Countdown to Annular Eclipse

Sequence from 2012 Annular Solar Eclipse
Photo by Nathan Eaton

We are just a month away from the next solar eclipse! On October 14th, 2023, people from the upper west coast down across Texas and in parts of Central and South America will have the opportunity to see an annular solar eclipse. As with all solar eclipses, this one involves the Moon passing between the Earth and the Sun. This time, the Moon will be farther out in its orbit around the Earth, making its angular size just a bit smaller than that of the Sun. In such a case, the Moon doesn't quite cover the face of the Sun. Consequently, at mid-eclipse viewers along the centerline of the eclipse track will get to see the Ring of Fire, a view in which the Moon is centered over the Sun with a fraction of the solar surface showing all round the Moon. 

The image sequence above is one I took during an annular eclipse in 2012 when we were visiting Albuquerque, New Mexico. Shot with a Nikon D7000 DSLR through a telephoto lens and a Baader film white light filter, it shows the eclipse from near the start at top left as the Moon has begun to cover the Sun, progressing across and down to the Ring of Fire view at bottom left then on through near the eclipse end at bottom right. For that eclipse, the Sun set before the eclipse was over so I wasn't able to get any shots of the final phase. The reason for the yellow color in that last shot is partly because the Sun was close to sunset and partly due to heavy smoke in the atmosphere from fires west of Albuquerque. Basically, the image came out yellow for the same reason a sunset with heavy clouds to the west looks yellowish to reddish; the denser atmosphere with heavy particulates (in this case, smoke) filters out shorter wavelengths, only letting the longer (and redder) wavelengths through. 

To see how close the path of the eclipse comes to where you live and to see where and when you can view it in person, check out Xavier Jubier's interactive map for the eclipse. Click anywhere on the map and it will give you details of the eclipse for that location including when it starts and ends, how long it lasts and how much of the Sun will be eclipsed by the Moon. Note that the path of the eclipse on the map represents the areas where the annular form of the eclipse will be visible. In the Dallas area, we'll experience a partial eclipse since the Moon never moves completely over the face of the Sun from our vantage point. Unfortunately, we won't get to see the Ring of Fire here. 

Where will you be during the eclipse? 

See the NASA eclipse page for more information on this eclipse as well as the total solar eclipse next April. 

WARNING

Be sure to NEVER look directly at the sun without appropriate eye protection. Read up on safe viewing techniques before viewing the eclipse. 

Demystifying Light Streaks in Night Sky Images: Meteor, Plane, Satellite, or Space Station?

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. 

Geek Break: Calculating Elevation of Jet Transiting the Sun

Brandon Ghany on Flickr

After my last post about capturing a jet flying in front of the Sun, I saw this image Brandon Ghany posted on Flickr of a jet crossing in front of the Moon and what caught my attention is how much smaller the jet is than in my video. 

From Brandon's description, this jet was flying at about 12,500 feet. Clearly, the jet I caught was flying much lower. But how low? I was curious enough to try figuring it out. 

The first factor to consider is that the Sun and Moon are similar but not exactly the same size from our viewpoint. Actually, the distance to the Moon varies a little at different points in its orbit so sometimes it is closer and thus a bit larger than the Sun (necessary to have a total solar eclipse) and sometimes it is farther away and thus a bit smaller than the Sun (leading to views like the Ring of Fire of an annular eclipse such as the one on October 14th this year). But in general terms, they are both roughly a half a degree across viewed from Earth. That means if Brandon had captured a jet at 12,500 feet passing in front of the Sun instead of the Moon, his image would have looked similar, at least in terms of the relationship between the size of the jet and the size of the celestial orb it passed in front of. 

The second factor is that while I know the angular size of the sun (reported by Stellarium as 31.68 arc minutes at the time of my video), for me to determine things like the jet's altitude, I need to know the jet's angular dimension, too. 

To calculate this, I captured a single frame from my video, cropped and rotated it then used drafting software (FreeCAD) to measure the relative difference between the width of the jet's fuselage and the width of the Sun. The ratio came out to 55.25% which means the width of the jet body is 17.5 arc minutes. 

So, how did you do in high school or college trig class? I muddled through but that was also so long ago, I just don't recall the exact formula to use. Thanks to Google, though, it was pretty easy to find what I needed.

d = ( w / θ ) * 60

Simple, right? Well, except I'm missing one variable: w, or width. Google to the rescue again. If I assume that this jet was something like a Boing 737 or an Airbus 320, both pretty common models used by airline companies flying out of DFW and Love Field, then the width of the fuselage is approximately 13 feet. Plugging that into the formula as w and converting arc seconds to degrees and using that as θ, that tells me that the jet was approximately 2,674 feet away when it passed over our neighborhood. 

But was that how high it was flying? Not exactly since it wasn't directly over my backyard. Again checking Stellarium, I found that the Sun's altitude (its angle above the horizon) was about 62.65 degrees.

Dusting off more trigonometry, that gives me two variables of a right triangle, the angle and the hypotenuse. From those, I can calculate the jet's elevation (the opposite side of the triangle) and the distance over ground to the jet's position when it transited the Sun. 

o = h * sin(θ)

a = h * cosin(θ)

Solving these is pretty straightforward with a modern calculator or using spreadsheet software like Google Sheets. Note that these formulas require the angle to be expressed in radians but spreadsheets have a function for that, too, so you don't have to remember the formula for converting degrees to radians:

θ radians = θ degrees * π / 180

So what's the answer? Based on the numbers above, at the time it passed in front of the Sun that jet was flying at an elevation of approximately 2,377 feet and its position over land was about 1,225 feet from my backyard, about a quarter of a mile to the southwest.

I would have thought the jets flying over East Dallas would be higher than that but the data and formulas say it has to be somewhere between 2,000 and 3,000 feet up. To see for yourself, make a copy of my spreadsheet and play around with what happens when you change the jet fuselage width (cell B7). Even if you change it to the size of a jumbo 747, 21.3 feet, the calculated elevation of the jet is still well under 4,000 feet.

Another interesting exercise would be to play around with the ratio of the jet fuselage width to the orb size to see what it has to be for the jet in Brandon's image to be flying at 12,500. Give it a try.

Remind you of one of those math word problems you hated solving in school?

Yeah, but pretty cool, huh?

Note

It's been decades since I was in school so if you find that I've made a mistake in my calculations, I'd be happy to hear about it in the comments.