Wind & Sky: Earth's Shadow

Updated January 6, 2025 (new content).

Long exposure night sky photography can capture the beauty of the cosmos, whether it depict stars, solar system objects, the Milky Way or deep sky objects such as galaxies and nebulae. Sometimes though, such images include streaks of light that were unexpected. In some cases, we’ve luckily captured a speck of dust streaking through Earth’s upper atmosphere, glowing as it burns up, what we know as meteors. But often that streak across the photo is something else. For example, what about this one?

Photo by Nathan Eaton

Well, this one is more of a smudge than a streak. A shot of Comet NeoWise (C/2020 F3), it's not likely to be mistaken for a meteor but I'm sharing it both because comets do trace a path across our night sky images but also as I was up at 5:00 a.m. this morning, out by the lake trying to capture a shot of Comet Nishimura (C/2023 P1) just as I am sure so many other people were. Also, of all the things I could think of that leave streaks of light across night sky images, comets seemed at least worth a mention. I'll leave UAPs for someone else to cover. 👽

Anyway, below I’ll share examples of other types of objects that may show up in night sky images and ways to tell when you’re seeing them.

First, a distinction to make. My focus here is on things that appear in night sky images taken with exposures at least a few seconds long. Obviously, when viewing the night sky directly, if you see a streak of light across the sky it’s typically going to be a meteor while the other objects I’ll discuss would appear to the eye as a relatively slowly moving point (or points) of light. It’s when these things are photographed over the length of seconds or minutes that they appear as a streak of light.

Meteors

A bright meteor passing below the Milky Way
Photo by Martha Dean via EarthSky.org

Although many streaks of light on long exposure images are taken for a meteor at first glance, there are a few key characteristics that can help confirm if that’s what you’re looking at. Generally, meteors in photographs (individually or from a meteor shower) usually:

appear wider in the middle narrowing to a point on each end,
may vary in color from end to end depending on the chemical elements released as the meteor burns up,
in some cases may have a wispy, smokey appearance along its length,
due to how fast they move, appear in just one frame of a sequence.

Of course there are exceptions. You might find that a streak that otherwise appears to meet the characteristics listed above for a meteor but has a blunt end. In this case, your exposure may have started or ended during a meteor mid-flight. Another possibility is when the meteor is a bolide or a fireball. In this case, the streak will grow much larger and brighter at one end. It may also last long enough to appear in more than one frame of a sequence.

Aircraft

Stack of 8 second exposures capturing DFW-area aircraft
Photo by Nathan Eaton

Of all the things that might be mistaken for a meteor in a night sky photo, aircraft are usually the easiest to exclude. We have a number of things going for us in identifying when that streak of light is an aircraft:

they most often have multiple lights so on close inspection the streak is actually multiple streaks,
the different lights on aircraft are different colors, usually a combination of white, red and green,
the navigation lights (red on left wing, green on right) are constantly on so will appear in a long exposure image as solid streaks,
the red beacon lights and white anti-collision strobe lights flash at different intervals so they will appear in a long exposure image as a series of dashes or dots,
if flying at high altitude where you're unable to distinguish separate streaks, there will still be dots as the navigation lights blink on and off in flight,
in image sequences, an aircraft will appear in multiple, consecutive images (though as in the image above, not always in a consistent direction).

The image above was taken with a wide-angle lens but aircraft (and other objects described here) may also appear in telescopic images which have a much narrower field of view. The image below was taken by photographer Bill Brownlie while imaging the Andromeda Galaxy (M31) with his Williams Optics GT71. As he reviewed the images, he found that in one the galaxy was photo-bombed by a commercial jet.

Photo by Bill Brownlie

The various aircraft lights are clearly visible:

Green navigation light on the right wingtip
Red navigation light on the left wingtip
Red beacon lights on the aircraft's centerline (top and bottom) that flash periodically - in this case the bottom beacon is illuminating the engine cowlings on each wing
White anti-collision strobes outboard of the navigation lights on each wing, appearing as intermittent white streaks

The position of the red and green navigation lights indicates the jet was flying from left to right across the frame.

Bill shared this image in a private Facebook group, Astrophotography. If you belong to the group or decide to join it, you can see his original post here.

Satellites

Starlink Satellites
Photo by Jason Parnell-Brookes

With the dramatic increase in satellites launched into Earth orbit in recent years (nearly 38% between early 2022 and mid-2023), capturing a satellite in a long-exposure photograph of the night sky is becoming more and more likely. Although they vary in speed, altitude and direction, they often appear as solid streaks of light having a consistent brightness and color front end to end and the ends appear blunt rather than tapered like a meteor. As with aircraft, in image sequences a satellite will typically appear in multiple, consecutive images (though in a consistent direction).

Satellites show up to us on the ground not because they have their own lights (like aircraft) but because they reflect sunlight. Sometimes, as a satellite passes over, the angle of its reflective surfaces may change, causing a brightening along part of the streak it leaves in your photograph.

Another characteristic of satellites: when they move into or out of the Earth's shadow, they no longer reflect sunlight. Instead of having a blunt end (caused by the abrupt start or end of your exposure), the end may taper. This can happen on either or both ends. Although this may look similar to the tapered ends of a meteor, a satellite trail will typically be longer, appear in multiple frames, and lack the distinctive color associated with meteors.

These days, that brightening is likely to be relatively minor. Prior to 2019, a constellation of 66 Iridium satellites were still in operation. Having highly reflective surfaces they commonly produced a streak of light in the sky called an Iridium flare, visible not just in photos but to viewers on the ground, much like a meteor. Often occurring shortly after sunset, they were bright enough to be visible even in the twilight sky. Since the last Iridium satellite was decommissioned in 2019 (burning up in the Earth’s atmosphere), there aren’t any other satellites that produce streaks of light across the sky quite so brightly.

