Your Birth Sky Star Map

Sirius enters the catalog as three numbers: 101.29, −16.72, −1.4 — right ascension, declination, magnitude. It is the first of 1,627 entries, every star in the HYG database down to magnitude 5: naked-eye stars, though not the faintest a genuinely dark sky would hand you. That list never changes. What changes is where those numbers land, and working that out is the whole job of this star map. Your birth minute and the coordinates of your birthplace fix a local sidereal time, printed in hours in the caption under the plate, and every entry is resolved against it.

Two angles come out of that: altitude and azimuth. Altitude is height above the horizon — 0° at the rim of the world, 90° straight overhead. Azimuth is the bearing, counted from north and running east, which is why the ring carries N, E, S and W with a tick every 15°. Any star whose altitude comes out at or below zero was under the ground at your birth minute, and the star map drops it: not dimmed, not greyed, absent. That is close to half the catalog on any given plate, because a horizon is a great circle and always cuts the sky in two. Stand in the other hemisphere at the same instant and the other half is what survives.

The survivors are laid down azimuthal equidistant — zenith at the centre, horizon at the rim, each star placed at a radius of (90 − altitude) ÷ 90, so a degree of height buys the same distance anywhere on the disc. The two dashed circles mark 30° and 60°. Brightness is written twice: a brighter star takes both a wider dot and heavier ink, so a first-magnitude star reads as solid gold and a fifth-magnitude one as barely a speck. Mercury, Venus, Mars, Jupiter and Saturn are computed separately, in the same apparent frame the rest of this almanac uses, and appear with their glyphs only if they too stood above your horizon that minute.

Magnitude runs backwards — the smaller the number, the brighter the star, which is why Sirius sits at −1.4 and the faintest entries here sit at 5. Magnitude 5 is roughly the limit of unaided eyes on a genuinely dark night, once they have had time to adjust: country dark, no streetlight in view. A properly black sky reaches fainter still, so this is a naked-eye sky rather than every star there is. The other half of that fact is less comfortable. Town light scatters in the air overhead and takes the faint end first, so for most people the plate holds more stars than they have ever seen at once.

Being computed rather than illustrated makes this star map unforgiving about one thing. The Earth turns the sky 15° every hour, so an hour of uncertainty swings the whole plate a full compass point off where it belongs — which is why the form asks for an exact time and will not go on without one. The arithmetic answers to an outside judge in the test suite: sampled catalog stars must land within a tenth of a degree of an independent horizon model, and Polaris must sit within a degree of your latitude, due north, the way sailors have always used it. All of it runs in your browser — the star map, the export, the lot — and none of your birth details leave it.