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Ed Lu letter from space #5

25 Jun 2003

Watching the World Go By

Part 1 of 2

One of my favorite things to do when I have time off is to just watch
the world go by. Whenever I get a chance, I spend time just observing
the planet below. It turns out you can see a lot more from up here
than you might expect. First off, we aren't as far away as some
people think - our orbit is only about 240 miles above the surface of
the Earth. While this is high enough to see that the Earth is round
(believe me, it is), we are still just barely skimming the surface
when you consider that the diameter of the Earth is over 8000 miles.

So how much of the Earth can we see at one time? When you are
standing on the ground, the horizon is a few miles away. When in a
tall building, the horizon can be as far as about 40 miles. From the
ISS, the distance to the horizon is over 1000 miles. So from horizon
to horizon, the section of the Earth you can see at any one time is a
patch about 2000 miles across, almost enough to see the entire United
States at once. It isn't exactly seeing the Earth like a big blue
marble, it's more like having your face up against a big blue beach
ball. When I look out a window that faces straight down, it is
actually pretty hard to see the horizon - you need to get your face
very close to the window. So what you see out a window like that is a
moving patch of ground (or water). From the time a place on the
ground comes into view until when it disappears over the horizon is
only a few minutes since we are traveling 300 miles per minute.

When looking out a sideward facing window, you can see the horizon of
the Earth against the black background of space. The horizon is
distinctly curved, so as I said earlier, I can see that the Earth is
not flat. The edge of the Earth isn't distinct but rather is smeared
out due to the atmosphere. Here you can get a feel for how relatively
thin the atmosphere is compared to the Earth as a whole. I can see
that the width of the atmosphere on the horizon is about 1 degree in
angular size, which is about the width of your index finger held out
at arms length. For those of you who are farsighted, it is also about
the height of a person when seen from about 100 yards away (the
length of a football field). At a distance of 1000 miles, that
translates into a height of about 20 miles. There really isn't a
sharp boundary to the atmosphere, but it gets rapidly thinner the
higher you go. Not many airplanes can fly higher than about 10 miles,
and the highest mountains are only about 6 miles high. Above about 30
miles there is very little air to speak of, but at night you can see
a faint glow from what little air there is at that height.

Since we orbit at an altitude about 40 times higher than the tallest
mountain, the surface of the Earth is pretty smooth from our
perspective. A good way to imagine our view is to stand up and look
down at your feet. Imagine that your eyes are where the ISS is
orbiting, and the floor is the surface of the Earth. The atmosphere
would be about 6 inches high, and the height of the tallest mountain
is less than 2 inches, or about the height of the tops of your feet.
Almost all of the people below you would live in the first one
quarter of an inch from the floor. The horizon of the Earth is a
little over 20 feet away from where you are standing. If you are
standing on top of Denver, then about 15 feet to one side you can see
San Francisco, and about 15 feet to the other side you can see
Chicago. At this same scale, the Earth that you are standing on would
be a sphere with a diameter of about 160 feet. If you want to
complete the effect, you can start walking and take a step about
every 20 seconds. 

Now I thought I'd take you on a guided tour of an orbit around the
Earth. Take a globe and imagine a hoop representing our orbit around
the equator. Now tilt the hoop by 51.6 degrees, and that is what our
orbit track looks like. The Earth rotates on its axis every 24 hours
inside the hoop, while we go much faster around the hoop, making a
lap every 90 minutes. I'll write more about the mechanics of orbits
and ground tracks in a later installment. For now though you can see
that if we start our tour along the inclined hoop from over the
equator, we will at first be moving in a northeasterly direction, and
by a quarter of an orbit later will be at the northernmost point of
our orbit traveling in an easterly direction. The orbit then becomes
southeast, we cross the equator into the Southern Hemisphere and by
the time we reach the southernmost point of our orbit we are again
traveling due east. The final quarter of an orbit takes us back to
the equator, but not over the original point since by then the Earth
will have rotated 1/16th of a revolution in the 90 minutes (or 1/16th
of a day) it took us to travel once around. If our orbit hoop was
completely fixed in space (which it is not quite exactly), then we
will see 16 different orbit tracks each day with the pattern
repeating itself every day. 

One of my favorite orbit tracks starts over the equator southwest of
Hawaii. At this point, looking down you will just see water and
clouds. The Pacific Ocean is a deep bright blue color, and typically
over the equator there are scattered bright white clouds. In about
3minutes, off to the left of our track you can see the islands of
Hawaii. You can easily see the standing clouds over the mountains as
the trade winds blow up the mountainsides. When you fly right over
the top of the islands you can look down and see the city of Honolulu
near Pearl Harbor. If the weather is good and the air is clear (and
it almost always is in Hawaii) you can see objects as small as maybe
a quarter of a mile in size with your bare eyes. What matters most
for spotting objects is usually the contrast with surrounding areas.
For instance there is a large runway (8R if you care to know the
name) at Honolulu International Airport that has been built out on
the edge of the water that you can very easily see from space since
it is easy to pick out against the blue color of the water. With
binoculars you can see much smaller objects like ships and individual
buildings! 

After Hawaii passes off to the left, again you see mostly ocean for a
few minutes as we head northeast towards the California coast. We
cross the coastline just north of San Francisco, and looking down you
can see the cities of San Francisco, Oakland, and San Jose
surrounding San Francisco Bay. Cities have a grayish color, probably
because of all the asphalt and buildings. They are not always easy to
spot unless they are located near an easily recognizable feature
(like San Francisco Bay) or the surrounding areas have a very
different color or brightness (cities surrounded by forests for
instance). In the bay near Fremont (where my parents live) are huge
maroon red ponds which are very easy to spot from space. This color
is from bacteria growing in the ponds where they evaporate water to
collect salt. The Great Salt Lake in Utah has a similar color.
Looking to the left of our track you can see a line of white snow
capped volcanoes running up the Cascade mountain range and Washington
State in the distance. To the right you can look down the central
valley of California to the Baja Peninsula in Mexico.

We continue northeast heading over the Rocky Mountains, over
Yellowstone Park, and up into Canada. When I lived in Colorado I
remember the big afternoon thunderstorms that we would get in the
summertime. From space, you see that this area is covered with
isolated thunderhead clouds that pop up like mushrooms in the late
afternoon. Actually, thunderstorm clouds look more like flattened
cauliflower heads when viewed from above. 

 - Continued -

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