Reading the Pass Report

Your sky can be represented as a circle, where the center is your zenith (straight up) and the outermost circle is your horizon. Sky charts, such as the one below, are meant to be held over your head to read, which means the compass points look different than on a ground map or atlas. 

Sample Sky Chart

The Satellite Prediction Report

At the top of the report you will find a list of changes to the system, any satellites added, and a summary of NASA science headlines.

The report proper begins with the start and end dates of the predictions, and your latitude and longitude. Each satellite name and estimated visual brightness starts a new section of the report in a pink block.

Here is an example entry from your pass report.

General

Each section is headed by a pink block containing the satellite's name and an estimated visual magnitude. Any satellite with a magnitude of 1.0 or less (the lower the magnitude number, the brighter the object) will be readily visible. 

If you choose to see only visible passes in the report, all the pass information will show up in alternating white and gray blocks, for easy reading. If you choose to see all passes, those with a high rating for visibility will show up in a yellow-green block.

Date, Rise and Set

These three columns tell you when and where to look for the satellite. The rise time is when the object will first be above your horizon, and the compass designation tells you where it will rise. Using the first pass in the sample report above, the object will rise at 6:41:15pm on January 8th in the North East and set in the South. 

Duration (mm:ss)

The total length of time the satellite will be above the horizon is given in minutes and seconds. In our example, the duration is 9 minutes and 40 seconds. 

Sun Lit (mm:ss)

Satellites are lit by reflecting sunlight.  You can see a satellite best when you are in darkness on the ground but it is in sunlight.  This is most likely around dusk and dawn.  The sun lit duration indicates how long the satellite will be in sunlight during its pass.  Usually, at dusk it will most likely be lit during the first part of the pass, and at dawn it will be lit during the last part of the pass. This is due to the rotation of the earth, which causes the sun to rise in the East and set in the West. The earth turns underneath the satellite, making its set position generally further to the West of its rise position. (There are exceptions, though, with retrograde satellites!)

In our example, the sun lit duration is 5 minutes and 24 seconds. Since this is a dusk pass, the sun lit portion of the pass is probably toward the beginning, while the satellite is rising in the North East.

The best way to know exactly when the satellite will be lit, is to visit the J-Pass 2.0 applet and look at the generated sky chart for the pass.  Then you will know for sure!

Maximum Elevation

Elevation is a measurement of how high above the horizon a point in the sky is. Point your finger straight out at your horizon, this is approximately zero degrees elevation.  If you point straight up to the zenith, that is 90 degrees elevation.  Notice the circle on the sky chart below labeled "60." This circle represents 60 degrees elevation, and any object within that circle will be at least 60 degrees above the horizon. 

If you live on a plain, with few or no trees and hills, you will be able to see objects very close to the horizon. Most people, however, live in the midst of buildings, vegetation, and hilly terrain. It's a good idea to estimate the lowest elevation you can see from your location and ignore any passes in the report that are lower.

In our example, the satellite only reaches a maximum elevation of 27 degrees. In many areas, it might not rise above the location terrain effects. 

Our Example

Now we know enough to visualize the satellite's path through the sky. If we use a sky chart like the image above, we can mark the rise and set positions, and estimating the maximum elevation to be halfway between them, we can draw an arc representing the path. Practice doing this and you will eventually be able to quickly locate satellites using the J-Pass Generator report.

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