How to use it
Pick the month, the time on your clock and the part of the UK you are in. The chart redraws straight away. It shows the sky in the middle of that month, on the night of the 15th. Press Print this chart for a black-on-white A4 sheet that uses very little ink.
Take it outside with a red torch if you have one, because red light keeps your eyes used to the dark. Give your eyes twenty minutes and far more stars appear.
How to read the chart
The middle of the circle is the point straight above your head, and the rim is the horizon all the way round. Hold the chart over your head, face down, and turn it until the N is towards north. East is then on your left, which is why the chart looks back to front on a table.
Bigger dots are brighter stars. The lines join the main stars of each constellation into the shapes star charts have used for a long time, and the names sit beside them. A faint ring marks 30° above the horizon and another 60°. The list under the chart says where to look for the brightest named stars.
Planets and the Moon are not on it. They move against the stars from week to week, so a bright "star" that is not on the chart is very often a planet.
How it is worked out
Each star's position comes from the same catalogue as our personalised star map: the d3-celestial star list by Olaf Frohn. The constellation lines and names come from the same open project. Astronomy Engine, an open astronomy library, then works out where each star sits in your sky at that moment, allowing for the slow wobble of the Earth's axis since the year 2000.
The chart is drawn for longitude 1.5° west, roughly the middle of England. The sky turns one degree every four minutes, so in Norfolk the same sky arrives up to about 13 minutes earlier, and in the far west (Cornwall, Northern Ireland, the Hebrides) up to about 25 minutes later. For a star chart that is close enough.
We check the sums against published figures. Polaris stands as high as your latitude, to within the 0.74° it sits from the pole. Sirius crosses due south at about ten past midnight GMT on New Year's night 2027, the same minute a hand sum gives from its catalogue position.
Where to go next
The Plough is the easiest place to start: find the Plough, then follow its pointer stars to Polaris, the North Star. In winter look for Orion in the south; in summer, the Summer Triangle high in the east and overhead.
Written by Craig Fearn, Aphelion Prints. Last updated .