What's that bright light in the sky?
Say where you are and roughly where the light is. This works out every planet and bright star above you from the published formulas, dims each one by the air it shines through, and tells you which one you're looking at, how sure that is, and how to check it with your own eyes.
1Where are you, and when did you see it?
Or type coordinates
Waiting for a place.
Typing a place sends what you type to OpenStreetMap's place search, which finds its coordinates; that is the only thing this page ever sends. Using your device's location sends nothing: the position is written after the # in the address (browsers never send that part to any server), rounded to about a kilometre, so you can reload or share it.
What this cannot see
- Anything that moves across the sky in a minute or two. Aircraft, drones, balloons, satellites and meteors are not in it. Planets and stars keep their places among each other and drift slowly west together, so if your light visibly moves against the stars, it is none of the lights listed here. A light on its own can seem to wander when it is still, though: small movements of your own eyes do it, so hold it against a roof edge, a branch or a star beside it before deciding. How to tell what it might be.
- The weather and the glow of a town. The dimming it applies is a standard clear-night model for the air at sea level, or at the height a link gives. Haze, smoke or humidity dims low lights more, cloud hides them, and city light washes out the fainter ones.
- Your horizon. It assumes a flat, open horizon. Hills, trees and buildings hide lights it says are up, and set them earlier than it says.
- Anything fainter than the brightest stars. It carries the five naked-eye planets, the Moon and the 62 brightest points among the stars (everything brighter than magnitude 2.15), but not a comet, a new star, or the fainter stars around your light.
If it moves. Aircraft carry a red light on the left side, a green one on the right and a white one at the rear, with flashing red or white anticollision lights (that is the US rule for airliners). The Space Station looks like a very bright star moving across the sky, without flashing or changing direction. A line of lights moving together can be a string of Starlink satellites soon after their launch. For satellites, past ones included, use azmth, Heavens-Above or Astroviewer, which compute them.
Lift the hood: the numbers, and where they come from
| Light | Height ° | Bearing ° | Magnitude | Airmass | Dimming | As seen |
|---|
Positions. The Sun, the Moon and the planets come from Sky Now's engine (Meeus's Astronomical Algorithms and JPL's published planetary elements), already checked against JPL Horizons. The stars are Sky Now's too: the brightest of the Hipparcos catalogue, each moved along its own motion to the moment shown. This page adds the Moon's parallax (up to about a degree, which matters when the question is "the light next to the Moon") and applies refraction at every height.
Brightness. Each planet's magnitude comes from the equations Mallama and Hilton published in 2018 for The Astronomical Almanac. Run on this engine's positions, they reproduce JPL Horizons' own magnitudes for Mercury, Venus, Jupiter and Saturn (rings included) to within a hundredth of a magnitude on every date checked from 2020 to 2032. Mars is computed without the paper's two small corrections for its rotation and season, which together span about a quarter of a magnitude, so it can be off by a little over a tenth. A star's magnitude is the Hipparcos catalogue's.
The air. Light from low in the sky crosses far more air than light from overhead: about 2 times as much at 30°, 5.6 times at 10°, 38 times at the horizon. Each light is dimmed by its airmass times an extinction coefficient from Green's 1992 model (0.28 magnitude per airmass at sea level), the same calculation JPL Horizons makes for its own extinction column.
How it decides. It never guesses a probability, because it knows nothing about your eyes or your weather. It ranks the lights where you pointed by how bright each looks from where you are, and its words follow a stated rule: almost certainly when the brightest outshines the next by at least 1.5 magnitudes (four times the light) and nothing else there is within one magnitude of it; most likely when the gap is at least 0.75 magnitude (twice the light); and otherwise it says brightness alone cannot decide, and tells you how to tell them apart.
Hand it to someone, or to an assistant. Everything is in the address: ?lat=51.51&lng=-0.13&t=2026-11-06T19:30Z&dir=SW&alt=low, or near=moon, or brightest=1. The same parameters work after a #, which this page uses itself so a location never reaches a server. Without t it means now. An assistant can also call the same engine as the which_light tool on this site's MCP server.
The check. research/bright-light/verify.mjs compares this engine with JPL Horizons for every planet every five days from 2020 to 2032, and for positions, airmass and dimming at five sites, and fails on purpose when its own checks are broken.