By Alex Carter · SatellitePlus / CannedRocket. Independent historical calculation; no live forecast or observed sighting is claimed.
Three different questions
- Where is the satellite? Its elevation tells us whether it clears a chosen horizon or obstruction threshold.
- Is it illuminated? A satellite above the horizon can still be in Earth's shadow.
- How bright is the observer's sky? Sunlight at the satellite is not the same as darkness on the ground.
We evaluated an archived ISS orbit near its epoch, at a fixed Greenwich reference site. The data window is 4 July 2018 UTC; the element epoch is 3 July 2018, 19:25:57 UTC. These historical dates make the example repeatable.
Four geometric passes. Zero intervals meeting this screen.
Skyfield 1.54 found 4 culminations above 10°. Every peak was sunlit, but the Sun was also above the observer's horizon. None met our selected combined screen: elevation at least 10°, Sun below −6°, and a sunlit satellite.
| Peak time, 4 July 2018 UTC | Satellite elevation | Sun elevation | Satellite sunlit |
|---|---|---|---|
| 04:31:59 | 36.8° | 4.5° | Yes |
| 06:08:19 | 89.2° | 18.3° | Yes |
| 07:44:50 | 79.3° | 33.2° | Yes |
| 09:21:04 | 27.2° | 47.6° | Yes |
The separate ten-second sampling calculation also found 0 qualifying intervals. That is a computed result for this fixture, not an absence of reported data. A peak check alone would not describe illumination throughout a pass.
Download the inputs, code and actual output
README and reproduction instructions · Calculation source · File checksums
Tested with CPython 3.14 and the pinned dependencies. Once those dependencies are installed, the bundled inputs support an offline run. The script checks the ephemeris hash before calculating.
A screening example, not a brightness model
The −6° Sun threshold and 10° satellite elevation are explicit choices for this example. Weather, light pollution, phase angle, satellite size and attitude, your eyesight and the actual skyline are not modelled. Passing these checks would not guarantee naked-eye visibility. They also do not describe every kind of optical instrument or radio reception.
This is not a benchmark of SatellitePlus, current orbital data, or a physical-device test. Event timestamps show numerical output; they do not imply equivalent physical accuracy. Ten-second samples can miss shorter intervals.
Method: Skyfield satellite documentation. The archived TLE source commit, upstream MIT notice, NASA/JPL DE421 source and permitted unmodified-kernel redistribution are recorded in the package. No third-party endorsement is implied.
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