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Spotting the space station, The clock beats slowly
In
honor of our resumption of space shuttle flight, we are dedicating this column
to space. Here are two of my favorite articles from the past — the first
published originally in the year 2000 and the second in 2002 — updated to the
present day. By the way, the ISS is about 10 miles (16 km) closer to Earth now
than then and flies around in 91.61 minutes instead of 91.71.
ISS at sunrise — the best time to see it is dawn or
dusk. [NASA]
* * *
Q: It seems that the space station, with its newly
installed solar arrays should be visible to the naked eye.
Is it visible to the naked eye?
How bright, in comparison to the brightest planets
Venus, Mars and Jupiter, is the space station?
What is the celestial path of the space station and how
can we calculate the next possible observation of it?
When will the next sighting be possible from the
northern hemisphere?
A: Yes, it is visible to the naked eye and best seen near dawn
or dusk when the Sun lights the International Space Station (ISS) and you are in
near darkness.
The Station is brighter sometimes than others, depending on
its illumination and distance from us. It has a maximum brightness of -3.0 when
it's fully lit and nearest (about 230 miles, i.e., 370 kilometers) to Earth.
That's a little brighter than Jupiter (-2.5). When the Station is only halfway
lit and 620 miles (1,000 km) distant, it's brightness is only +1.5--about as
bright as a dim Mars.
Mars varies in brightness from -3.0 (outshining Jupiter) to
+1.6. Venus has a magnitude of -4.4, which is the brightest object in the night
sky, except for the full moon (-12.7). We measure star brightness with a
logarithmic scale, similar to the way we measure earthquake magnitudes.
The Station's celestial path is an oval-shaped path--an
elliptical orbit, which varies from 218 miles (351 km) to 221 miles (356 km)
from Earth. Going about 17,200 miles per hour, the Station circles Earth 16
times a day at an inclination (a measure of the tilt of the ISS’ orbital plane)
of 51.6 degrees to the equator. The Station eventually crosses almost every
point on Earth: flying over 85 percent of the globe and 95 percent of its
population.
Actually, "the height of the ISS varies quite a bit over time
due to air drag and corrective reboosts," says
Christ Peat of
Heavens Above. He built a
chart
so we can see this effect. Over last year, the height varied between about 217
to 226 miles (350 to 364 km) — repeatedly jumping up with a reboost and decaying
back down due to the drag of the atmosphere. The decay rate, however, is not
constant due to changes in the density of the "tenuous outer atmosphere", caused
mainly by solar activity.
You can get tracking software to calculate the next sighting
(see Further Reading) but it's easier to surf the web to any of several tracking
sites. I like the
Heavens Above site because it not only gives you the next sightings but it
also grades the sightings, telling which is brightest.
You just enter the name of your city (or its latitude and
longitude) and... Presto! The program displays screens giving several days of
sighting times, how to locate the Station each time it passes, and how visible
the Station is during the pass.
For example, I live in Albuquerque, New Mexico. I just had a
good sighting on the 18th of July at about 21:45 MDT (9:45 pm). I looked to the
northwest (308 compass degrees) to see the Station rise above the horizon at
21:43:08. I followed the rising satellite to the northwest where it reached its
maximum elevation (55 degrees up from the horizon — where 90 degrees is straight
up, overhead) for this pass at 21:48:02, and then watched it set in the south at
21:48:43. The program provides all this information so I can tell where and when
to look.
The next good one in Albuquerque will be on the 8th of August
about 05:35 (5:35 am), rising in the southwest. I can hardly wait.
Further Reading:
Aerospaceweb.org:
Emergency ISS-Shuttle Rendezvous
Discovery Channel:
Inside the space station
Heavens Above, tracking site
NASA
orbit trajectories and tracking software
Austin Astronomical Society:
How to estimate
angles in the sky
ESA:
Space
station and useful terminology
NASA:
International
Space Station press kit
NASA:
International Space Station
Q: Since time and speed is relative,
according to Einstein's theory of relativity, will an atomic clock on the
International Space Station be slower than a synchronized atomic clock on the
ground? (Yiu Wai Chan)
A:
Yes, the clock on the International Space Station (ISS) will tick slower than
the earthbound clock because of relativistic effects. However, two effects
compete and complicate the picture.
Position (arrow) of the atomic clock aboard the ISS.
[NASA/JPL]
The dominant effect on the ISS clock is, as you surmised, a
slow down because the space station zings around Earth. Einstein figured out,
back in 1905, that the speed of light is a constant for all frames of reference.
Given that, an observer on Earth peering through a telescope at the ISS clock
would see it running slow compared with his own clock. This is the same effect
(the "time dilation" effect) that allows a returning space farer twin to be much
younger than her earthbound sister, after the astronaut spends years blasting
through space at near light speeds.
Our genius of the last century, Einstein, however, didn’t stop
with the Special Theory of Relativity. He went on to develop his General Theory
of Relativity, which includes gravity effects. In 1915, he predicted that clocks
run slower in high gravity fields. A clock on the Sun would run slower than an
Earth clock, for example. Thus, the gravity effect causes the ISS clock to speed
up, since the gravity field is slightly less for an object in orbit (more
distant) than at the Earth’s surface. We have competing effects.
The question is: which effect wins out–the speed or the
gravity effect? Interestingly enough, the two effects cancel if you orbit Earth
at a radius of 1.5 times Earth’s radius. This is pretty far out: about 1,900
miles (3,100 kilometers) high. If the space station were to orbit this high, an
observer on Earth peering through his telescope would see the ISS clock and his
clock agreeing.
The lower the ISS, the faster it must orbit. Increased speed
means a slower clock. Thus, orbiting below that 1900-mile height, speed
dominates: the ISS clock ticks slower. Orbiting above 1900 miles high, gravity
dominates: the ISS clock ticks faster. The ISS actually orbits only 220 miles
(353 kilometers) high so the ISS clock does run slow: About 0.0000000014 %
slower.
Further Reading:
NASA/JPL:
Primary Atomic Reference Clock in Space
(Answered July 29, 2005)
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