Many people have a difficult time comprehending the massive proportions of the International Space Station (ISS). Weighing almost one million pounds, and filling the footprint of a football field, it is by far the largest man-made object in space. The ship has an acre of reflective solar arrays that provide power for the crew and also help make the ISS the third brightest object in the night sky (behind the Moon and Venus). It is easily viewed with the naked eye. You just need to know where to look and what to look for.
Where It’s Going
Before we talk about how to find the ISS in the sky, let’s take minute to review some basic orbital mechanics. The ISS has a roughly circular orbit (as opposed to elliptical) at an altitude of about 260 miles. The plane of orbit is tilted 51.6 degrees from the plane of the equator. If you flatten the Earth onto a map, one orbital path takes on the shape of a single sine wave. That is often the image seen on the large wall displays in photos of the Mission Control Center.
Each orbit takes roughly 90 minutes to complete. During that time, the Earth is rotating as well. Due to this relative movement, every orbit of the ISS overflies a path that is a little west of its previous orbit. When the paths of multiple orbits are displayed on a flattened Earth, the image is a series of identical sine waves with a slight and equal offset. The real advantage to this constant path shifting is that the ISS overflies pretty much all of the Earth between 51.6 degrees latitude north and south. This is great for science experiments aboard the ISS that require Earth observation. It is also a boon for those of us stuck on the ground who want to catch a glimpse of this enormous machine.