Astrophotography planning
The ISS crosses the Moon in one second. Be in the right place.
A spacecraft transit is visible only from a corridor on the ground a few kilometres wide, and it lasts about a second. Miss the corridor by five kilometres and you see nothing at all. Transit Hunter works out where that corridor falls near you, ranks every opportunity, tells you whether it is worth the drive — and watches for the good ones so that you do not have to.
Free to use. Monitoring and alerts included, not upsold. No account needed to start.
The problem
Getting the astronomy right is the easy half
Photographing a spacecraft against the Sun or the Moon is not a matter of pointing a camera upwards and waiting. It is a logistics problem with a one-second deadline, and almost everything that decides whether you get the picture happens before you leave the house.
The window is about one second
A spacecraft crosses the face of the Sun or Moon in roughly a second, sometimes less. There is no second attempt and no adjusting between frames — everything has to be right before it begins.
The corridor is a few kilometres wide
Stand five kilometres to one side of it and the spacecraft passes near the disk rather than across it. You will have driven out, set up, and seen nothing.
The corridor moves
Fresh orbital elements arrive several times a day and the centre line shifts with them — by more than the corridor is wide after a station reboost. A spot that was right last week can be wrong tonight.
Most events are not worth the drive
A month of predictions might hold forty events. Most are grazing passes, or too low, or at an hour that costs a night's sleep for a poor result. Deciding which one deserves an alarm is the actual work.
The good ones are rare and easy to miss
A high-quality lunar transit close to home happens a handful of times a year. Nobody opens an app often enough to find those by chance, so in practice they pass unnoticed.
Existing tools stop at the astronomy
The dedicated transit finders compute the geometry well, then hand back a chronological table. Which event to choose, where exactly to park, when to leave and whether the sky will cooperate are all left to you.
What it does
The layer above the astronomy
The geometry is a solved problem, and Transit Hunter solves it the same way everyone else does. What it adds is everything between a correct prediction and a photograph you actually took.
A quality score you can interrogate
Nine weighted factors — how centrally the spacecraft crosses, how high the target sits, duration, apparent size, travel, corridor width, prediction reliability, sky and hour. Each is reported separately, so you can see exactly why an event scored what it did.
Worth the trip, or not
A plain verdict, with the reasons for and against it. Travel is judged against the budget you set, because ten kilometres means something different to someone who chose "where I am" than to someone who chose two hundred.
It watches so you do not have to
Standing rules run in the background and alert you when a transit worth your attention appears within your radius — with a lead time you choose, so a ninety-minute drive is not announced twenty minutes beforehand.
Alerts when the corridor moves
Fresh orbital data can shift the centre line after you have already been told where to stand. When that happens to an event you were alerted to, you are told again. Sending someone to a spot and staying quiet when the spot changes is worse than never sending them.
The corridor mapped, and any point on it evaluated
The centre line and the corridor either side of it are drawn on the map. Tap anywhere and the geometry is recomputed for that exact spot — the crossing you would see from the field you can park beside, not from the ideal point you cannot reach.
Framing for the equipment you own
Field of view and the number of pixels the spacecraft will span, worked out for your sensor and focal length, with the sampling limit and the diffraction limit treated as the separate constraints they are.
The forecast folded into the ranking
Cloud cover for the observation point is weighted into the score rather than bolted on beside it. It never touches the astronomy, and when no forecast is available its weight is redistributed rather than assumed clear.
A countdown that refuses to lie
Before it starts counting, the app recalculates the event from the point you actually chose, using the orbital data on the device now. If the time has moved, the quality has dropped or the data has gone stale, it stops and says so rather than counting towards a number that stopped being true.
How it works
From "where am I" to "this one, at this spot"
Say where you are, and how far you will go
An approximate location and a travel budget — from "where I am standing" to a few hundred kilometres. Nothing else is needed to begin.
It finds and ranks every transit
The search runs on your own device and returns the crossings of the Sun and Moon within reach, ordered by how good a photograph each one is actually likely to make.
It tells you whether it is worth the trip
A verdict with its reasons, an estimated drive, a departure time, and the exact point to stand — shown on a map with the corridor drawn around it.
