julian weßel - astrophotography

How to calculate an ISS Transit

Many of you asked me how I calculate and plan my transits. So I wanted to help you with the first steps.

The website I use for calculating ISS transits is calsky.com. It's a great tool for everything related to astronomy. You can use it to get information about the nightsky,
locations of the planets and data for several satellites. The big advantage of calsky.com is that it's always very acurrate and precise when it comes to timing and location.
I've planned all my transits with this website and it has never disappointed me.

At the right of the toolbar you can find the button "Satellites". This is the chapter we need to use.

Now you  have to click on Sun/Moon crossers, Occultations and central path of selected satellite.
In our case the selected satellite is the ISS.

1) The first step is to set your location. Make sure your coordinates are correct otherwise you'll get an error in your predictions.
After this step calsky can use the coordinates as a reference for every following step.

2) Set your time you want the event to happen. In between 3 weeks the prediction is acurrate.
At dates further than 3 weeks the calculations can't be precise due to orbit corrections of the ISS.

Calsky can calculate a prediction for every day in one week.
You can set you predictions for the sun, the moon and every planet of the solar system. 

After that chose your maximum distance from your location.

Now you have to click on go! and the website will calculate a map with your parameters. This can take up to several minutes depending on the time range you want to calculate.

The website will give you a map with a purple line which is the transit line. The red black squares are reference points on this line.

Everytime you click on those points it will show you specific data for this location.

Below the map is a table with information referred to the reference points on the map. The data is listed in chronological order.
This data shows specific information about transit time, distance of the ISS to the observer, diameter of the ISS and transit path width at this location.

After you have checked the data and chose your favorite time and location for the transit you have to look up the location features.
At this time I use stellarium to visualize the event. You need to have a free view at the event direction.

In this example the event will take place in the western part of the sky.

The red dot shows the ideal location to set up your equipment.

Make sure your observation side is near the transit line. Especially when doing a planetary transit the transit line width can be less then 100 meters.

When everything is planned you have the chance to capture a good transit shot. If you want to know how it looks like when everything is going well you can see some of my transit pictures here.

Good luck and clear skies!

Links: calsky.com


I've made a short video about calculating an ISS Transit. You can see it here: