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Ed Lu Letter from Space #6

4 July 2003

Orbits

Part 2 of 2

I've included a picture here of a computer program we use to give us
our current position and show the track of our orbit across the
ground. In the middle of the screen, the two small red rectangles
with the little white circle between them is supposed to represent
the Space Station. The white circle around it is roughly the patch of
ground you can see if you look down. Right now the Space Station is
over Western Sahara (you can see the zoomed insert in the lower left)
and moving southeast towards the bottom right hand corner. The white
dotted lines show the path that the Space Station will follow in this
orbit, as well as the next two orbits. The reason the 2nd and 3rd
orbits are displaced to the left is that during the 90 minutes it
takes us to complete a lap around the hoop, the Earth has rotated by
one 16th of a revolution to the right, so our orbit track is
displaced to the left by that much each time. You can see that we
never cross over any point with latitude greater than 51.6 degrees,
so we never get to see the North or South poles from here. You can
also see that if we do go over a point, we go over it twice a day:
once going northeast, and once going southeast.

The shaded areas of the map are the areas in darkness, and the rest
of the map is in daylight. If you are wondering why the day-night
line curves up and down, it is for the same reason that our orbit
curves up and down - namely the sun isn't over the equator so that
while half the Earth is lit up that half doesn't line up with the
equator or one of the lines of longitude. You can see that now in the
summertime, very far northern points will always be in daylight.

From the map, you can also see that part of the solution to our
puzzle is that our orbit tracks at the far northernmost and
southernmost parts run due East. That means that on repeated orbits
we'll get to see the same point again. That doesn't happen for points
near the equator since as you can see our orbit path is southeast or
northeast so if you see a point once, you won't see it again on the
next orbit, at least not very well. So you are likely to see points
near the southernmost and northernmost parts of our orbit more
frequently. But then why Canada and not the Andes now, and why the
opposite a few weeks ago? 

The answer has to do with timing. The shaded portion of the map and
our orbit track both move left together over the course of the day as
the Earth rotates underneath us, so picture the map as sliding to the
right while the dotted lines and shaded area of the screen stay in
position. From the map you can see that when Canada crosses under our
orbit, it will be roughly in the middle of the bright region, meaning
in the middle of the day in Canada. When we are crossing the
southernmost part of our orbit (i.e. the Andes) it is nighttime. At
the time this picture was taken the "Oh Canada" effect was in full
force. During our workday (we live on a timezone roughly halfway
between Houston and Moscow - Greenwich Mean Time), we tend to look
out and see Canada, especially since we have free time in the
evening, which is the middle of the day in Canada.

It turns out though that our orbit hoop slowly rotates due to the
fact that the Earth isn't quite a perfect sphere. This turns out to
apply a torque to our orbit which makes it slowly shift westward with
respect to the sun, and therefore the lighted part of the screen. It
is the same effect that makes a spinning top wobble. In effect our
orbit is like a large top, and the fact that the Earth has a bit of a
bulge around the equator causes our orbit hoop to wobble slowly. In a
few weeks, the southernmost part of the orbit will be in the fully
lit section of the orbit, and we will be back to seeing lots of the
Andes again. Actually I'm looking forward to our orbit track shifting
a bit since I am trying to take a photograph of the Great Wall of
China. Right now, of the two times a day we cross over it, once is
during our sleeptime when we are heading southeast. The
northeastbound crossing is during our awake hours, but it is very
close to sunrise in Beijing, so the lighting is bad and it is
difficult to take a photo. 

This is a screen shot of the computer program we use to tell where we
are. The places labeled EOS are locations that scientists have
requested photos of. Godzilla is shown for scale.

-Ed Lu-

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