It keeps watching after you close it
Set a standing rule and it checks on your behalf, alerts you when something good appears, and tells you again if the corridor moves after you have been told where to go.
Measured, not asserted
We checked, event by event
419 events were collected from an established reference tool and compared one at a time. The propagator was separately measured against NASA's own published ISS ephemerides.
What we do not claim. We do not claim to be more accurate than the established transit finders. We claim to agree with them closely, to have measured that agreement rather than assumed it, and to tell you honestly how uncertain any individual prediction is. That comparison found three real defects in our own calculations, and all three were fixed before any of this was said out loud.
Who it is for
Anyone who has to decide which night to go out
Astrophotographers
Anyone who has driven out for a transit and found the corridor was somewhere else. The score, the corridor map and the framing figures exist for exactly that trip.
Astronomy societies and clubs
A shared, explainable ranking makes it possible to agree which of the coming month's events the group should organise around — and where everyone should stand.
Educators and outreach programmes
A spacecraft crossing the Moon is a rare thing to be able to schedule. Knowing well in advance which events are worth planning a session around is what makes it teachable.
Science communication and media teams
Planning a shoot around a one-second event needs a defensible answer to "is this the right night, and where do we send the crew?" — with the uncertainty stated rather than hidden.
Security & privacy
Where you photograph the sky is where you live
An application that collects observing locations is collecting home addresses. This one was built by people who noticed that, and the consequences are in the architecture rather than in a policy document.
- Searches never leave your device. The orbital mechanics engine is in the app itself. Orbital elements are public data, identical for everyone, so the search runs where you are — and your coordinates are not transmitted in order to run it. The app states this beneath every set of results.
- Saved places are stored deliberately coarsely. A saved location is held to roughly two and a half kilometres — wider than a transit corridor. Precise enough to decide whether an event is worth telling you about; not precise enough to identify where you live.
- The Android app asks only for approximate location. Fine location is never requested, so Android's own permission dialogue offers approximate location and nothing finer. The position is visible in the application manifest, not only in a policy document.
- No account is needed to start. The first session is anonymous. You can save a place and set a standing alert without giving an email address. Signing in later uses a single-use link that expires in fifteen minutes, and session tokens are stored hashed rather than in the clear.
- Deletion means deletion, and you can look first. You can export everything held about you before deciding. Deleting the account removes it and everything that references it — saved places, alerts, notifications, devices and sessions — immediately, rather than marking a row inactive.
- No advertising, no third-party analytics, no session recording. Product analytics exist and are structurally unable to carry a location or an identifier: those fields are stripped at the point of recording rather than by convention.
The company behind it
Built by an engineering company that has been getting positions right since 1994
Transit Hunter is an Embarc product — designed, built, tested and maintained by the same team behind the Find'n'Secure fleet and asset tracking platform.
Three decades of position-critical software
Fleet and asset tracking platforms deployed across more than thirty countries, where a wrong position is not a cosmetic defect. The same discipline applied here.
Uncertainty engineered, not glossed over
Every prediction carries a reliability grade and the sentence explaining it, derived from measured residuals rather than from a figure chosen because it sounded reassuring.
Maintained as a product
Built, tested and maintained by Embarc as a product — not shipped as a demonstration and abandoned.
The data it relies on
| Source | What it provides |
|---|---|
| CelesTrak | Orbital elements, including the supplemental sets that track manoeuvres |
| NASA public ISS ephemerides | Manoeuvre schedule, and the reference for absolute accuracy |
| Open-Meteo | Cloud-cover forecasts for the observation point |
| OpenStreetMap contributors | Map tiles, via OpenFreeMap |
⚠ Solar observing safety
Never point a camera, telescope, binoculars or finder scope at the Sun without a certified full-aperture solar filter fitted over the front of the optic. Unfiltered sunlight through any optical instrument causes permanent blindness in a fraction of a second, and will destroy the camera. Filters that fit at the eyepiece end are not safe, and neither is an optical viewfinder.
The application enforces this: the warning is the first thing shown on every solar event, cannot be dismissed, and travels into the calendar export. No exposure advice is given that could be followed without a filter fitted.
Go and photograph something
Free to use, with monitoring and alerts included. Plus extends the search window, the travel radius, and the number of places you can watch.