A pass is one crossing of your sky by a satellite; this site counts it only while the satellite is above 10°, sunlit, bright enough, and your sky is dark.
A satellite in low Earth orbit circles the Earth in about an hour and a half, while the Earth turns beneath it. So from any one place, a satellite comes into view, crosses part of the sky and disappears — a pass — then is gone until a later orbit brings it back over you.
Many trackers call any moment the satellite is above your horizon a pass. This site is stricter. It lists a pass only when you could actually see it with your eyes. Four things must all be true at once:
Each pass on satellites visible tonight shows:
Some cards add a note: Fades out mid-sky as it enters Earth's shadow. That pass ends not at the horizon but when sunlight stops reaching the satellite.
Suppose a card for Delhi reads: ISS — Tonight 7:12 pm IST · Look W → SE · Max height 64° · mag −3.0 Very bright · Visible for 5m 40s. Go out a few minutes early. At 7:12 pm, face west and watch just above the rooftops. A steady, bright point will climb to more than six fists high and sink towards the south-east, all in under six minutes.
Each card shows the IST start time, Look direction, Max height, Brightness and how long the pass is visible. See tonight's passes over Delhi
The pass finder (findPasses in the site's visibility code) works in two stages.
Coarse scan. Starting now, it evaluates the satellite every 30 seconds for 24 hours. Each evaluation runs SGP4, computes look angles for the observer, computes the Sun's position, tests Earth's shadow, estimates magnitude and checks the four conditions in a fixed order: below horizon → in shadow → daylight → too dim. The first failing condition is kept as the reason, so the interface can say why something is not visible.
Refinement. When the state flips between visible and not visible across a 30-second step, the code bisects the interval — halving it repeatedly and keeping the half where the flip happens — until the gap is under one second. Bisection needs only about five halvings: 30 → 15 → 7.5 → 3.75 → 1.9 → 0.94 s.
For each pass it records start, peak (the sample with the highest elevation) and end times, the start, peak and end azimuths, the brightest magnitude reached, the duration, and a flag for passes that end because the satellite entered Earth's shadow while still above 10°.
To keep this fast for a catalogue of thousands, the site first discards objects that can never be naked-eye visible: anything with mean motion below 11 revolutions per day (from TLE line 2, columns 53–63) and anything whose standard magnitude is more than two magnitudes fainter than the limit. Mean motion 11 rev/day means a period of 1440 / 11 ≈ 131 minutes; by Kepler's third law that is a circular orbit about 2,200 km above the ground.
The sky is not dark — the Sun is above the horizon — so the satellite cannot be seen with the naked eye even though it is geometrically overhead.
The satellite flies into Earth's shadow while still above the horizon, so it stops reflecting sunlight and vanishes instead of setting.
The ISS completes one orbit in about 93 minutes, so on the next orbit it can pass over the same region again, shifted west because the Earth has turned.