Accuracy

How we draw your sky

The chart is the sky over the town you pick, at the minute you pick. It is calculated on your device.

Stars

Star positions come from the HYG Database v4.0. The site draws the stars above the horizon and, when you ask, the lines of the constellations and the names of the bright ones.

Moon, planets and the clock

The Moon’s phase and the planets Mercury to Saturn are calculated with astronomy-engine for that instant. The town’s time zone is used, so a time in Britain in summer is British Summer Time, not a fixed offset from Greenwich.

The disc is drawn with north at the top and east to the right. The horizon is the ring. A star near the middle of the ring is high overhead.

A worked example

London on 21 June 2019. This site’s clock for that day is sunrise at 04:43 and sunset at 21:21. A published almanac for the same day gives 04:44 and 21:20. The difference is about a minute.

Two older checks, from published sources: 20 July 1969 at 20:17 UTC is a waxing crescent, about 33% lit. 11 August 1999 at 11:03 UTC, the eclipse, is a new moon.

What the chart leaves out

Cloud, weather and street light are not in the calculation. If the Sun is up, the stars are still placed where they sit above the horizon, and the page says the Sun is up. The Moon is drawn larger than it would look, so it survives on paper.

Download a watermarked sample PDF. The paid file is the one to print. The print guide explains actual size.