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Binocular Universe: Project Moonwatch 2015

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Binocular Universe: Project Moonwatch 2015

May 2015

Phil Harrington


Ten years ago, I gave a talk at an astronomy convention in western PA called Astroblast. Astroblast is hosted each year by the Oil Region Astronomical Society, based in Franklin. One of the other speakers that year was Eric Fischer from Allison Park, PA, who gave a fascinating presentation entitled "If Sherlock Holmes Observed Artificial Satellites."

Earth-orbiting satellites pass overhead every night, but few of us pay them much mind apart from some idle curiosity. In his presentation, Fischer showed how you can deduce what kind of satellite you’re seeing by the power of observation. I found it intriguing.

Of course, satellite-watching is nothing new for amateur astronomers. Half a century ago, more than 5,000 amateurs around the world banded together to conduct “Project Moonwatch.” Their mission, in those days before a global satellite-tracking camera network, was to spot and record the passages of the earliest satellites across the sky. Observations were then compiled and used to calculate the precise orbit of each satellite.


Here is a vintage Moonwatch telescope cobbled from an old pair of 7x50 binoculars. Moonwatch telescopes were often mounted on adjustable stands aimed down toward a flat mirror, the thought being that looking down actually made looking up easier!  Photos from Neale Howard's Handbook for Observing the Satellites published in 1958.


Project Moonwatch has long been superseded by the North American Aerospace Defense Command’s sophisticated network that monitors the whereabouts of more than 9,000 satellites circling Earth. So, while the scientific usefulness of observing satellites has left the realm of the amateur astronomer, the fun and excitement of spotting them has not.

Then, as now, the best time to view satellites is during and immediately after sunset or just before dawn. Either way, the observer is in darkness, but satellites, orbiting far above, are still in sunlight. The high angle of the Sun in the sky during the summer makes this the best season for viewing artificial satellites. Depending on its altitude, a satellite may still be illuminated by sunlight several hours after sunset.

Although there are several on-line resources for today’s satellite hunters to positively identify a sighting, and even predict when a specific target will pass through the local sky, Fischer’s talk emphasized that it’s much more fun to determine that for yourself based on a mystery object’s appearance. So, let’s put on our detective caps and investigate a few sightings.

First, you need to know where to look. Although they can appear in just about any direction, in general, you’ll do best to face away from the Sun. That means that during the evening, you want to focus on the eastern half of the sky, and the western half in the early morning. Concentrate 30° or more above the horizon to avoid haze and clouds. And get comfortable; a lounge chair tilted back is perfect for satellite sweeping.

You’re probably going to pick several imposters along the way. These may be anything from bright stars like Vega, rising in the northeast in the late evening this month, and airplanes. If you aren’t sure whether you’re seeing a satellite or some other phenomenon, keep in mind that satellites move, but never have landing lights!

Once you spot a real satellite, and there are probably going to be at least a dozen bona fide candidates every clear night at this time of year, take note of its direction of travel, speed, brightness, and light curve.

If a satellite is moving west to east, it is likely related to astronomical studies, such as the Hubble Space Telescope. A satellite following a path more southwest-to-northeast or northwest-to-southeast is probably a manned mission, such as International Space Station. The ISS can shine as brightly as magnitude -2, making it tough to miss. 


The International Space Station flies through Orion and over the Westport Astronomical Society's Rolnick Observatory in Westport, CT.  Photo © 2015 by Dan Wright.  Used with permission. 


A north-to-south or south-to-north path probably means the satellite is related to meteorological or climatological studies. Finally, if a satellite is following a retrograde, or east-to-west, path, it may be a reconnaissance, or spy, satellite. Shhh!

The speed of the passage will give you some idea of its orbital altitude. The higher above Earth’s surface, the longer a satellite will take to traverse the sky. Tied into this is an object’s magnitude. If a satellite appears very bright, but moves very slowly, then it is probably quite large and in a high orbit. An equally bright satellite that moves quite quickly, however, is probably comparatively small.

Lastly, is it shining steadily or does it flicker? If it is shining steadily, then it is probably a functioning satellite. But if it flickers, then it is most likely tumbling. That could be because of sunlight glinting off solar panels, or the sign of a spent rocket stage.

A fun sight to hunt for is a formation of Naval Ocean Surveillance System (NOSS) satellites. Here, we see not one, but two or even three satellites moving in formation. These throwbacks to the Cold War were originally launched to track movements of Soviet ships by locating and locking onto radio transmissions. Three generations of these satellites were launched over the span of three decades. Eight NOSS triplets were orbited between 1976 and 1987, followed by three triplets between 1990 and 1996, pairs of satellites in September 2001, December 2003, February 2005, June 2007, April 2011, and September 2012. Published orbits are no longer available, but they have been tracked ever since by amateur satellite spotters.

And of course, spotting an “iridium flare” is always exciting. Born in the 1990s, the original concept behind iridium satellites was to provide voice and data coverage to satellite phones and pagers around the world. In all, 66 satellites make up the "Iridium constellation." The concept actually called for 77 satellites -- the atomic number of iridium, hence the name. But in the end, 66 were launched into earth orbit at a variety of angles to ensure global coverage.

Each orbiting Iridium satellite has three large reflective panels. Normally, the satellites appear just above the naked-eye limit. But when sunlight strikes one of the reflective antennas at just the right angle, an observer on the ground will see short-lived flare that can be as bright as magnitude -8. That’s more than two dozen times brighter than Venus at its best and easily visible in broad daylight!

But you have to be in the right place at the right time. Fortunately, we can do that thanks to the Internet. The best resource for predicting when low earth-orbiting satellites will be visible from a given location, including Iridium flares, is Heavens Above. By inputting your location, you can retrieve customized predictions of what satellites will be visible over the next several nights.

If you get bitten by the satellite bug as I was, be sure to visit SeeSat-L mailing list for visual satellite observers and SatBuster for more tips and tricks.  Howard's Handbook for Observing the Satellites is also a fascinating read, especially since it was written at the dawn of the space age when successfully launching any satellite into Earth orbit was front page news.  Although it's not as popular as Howard's better known Telescope Handbook and Star Atlas or his Standard Handbook for Telescope Making, copies can be found through on-line book dealers.

Finally, as Fischer emphasized in his talk, “deductive observation is only intended to sharpen your powers of observation, not make definitive identifications.  What looks like a tumbling rocket stage could very well be something else.”  That’s exactly what makes this so much fun.

Till next month, remember that, whether searching for objects both near and far, two eyes are better than one.


About the Author:

Phil Harrington has written 9 books on astronomy, including Star Ware, Star Watch, and Touring the Universe through Binoculars.  Visit his web site, www.philharrington.net, for more information.


  • okiestarman56 and John O'Hara like this


John O'Hara
May 01 2015 08:24 PM

Hi Phil,


Thanks for the mentioning the Astroblast as the party where you heard this! 


Great article.


John O'Hara

    • PhilH likes this
Dave Mitsky
May 02 2015 11:47 AM

That was another interesting edition of Binocular Universe, Phil.  I recently caught a pass of the mysterious Chinese satellite, if that is indeed what it is, known as Object C thanks to Heavens Above.




Dave Mitsky

    • PhilH likes this

I once spotted a geosynchronous satellite using a telescope with no drive.  All the stars drifted through the field except one (the satellite).

    • PhilH likes this

Incidentally, I should have also mentioned the Spaceweather app for satellite passes, found here: http://spaceweather.com/flybys/  Very handy to have on your phone!

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