Visibility methodology
Every visibility label in OrbitWatch comes from the rules below. They are deliberately simple and deliberately conservative, and the section on what this method cannot tell you is as important as the rest.
The rule
A satellite is visible to the naked eye when it is lit by the Sun while the observer is in darkness. It shines by reflected sunlight, so it must be illuminated; and the sky must be dark enough for that reflection to stand out. In practice this makes the hours after dusk and before dawn the only useful ones — in the middle of the night, satellites in low orbit are inside Earth’s shadow.
Each pass is classified as one of:
- Likely visible
- The spacecraft is in full sunlight, the observer’s sky is below civil twilight, and the pass reaches at least 30° above the horizon.
- Possibly visible
- The spacecraft is lit — fully or in penumbra — and the sky is dark, but the pass stays low or the object is entering shadow. Worth looking for; easy to miss.
- Daylight
- The Sun is higher than 6° below the observer’s horizon. Only a handful of exceptionally bright objects are visible in these conditions, so no pass is reported as visible however good its geometry.
- Satellite in shadow
- The object is inside Earth’s umbra and reflects no sunlight, however dark the sky is.
- Not optically favourable
- Anything else — the conditions do not meet the rule above.
The numbers
| Quantity | Value | Why |
|---|---|---|
| Minimum elevation for a pass | 10° | Below this, terrain, buildings and atmospheric extinction make a pass largely academic. |
| Minimum elevation for “likely visible” | 30° | Low passes sit in haze near the horizon and are usually blocked by something. |
| Civil twilight | Sun 6° below the horizon | The point at which the brighter satellites begin to stand out against the sky. |
| Astronomical twilight | Sun 18° below the horizon | Full darkness; fainter objects become reachable. |
| Search window | Dusk to dawn | “Tonight” is the next period below civil twilight, not the next 24 hours. |
Which objects are searched
Visible tonight searches CelesTrak’s visual group — a curated list of roughly 150 objects bright enough to look for with the naked eye — and not the full catalogue of more than 16,000.
This is a real restriction, and it exists because the orbital elements OrbitWatch ingests contain no information about brightness at all: no size, no albedo, no shape, no attitude. Applying the lighting rule to the whole catalogue is perfectly possible, and produces roughly 3,600 “optically favourable” passes over a single location in a single night — nearly all of them Starlink satellites and spent rocket debris that nobody could pick out of the sky. A list like that looks authoritative and is useless. Group membership is the only published statement about which objects can actually be seen, so it is what we use.
Reading the sky chart
Expanding a pass draws it on the hemisphere of sky above you. The centre is the zenith, straight overhead; the rim is the horizon; and the radius is linear in angle, so a point halfway to the centre is 45° up. The rings are drawn at 30° and 60°.
North is at the top and east is at the right — the orientation of a compass held flat. Star charts use the opposite convention, mirroring east and west, because they depict the sky as seen looking up rather than the ground as seen looking down. Both are correct; this one is chosen because the action that follows reading the chart is turning to face a bearing. The chart labels its cardinal points so the convention never has to be guessed.
The arc is drawn solid where the spacecraft is sunlit and dashed where it is in Earth’s shadow, with the crossing point marked. This matters more than it sounds: a satellite entering the umbra partway across simply disappears while still high in the sky, and a chart drawing one unbroken arc would be showing a pass that does not happen.
What this method does not tell you
- No predicted brightness. OrbitWatch does not quote a magnitude. Doing so needs a per-object model of size, shape, surface and orientation that public catalogues do not publish; a number derived from orbital elements alone would be invented precision.
- No weather. Cloud, haze and light pollution are not modelled. A “likely visible” pass under overcast is not visible.
- No local horizon. Elevations are geometric, measured from an ideal flat horizon. Hills, buildings and trees are not accounted for, and atmospheric refraction — which lifts objects near the horizon slightly — is not modelled either.
- No flares. Brief specular glints from flat surfaces can make an otherwise invisible object momentarily obvious. They depend on attitude data we do not have.
- Predictions degrade with element age. Positions come from SGP4/SDP4 propagation of published elements, and accuracy falls as the elements age. Every panel in the app states the age of the element set it used; a pass computed from three-day-old low-orbit elements can be out by enough to matter.
Sources
Orbital elements and the visual group are courtesy of CelesTrak. Propagation uses SGP4/SDP4 as specified in Spacetrack Report #3 and its 2006 revision. Solar position and illumination are computed locally; nothing on this page depends on a third-party visibility service.