How to photograph an ISS transit across the sun or moon

Why it is hard

Three things conspire against you, and it is worth understanding each because they decide every choice you make afterwards.

It is fast. The Station moves at roughly 7.7 km per second. Against the half-degree disc of the sun or moon that works out at somewhere between a third of a second and about a second and a half, depending on the geometry. There is no second attempt and no time to react — the shutter has to already be running.

The corridor is narrow. A transit is visible only from the strip of ground where the satellite’s shadow of that body actually falls. That strip is typically a few kilometres wide. Ten kilometres away and the Station simply passes near the disc instead of across it, which photographs as a dot in an empty sky.

Most days it falls nowhere useful. This is the part that surprises people. The shadow only reaches the Earth when the satellite sits well inside the cone pointing at the body — it is not enough to be on the correct side of the planet. So the Station can spend a week on the moon-facing side of the Earth without producing a single lunar transit anywhere at all.

Put together: an empty result is usually the correct answer, not a failure of the tool. Expect to wait for a good one, and expect to drive to it.

Finding your next one

The three things to read off any prediction are when, where you have to stand, and how fresh the orbit data is.

That last one gets overlooked. Orbits drift, and the two-line element sets that describe them go stale — along-track error grows roughly a kilometre a day. Against a corridor a few kilometres wide, a week-old prediction can put the centre-line somewhere you are not. Check again the morning of the shoot; if the data has been refreshed since you planned, the corridor may have moved.

Getting to the corridor

The centre-line is the path from which the Station crosses the exact middle of the disc. Either side of it, the chord it cuts gets shorter until, at the edge of the band, it grazes the limb and then misses.

Aim for the centre-line, not the edge. Two reasons: the crossing is longest there, so you get the most frames; and your position is never as exact as you think. GPS in a valley or among buildings can be off by tens of metres, which does not matter, but a misread map or a blocked farm track can put you a kilometre out, which does.

Arrive early enough to set up completely and then wait. Fifteen minutes is a sensible minimum. You want the camera focused, the exposure set, the framing locked and the burst already running before the predicted time — not because the prediction is unreliable, but because a sub-second event gives you no chance to correct anything.

Looking at the sun

Never point a camera at the sun without a certified solar filter, and never look through an unfiltered optical viewfinder at it. Damage to your eyes and to the sensor is immediate and permanent. This is not a caution to weigh up — it is the one part of this guide with no judgement call in it.

Use a purpose-made solar filter that fits over the front of the lens. Filters that screw in behind the lens concentrate the heat and can fail. Live View rather than the optical finder. Fit the filter before you point the camera anywhere near the sun, and leave it on for the whole session.

A lunar transit needs no filter at all, which makes the moon the easier place to start.

Gear and settings

Focal length. The Station’s silhouette is small — roughly 45 to 60 arcseconds across at typical range, against a disc about 1,900 arcseconds wide. At 400 mm on full frame the disc fills a useful part of the frame and the silhouette is recognisable. At 800 mm and beyond you start to resolve the solar panels and the truss. Below about 300 mm it is a speck.

Shutter speed. Fast. The Station crosses the disc in around a second, so at 1/2000 s it still moves a little; 1/4000 s and shorter freezes it cleanly. On the sun with a filter you have the light for it. On the moon you are trading against a much dimmer subject — expect to open up and raise ISO rather than slow the shutter, because motion blur is what ruins these frames.

Drive mode. The highest continuous rate your camera has, started before the predicted moment. Electronic shutter if it gives you more frames and your camera’s rolling shutter is fast enough. Many people come home with the Station in exactly one frame out of forty; that is normal and it is why you shoot the burst.

Focus. Manual, set on the limb of the sun or the terminator of the moon beforehand, then left alone. Autofocus will hunt at the worst possible moment.

Exposure. Expose for the disc, not the sky. The sky around it should be black. Meter and lock before the pass, and do not touch anything once the burst starts.

What you actually get

A dark, angular silhouette crossing a bright disc, sharp-edged and unmistakably artificial against the granulation of the sun or the craters of the moon. In a good frame at long focal length the solar arrays read as a distinct H shape.

Some photographers stack the sequence into a single frame showing the whole track across the disc; others keep the single sharpest frame. Both are honest presentations as long as you say which you did.

Related reading

The same planning discipline — a narrow window, a precise place, and no second chance — applies to eclipses, and the geometry is closely related. If you have not photographed the sun before, start with the moon: no filter, more forgiving, and the same technique.