That said, today there is a new type of satellite that may make a big appearance in your night sky images: Starlink. Although individual Starlink satellites would appear as any other satellite once on-station 342 miles up, two factors make Starlink more likely to show up:

sheer numbers - as of last month, there are over 5000 of them in low Earth orbit (out of a grand total of about 12,000 total satellites in orbit, active or inactive),
the number launched at once, usually about 20 or so at a time - within the days after a launch and before they achieve full orbit, all of a group of Starlink satellites launched together are flying in a line, appearing in the sky as a series of dots, often referred to as a “train”.

These same points are also part of the reason Starlink is of particular concern for astronomers as are other new satellites now being deployed such as BlueWalker 3.

The good news is that there are tools that can help identify whether that streak in your photo is a satellite. My favorite is Stellarium (available as free open source software for Windows, Mac and Linux or as paid software for iOS, Android and Symbian). By setting the location where your shot was taken and the date/time of the image in Stellarium, you can zoom in on the area where the streak appears in your image and increase the time rate then watch for what crosses the same section of the sky. Sometimes it’s difficult to find what it was but I can usually confirm there was a satellite and which one it was. Another useful tool is Heavens Above.

While those tools are great for looking back in time at when/where a satellite was visible or for looking ahead at when they might be visible, another tool you can try if your goal is finding when you can see a Starlink satellite, give the SpaceX Starlink Satellites Tracker a try. Thanks to Stephen Buck for his comment on the Facebook group Astrophotography for Beginners for that link.

Space Stations

Stack of 8 second exposures of ISS fly-over
Photo by Nathan Eaton

For the most part, craft like the International Space Station (ISS) would appear in a long exposure night sky image as a long streak similar to any other satellite. In the case of the ISS and eventually the China space station Tiangong, given their large size relative to other satellites they can sometimes appear brighter and result in a wider streak across your photo. Similar to other satellites, as a space station passes overhead it may pass into the Earth’s shadow before reaching the horizon. When this is captured in an image such as the one above, it may appear to fade out, perhaps even with a tapered appearance, rather than the light streak having a blunt end representative of your camera’s shutter closing.

Another similarity space stations have to other satellites is that their orbits are well understood. This makes their appearance predictable. There are many tools available to forecast these appearances including NASA's Spot the Station. One of my favorites is Heavens Above. With it, you can see when various satellites and space stations are expected to be visible from your location.

Variation on a Theme: Solar and Lunar Transits

ISS Transiting the Moon
Photo by Andrew McCarthy

If having the ability to tell where satellites and space stations are in the sky piques your interest beyond just figuring out what that light streak across your long exposure night sky photo is, if you ever take solar or lunar photos, then another tool you should check out is Transit Finder. With it, you can identify when the ISS, the Hubble Space Telescope (HST) and other large craft in space will fly across the face of the Sun or Moon, giving you a chance to try your hand catching them in a photograph. Follow the link of the image above for information on what's involved.

Last month, my granddaughter Harper and I went out to capture an image of the International Space Station (ISS) flying over Dallas. As described in my post about that effort, we were only partially successful, foiled by clouds. This past Tuesday evening, granddaughter Olivia and I headed up to the north shore of White Rock Lake to try again. In spite of being a little late getting to the lake and rushing to set up cameras, we managed to catch the last 90 seconds or so of the ISS flyover as it traveled from the northwest, overhead and down to the southeast. Here is the result.

This is a composite image created by stacking 9 images taken with a Nikon D750 and a Rokinon 14mm wide angle lens mounted on a tripod. Each shot was 8 seconds long with 2 seconds between each shot. What Olivia and I saw was an incredibly bright "star" moving across the sky, too steady and too fast to be an airplane. It appears as a streak in this image due to the long exposure of each original image; the gaps are due to the delay between each shot while the camera saved the image files.

This photo demonstrates an interesting aspect of ISS flyovers. We can see the ISS fly by only at certain times and places, its visibility due to the viewer being in a position to see sunlight reflected off of the ISS. For this flyover, as it was traveling "away" from the sun (which had set just over an hour earlier) its orbit carried it into the Earth's shadow. Notice how the lowest trail fades off and disappears.

Another interesting object in the sky that night was the Sturgeon Moon, the first of two "supermoons" this month. It was cloaked in clouds until just as we were getting in the car to head home so it just appears in this image as a bright glow to the lower left. Note all the people on the dock though; there was quite a crowd out that night with people all up and down the shoreline to watch the full moon rising. I wonder how many of them also realized they were getting a chance to see the ISS, too.

We also tried to capture a flyover of a satellite, BlueWalker 3, which passed over this same view looking south across the lake about a half-hour before the ISS flyover. Unfortunately, even having two cameras shooting during its flyover, we didn't capture it. I'm not sure whether it was due to its lower elevation, the haze to the south east or a miscalculation on my part about where and when we would see it. That'll be a target to try for another night.

Thanks to the long exposures taken, the composite includes more than just the ISS and the Moon. Stars of the constellation Sagittarius (the Archer) and Scorpio (the Scorpion) are visible including Antares, the red super-giant star that is known as the Heart of the Scorpion. Here is a labeled version of the image above.

The images above were edited to remove all the other stuff we saw in the sky that night: aircraft. The following is a composite of 12 images stacked "as is" in Photoshop. It shows not only the ISS but all the aircraft trails captured. Having simply stacked the images, stars appear as streaks and headlights and taillights of cars on the far shore of the lake appear as streaks.

One final image, this time an edited stack of all the images, showing not only the ISS but all the aircraft trails captured. Unlike the unedited stack, in this view the sky (Moon and stars), lake and even the car headlights on the far shore are all from a single image. The aircraft and ISS trails from the other 11 images were masked to overlay that single image.