# NASA - National Aeronautics and Space Administration

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Michael Lopez-Alegria's Mission Log
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Like Night and Day

We’re going fast up here. Really fast. Our speed relative to the surface of the Earth is about 17,500 miles per hour, or 5 miles or so per second. At that speed, if the Dodgers and the Yankees would ever make it to the World Series they could get back and forth between stadiums much faster than they could when the Dodgers were still playing in Brooklyn, even without the traffic.

Image to left: This view of the International Space Station, backdropped against a blue and white Earth, was photographed shortly after the Space Shuttle Atlantis undocked from the orbital outpost on Sept. 17, 2006. Image credit: NASA

Why so fast? Physics (which seems to be the answer to a lot of questions up here). If we weren’t going so fast, we would fall out of orbit like a safe. Contrary to popular belief, there is plenty of gravity 240 miles up. I don’t have the formula handy, but if you do the math you’ll see that the gravitational attraction pulling us earthward here is about the same as it is down there. So how do we stay up here? Speed. To simplify things a little, imagine taking a cannon to a nearby hill, aiming the barrel horizontally, loading the gunpowder and lighting the fuse. The cannon ball would leave the cannon with a certain velocity, start out nearly horizontally, and would fall to the ground some distance away. Put in more powder, the cannon ball leaves the cannon with more speed, and travels further before falling to the ground. More gunpowder, more velocity, more distance. But the cannon ball has two forces acting on it; one is gravity, which is constantly pulling it downward, and the second is friction from the air slowing it down (recall the hand-out-the-window-of-the-moving-car example). If the hill is high enough to be out of the atmosphere (about 50 miles high), the resistance is minimal and you “only” have to deal with gravity. Load enough gunpowder and you’ll be struck in the back of the head by your cannon ball after it circles the Earth.

For every orbital altitude there is a speed that will exactly balance the force of gravity; the higher the orbit, the slower the speed. At so called “geosynchronous” orbital altitude (about 21,800 miles up) the speed is conveniently the same as the rotation speed of the Earth, so that a satellite in that orbit stays above the same point on the Earth as the Earth turns beneath it. That is why the antenna for your satellite television system points at the same point in the sky all the time.

At 17,500 miles per hour, our speed is just fast enough to keep us in orbit. We are actually constantly falling; the ISS is falling, and we’re falling within it, but not to Earth. Imagine being in an elevator in a very tall building when suddenly the cable breaks and the elevator, with you in it, starts falling toward the ground. After a while you would be floating inside the elevator, even though you are most definitely under the influence of gravity (just wait), and for a few moments it would be fun. Happily, our freefall never ends, because we are falling around the Earth instead of at it.

Image to right: A setting sun and Earth's horizon are featured in this image photographed by a crew member on the International Space Station. Image credit: NASA

We make a revolution around the Earth about once every 90 minutes. All the while, the Earth is turning underneath us, so that we fly over a different swath with each revolution. At any given moment, half of the Earth is sunlit; the other is in darkness. Day/night cycles on the ground are caused by the Earth’s rotation, which brings you into the sunlight at dawn as your house enters the half of the planet that’s facing the sun, and then, about a half revolution later (this time of year) brings it back into darkness as you enter the part of the globe that is in shadow. Our day/night cycles have nothing to do with the Earth’s rotation; they are caused by us going around the planet and passing from the sunlit to darkened sides of the globe. For us there are sixteen dawns and sixteen sunsets every day (if you assume 24 hours in a day). If we were to strictly follow the hours associated with each time zone, we’d be setting our watches ahead an hour every eight minutes.

So what time is it up here? Rather than trying to keep up with the hour according to the position of the sun in “our” sky (which is pretty much how it’s done on Earth), it was decided to pick a time zone (somewhat arbitrarily) and stick to it. We chose to use Greenwich Mean Time (GMT), also called Universal Time. That’s the time in Greenwich, England, which is approximately at zero degrees longitude. This time zone is four hours ahead of Eastern Daylight Time, seven ahead of the Pacific Daylight Time, two behind most of Europe (except, of course, Greenwich) and four behind Moscow. Since all of these places observe daylight savings time, when that ends all of the differences will change by one hour. You can do the math; spring forward, fall back.