Sun & Moon transit finder

ISS Transit Calculator

Q: What is an ISS transit?

An ISS transit is the moment the space station passes directly in front of the Sun or the Moon as seen from one spot on the ground. The station appears as a small dark silhouette crossing the bright disc, usually for well under two seconds.

Q: How rare are ISS transits?

From any single location a true transit of the Sun or Moon happens only a few times a year, because the visible path is a strip of ground roughly 5 to 15 kilometres wide. Near misses, where the station passes within a fraction of a degree of the disc, are much more common.

Q: How accurate are the predictions?

The timing is typically accurate to a few seconds and the ground path to a few kilometres when the orbital elements are only a day or two old, but the ISS reboosts and drag change its orbit, so predictions more than three or four days ahead are provisional. Re-run the calculation within a day or two of the event and again on the day.

Exact-location prediction · computed in your browser · free

Once in a while the International Space Station passes directly in front of the Sun or the Moon. For well under two seconds it shows up as a tiny dark silhouette racing across the bright disc — one of the most sought-after shots in amateur astrophotography. This calculator finds those moments for your exact coordinates: it scans the next days of ISS passes, works out where the Sun and Moon are at every instant, and reports each crossing along with how close you are to the centerline of the shadow path.

Eye and equipment safety. Never look at the Sun, or point a camera, binoculars or telescope at it, without a certified solar filter fitted over the front of the optics (and certified solar glasses for your eyes). A Moon transit needs no filter. If you are not sure your filter is safe, do not observe the Sun.

Latitude Longitude Search window

Coordinates are used only in your browser to run the calculation; observer height is assumed to be sea level.

How the calculator works

The station’s position comes from the latest NORAD two-line orbital elements (see what a TLE is), propagated with the same SGP4 model that drives the live tracker. For the Sun and Moon the page uses a full astronomical ephemeris (the open-source Astronomy Engine), accurate to about an arcminute — far smaller than the half-degree discs it has to hit. Each ISS pass is sampled every two seconds, the closest approach to the Sun or Moon is refined to a few milliseconds, and the result is classified by comparing the angular separation with the body’s apparent radius at that moment.

Two kinds of events are reported. A transit means the station’s centre crosses inside the disc from your location. A near miss means it passes within a quarter of a degree outside the edge — close enough that a small move sideways could turn it into a transit. For every event you also get the distance between you and the centerline of the shadow path, and the width of the corridor from which a transit is visible.

Why your exact position matters

The ISS is only a few hundred kilometres up, so it is close enough that its apparent position against the Sun or Moon shifts noticeably as you move on the ground. The strip of ground from which the station is seen in front of the disc is typically only 5 to 15 km wide, and it is a long thin line running across the planet in the direction of the orbit. Stand a few kilometres outside it and the station slides past just beside the disc instead. That is why transit chasers drive to a spot computed to the nearest hundred metres — enter coordinates to four decimal places for the best result and use the offset shown on each result card to decide whether to relocate.

Planning a transit photo

Re-run the numbers. Predictions made a week out are provisional: the station is occasionally reboosted and drag changes its orbit. Recheck two days before and again on the day, when the elements are freshest.
Be early. Set up at least 20 minutes ahead, find the Sun or Moon in your frame, and focus on the disc’s edge or surface features.
Use a fast shutter. The crossing lasts around a second, so use a high shutter speed (for the Sun with a proper filter, 1/1000 s or faster) and burst or video mode, then pick the frames afterwards.
The Sun needs a certified filter. The Moon does not, and Moon transits can happen at night or in daylight, so they are the easier place to start.
Check the weather. Thin cloud is fine for the Sun; for the Moon, look at the forecast near the time of the transit.

Frequently asked questions

What is an ISS transit?

It is the moment the station passes directly in front of the Sun or the Moon as seen from one spot on the ground, appearing as a dark silhouette on the disc for typically half a second to two seconds.

How rare are ISS transits?

From a single fixed location a true transit happens only a few times a year, because the visible path is so narrow. Near misses are much more common, and travelling to the centerline makes success far likelier.

Is it safe to photograph an ISS transit of the Sun?

Only with a certified solar filter fitted over the front of the telescope or lens, and certified eclipse glasses for your eyes. Never look at the Sun, or point a camera or telescope at it, without proper solar filtration. The Moon does not need a filter.

How accurate are the predictions?

With fresh orbital elements the timing is good to a few seconds and the ground path to a few kilometres. Accuracy degrades with time, which is why the 3-day window is labelled the most accurate and the 14-day window provisional.

Why does it show a Moon event when I cannot see the Moon?

Only events where the Moon is at least three degrees above your horizon are listed. A thin crescent or a bright daytime sky can make the Moon hard to see, and the card shows the Moon’s illuminated fraction to help you judge.

Want to see the station itself first? The live tracker shows where it is right now, and the visible-tonight finder lists the next passes you can see with your own eyes.

Open the live tracker Is the ISS visible tonight?