How to See the Space Station Fly in Front of the Moon

A beautiful ISS transit on June 19 2015 recorded at Biscarrosse, France. Credit: David Duarte
What strange creature is this flitting across the Moon? Several members of the European Space Agency’s Astronomy Center captured these views of the International Space Station near Madrid, Spain on January 14 as it flew or transited in front of the full moon. Credit: Michel Breitfellner, Manuel Castillo, Abel de Burgos and Miguel Perez Ayucar / ESA

One-one thou… That’s how long it takes for the International Space Station, traveling at over 17,000 mph (27,300 kph), to cross the face of the Full Moon. Only about a half second! To see it with your own eyes, you need to know exactly when and where to look. Full Moon is best, since it’s the biggest the moon can appear, but anything from a half-moon up and up will do.

The photo above was made by superimposing 13 separate images of the ISS passing in front of the Moon into one. Once the team knew when the pass would happen, they used a digital camera to fire a burst of exposures, capturing multiple moments of the silhouetted spacecraft.

The ISS transits the Full Moon in May 2016

The ISS is the largest structure in orbit, spanning the size of a football field, but at 250 miles (400 km) altitude, it only appears as big as a modest lunar crater. While taking a photo sequence demands careful planning, seeing a pass is bit easier. As you’d suspect, the chances of the space station lining up exactly with a small target like the Moon from any particular location is small. But the ISS Transit Finder makes the job simple.

This is a screen grab from the homepage of Bartosz Wojczy?ski’s most useful ISS Transit Finder. Credit: Bartosz Wojczy?ski

Click on the link and fill in your local latitude, longitude and altitude or select from the Google maps link shown. You can always find your precise latitude and longitude at NASA’s Latitude/Longitude Finder  and altitude at Google Maps Find Altitude. Next, set the time span of your Moon transit search (up to one month from the current date) and then how far you’re willing to drive to see the ISS fly in front of the Moon.

When you click Calculate, you’ll get a list of events with little diagrams showing where the ISS will pass in relation to the Moon and sun (yes, the calculator also does solar disk crossings!) from your location. Notice that most of the passes will be near misses. However, if you click on the Show on Map link, you’ll get a ground track of exactly where you will need to travel to see it squarely cross Moon or Sun. Times shown are your local time, not Universal or UT.

A beautiful ISS transit on June 19 2015 recorded at Biscarrosse, France. The photographer used CalSky, another excellent satellite site, to prepare a week in advance of the event. This composite image was made with a Canon EOS 60D. Notice how bright the space station appears against the moon due to the lower-angled lighting across the lunar landscape at crescent phase compared to full, when the ISS appears in silhouette. Credit: David Duarte

The map also includes Recalculate for this location link. Clicking that will show you a sketch of the ISS’ predicted path across the Moon from the centerline location along with other details. I checked my city, and while there are no lunar transits for the next month, there’s a very nice solar one visible just a few miles from my home on Feb. 8. Remember to use a safe solar filter if you plan on viewing one of these!

The ISS transits the Sun on May 3, 2016. Click for details on how the photo was taken. Credit: Szabolcs Nagy

While you might attempt to see a transit of the ISS in binoculars, your best bet is with a telescope. Nothing fancy required, just about any size will do so long as it magnifies at least 30x to 40x. Timing is crucial. Like an occultation, when the moon hides a background star in an instant, you want to be on time and 100% present.

Make sure you’re set up and focused on the moon or sun (with filter) at least 5 minutes beforehand. Keep your cellphone handy. I’ve found the time displayed at least on my phone to be accurate. One minute before the anticipated transit, glue your eye to the eyepiece, relax and wait for the flyby. Expect something like a bird in silhouette to make a swift dash across the moon’s face. The video above will help you anticipate what to expect.

The next lunar transit nearest my home is an hour and a half away in the small town of Biwabik, Minn. according to the ISS Transit Finder. On Jan. 30 at 8:00:08 p.m local time, the ISS will cross the crescent moon from there. Once you know the time of the prediction and the exact latitude and longitude of the location (all information shown in the info box on the map using the ISS Transit Finder), you can turn on the satellites feature in the free Stellarium program (, select the ISS and create a simulated, detailed path. Created with Stellarium

Even if you never go to the trouble of identifying a “direct hit”, you can still use the transit finder to compile a list of cool lunar close approaches that would make for great photos with just a camera and tripod.

The Transit Finder isn’t the only way to predict ISS flybys. Some observers also use the excellent satellite site, CalSky. Once you tell it your location, select the Lunar/Solar Disk Crossings and Occultations link for lots of information including times, diagrams of crossings, ground tracks and more.

I use Stellarium (above) to make nifty simulated paths and show me where the Moon will be in the sky at the time of the transit. When you’ve downloaded the free program, get the latest satellite orbital elements this way:

* Move you cursor to the lower left of the window and select the Configuration box
* Click the Plugins tab and scroll down to Satellites and click Configure and then Update
Hover the cursor at the bottom of the screen for a visual menu. Slide over to the satellite icon and click it once for Satellite hints. The ISS will now be active.
* Set the clock and location (lower left again) for the precise time and location, then do a search for the Moon, and you’ll see the ISS path.

