a plummeting physicist. I don't think that I really breathed during the 40 seconds of freefall. Then the parachute opened, everything slowed down, and my brain stopped panicking and started appreciating what was going on around it.

    Seeing the layers of the sky is fascinating. Floating down past a cloud is amazing - a fluffy cumulous cloud is telling you that there's been a little puff of air upwards in that location. We can't really see the structure of the atmosphere, but seeing a cloud from the side makes it easy to imagine the turbulent swirls that are mixing all that air up.

    After five minutes of sharing the three dimensions of the sky with the clouds, we arrived at the landing zone and my feet touched the ground again. I was very happy to feel something solid under my feet, but there was also a small sense of loss. I was back to crawling around on the bottom of our fabulous three-dimensional atmosphere, and my understanding of the depth of the atmosphere was again limited to hints given away by the clouds. But I remember what it felt like, and my view of the sky will never be quite the same again.

    Can an earthquake shift the Earth's axis?

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    Bethan Harris Meteorologist Bethan Harris Meteorologist | 18:11 PM, Monday, 14 March 2011

    d ~ 187'814'400 km: day 73

    The short answer to this is yes. But the effects of such a shift are tiny. The Earth's tilt and rotational spin on its axis as it travels around the Sun causes our seasons. The earthquake in Japan moved the axis of rotation by around 16 cm. That might sound like a lot, but it's small compared to the size of the Earth. 1 degree change to the tilt of the axis of the Earth would mean moving it by around 110 km.

    But the quake's interference with our axis doesn't stop there. The Japanese landmass was moved around by as much as 4m. This redistribution of mass on the surface changes our moment of inertia. In order to conserve angular momentum, the changes in inertia are compensated by changes in the rate of rotation of the Earth about the axis. After the earthquake it's quite possible that our days will be 1.8 millionths of a second shorter because of this shifting.

    We can see differences in the average length of the day due to other changes in the Earth and atmosphere. The plots below show that there is a significant seasonal variation, with the day length (speed of rotation) being shortest (fastest) during the boreal summer. This happens because the northern hemisphere winds slow down in the summer and the momentum they lose - half the momentum of the atmosphere - is transferred to the Earth. This increase in momentum makes the Earth spin faster and our days become slightly shorter by 1-2 milliseconds.

    Deviation of day length from SI day (86'400 s) 1962-2010: daily, moving 365-day average and cumulative.

    So while the changes brought to our planet by the earthquake are unique and collosal enough to affect the Earth; they aren't big enough that we will notice them any more than we notice the milliseconds we lose each summer.

    Can we use the Moon to tell time?

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    Helen Czerski Helen Czerski | 15:01 PM, 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!

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