If you're out watching the twilight sky in the time framefrom 45 to 90 minutes before sunrise, or 45 to 90 minutes after sunset, you'll mightsee a few "moving stars." They are most likely artificial satellites.
The brightest of allis the International Space Station, and this month provides some great opportunitiesto see it from just about anywhere.
Satellites are seen at night because they are illuminated athigh altitudes by reflected sunlight and can be seen against a dark sky. Asatellite entering the Earth's shadow immediately vanishes from view andpursues an unseen path until it again emerges into full sunlight. There are nearly10,000 satellites now in orbit around Earth, and typically it should not takemore than 15 minutes of skywatching to spot one.
The biggest and brightest
The International Space Station (ISS) is by far the largestand most brilliant of all the man-made objects orbiting the Earth. In earlyJune, the station got its biggest live-in additionyet, a billion-dollarJapanese lab stretching 37 feet, named Kibo, which means "hope." Currentlymore than four times as large as the defunct Russian Mir space station, the InternationalSpace Station — when fully completed — will have a mass of about 1,040,000pounds (520 tons). It will then measure 356 feet across and 290 feet long, withalmost an acre of solar panels to provide electrical power to sixstate-of-the-art laboratories.
Circling the Earth at an average altitude of 240 miles andat a speed of 18,000 mph, it can appear to move as fast as a high-flying jet airliner,sometimes taking upwards to four or five minutes to cross the sky.
Nominally, its visual magnitude from the ground can make itappear as bright as the planets Jupiter and Venus, although in recent days someobservers have seen the ISSbriefly "flare" to dazzling brilliance, thanks to sunlightglinting off one of its many solar panels. In fact, some have even been able toglimpse the ISS while the sun was just above the horizon!
Windows of opportunity
During the next couple of weeks, North Americans andEuropeans will have many opportunities to see the ISS flying over their homes,due chiefly to a seasonal circumstance.
Right now, the nights are still rather short and thetime that a satellite in a low Earth orbit (like the ISS) remainsilluminated by the sun can extend through much of the night, a situation thatcan never be attained during other times of the year. Because the ISS circlesthe Earth about every 90 minutes on average, this means that it's possible tosee it not just once, but for several consecutive passes.
Moreover, because the ISS revolves around the Earthin an orbit that is inclined 51.6 degrees to the equator, there are two typesof passes that are visible.
In the first case (we'll call it a "Type I"pass), the ISS initially appears over toward the southwestern part of the skyand then sweeps over toward the northeast. About seven or eight hours later, itbecomes possible to see a second type of pass (we'll call it "Type II"),but this time with the ISS initially appearing over toward the northwesternpart of the sky and sweeping over toward the southeast.
And between roughly July 17 through 24, thanks to theshortness of the nights, North Americans will get a chance to see the ISSundergoing a series of Type I passes after sunset in the evening sky, and thensee it again the following morning before sunrise, undergoing a series of TypeII passes.
For some locations, there may be as many as sixchances to see the ISS during a single night! For much of North America and Europe, the "prime viewing period" for both evening and morning passes will runroughly from about July 17 through 21. After July 21, the window of opportunityfor the Type II morning passes will close and only Type I evening passes willbe possible, lingering into the early part of August.
Some passes are superior to others. If the ISS is notpredicted to get much higher than 20 degrees above your local horizon, odds arethat it will not get much brighter than a moderately bright star (10 degrees isroughly equal to the width of your fist held at arm's length). In addition,with such low passes, the ISS will likely be visible for only a minute or two. Conversely,those passes that are higher in the sky — especially those above 45 degrees —will last longer and will be noticeably brighter.
The very best viewing circumstances are those thattake the ISS on a high arc across the sky about 45 to 60 minutes after sunset,or 45 to 60 minutes before sunrise. In such cases, you'll have it in your skyupwards to four or five minutes; it will likely get very bright — perhaps evenbriefly "flare" in brilliance — and there will be little or no chanceof it encountering the Earth's shadow.
While the ISS looks like a moving star to the unaidedeye, those who have been able to train a telescope on it have actually been ableto see and even photograph its T-shape as it whizzed across their field ofview. Some have actually been able to track the ISS with their scope by movingit along the projected path. Those who have gotten a good glimpse describe thebody of the space station as a brilliant white, while the solar panels appear acoppery red.
For the upcoming series of evening and morningpasses, take note of the fact that, for those occurring in the evening, the ISSwill usually start out rather dim, then tend to grow in brightness as it movesacross the sky. In contrast, for the morning passes, the ISS will already bequite bright when it first appears and will tend to fade somewhat toward theend of its predicted pass. This is due to the change in the angle of sunlighthitting the vehicle.
Lastly, remember that in certain cases, the ISS will either quickly disappear when it slips into the Earth's shadow (during eveningpasses) or quite suddenly appear when it slips out of the Earth's shadow(during morning passes). This becomes increasingly more likely for thosepredicted passes that take place more than 90 minutes after sunset or more than90 minutes before sunrise.
When and where to look
So what is the viewing schedule for your particularhometown? You can easily find out by visiting one of these three popular websites:
Each will ask for your zip code or city, and respondwith a list of suggested spotting times. Predictions computed a few days aheadof time are usually accurate to within about a minute. However, they can changedue to the slow decay of the space station's orbit and periodic reboosts tohigher altitudes, so check frequently for updates.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and otherpublications, and he is also an on-camera meteorologist for News 12 Westchester, New York.