Hold it up, not down
To read a star map, turn it so the direction you are facing is at the bottom edge, then hold it up against the sky. The center of an all-sky chart is the point straight overhead, the edge is your horizon, and bigger dots are brighter stars. That’s most of it. The rest is learning to translate distances on paper into distances on the sky.
One thing confuses almost everyone at first: on a star map with north at the top, east is on the left. That’s the reverse of a road map. A road map shows the ground as seen from above; a star map shows the sky as seen from below. Lie on your back with your head to the north and your feet to the south, and east really is on your left.
Reading an all-sky chart
The most useful chart for beginners is a round all-sky map drawn for a particular date, time and latitude. It shows everything above your horizon at that moment, with the four compass points around the rim.
- Check the chart matches your night. A chart drawn for 10 p.m. in early month is also roughly right for 9 p.m. at the end of the month, because the stars rise about two hours earlier each month. It should also be drawn for a latitude within about 10° of yours.
- Face a direction and rotate the chart. If you face south, hold the chart so “South” is at the bottom. The lower part of the chart now matches the sky in front of you, from the horizon up.
- Find the brightest stars first. Match the two or three largest dots on that part of the chart to the brightest stars in front of you, then work outward along the constellation lines.
Remember that the chart’s center is overhead. Stars printed halfway between center and edge are about halfway up the sky, which is higher than it feels when you’re looking.
Dots, lines and symbols
| On the map | Means |
|---|---|
| Larger dot | Brighter star (lower magnitude number) |
| Thin lines between stars | A constellation figure, drawn for convenience; the lines are not in the sky |
| Dashed or curved line across the chart | Usually the ecliptic, the path the Sun, Moon and planets follow |
| Shaded band | The Milky Way, visible only from darker sites |
| Small circles, ovals, squares | Star clusters, galaxies and nebulae; the key on the chart says which is which |
Dot size follows the magnitude scale, where lower numbers are brighter. Most charts show stars down to about magnitude 5 or 6, the limit of the eye under dark skies. From a city you will see far fewer stars than the map shows, so rely on the biggest dots.
Constellation figures differ from map to map. Some draw Ursa Major as a whole bear, others only the Big Dipper. The official constellations are 88 regions with fixed boundaries; the stick figures are just a memory aid. Browse them all on constellations.
From the page to the sky
Distances on a chart look small, but the sky is huge. Use your hand at arm’s length as a ruler:
- The width of your little finger covers about 1°, roughly twice the width of the full Moon.
- Three middle fingers together cover about 5°, close to the gap between the two “pointer” stars of the Big Dipper.
- A closed fist covers about 10°.
- From the horizon to straight overhead is 90°, or about nine fists.
Patterns are almost always bigger in the sky than people expect. Orion spans roughly two fists from top to bottom, and the Big Dipper is about the length of a spread hand.
Using a planisphere
A planisphere is a rotating star wheel: a disk of stars behind a window cut to the shape of your horizon. Line up today’s date on the edge with the current time, and the window shows the sky right now. It never needs batteries and works for any night of any year.
Two things to know. Planispheres are made for a band of latitude, so buy one for yours. And most use standard time; during daylight saving time, subtract an hour from the clock before setting the dial. Because the sky is squeezed onto a flat disk, constellations near the edge of the window appear stretched; that’s normal.
What a star map won’t show
Planets and the Moon. They move along the ecliptic from night to night, so printed charts usually leave them out or show them for a single date. If you see a bright “star” near the ecliptic that isn’t on your map, it’s probably a planet. Check planets visible tonight or the planets page.
Your local horizon. The chart assumes a flat, open horizon. Buildings and trees hide the lowest part of the sky, so stars printed near the edge may be out of view, and they are dimmed by the thicker air near the horizon even when they’re not blocked.
Satellites, meteors and passing events. These change constantly. For what’s happening tonight, see tonight’s sky and events. When you’re ready to put the map to work, the stargazing for beginners guide walks you through three signpost patterns.