Dozens of satellites are busy day and night, beaming your favorite TV and radio programs from more than 35,000 kilometers away. Here's how to tune into them.
Last week, I pointed my camera at a random spot in Aquarius along the the geosynchronous satellite belt, made a 3-minute exposure at f/2.8 and ISO 400, and then pressed the back screen button to check for anything interesting. Wow, wow, wow. There they were — not one, not two, but 11 pinpoint satellites in a row! I did the same a couple nights later, and although the stars had shifted to the west, the satellites stood in exactly the same place. I was hooked.
From Earth, a satellite in geosychronous orbit appears to hover over one spot of the equator, matching Earth's rotation rate. To a ground observer, they appear almost motionless, but they're zipping along at 11,300 kph (7,000 mph) to keep up with Earth's spin. As of August 10, 2017, there were 447 active geosynchronous satellites in a ring around the planet with space remaining for about 1,400 more.
Each satellite occupies a orbital slot far enough away from the others to avoid communication interference or a potential collision. Geostationary satellites remain exactly at one spot above the equator, with their positions maintained by thruster burns. Geosynchronous satellites' orbits are slightly inclined and describe a north-south-inclined figure-8 or analemma during the course of the day. Anomalies in Earth's gravitational field combined with the tug of the Moon cause all geosats to drift unless repositioned by thrusters. Satellite controllers make sure that any drift doesn't extend outside the bounds of the predetermined "box" within a slot.
Most geosats are used for communications. A dish on the ground can grab a TV transmission by simply locking onto a geosat instead of having to track a satellite across the sky. Thanks to these distant birds, our favorite TV shows and other communications can be transmitted around the planet. Geosynchronous orbit also makes a perfect haunt for spy satellites. The military can park over the point of interest and monitor transmissions 24 hours a day as well as track enemy missiles. The nightly weather photos, the same we check on our mobile phones when planning an observing session, are sent every 15 minutes by the geostationary satellites, GOES-East and GOES-West, positioned at 75° west and 135° west longitude respectively. Together, they provide complete views of both hemispheres of Earth.
Many geosynchronous satellites shine between magnitudes 10–12, so you can spot them in telescopes as small as 4 inches. They're also easy to photograph. High ISOs and fast, low light lenses aren't necessary, just a camera capable of a several-minute-long time exposure — long enough for the stars to trail, so you can easily tell them apart from the satellites. Set your shutter speed to "B" and ISO at 400. You can hold the shutter button down with your finger, but a shutter release cable is much better and vibration-free. Use a 100–200-mm telephoto lens, focus sharply, and expose for 2–4 minutes. When you enlarge the image, you'll should see long trails and a line of pinpoint dots — satellites!
Of course, you'll need to know the location of the geosat belt from your latitude. Northern hemisphere skywatchers see the belt several degrees south of the celestial equator because of parallax. For instance, from my latitude of 47°N, the belt arcs across the southern sky at declination –7°, or about 36° high at the meridian. The table below will help you know where to look for your latitude; reverse the sign if you're observing from the southern hemisphere.
When I'm in a geosynchronous-satellite-watching mood, I open up a sky-mapping program like Stellarium and have it draw the celestial equator. Then I click below the equator along an arc of sky at declination –7°, looking for brighter stars and easy deep-sky objects. Outside at the telescope, I center that star or object in a low magnification (64×) field of view and wait. Be sure to turn off your clock drive or tracker for the best effect. What you'll see next is one of the best illusions in the night sky.
Assuming geosats are in the field of view, they'll appear at first to drift to the east, until it dawns on you that no, the satellites are fixed and the stars are doing the moving, dragged across the field by Earth's rotation. Your instinct is to push the scope along to follow the satellites, but no need. They're not going anywhere. The other night I centered my 10-inch on a spot near Theta (θ) Aquarii and within a minute swept up four geosats between magnitude 10–11 in a compact group about 12′ across.
To identify the satellites you see or photograph, go to CalSky, let it find your location (automatic), then under the topic headings, click Satellites and then the Geostationary link. You'll be shown a list of geosats, with their magnitudes and positions (in R.A. and Dec.), that are visible from your site that night. Click the Star Chart link under the geosat's name for a map showing its position on a star map. Star names and data appear when you place your cursor over the star symbols.
To see pictures and learn more the purpose and other details of each satellite, stop by Gunter's Space Page, a fantastic resource. Hint: the search button is down to the lower left of the page.
Unlike the ISS and the many objects in low Earth object, geostationary satellites are visible all night long every night of the year. They only disappear for up to 70 minutes a day when entering Earth's shadow about two weeks either side of each equinox. At that time, their orientation in relation to the Sun (which is directly at your back) favors flaring.
If you make a point to look for geosats shortly before they enter or shortly after they leave the shadow, you might catch a flare, some of which are bright enough to see in binoculars and rarely, with the naked eye.
Listen to Sirius Radio? Get your TV via satellite? It's just cool to see where it all comes from — outer space. For more on satellites, please see my companion blog on low-Earth orbiting satellites published last month.