Skill guide
How to use
a planisphere
A planisphere is a rotating star chart that shows the visible sky for any date and time of year. No battery, no screen glare, no monthly subscription.
This page contains affiliate links. As an Amazon Associate I earn from qualifying purchases.
What a planisphere is
A planisphere consists of two discs held together at the centre. The lower disc is a star chart showing all stars visible from a given latitude across the full year. The upper disc is an oval window cut to show only the portion of sky that is above the horizon at any given moment. Rotating the upper disc against the lower one changes which stars are visible through the window.
The edge of the upper disc is marked with hours of the day. The edge of the lower disc is marked with the months of the year. Aligning the current time against the current date shows exactly which stars and constellations are above the horizon right now.
The oval shape of the window represents the horizon. The centre of the oval is the point directly overhead (the zenith). Stars near the edge of the oval are near the horizon; stars near the centre are high in the sky.
Setting it for the current date and time
Hold the planisphere with the current month at the bottom. Rotate the upper disc until the current time aligns with the current date on the outer rim. The window now shows the sky as it appears at that exact moment.
Use the 24-hour clock if your planisphere offers it - it avoids the AM/PM ambiguity that matters on long winter nights when midnight falls in the middle of an observing session. Most UK planispheres mark both.
Note that the time on a planisphere is astronomical time (based on sidereal time, not exact clock time), but the difference for casual observing is negligible. Set it to your current local time and it will be accurate enough to identify any constellation above the horizon.
Matching the planisphere to the sky
Hold the planisphere above your head with the direction you are facing at the bottom. The four cardinal points (N, S, E, W) are marked around the oval window. If you are facing north, hold the planisphere with N at the bottom; the stars shown in that portion of the window will match the stars in front of you.
This orientation feels counterintuitive at first - the planisphere appears upside down relative to how you would read a map. This is correct. A sky chart is viewed from below (looking up), not from above (looking down), so east and west appear reversed compared to a ground map.
Find a bright, recognisable pattern first - Orion in winter, the Plough year-round - and orient from there. Once one constellation is confirmed, the rest follow from it.
Why it beats a phone app
Astronomy apps are useful tools, but they have a practical disadvantage in the field: a bright screen destroys night vision. Human eyes take around 20-30 minutes to adapt fully to darkness. A single glance at an unfiltered phone screen resets this process.
A planisphere needs no light of its own. Under a dark sky, moonlight or a dim red torch is enough to read it. Red light does not reset night-adapted vision, which is why astronomers use red torches and why a physical chart beats a glowing screen for serious observing.
A planisphere also has no battery to run flat, no software to update, and no notification to interrupt a clear night.
Latitude and the UK
A planisphere is manufactured for a specific latitude. At different latitudes, different portions of the sky are permanently above or below the horizon. A planisphere made for latitude 51.5° North - covering London, and most of England and Wales - will not show the correct sky for an observer in Edinburgh (55.9° N) or in Australia.
For observers anywhere in England, Wales, or northern France, a 51.5° N planisphere is accurate enough. Scottish observers may prefer a 57° N version. The difference becomes most apparent at the southern horizon, where a planisphere calibrated further south will show stars that remain below the horizon for the actual observer.
Which planisphere to buy
Philip's produces the standard planisphere for British and Irish observers. The hardback construction and clear acetate window are durable enough for regular field use. The star chart is clean and well-calibrated for 51.5° N. Instructions are printed on the back. It fits easily in a jacket pocket and costs less than a round of drinks.
