Three things decide whether a star map is right: the place, the moment, and which stars it chooses to draw. Get the first two right and the stars fall where they really were. Get the time zone wrong and it doesn't matter how pretty the print is.
How many stars can you actually see?
The faintest stars the naked eye can pick out on a really dark night are around magnitude 6 to 6.5. There are roughly 5,000 to 9,000 stars that bright across the whole sky, depending on where you draw the line, and half of them are below the horizon at any moment. From a town, light pollution cuts that to a few hundred.
Our chart plots the 5,044 catalogue stars brighter than magnitude 6.0, which is the sky a person could have seen from a dark place. Some star maps advertise a million or more stars. Those come from survey catalogues full of stars you'd never see without a telescope; on a print they add a fine texture, not accuracy.
The time zone is where charts go wrong
The sky turns about fifteen degrees an hour, so a chart an hour out is a different sky. Local time has to be turned into universal time first, and that depends on the place: British Summer Time, a wedding abroad, a birth in another time zone. Our maps look up the time zone from the place you choose, so the time you type is the time on the clock there.
Dates a long way back or ahead
The stars themselves barely move against each other in a human lifetime, so the same catalogue serves for any date in the 1900 to 2100 range our maps support. We check our charts' star positions against independent references for selected dates in 1900, 2000, 2026 and 2050.
How accurate is a star map? Four checks before you buy
- Does it ask for the time, and say whether that's local time?
- Does it show you a preview of your actual chart before you pay?
- Can it explain which stars are drawn, and from which catalogue?
- Does the place and date print beneath the chart, so the recipient can see what it is?
Ours does all four. The personalised star map shows a free watermarked preview of the exact chart, and what a star map print is covers the basics.