How do you use a planisphere, and is an app better?

To use a planisphere, turn the star disc until tonight’s date lines up with the current time, then hold it up with the direction you’re facing at the bottom. The window shows the stars above your horizon. It’s quick, needs no battery and won’t spoil your night vision, but it can’t show planets, the Moon or satellites. A sky app does those.

Using a planisphere

1. Check it’s made for your latitude, printed on the front (for example “40° N”).
2. Find tonight’s date on the outer rim.
3. Turn the disc so the date lines up with the time on the cover. Use standard time; subtract an hour during daylight saving time.
4. Face a direction and hold the planisphere with that compass point at the bottom.
5. Match the brightest stars first, then fill in.

Use a dim red light to read it.

Where it falls short

• Planets: they wander, so they’re not printed.
• Distortion: the edges of the window are stretched, so shapes near the horizon look wrong.
• Latitude: wrong latitude, wrong sky.

Where an app does better

	Planisphere	Sky Map app
Planets, Moon	No	Yes
Satellites	No	Yes
Point at the sky to identify	No	Yes, live AR view
Battery, screen glow	None	Yes
Works offline	Always	Core sky map works offline

Sky Map (my app) covers the gaps above: planets, satellites and events. Many observers use both, the planisphere for planning and dark-adapted eyes, the app for “what is that bright one?”

Related: keeping your night vision with a phone, altitude and azimuth and the Sky Map guides.