The Ordered Cosmos

Right now

Tonight's sky

The dome over the coast, computed as you read

The whole sky over the Delmarva coast at this minute, in three dimensions. The stars, the constellations, the Milky Way, the road of the ecliptic with the signs along it, the planets, and the Moon lit the way she is tonight. Drag it to look around.

The strip on the front page shows the planets along their road. This is the road itself, seen from the only place anyone has ever seen it from: standing on the ground, looking up, with the whole dome overhead. Drag it to look around. Scroll to come closer. Slide the dial and the night turns over you the way it will.

How to read it

The white ring at the foot of the dome is the horizon, with the four directions on it. The dashed gold line is the ecliptic, the road the Sun walks in a year and the planets keep to, and the twelve names along it are the signs, set where the tropical zodiac puts them, which is the coordinate system your chart is drawn in. The brighter a star, the larger its point; the hotter it is, the bluer, so Rigel is blue-white and Betelgeuse is amber, the way they are outside. The faint gold threads are the constellations, drawn the way the astronomers' union draws them. The haze is the Milky Way, the edge of our own galaxy seen from inside it. The planets are named. The Moon is drawn with her true phase for this minute, so the lit side faces where the Sun is, even when the Sun is under the ground.

When the Sun is up, the sky goes blue and the stars fade, because they do. Push the dial forward and watch them come back.

What it is computed from

The same ephemeris the chart next door uses, run in your browser, places the Sun, the Moon and the planets for this minute and this spot on the Earth. The sixteen hundred stars are every star to the fifth magnitude in the HYG catalogue, by way of the d3-celestial star atlas, with the constellation lines and the names from the same source. The dome is drawn for the Delmarva coast unless you give it your own spot, in which case it is drawn for your sky and the stars rearrange themselves accordingly.

The honest part

What you see outside has fewer stars than this. The fifth magnitude is the limit of a good eye under a dark sky, and a coast with towns on it does not offer one; on an ordinary night you will see the first three magnitudes and be glad of them. The stars near the horizon are drawn where the air bends them, roughly, not exactly. The constellation lines are one convention among several, and the names are the ones in use, not the only ones there have been.

Two shapes are in this picture before anything else is drawn. The dome is half a sphere and the horizon is a circle, and neither of them is in the sky at all: they are what looking equally far in every direction makes. Why that keeps happening is on the shelf.