How Satellite Pass Predictions Work
Orbital Elements and Propagation
Satellite pass predictions use Two-Line Element sets (TLEs) — compact mathematical descriptions of a satellite's orbit — to calculate exactly where each object will be at any future time. The SGP4 propagation algorithm takes these orbital elements and models the satellite's position second by second, accounting for Earth's gravitational field, atmospheric drag, and solar radiation pressure.
Visibility Conditions
To predict a visible pass, the algorithm must satisfy three conditions simultaneously: the satellite must be above your local horizon (typically 10° elevation or higher), the satellite must be illuminated by the Sun (not in Earth's shadow), and your observing location must be in darkness or twilight (so the sky is dark enough to see it). This is why satellites are best seen within 1–2 hours after sunset or before sunrise.
Brightness and Look Angles
Orbital Radar's predictions compute look angles (azimuth and elevation), solar illumination state, and estimated visual magnitude (brightness) for thousands of satellites. The ISS is the easiest to spot — it can reach magnitude −4, brighter than any star. Recently launched Starlink trains are also spectacular for a few days after deployment. The Tiangong space station is another bright target, typically reaching magnitude −2.
When Can You See Satellites?
The Twilight Window
The optimal viewing window is during astronomical twilight — when the Sun is 6–18° below the horizon. At this time, the sky is dark enough for satellites to be visible, but objects in low Earth orbit (200–600 km altitude) are still catching sunlight. During the middle of the night, most LEO satellites pass through Earth's shadow and become invisible. Geostationary satellites at 35,786 km remain sunlit all night, but they appear stationary and are very faint.
Seasonal Variation
Season matters too: around the summer solstice at high latitudes, the Sun never dips far below the horizon, meaning satellites can be visible for much of the short night. Winter offers fewer but higher-quality passes in dark skies. See our guides on how to see the ISS tonight and how to see Starlink tonight for detailed tips. You can also check the space debris map to see all tracked objects orbiting overhead.
What Makes a Good Pass?
Elevation, Brightness, and Duration
The best passes have three qualities: high maximum elevation (the satellite passes high overhead, not just skimming the horizon), bright magnitude (easily visible without straining), and long duration (more time to watch and photograph). A pass that reaches 70–90° elevation, lasts 4–6 minutes, and has a magnitude below 0 is spectacular. The brightest satellites page ranks the easiest objects to spot.
The Pass Rating (Spectacle Score)
Orbital Radar assigns each pass a Pass Rating (also known as the Spectacle Score) that combines these factors into a single 0–100 rating, making it easy to find the best passes at a glance. The score also considers whether conditions favour viewing — factoring in twilight timing and solar elevation. For photography tips, see our satellite photography guide.
Understanding the Sky Chart
Reading the Polar Projection
The animated polar sky chart above shows your sky as a circular map. North is at the top, east is on the right, south at the bottom, and west on the left. The outer edge represents your horizon (0° elevation) and the centre is the zenith — the point directly above your head (90° elevation).
Colour Coding and Markers
Each moving dot on the chart represents a satellite currently making a visible pass. The dot's colour indicates brightness: gold for bright passes (magnitude < 2), cyan for medium brightness, and grey for faint objects. The ISS gets a distinct marker. Arcs show the predicted path each satellite will trace across your sky.
How Many Satellites Are in Orbit?
As of , there are approximately 14,200 active satellites in orbit, with over 7,000 belonging to SpaceX's Starlink constellation. Of these, several hundred are potentially visible from any given location at twilight. Use the satellite directory to explore individual objects, or browse satellites by country or by operator. To understand the different orbital paths these satellites follow, see our guide to types of orbits.
Start tracking now: Set your location above to see what's visible tonight, or explore the Space Library for 175+ guides covering everything in orbit. Check the launch schedule for upcoming missions that will add new visible objects to the sky. To deepen your understanding of orbital mechanics and how satellites move, visit the Orbital Academy — free interactive lessons from beginner to intermediate.