There you have it — lots of options. Or you can simply use the Transit Finder and call it a day! I hope you’ll soon be in the right place at the right time to see the space station pass in front of the Moon. Checking my usual haunts, I see that the space station will be returning next weekend (Jan. 27) to begin an approximately 3-week run of easily viewable evening passes.

How to Spot and Track Satellites

A 10 second exposure of a bright pass of the International Space Station. (Photo by Author).

It’s a question we get all the time.

Watch the sky closely in the dawn or dusk hours, and you’ll likely see a moving “star” or two sliding by. These are satellites, or  “artificial moons” placed in low Earth orbit. These shine via reflected sunlight as they pass hundreds of kilometres overhead.

Many folks are unaware that you can see satellites with the naked eye. I always make an effort  to watch for these during public star parties and point them out. A bright pass of the International Space Station if often as memorable as anything that can be seen through the eyepiece. But after this revelation, “the question” soon follows- “What satellite is that?”

Welcome to the wonderful and highly addictive world of satellite tracking. Ground observers have been watching the skies since Sputnik 1 and the first satellite launch in October 1957. Armies of dedicated volunteers even participated in tracking the early launches of the Space Age with Operation Moonwatch.

Depiction of the apparent motion of a typical satellite overhead with respect to the observer. (Graphic created by author).
Depiction of the apparent motion of a typical satellite overhead with respect to the observer. (Graphic created by author).

The Internet has offered a wealth of information for satellite hunters. Every time I write about “how to spot the ISS,” someone amazes me with yet another new tracker App that I hadn’t heard of. One of my favorites is still Heavens-Above. It’s strange to think that we’ve been visiting this outstanding website daily for a decade and a half now. Heavens-Above specializes in satellites, and will show you a quick listing of passes for brighter satellites once configured with your location. A nifty “quick check” for possibly resolving a mystery satellite is their link for “Daily Predictions for brighter satellites” Which will generate a list of visible passes by time.

Screenshot of a typical list of bright satellite passes from Heavens-Above.
Screenshot of a typical list of bright satellite passes from Heavens-Above filtered by brightness, time and location .

Looking at the time, direction, and brightness of a pass is crucial to satellite identification. No equipment is needed to start the hunt for satellites tonight, just a working set of eyes and information. We sometimes use a set of Canon image-stabilized 15x 45 binoculars to hunt for satellites too faint to see with the naked eye. We’ve seen the “Tool Bag” lost during an ISS EVA a few years back, as well as such “living relics” of the early Space Age as Canada’s first satellite Alloutte-1, and the Vanguards (Yes, they’re STILL up there!) using binocs.

A comparision of typical satellite orbits. (Credit
A comparison of typical satellite orbits. (Credit: Cmglee, Geo Swan graphic under a Creative Commons Attribution -Share Alike 3.0 unported license).

The trick to catching fainter satellites such as these is to “ambush” them. You’ll need to note the precise time that the selected satellite is going to pass near a bright star. Clicking on a selected satellite pass in Heavens-Above will give you a local sky chart with a time-marked path. I use a short wave portable AM radio tuned to WWV out of Fort Collins, Colorado for an accurate audible time signal. Just sit back, listen to the radio call out the time, and watch for the satellite to pass through the field of view near the target star.

Another great site for more advanced trackers is CALSky. Like Heavens-Above, CALSky will give you a customized list for satellite passes over your location. One cool extra feature on CALSky is the ability to set alerts for passes of the ISS near bright planets or transiting the Sun or Moon. These are difficult events to capture, but worth it!

The International Space Station transiting the Moon as captured by Mike Weasner from Cassiopeia Observatory in Arizona.
The International Space Station transiting the Moon as captured by Mike Weasner from Cassiopeia Observatory in Arizona.

A great deal of what’s up there is space junk in the form of discarded hardware. Many satellites are on looping elliptical orbits, only visible to the naked eye when they are near perigee. Many satellites are located out at geosynchronous or geostationary orbits 35,786 kilometres distant and are invisible to the naked eye all together. These will often show up as streaks in astrophotos. An area notorious for geosynchronous satellites exists near the direction of M42 or Orion Nebula. During certain times of year, satellites can be seen nearby, nodding slowly north to south and back again. Around the March and September equinox seasons, geostationary satellites can be eclipsed by the shadow of the Earth. This can also cause communications difficulties, as many geo-sats also lie sunward as seen from the Earth around these times of year.

Probably one of the simplest satellite trackers for casual users is Space Weather’s Satellite Flybys page. North American users simply need to enter a postal code (worldwide users can track satellites via entering “country-state-city”) and a list of passes for your location is generated.

It’s a basic truism of satellite tracking that “aircraft blink; satellites don’t”. Know, we’re going to present an exception to this rule.

Some satellites will flash rhythmically due to a tumbling motion. This can be pretty dramatic to see. What you’re seeing is an expended booster, a cylinder tumbling due to atmospheric drag end-over-end. Some satellites can flash or flare briefly due to sunlight glinting off of reflective surfaces just right. Hubble, the ISS and the late NanoSail D2 can flare if conditions are just right.

