How to read a star chart
East and west are swapped, and once you know why, the whole thing makes sense.
4 min read · Published 23 August 2026
The short answer
Hold the chart above your head with the printed direction matching the direction you are facing. East and west appear reversed compared to a ground map because you are looking up at the sky rather than down at the ground. Dot size indicates brightness, not physical size, with larger dots for brighter stars. Lines between stars are conventional constellation figures, not anything physically present.
Why east and west are swapped
This confuses everyone at first and the explanation is simple.
A ground map is viewed from above, looking down. A star chart represents the sky, which you view from below, looking up. Flipping your viewpoint flips the east-west axis while leaving north-south alone.
The practical consequence: hold the chart overhead, not in front of you, with the edge marked with the direction you are facing pointing that way. Do that and the chart matches the sky. Hold it like a road map and everything is mirrored.
What the symbols mean
| Symbol | Meaning |
|---|---|
| Dot size | Brightness (magnitude) — larger dots are brighter stars, not bigger ones |
| Lines between stars | Conventional constellation figures, drawn for recognition; not physical |
| Dashed boundaries | Official IAU constellation borders, which divide the entire sky into 88 regions |
| Open circles | Star clusters |
| Squares or rectangles | Galaxies |
| Circles with a cross | Planetary nebulae |
| Dotted band | The Milky Way |
| A curved line across the chart | The ecliptic — the Sun's path, and roughly where planets are found |
The ecliptic line is the most practically useful. Planets are always close to it, so if you see a bright steady object well away from that line, it is not a planet.
Magnitude, which runs backwards
Star brightness uses the magnitude scale, and it is counterintuitive in two ways.
Lower numbers are brighter. Magnitude 1 is bright; magnitude 6 is at the limit of naked-eye visibility under dark skies. The very brightest objects go negative — Sirius is about −1.5, Venus can reach about −4.5.
The scale is logarithmic. Five magnitudes is exactly a factor of 100 in brightness, so each step is about 2.5 times. A magnitude 1 star is 100 times brighter than a magnitude 6 one.
The scale descends from ancient Greek practice, where the brightest stars were called 'first magnitude' and the faintest visible 'sixth'. It was later formalised mathematically rather than replaced, which is why we are stuck with a backwards scale.
Coordinates, if the chart has them
Detailed charts carry a coordinate grid, and it maps directly onto Earth's own.
Right ascension is celestial longitude, measured in hours rather than degrees — 24 hours around the full circle, so one hour equals 15 degrees. Declination is celestial latitude, in degrees, from +90 at the north celestial pole to −90 at the south.
The system is Earth's latitude and longitude projected onto the sky. A star at declination +51 passes directly overhead from latitude 51 north, which is a neat consequence worth knowing.
AsterNym's star catalogue gives right ascension and declination for every named star, which is what you would use to find one on a detailed chart.
Charts versus apps
Apps are extremely good at answering 'what is that?' — point the phone and it tells you. They are poor at teaching the sky, because they remove the need to work anything out.
The people who learn the sky fastest tend to use a paper chart or planisphere to plan, then an app to check. The chart forces you to find the pattern; the app confirms it.
Whichever you use, keep the screen dim and red. A phone at full brightness undoes twenty minutes of dark adaptation in one glance.
Common questions
- Why are east and west reversed on a star chart?
- Because a star chart shows the sky viewed from below, while a ground map shows the earth viewed from above. Flipping the viewpoint reverses the east-west axis. Holding the chart above your head rather than in front of you resolves it.
- What does dot size mean on a star chart?
- Brightness, not physical size. Larger dots indicate brighter stars, following the magnitude scale in which lower numbers are brighter and each step of one magnitude is about 2.5 times in brightness.
- What is a planisphere?
- A rotating two-disc star chart where you dial in the date and time and a window shows the sky at that moment. It teaches how the sky moves better than an app does, because you can spin it forward and watch the rotation.
- What are right ascension and declination?
- Celestial coordinates equivalent to longitude and latitude projected onto the sky. Right ascension is measured in hours, with 24 hours around the full circle, and declination in degrees from +90 at the north celestial pole to −90 at the south.