About the ISS & Satellite Pass Predictor
Most “where is the ISS” pages show you a dot moving on a world map, which tells you almost nothing about whether you can actually see it. The question that matters is narrower and harder: at your exact latitude and longitude, when does the station cross a dark sky while it is still catching sunlight — and which way should you face?
This tool answers that properly. It pulls current orbital elements from Celestrak, refreshed several times a day, and propagates them with a full SGP4 implementation — the model those elements are specifically constructed for, rather than a circular-orbit approximation. From the resulting positions it derives your topocentric look angles, tests whether the satellite is inside the Earth’s shadow, checks that your own sky is dark enough, and estimates brightness from range and solar phase angle.
What you get is a pass list you can act on: the compass bearing to face, the peak elevation, how long it stays up, and an honest visibility verdict that marks daylight and eclipsed passes as unwatchable instead of padding the list. Our SGP4 was validated against the standard Spacetrack test vectors and against independent live ISS telemetry, agreeing to well under a kilometre. Your location is used only to compute look angles and is never stored.
The circle is your whole sky: the outer ring is the horizon, the centre is straight overhead. A dashed segment means the satellite is in the Earth’s shadow and invisible.
Each square counts visible passes that day; the number below is the best peak elevation. Filled squares are days with a pass above 40°.
Scans the next 24 hours for every pass above 20° that is genuinely visible from your location.
How to Use the Satellite Pass Predictor
Confirm the location at the top — it is detected from your connection, but a pass prediction is only as good as the coordinates behind it, so override it if you are planning to observe from somewhere else. Pick a satellite, and read the hero: it gives the next pass that is actually visible, not simply the next time the satellite is above your horizon. Then work down the pass list. The three numbers that decide whether a pass is worth going outside for are peak elevation, magnitude, and the compass bearing you need to face.
The Visibility Window: Why Timing Beats Position
Satellites in low Earth orbit do not glow. They are visible only as reflected sunlight, which creates a strict three-part condition: the satellite must be above your horizon, it must be in sunlight, and you must be in darkness. At an altitude of roughly 420 km the ISS stays lit well after the ground below it has gone dark — that is the whole trick. The result is two viewing windows per day, each opening around 90 minutes either side of sunrise and sunset. At local midnight the station is deep in the Earth’s shadow and completely invisible even when it passes directly overhead. This tool applies all three tests and labels any pass that fails one as daylight or eclipsed, rather than listing it as if you could see it.
Reading a Pass: Elevation, Bearing and Duration
- Peak elevation is the angle above the horizon at the pass’s highest point, where 0° is the horizon and 90° is the zenith. Below 20° you are looking through a lot of atmosphere and probably a lot of trees. Above 45° is a good pass; above 70° is excellent.
- Compass bearing tells you where to stand and which way to face. Passes are listed rise → peak → set, so “WSW → S → NE” means it appears low in the west-southwest, climbs through the south, and leaves to the northeast.
- Duration is how long the satellite stays above 10°. A high overhead pass typically lasts about six minutes; a low grazing pass may only give you 90 seconds.
- Magnitude runs backwards — smaller is brighter. The ISS peaks near −3.5, brighter than any star or planet. Hubble is far dimmer at around magnitude 2, and needs a genuinely dark sky.
Shadow Entries: The Best Thing to Watch For
On evening passes the station frequently flies into the Earth’s shadow partway across your sky. It does not set; it fades, dimming from unmistakably bright to invisible over about ten to twenty seconds, often high above the horizon. Morning passes run the process backwards: the station materialises out of apparently empty sky as it exits the shadow. Because the Earth has an atmosphere rather than a hard edge, the fade often carries a faint orange cast — you are watching the station lit by sunlight refracted through every sunrise happening on the planet at that moment. The sky map marks shadowed portions of the track with a dashed line so you know to expect it.
Why SGP4, and What Limits the Accuracy
Two-line element sets are not raw positions — they are mean orbital elements fitted for one specific propagator, SGP4, with the periodic effects of Earth’s oblateness and atmospheric drag deliberately averaged out. Feeding them to a simple Keplerian model introduces errors of tens of kilometres within a day, which is the difference between a pass overhead and a pass you cannot see. This tool runs the real algorithm, including the J2 and J4 zonal harmonic terms and the drag model driven by the element set’s own B* coefficient.
The practical limit is not the maths but the freshness of the elements and the unpredictability of the upper atmosphere. Solar activity inflates the thermosphere and changes drag from day to day, and the ISS additionally performs periodic reboost burns that no propagator can anticipate. Expect timing accurate to a few seconds one to two days out, tens of seconds at a week, and treat anything past ten days as a planning sketch that will shift. Re-check the day before you plan to observe.
Practical Observing Notes
Get outside two or three minutes early and face the rise bearing — the station appears low and already moving, and it is easy to miss the first thirty seconds. No equipment helps: the ISS moves too fast for a telescope to track by hand, and binoculars narrow your field of view for no gain in brightness. What does help is a clear horizon in the rise direction and letting your eyes adapt for a few minutes. The station shows a steady light with no flashing — if what you are watching blinks, it is an aircraft. Pair this with the Golden Hour Planner to know exactly when your sky goes dark, or check the Aurora & Space Weather Forecast for the geomagnetic conditions that also drive orbital drag. More in Math & Science tools.
Frequently Asked Questions
Why can I only see the space station near sunrise and sunset?
A satellite is visible only when it is still lit by the sun while the ground beneath it is already dark. That geometry occurs in a window roughly 90 minutes either side of your local sunrise and sunset. In the middle of the night the station passes through the Earth's shadow and vanishes even though it is directly overhead.
What does the magnitude number mean?
Magnitude is the astronomical brightness scale, and it runs backwards: lower numbers are brighter. The ISS at a high pass reaches about magnitude -3, brighter than Venus and far brighter than any star. Magnitude 6 is roughly the naked-eye limit under a dark sky, so anything above about 4 needs binoculars.
How accurate are these predictions?
The orbital elements come from Celestrak and are refreshed several times a day, and the propagation uses SGP4, the same model the elements are designed for. Timing is typically accurate to a few seconds for a pass in the next day or two, degrading gradually over a week as atmospheric drag deviates from the model. Predictions more than about ten days out should be treated as provisional.
Why does the station suddenly disappear partway across the sky?
It has entered the Earth's shadow. This is called a shadow entry, and it is one of the more striking things to watch: the station fades from full brightness to nothing over about fifteen seconds rather than setting below the horizon. The reverse, a shadow exit, makes it appear out of empty sky mid-pass.
What elevation counts as a good pass?
Peak elevation is measured in degrees above the horizon, where 90 is straight overhead. Passes peaking below about 20 degrees are low, brief and often blocked by trees or buildings. Anything above 45 degrees is a genuinely good pass, and above 70 degrees the station tracks almost directly overhead and is at its brightest because it is closest to you.
