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Royal Sighting Opportunities; Station Crew Sends Congratulations
Flight Engineers Cady Coleman, Paolo Nespoli and Ron Garan

Flight Engineers Cady Coleman, Paolo Nespoli and Ron Garan send a congratulatory message to the royal couple on behalf of the Expedition 27 crew. Photo credit: NASA
› View station crew’s message

ISS006-E-22939 -- London at night

City lights of London, England were captured with a digital still camera by one of the Expedition Six crew members on the International Space Station. Photo credit: NASA

Crowds gathering around Buckingham Palace for the Friday wedding of Prince William and Kate Middleton may have a special treat this week – royal sightings of the International Space Station, and if it launches, space shuttle Endeavour.

› View video of England from the International Space Station

If weather conditions in London cooperate, there are excellent sighting opportunities Thursday and Friday as the space ships fly 220 miles above the United Kingdom. Two of the best opportunities for Londoners to see the space station are tonight at 9:09 p.m. London time and Friday at 9:32 p.m.

If Endeavour launches as planned Friday evening, the shuttle should be visible from London at 8:50 p.m. Saturday. Again, it’s a perfect pass and should be quite a sight.

Since the distance between the station and shuttle is so large at launch, the two will not be visible together until docking.

› Get help with sighting opportunities

In order for the space station, or any other orbiting spacecraft, to be seen by a ground observer, there are four conditions that must be met all at the same time. First, the satellite must be above the horizon with respect to the observer. People cannot see the satellite if it is below the horizon and blocked by the Earth. Second, the observer must be in darkness - that time when the sun is more than four degrees below the horizon. If the sun is any higher than this, the sky is simply too bright for the spacecraft to be seen. Third, the spacecraft must be lit by the sun. In other words, the sun must be above the horizon with respect to the spacecraft and light must be shining on the spacecraft. Since satellites rarely provide their own lighting (at least lighting that can be seen on the ground), the sighting is made by the spacecraft reflecting sunlight toward the observer. Finally, even if sunlight is shining on the spacecraft, the fourth condition is that the side of the satellite that has sunlight on it must be facing roughly in the direction of the observer. If this condition is not met, then the observer is looking at the dark side of the spacecraft and it won’t be visible.

So when do all of these conditions come together? Since the satellite must be in daylight and the observer must be in darkness, this only happens near sunrise and at sunset. The rest of the conditions vary and depend on the positions of the observer, the spacecraft, and the sun. Many times, sightings can occur at both sunrise and at sunset on the same day depending on the geometry of the spacecraft with respect to the observer and the time of year. Sightings during the day are generally not possible without a telescope since the sky is too bright. Only very bright objects can be seen during the day (like the sun and the moon). Unfortunately, there is no Earth-orbiting spacecraft that can be seen unaided from the ground during daylight hours.