Your guide to spotting the space station

By Joe Rao

Skywatching columnist

updated 4:10 p.m. ET July 11, 2008

If you're out watching the twilight sky in the time frame from 45 to 90 minutes before sunrise, or 45 to 90 minutes after sunset, you'll might see a few "moving stars." They are most likely artificial satellites.

The brightest of all is the international space station, and this month provides some great opportunities to see it from just about anywhere.

Satellites are seen at night because they are illuminated at high altitudes by reflected sunlight and can be seen against a dark sky. A satellite entering Earth's shadow immediately vanishes from view and pursues an unseen path until it again emerges into full sunlight. There are nearly 10,000 satellites now in orbit around Earth, and typically it should not take more than 15 minutes of skywatching to spot one.

The biggest and brightest
The space station is by far the largest and most brilliant of all the artificial objects orbiting the Earth. In early June, the station got its biggest live-in addition yet, a billion-dollar Japanese lab stretching 37 feet, named Kibo, which means "hope." Currently more than four times as large as the defunct Russian Mir space station, the international space station — when fully completed — will have a mass of about 1,040,000 pounds (520 tons). It will then measure 356 feet across and 290 feet long, with almost an acre of solar panels to provide electrical power to six state-of-the-art laboratories.

Circling the Earth at an average altitude of 240 miles and at a speed of 18,000 mph, it can appear to move as fast as a high-flying jet airliner, sometimes taking up to four or five minutes to cross the sky.

Nominally, its visual magnitude from the ground can make it appear as bright as the planets Jupiter and Venus, although in recent days some observers have seen the space station briefly "flare" to dazzling brilliance, thanks to sunlight glinting off one of its many solar panels. In fact, some have even been able to glimpse the station while the sun was just above the horizon!

Windows of opportunity
During the next couple of weeks, North Americans and Europeans will have many opportunities to see the station flying over their homes, due chiefly to a seasonal circumstance.

Right now, the nights are still rather short, and the time that a satellite in a low Earth orbit (like the station) remains illuminated by the sun can extend through much of the night, a situation that can never be attained during other times of the year. Because the station circles Earth about every 90 minutes on average, this means that it's possible to see it not just once, but for several consecutive passes.

Moreover, because the station revolves around the Earth in an orbit that is inclined 51.6 degrees to the equator, there are two types of passes that are visible.

In the first case (we'll call it a "Type I" pass), the station initially appears over toward the southwestern part of the sky and then sweeps over toward the northeast. About seven or eight hours later, it becomes possible to see a second type of pass (we'll call it "Type II"). During a Type II pass, the station initially appears in the northwestern part of the sky and sweeps over toward the southeast.

Between roughly July 17 and 24, thanks to the shortness of the nights, North Americans will get a chance to see the station undergoing a series of Type I passes after sunset in the evening sky, and then see it again before sunrise, undergoing a series of Type II passes.

For some locations, there may be as many as six chances to see the station during a single night! For much of North America and Europe, the "prime viewing period" for both evening and morning passes will run roughly from about July 17 through 21. After July 21, the window of opportunity for the Type II morning passes will close, and only Type I evening passes will be possible, lingering into the early part of August.

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