The ISS Transit Finder tells you when the International Space Station crosses the Sun or the Moon as seen from near you, and how far to go to stand on the centerline, where it crosses the middle of the disc. It searches the coming 7, 14 or 30 days and lists every transit within your travel radius. Never look at the Sun without a certified solar filter: watch and photograph a solar transit only through one.
How to use it
- Set your location: Use my location, a street address or a place under Address, Latitude, Longitude and Elevation in metres, Pick on the map, or drag the white marker.
- Set the First day and the number of Days to search: 7, 14 or 30.
- Set the Travel radius, 5 to 300 km and 60 km at first: how far you will go to reach a centerline. Transits whose centerline passes farther are left out.
- Under Transits of, choose the Sun, the Moon or both. Before you point anything at the Sun, fit a certified white-light or H-alpha solar filter to your telescope or camera lens.
- Check the Orbit panel: the TLE the search uses, with its epoch in UTC. It is read live from CelesTrak, with a built-in copy if CelesTrak is silent. Paste a newer TLE to use your own.
- Read the Transits list, soonest first: body, date, time, then, as seen from the centerline, altitude, duration, the ISS’s angular size and the disc’s diameter, and last the distance and direction of the centerline, or Here inside the transit band. Switch Times between Local and UTC.
- Pick a transit to see its centerline and band on the map. Under Seen from, choose Your location or Centerline: the disc shows the Sun or the Moon at its true angular size, zenith up, with the ISS’s track across it.
How it works
The ISS’s orbit comes from its TLE, propagated with SGP4; the Sun and the Moon come from astronomy-engine. Every half second the tool follows the line from the Sun or the Moon through the ISS down to the ground. The points where it lands make the centerline: from there the ISS crosses the middle of the disc.
The transit band is the strip either side of the centerline from which the ISS still crosses the disc. The Sun and the Moon are each about half a degree across, so it is narrow: roughly 5 to 20 km wide when the body stands above about 25°, and tens of kilometres or more when it is low. A crossing usually lasts about a second, and several seconds with the body low and the ISS far away.
All of it is only as good as the TLE. The further the search runs from its epoch, the more a centerline can shift, by kilometres in a few days. The tool warns when the search ends more than 4 days after the epoch or starts more than 2 days before it. It knows nothing of clouds or of your horizon.
Formula
θ = S / (1000 · d) × 206 265
θ is the ISS’s angular size in arcseconds (″). S is its width across the truss, 109 m. d is your distance to the ISS in km, and 1000 converts m to km. 206 265 is the number of arcseconds in a radian.
Worked example
With the ISS 619 km away: θ = 109 / (1000 × 619) × 206 265 = 0.0001761 × 206 265 = 36.3″. The Sun’s disc is 31.9′, or 1 915″, across, so the ISS spans 36.3 / 1 915 = 1.9% of it. At 0.62°/s, the 0.532° disc takes 0.532 / 0.62 = 0.86 s to cross.
Common questions
What is an ISS transit, and how long does it last?
It is the ISS crossing the disc of the Sun or the Moon, seen from one place as a small dark silhouette. It is not a pass, when the ISS only crosses your sky. It lasts about a second, and only a narrow strip of ground sees it.
How far do I have to travel to see an ISS transit?
Each transit gives the distance and direction from your location to the nearest point of its centerline, and its panel says how far to move. The Travel radius only decides which transits are listed: raise it to 300 km to see more.
How do I photograph an ISS solar transit safely?
Never look at the Sun without a certified solar filter, by eye or through binoculars, a telescope or a camera. Fit a certified white-light or H-alpha filter over the front of the instrument before pointing it at the Sun, and keep it there. The tool repeats this warning under every solar transit.
How accurate are the times and the centerline?
Only as accurate as the TLE: read a fresh one close to the event, or paste a newer one the day before. Times show tenths of a second, which is only how they are displayed, not how well the orbit is known.
Does it need an account, and does it store my location?
No account, and no server of our own: your location stays in your browser for the visit and is never stored. What you type under Address goes to Photon, the TLE comes from CelesTrak and the map from OpenStreetMap, all straight from your browser.
Why are no transits listed?
No centerline passes within your travel radius in the days searched, or neither body is selected. Raise the Travel radius, search 14 or 30 days, or pick both the Sun and the Moon.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: