Satellite transits

The International Space Station crosses the face of the Sun or the Moon somewhere on Earth all the time. Catching it is a different matter: the crossing lasts under a second, and it’s only visible from a corridor a few kilometres wide. Miss the corridor and you see nothing at all.

Satellite transits tells you when one happens near you, exactly where you have to be standing, and which way to point.

Why the corridor is so narrow

It comes down to how close the ISS is next to what it’s crossing in front of. The Sun and the Moon each cover about half a degree of sky — roughly a fingertip held at arm’s length. The ISS orbits only about 400 km up: next to nothing beside the Moon’s 384,000 km, and nothing at all beside the Sun’s 150 million. At that altitude, half a degree of sky works out to only a few kilometres on the ground. Step outside that band and the ISS’s position against the Sun or Moon has shifted by its own width — it isn’t fainter or off to one side, it just isn’t there.

That’s also why a transit is nothing like an ordinary ISS pass — the bright dot crossing the evening sky that an entire country can watch at once. A transit isn’t a rarer version of that. It’s a different kind of event: two things lined up to within a few kilometres, not just something overhead.

What it finds

Three satellites, because they’re the ones big enough and bright enough to make a silhouette worth photographing:

• The ISS — by far the most productive. Its orbit is steeply inclined, so it reaches most inhabited latitudes.
• Tiangong — the Chinese space station.
• Hubble — a low-inclination orbit, so it stays much closer to the equator.

For each one it searches the dates you choose and reports every pass that crosses the Sun or the Moon near you, or comes close enough that a short drive would put you on the line.

What you tell it

Four things, and it remembers sensible defaults for all of them:

• Where you are. Search for a place, use your current location, or — on the web — tap the map. The results say which location they are for, so you always know what you are looking at.
• How high you are. A corridor is only a few kilometres wide, and standing a thousand metres up moves where it falls. In metres or feet, following your unit setting.
• Which dates. A start and an end, not a vague “next few days”.
• How far you would travel. From 1 km upward, in kilometres or miles. This is what decides whether a corridor 40 km away is a result worth showing you or noise worth hiding.

Reading a result

Each result leads with a sketch of the pass: the disc of the Sun or the Moon, with the satellite’s track drawn across it. Dead centre, a graze, or a clear miss — you can see which at a glance, without doing the arithmetic yourself.

Alongside it:

• Bearing — which way to point, in degrees and as a compass direction.
• Altitude — how high above the horizon, so you know whether a treeline or a building is in the way.
• Corridor width — how much room for error the drive has.
• Sunlight — whether the satellite is lit, in the Earth’s shadow, or partly lit.

You also get the moment it enters and leaves the disc, and a be set up by time. That last one matters more than it sounds: the event is over in a fraction of a second, so arriving on time means arriving early.

Getting to the corridor

The map draws the centre-line — the path along which the satellite crosses the exact centre of the disc — and the visibility band either side of it. Your own position is marked, so the question “is this worth driving to?” answers itself.

If you’re not already on the line, one tap gives you directions to the nearest point on it. If the line already runs over you, it says so.

From any result you can turn it into a shoot plan, with the time and the place already filled in, or add it to your calendar with a reminder set for when you need to be in position.

Looking at the Sun

Never look at or photograph the Sun without a certified solar filter. Eye and sensor damage is immediate and permanent. Every solar result in the app carries this warning, and it cannot be dismissed.

A lunar transit needs no filter.

Why it sometimes finds nothing

An empty result is often the correct answer, not a failure.

A transit exists only where the satellite’s shadow of that body actually lands on the Earth — and frequently it lands nowhere at all, passing beyond the edge of the planet into space. So a satellite can sit squarely on the Moon-facing side of the Earth for a week without producing a single lunar transit anywhere.

Latitude matters too. Hubble’s orbit doesn’t reach far from the equator, so if you’re in northern Europe you’ll almost never see a Hubble transit however long a range you search. The app tells you when that’s the reason, rather than leaving you to widen the dates forever.

How fresh the numbers are

Satellite orbits aren’t fixed. Drag from the thin upper atmosphere slowly changes them, and the ISS is also nudged with an engine burn every so often to correct its altitude — a reboost that can move its orbit outright. The data describing an orbit is only a snapshot, taken at one moment, and it goes stale as the real orbit drifts away from it. A prediction built on week-old orbit data can be off by several kilometres — which, for a corridor only a few kilometres wide to begin with, is enough to put you in the wrong field entirely.

So every result carries the age of the orbit data it came from, and anything more than a couple of days old is marked provisional — check again before you drive. When the app can’t reach the orbit-data service it says so and uses the last copy on your device rather than quietly giving you a stale answer.

There’s a second way a prediction can go soft, and it has nothing to do with the data being old: asking about a date too far ahead. You can set the search to start next summer, and it will be worked out from orbit data downloaded an hour ago — perfectly fresh data, and a corridor that is pure extrapolation, because the drift between now and then is many times the corridor’s width. So treat anything more than about a week out as a rough answer to “does this happen at all”, never as a place to stand. A far-off search that comes back empty isn’t telling you the sky is quiet either; it’s telling you the question is beyond what an orbit prediction can answer. Come back to it nearer the date.

Working offline

Only the orbit data comes from the network. Every calculation — the orbit propagation, the positions of the Sun and Moon, the corridor on the ground — runs on your device. Once the data is downloaded, the search itself needs no signal, which is the point at a trailhead.

Where to find it

Tools → Satellite transits, on iPhone and on the web app.

Related

• Alignment planner — the same idea for the sun, moon or Milky Way behind a landmark.
• Getting started