ISS Transit Finder

See the International Space Station in front of the Moon!


The ISS Transit Finder is a web application designed to aid planning the observations of lunar and solar transits of the largest artificial satellite of our planet, the International Space Station. It is designed to be intuitive and easy to understand even for non-astronomically inclined observers. This help page contains information about the features of this website and the interpretation of the prediction results.

Setting up the calculation

On the main page, you can specify the following parameters:

Interpreting the event predictions

After the calculation is complete, you will see a list of events about to occur in a requested time period. In case there are no results, return to the main page and try to specify a broader range of dates and/or a larger travel radius.

Each entry in the event list contains the following information:

Clicking the More information button will reveal additional technical details about the event:

It should be noted that the positional data (azimuth, altitude, celestial coordinates etc.) are calculated for the ISS, not for the target object, unless noted otherwise. All the values are valid for the observer's position and may be different across the visibility path.

The illustration below provides a visual representation of some observational parameters. Click to enlarge.

transit properties illustration

Visualization and quality rating

An additional piece of information is a small visualization of the apparent path of the ISS seen from the observer's location. It shows the brightness of the sky (blue or black background), the phase of the Moon (in case of lunar events) and the direction of the motion of the station in the horizontal reference frame. The path has a white color when the ISS is illuminated by the Sun.

Below the visualization there's a simple star rating of the observational quality of an event, determined as follows:

For lunar events occuring during the day, one star is subtracted when the event occurs at an angular distance of less than 90° from the Sun, and two stars are subtracted for a separation smaller than 30°. This rating is obviously highly subjective and may not be on par with your own judgment.

Using the map

The application includes a built-in map tool which comes in handy in pin-pointing the locations of potential observational spots.

In order to display the map, either click the Show all on map button on top of the results page or one of the Show on map buttons visible in each of the event entries on the result list. Doing the former will automatically revert the map center and zoom position so that the entire travel circle is shown. The visibility paths of all calculated events will be displayed. You can also choose to show only lunar (blue) or solar (red) transits by clicking an appropriate option in the bottom of the map. Choosing Only selected will display the currently highlighted event, if there's one. This is also the automatic option when the map is displayed after one of the Show on map buttons is clicked.

The top settings bar allows you to toggle the visibility of the center lines (where the ISS passes directly through the center of the target's object apparent disk), and the selected location along with its travel radius.

Clicking anywhere on the map will display a small information box containing basic information about the selected location: its geographic coordinates, name and the geodesic distance from the observer. You can use the Recalculate for this location shortcut to quickly obtain information about upcoming transits for the selected coordinates. If the clicked location lies on one of currently visible paths, the information box will also contain a short summary of the selected event, as seen from the chosen point.

A word on prediction accuracy

While the positions of Solar System bodies can be determined with near-perfect accuracy for many centuries into the past and the future, the orbit of the International Space Station is constantly changing in an unpretictable manner due to semi-random factors such as the atmospheric drag.

Because of this, accurate parameters of the transit events can only be reliably calculated for a short time into the future. Any predictions further than 10-14 days from today should be treated as a rough approximation - both the timing and the position of the event may change significantly.

In a few hours directly before any event, the predicted time is known with a precision of a fraction of a second, while the position of the center line is usually accurate down to 100 meters, which is approximately one width of the ISS.

Eye safety

Solar observing is dangerous. Never look at the Sun through any kind of optical instrument without an appropiate and reliable filtering, such as an ND5 mylar foil. Observing the Sun without eye protection will result in an instant, severe and irreparable damage or complete loss of your eyesight. Always inspect the surface of your filter for potential damage (such as punctures) before each observation. When using your telescope during the day, pay extreme caution not to accidentally point it at the Sun when the filter is not mounted.

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