Thursday, October 9, 2008

It's a Bird!

The days of the month surrounding Full Moon can sometimes feel like an astronomical “dead zone.” The floodlight effect of our lunar neighbor on the night sky makes everything from tracing out naked-eye star patterns to telescopic viewing of deep-sky objects difficult to impossible.

Certainly you can enjoy looking at the Moon itself. There are interesting craters to behold with your binoculars or telescope. Naked-eye moongazers with an extra dollop of imagination can look for pareidolia among the Moon’s dark maria or “seas.”

But here’s another diversion for those suffering from Full Moon fever: satellite watching. Many satellites are naked-eye targets, they’re generally bright enough to be seen on moonlit nights, and they travel in predictable orbits. What could be easier?

Image source: NASA

Satellites, colloquially known as “birds,” are man-made objects in orbit around Earth. Typically, they are unmanned communications, navigation, and observation stations. One notable unmanned craft is the Hubble Space Telescope, which regularly returns remarkable, detailed images of celestial objects, for scientific study and public enjoyment.

Some satellites are simply large pieces of space junk. These may include stations that are no longer operational but which continue to circle the planet until their orbits deteriorate, they enter Earth’s atmosphere, and they vaporize before impact (hopefully). Space junk may also include objects that have been jettisoned from manned craft.

Which brings us to the manned station circling the planet: the International Space Station (ISS). The ISS is a 16-country international collaboration: proof positive that human beings can work together for the advancement of the entire human race. This engineering marvel, with a resident multi-national crew on board, completes an Earth orbit every 90 minutes.

International Space Station
Image source: NASA

By the way, the word “satellite” is also used in astronomy as a synonym for “moon,” as in, “Saturn has 34 satellites.” The Moon is sometimes referred to as a natural satellite of Earth.

Image source: NASA

There are two terrific websites that can help you get started with man-made satellite watching. The first one I’ll mention, because it’s the easiest for beginners to use, is Satellite Tracker by Simply enter your zip code on the home page, click the “Go” button, and you’ll get a list of the best-and-brightest satellite flybys in your area over the next ten days. For each flyby, the rise time (when the satellite first appears over the horizon) and the transit time (when the satellite is at its highest in the sky) are both given in your local time. Nope, you won’t have to do any pesky conversions.

You’ll also be given the compass direction in which to look, as well as the predicted magnitude (brightness) of the satellite. You may recall from my previous discussion of the star Vega that objects with negative-number magnitudes are brighter than objects with positive-number magnitudes. However, you don’t need to remember that to use this website; the magnitude is also characterized for you as “very bright,” “visible,” or “dim.” Start with the satellites labeled “very bright,” as you’ll have the best chance of success with these. Once you have a few bright flybys under your belt, work your way up to the “visible” and “dim” objects.

Keep in mind that you’ll only see low-flying satellites such as the ISS (~250 miles above Earth) and the Hubble Space Telescope (~350 miles above Earth) shortly after sunset or shortly before sunrise. Satellites are visible to us only when they’re reflecting sunlight. A low-orbit satellite passing overhead in the dead of night would be in Earth’s shadow, with the Sun on the other side of the planet and no way to receive light from it.

Hubble Space Telescope
Image source: NASA

How can we tell a satellite from a plane? A satellite moves in a straight line and is usually visible for only a few minutes before winking out. A plane will typically be visible for longer, and you may see directional change. Also, a plane will usually have flashing lights and/or colored lights. If it’s close enough and the observing environment quiet enough, you’ll hear the plane’s engine(s). Satellites are silent. Several lights moving in formation are often military craft performing maneuvers.

The second website I’d like to recommend, for intermediate and advanced observers, is Heavens Above. On the home page, select your current observing site “from database” by selecting your country and typing in your town name. When it appears on the search results, click your town name to select it as your location. Then in the “Satellites” section, request your desired satellite predictions. This site also gives you pass results in local time, but it’s expressed in military time. I don’t know about you, but I don’t think in military time. Points taken off for making me do a pesky conversion!

Heavens Above gives prediction data for many more satellites than the Satellite Tracker site, and you can also request data for a specific satellite from their large database. Plus, the website offers much more than just satellite data. The “Astronomy” section has current data about visible comets and asteroids, and lots more.

On the evening of October 8, I took a last gulp of ice tea and hurried out of my local diner with four minutes to spare before a scheduled ISS pass. Soon, my partner, two other customers, a waitress, and a cook joined me in the parking lot to scan the northwestern horizon for the first appearance of the Space Station. Satellite spotting is a great activity to share with adults and kids, friends and perfect strangers.

We had a few false alarms as planes appeared over the horizon, heading for the international airport nearby. One of the customers was the first to whoop, “There it is!” Then we all spotted it, a steadily moving light already well above the horizon. The cook scurried back inside to flip some burgers and answer the phone, while the rest of us watched it climb and waved to the unseen crew inside.

Shortly after it reached the highest point of the pass, the reflected sunlight from the satellite became ruddy-colored. It was sunset on the ISS. From the astronauts’ vantage point as they rounded Earth, the Sun was sinking below Earth’s horizon. Then the satellite was swallowed by Earth’s shadow, and the light winked out.

Just think: if you were a sleepless astronaut on the ISS, over the course of 24 hours you could watch the Sun rise and set many times. Busy old fool, the poet called him.

…since thy duties be
To warm the world, that’s done in warming us.
Shine here to us, and thou art everywhere;
This bed thy center is, these walls thy sphere.
from The Sun Rising, by John Donne