Spotting the space station, and why its clock ticks slower

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.

Sunrise and the ISS — 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% of the globe and 95% 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 MT (9:45 p.m.). 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 a.m.), rising in the southwest. I can hardly wait.

Same clock, different times

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.

NASA/JPL The position (arrow) of the atomic clock aboard the ISS.

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:

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

NASA/JPL: Primary Atomic Reference Clock in Space

April Holladay, science journalist for USATODAY.com, lives in Albuquerque, New Mexico. A few years ago Holladay retired early from computer engineering to canoe the flood-swollen Mackenzie, Canada's largest river. Now she writes a column about nature and science, which appears Fridays at USATODAY.com. To read April's past WonderQuest columns, please check out her site. If you have a question for April, visit this informational page.