One degree per second: the Space Station with a phone

Almost every target in the sky has the decency to wait. A nebula sits where it is for hours, and if the tripod was crooked tonight you shoot the same thing tomorrow. The International Space Station behaves like a train: it leaves at the time printed on the timetable, crosses the sky in a few minutes and is gone, and the next decent pass might fall in two days or in two weeks. Turn up with the phone still in your pocket and you get to watch, nothing more.

A dot reflecting the Sun

The station gives off no light of its own. You see it because it reflects sunlight, the same reason you see the Moon, and NASA calls it the third brightest object in the sky: a steady white dot sliding between the stars without blinking, obvious to the naked eye, no instrument required.

That single fact sets the only usable window. During the day the background sky is too bright, and in the middle of the night the station flies inside Earth’s shadow cone with nothing left to reflect. Every visible pass falls in the hours around sunset and sunrise, when it is already dark down here while up there, four hundred kilometres up, the Sun is still shining. The same geometry explains how certain passes end. The dot does not always set behind the horizon: if it crosses into Earth’s shadow halfway up, it fades out right there, over a handful of seconds, and in your photograph the trail thins away into nothing. It is the finest ending this subject can give you, and the prediction tells you in advance which passes will do it.

The arithmetic that decides everything

Three NASA numbers are enough. The station orbits at roughly 28,000 kilometres per hour, at an average altitude of about 400 kilometres, and completes a lap of the planet every ninety minutes or so. One division gives you the rest: at the top of a pass, close to overhead, those 7.8 kilometres per second work out to a little over one degree per second of apparent motion.

A degree per second is a speed the sky almost never shows you. Stars drift at fifteen arcseconds per second, roughly two hundred and fifty times slower. A ten-second exposure, the sort that hands you a clean star field, will record an eleven-degree trail while the station is culminating: on a phone’s main wide camera, covering about 74 degrees horizontally, that is a good seventh of the frame width. Low down near the horizon it is far slower, because it sits much further away and the geometry foreshortens it. The fast part is the middle.

The planning happens at home, sitting down

NASA publishes the pass predictions through Spot the Station, free of charge, and every line contains exactly the four things a photographer needs: the time, how long it stays above the horizon, the maximum height in degrees (zero is the horizon, ninety is the zenith), and the compass points where it appears and disappears.

Maximum height is the first filter. A pass topping out at fifteen degrees gets tangled in rooftops, trees and the low haze that eats most of the light. Above forty degrees the dot comes through clean and shrugs off any streetlight. The directions are what you frame by, and this is where a constraint arrives that no telescope owner ever has to think about: a high pass can sweep a hundred and eighty degrees of sky, while your wide lens takes in seventy-four. The whole track will not fit in one frame. Decide beforehand which slice you are telling, and pick the one with something on the ground to sit underneath it, a church tower, a ridge line, a pylon. A white streak on a black field with no reference is a diagram. The same streak passing above a recognisable skyline is a photograph.

Get to the spot ten minutes early and spend all ten. Tripod planted, framing locked, focus set by hand on a bright star, white balance in manual, a test exposure already made on empty sky. Once the dot appears there is no time to touch anything: you press, and you let it run.

The settings are the star trail settings, with one difference

The method is borrowed almost wholesale from star trails. A continuous run of ten to fifteen second frames, the lowest ISO that still registers (the station is bright, the noise is not), RAW, and the shortest gap between frames your app will allow. At home the frames get combined by maximum, keeping the highest value for each pixel across the series, and the trail reassembles itself piece by piece over the same starry sky.

The difference is in what you must not do tonight. The everyday job of a stacking app like AstroStackerPro is to align on the stars and average the frames, which is the correct way to treat anything that holds still: add up dozens of exposures and the noise cancels while the signal stays. Point that same arithmetic at something covering eleven degrees in ten seconds and it does the opposite. It spreads the station’s light across every position it occupied, divides by the number of frames, and hands back a grey smear where the line should be. Maximum keeps it. Mean erases it.

There is a simpler route that works beautifully: one long exposure, twenty or thirty seconds, taken while the dot is at its highest. You come away with a single stretch of trail, continuous and unassembled, and it is the fastest way to turn a pass into something publishable the same evening.

The dashed line that star trails never show

Between the end of one frame and the start of the next, the phone writes the file, and during that second or two the sky keeps moving. On star trails the gap exists but stays invisible, because a star shifts barely half a pixel in two seconds. Run the same sum at a degree per second: two seconds of file writing costs you two degrees of sky, four lunar diameters, a hole you can spot on the phone screen itself.

A station trail assembled from a sequence comes out dashed. That is the subject’s signature rather than a flaw to hide, visible proof that the dot was moving fast. If you want it unbroken there is exactly one way, the single exposure described above. If you want the dashes even, keep the interval constant and keep your hands off the phone mid-pass.

The dashes also answer the question that always turns up when the files come home, namely whether that line really is the station or just another satellite going by. Steady brightness and a straight track are common to both, and the three signatures that separate meteors, satellites and aircraft are already laid out in the piece on shooting a meteor shower. The proof that settles it is the clock: if the line shows up at the predicted minute, on the predicted bearing, it is her.

What a phone will not give you

Worth being blunt about one thing before you drive anywhere. The station measures 109 metres end to end, the largest object humans have ever put in orbit, and seen from four hundred kilometres those 109 metres subtend less than an arcminute of sky: about 56 arcseconds. One pixel of a phone’s wide camera covers seventy. At the best moment of the best pass of your life, the International Space Station fits inside a single pixel. No solar arrays, no modules, no cross-shaped silhouette. Those images come from telescopes on fast tracking mounts, and they are a different sport.

The same applies to the transits across the Moon and the Sun that circulate online and look like an excellent idea. They last under a second, often half of one, and freezing the silhouette takes shutter speeds below a thousandth: out of reach for a phone, and in the solar case out of the question entirely without a certified solar filter over the lens.

What is left is the pass, which is plenty. A bright point climbing out of one horizon, cutting across the sky at a speed you can actually see, sometimes going out halfway, leaving a white line above the village bell tower in your photograph. Inside that pixel there are people watching the sixteenth sunset of their day.