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 the 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 International Space Station (ISS) is by far the largest and most brilliant of all the man-made 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 upwards 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 ISS 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 ISS 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 ISS 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 ISS) 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 ISS circles the 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 ISS 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 ISS 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"), but this time with the ISS initially appearing over toward the northwestern part of the sky and sweeping over toward the southeast.
And between roughly July 17 through 24, thanks to the shortness of the nights, North Americans will get a chance to see the ISS undergoing a series of Type I passes after sunset in the evening sky, and then see it again the following morning before sunrise, undergoing a series of Type II passes.
For some locations, there may be as many as six chances 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 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.
Some passes are superior to others. If the ISS is not predicted to get much higher than 20 degrees above your local horizon, odds are that it will not get much brighter than a moderately bright star (10 degrees is roughly 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 that take 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 sky upwards to four or five minutes; it will likely get very bright perhaps even briefly "flare" in brilliance and there will be little or no chance of it encountering the Earth's shadow.
While the ISS looks like a moving star to the unaided eye, those who have been able to train a telescope on it have actually been able to see and even photograph its T-shape as it whizzed across their field of view. Some have actually been able to track the ISS with their scope by moving it along the projected path. Those who have gotten a good glimpse describe the body of the space station as a brilliant white, while the solar panels appear a coppery red.
For the upcoming series of evening and morning passes, take note of the fact that, for those occurring in the evening, the ISS will usually start out rather dim, then tend to grow in brightness as it moves across the sky. In contrast, for the morning passes, the ISS will already be quite bright when it first appears and will tend to fade somewhat toward the end of its predicted pass. This is due to the change in the angle of sunlight hitting the vehicle.
Lastly, remember that in certain cases, the ISS will either quickly disappear when it slips into the Earth's shadow (during evening passes) or quite suddenly appear when it slips out of the Earth's shadow (during morning passes). This becomes increasingly more likely for those predicted passes that take place more than 90 minutes after sunset or more than 90 minutes before sunrise.
When and where to look
So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these three popular web sites:
Each will ask for your zip code or city, and respond with a list of suggested spotting times. Predictions computed a few days ahead of time are usually accurate to within about a minute. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher 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 other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.