Observer's guide
How to read a pass
A pass is one continuous trip of a satellite above your local horizon: it rises somewhere, reaches its highest point, and sets on the other side. Everything else on a pass listing is a description of that arc.
Max elevation — the number that matters most
Elevation is the angle above the horizon, from 0° (on the horizon) to 90° (straight overhead). A pass with a maximum elevation of 15° skims the treetops and is easily missed. Above 50° the satellite crosses high overhead, bright and hard to miss. Above 80° it passes almost directly above you.
| Max elevation | What to expect |
|---|---|
| 0–20° | Low and dim, hidden by buildings and trees, often not worth it |
| 20–40° | Clearly visible, comfortable to watch |
| 40–70° | Bright and fast, best kind of pass |
| 70–90° | Almost overhead, very bright, crosses the whole sky in minutes |
Azimuth — where to look
Azimuth is a compass bearing: 0° is north, 90° east, 180° south, 270° west. A pass listing usually gives three values — where it appears, where it peaks, and where it disappears. Stand facing the appearance bearing and let your eyes adjust for a minute before the predicted time.
Most ISS passes run roughly west to east, because the station orbits in the same direction Earth rotates. That is a useful default if you only remember one thing: look west first.
Magnitude — how bright
Magnitude is an inverted scale: lower numbers mean brighter objects. The naked-eye limit under a dark sky is about +6. For comparison:
| Object | Magnitude |
|---|---|
| Full Moon | −12.7 |
| Venus at brightest | −4.6 |
| ISS on a high pass | −4 to −2 |
| ISS on a low pass | −1 to +1 |
| Brightest star (Sirius) | −1.4 |
| Polaris | +2.0 |
| Faintest star visible in a city | +3 to +4 |
An ISS pass at magnitude −3 is brighter than any star and brighter than Venus — it is immediately obvious once you are looking in the right place.
Duration — why it is short
A typical visible pass lasts three to six minutes. The satellite is moving at 7.7 km/s at an altitude of a few hundred kilometres, so the geometry of "above your horizon" changes fast. A pass that lasts longer than seven minutes is rare and usually means a very high, near-overhead crossing.
Why there is no pass every night
Two conditions have to line up at once. The satellite must be above your horizon, and you must be in darkness while the satellite is still in sunlight. The station's orbital plane precesses around Earth, so the favourable geometry arrives in clusters: a week or two of good evening passes, then a gap, then a run of morning passes.
What you will actually see
- A steady point of light, brighter than the surrounding stars.
- Smooth, continuous motion — no blinking, no strobing, no navigation lights.
- It brightens as it climbs, peaks, then dims and can fade out abruptly at the shadow line.
- Aircraft look similar but blink, move slower across the sky, and usually have a visible contrail or sound. Satellites are silent and perfectly steady.