The most dramatic of these are Iridium flares. The Iridium constellation consists of 66 active satellites used for satellite phone coverage in low-Earth orbit. When one of their three refrigerator-sized  antennas catch the Sun just right, they can flare up to magnitude -8, or 40 times brighter than Venus. CALSky and Heavens-Above will also predict these events for your location.

Didn’t see a predicted satellite pass? Light pollution or bright twilight skies might be to blame. Keep in mind, passes lower to the horizon also fall prey to atmospheric extinction, as you’re looking through a thicker layer of the air than straight overhead.  Some satellites such as the ISS or the USAF’s X-37B spy space plane even periodically boost or modify their orbits, throwing online prediction platforms off for a time.

More advanced satellite trackers will want to check out Celestrak and SAT-Flare Tracker 3D.

A screenshot example of TLE's for the ISS & Tiangong-1 from Celestrak.
A screenshot example of TLE’s for the ISS & Tiangong-1 from Celestrak.

I use a free tracking platform created by Sebastian Stoff known as Orbitron. Orbitron lets you set your observing location and tailor your view for what’s currently over head. You can run simulations and even filter for “visual only” passes, another plus. I also like Orbitron’s ability to run as a stand-alone system in the field, sans Internet connection. Just remember, for it to work properly, you’ll need to periodically update the .txt file containing the Two-Line Element (TLE) sets. TLE’s are data element sets that describe the orbital elements of a satellite. Cut and paste TLEs are available from Heavens-Above and Celestrak.

Orbitron screenshot for visible satellites using 'radar' mode... there's lots up there! (Credit: Orbitron).
Orbitron screenshot for visible satellites using ‘radar’ mode… there’s lots up there! (Credit: Orbitron).

For serious users, NORAD’s Space-Track is the best site for up-to-date TLEs.  Space-Track requires a login and user agreement to access, but is available to satellite spotters and educators as a valuable resource. Space-Track also hosts a table of upcoming reentries, as does the Aerospace Corporation’s Center for Orbital & Reentry Debris Studies.

The SeeSat-L mailing list is also an excellent source of discussion among satellite trackers worldwide. Increasingly, this discussion is also moving over to Twitter, which is ideal for following swiftly evolving  action in orbit. @Twisst, created by Jaap Meijers,will even Tweet you prior to an ISS pass!

And there’s always something new or strange in the sky for the observant. Satellites such as those used in the Naval Ocean Surveillance System (NOSS) were launched in groups, and are eerie to watch as they move in formations of 2 or 3 across the sky. These are difficult to catch, and all three of our sightings thus far of a NOSS pair have been surreptitious. And we’ve only had the camera ready to swing into action once to nab a NOSS pair;

A NOSS pair captured by the author. The multi-colored trail bisecting the path is an aircraft. Note a bit of "jitter" at the beginning of the exposure- I had to swing the camera into action quickly!
A NOSS pair captured by the author. The multi-colored trail to the left of the path is an aircraft. Note a bit of “jitter” at the beginning of the exposure- I had to swing the camera into action quickly!

Another bizarre satellite to catch in action is known as the Cloud-Aerosol LiDAR & Infrared Pathfinder Satellite for Observations, or CALIPSO. Part of the “afternoon A-Train” of sun-synchronous Earth observing satellites, you can catch the green LiDAR flashes of CALIPSO from the ground with careful planning, just as Gregg Hendry did in 2008-2009:

A CALIPSO LIDAR pass imaged by Gregg Hendry in 2008. My Hendry mentions that, "The hollow nature of the spots is likely due to some spherical aberration in the camera lens coupled with imperfect focus and is not representative of the laser beam's optical quality."
A CALIPSO LiDAR pass imaged by Gregg Hendry in 2008. My Hendry mentions that, “The hollow nature of the spots is likely due to some spherical aberration in the camera lens coupled with imperfect focus, and is not representative of the laser beam’s optical quality.” (Credit: Gregg Hendry, used with permission).

NASA even publishes a prediction table for CALIPSO lidar passes. I wonder how many UFO sightings CALIPSO has generated?

Artist's depiction of the A-Train constellation of Earth-Observing satellites. (Credit: NASA).
Artist’s depiction of the A-Train constellation of Earth-Observing satellites. (Credit: NASA).

And speaking of photography, it’s easy to catch a bright pass such as the ISS on camera. Shooting a satellite pass with a wide field is similar to shooting star trails; just leave the shutter open for 10-60 seconds with a tripod mounted camera. Modern DSLRs allow you to do several test exposures prior to the pass, to get the ISO, f/stop, and shutter speed calibrated to local sky conditions.

You can even image the ISS through a telescope. Several sophisticated rigs exist to accurately track and image the space station through a scope, or you could use our decidedly low-tech but effective hand-guided method;

And that’s a brief overview of the exciting world of sat-spotting… let us know of your tales of triumph and tragedy as you sleuth out what’s going on overhead!