About

A star map that shows its working

Most sky apps will name a star for you. Few will say how they worked it out, how far off they might be, or what they are doing with your location. This one does.

What it is

A live map of the sky above wherever you say you are standing: 8,920 stars, 88 constellation figures, the Sun, the Moon and the planets, drawn for the minute on the clock rather than for a generic evening.

• Look. Drag and zoom, switch lines and boundaries on and off, set the light pollution where you live and watch the faint stars go.
• Plan. When it gets dark, when the Moon rises and how much it will ruin, what is up and how high, and how long before it sinks into the murk.
• Learn. 66 guides across 9 categories, each one moving the map while it explains, because paper does not rotate. Tap anything on the map for its story — the myth, who found it, what to look for — or read it first: every constellation, the named stars, the deep-sky objects worth the detour, the asterisms and the solar system each have a page of their own, and the field guide explains the words.

Why it exists

Knowing that Cassiopeia is a W will not help you find it. You also need to know which way up the W is tonight, from here, at this hour, and a static chart cannot tell you.

So this is a map to read rather than obey: the same sky an almanac would give you, from published methods, with the errors stated. Every figure on the credits page is measured, not asserted — the Moon within 8.1″ of JPL Horizons, the planets within 2.5″, twilight within a tenth of a second of an independent solver.

Who made it

Jesper Kiledal, as a side project. The same hands made What the Day, which teaches the Doomsday algorithm for working out the weekday of any date.

What it will not do

• Nothing follows you. There is no account to make, so your progress stays in this browser, and your location never leaves it unless you turn on the cloud forecast, which sends a point rounded to about 25 km. Privacy lists every key.
• No sky recognition from your camera. Orienting yourself is the skill.
• Not accurate enough to guide a telescope mount. Planets sit within 2.5″ of JPL, but stars are drawn at their mean places, without nutation or the aberration of starlight, so a star can be some 20″ off: fine for the eye, binoculars and an eyepiece, too coarse to plate-solve or guide against.
• No current comets, no asteroids, and only three satellites. Their element sets change faster than a static site can honestly ship; the ISS, Tiangong and Hubble fetch fresh ones. The great comets of the past are different — each pass's orbit is finished — so travel to March 1997 and Hale–Bopp is there.
• Not a photograph. Colour is the catalogue's B−V index, or what the spectral class implies where B−V was never measured; size is magnitude.

If something is wrong

It might well be. The credits list the known failure modes: where the series are extrapolating, and where this app and JPL disagree on purpose. A star drawn somewhere it is not is a real bug, and worth saying so.