The appearance of either the space shuttle or the ISS moving across the sky is not in itself unusual. On any clear evening and with no optical aid, one can usually spot at least one or two orbiting Earth satellites, and sometimes as many as a half dozen, creeping across the sky like moving stars.

Satellites become visible only when they are in sunlight and the observer is in deep twilight or darkness. This usually means shortly after dusk or before dawn.

What makes this Mondays passage so interesting is that an observer will able to see the two largest orbiting space vehicles in the sky at the same time. And what will make this situation rather unique will be the closeness of the two to each other, since only a few hours earlier, the shuttle will have undocked from the ISS.

At many previous shuttle missions to the ISS, the undocking procedure has usually occurred many hours before ground observers in North America could sight both objects. By the time observers finally could sight them, the two would appear rather widely separated as they moved across the sky. Not so on Monday!

According to the current NASA mission schedule, shuttle Endeavour is to undock from the ISS at 3:05 p.m. EST. Later, the shuttle will begin to slowly pull away from the station, but the two will still remain relatively close to one another for a few more hours, setting the stage for their spectacular pairing-off as darkness descends across North America on Monday evening.

There will be three passes of the shuttle/ISS over the United States and southern Canada on Monday. Each pass will be separated by roughly 92.5 minutes, and all three will take southwest-to-northeast trajectories. Many localities should be able to catch a glimpse of at least one and possibly even two of these three passes.

Unfortunately, for parts of the southern United States, no sightings will be possible primarily because the two vehicles will be in the Earths shadow when they are passing overhead and will not be illuminated by sunlight. Hence they will rendered invisible (check either the table below or the NASA Website provided at the end of this article to check viewing circumstances for your location).

Seeing both the shuttle and the ISS, one closely following the other, should make for an eye-catching, albeit rather brief sight.

Traveling in their respective orbits at 18,000 miles per hour and at an average altitude of 255 miles (410 kilometers), both should be visible for no more than three or four minutes at most as they glide with a steady speed across the sky. Appearing as a pair of very bright "stars," the ISS should appear as the somewhat brighter object and will appear to be trailing closely behind shuttle Endeavour.

The ISS is, in fact, slowly evolving into one of the brightest objects in the night sky as it continues to increase in size. The ISS assembly consists of the energy and control block FGB "Zarya," the U.S. connecting module "Unity", the Russian Service Module "Zvezda" and the first element, Z1, of the future solar-array-carrying truss, on top of the Node Unity.

For more sighting information for nearly 400 locations worldwide, go to this NASA site.