Make a Planisphere

I’ll show you how to build and use an astronomical computer. It’s not a digital computer, like you have on your desk at home, but an analog computer, like a slide rule. Analog computers have been around for hundreds of years. In fact, Galileo made a lot of his income from inventing and manufacturing an analog computer, then teaching people how to use it.


This analog astronomical computer is called a planisphere – literally a flattened sphere. There are two parts to it. The first is a
round map of the stars visible from our latitudes. The other is an oval window representing the horizon; it lets you see the stars that are visible at a particular time. Both are centered on the North Star, Polaris, and connected by a pin so the horizon window can be turned, showing stars visible at different times and hiding the rest. Each has markings on its rim to show dates or times. Planisphere parts can be downloaded from the internet, ( or you can buy a ready-made planisphere at a science store. If you do not like the constellation shapes drawn on it, carefully use permanent markers to change them.

Using a planisphere is simple. To start, you just align the date on one circle with the time on the other. For example, if you wanted to know what the stars will be like on November 10 at 9:00 p.m., you just find November 10 on one circle and 9:00 p.m. on the other and turn the horizon circle until the date and time line up. It’s a good idea to paper-clip the planisphere’s parts together, so it keeps its setting. Next, face the direction you want to observe, southeast for example. Hold the planisphere so its southeast horizon is pointing at your stomach, then lift it so it is above you. The planisphere’s southeast horizon now points southeast, its northern horizon now points north, its west points west, and so on. (Did you notice that its east and west horizon markers seemed reversed, at first? That’s because it’s made to be held overhead.) The stars and constellations you see on the planisphere now match the sky’s stars at the chosen date, time and direction.

You can learn the positions of the constellations from your planisphere, but not the positions of the Moon and planets. You can learn a lot about stellar motions, though. For example, set your planisphere to show the sky tonight at 8:00 p.m. Have a careful look at what stars and constellations are near the east and west horizons. Now turn the date and time circles to set it for 9:00 p.m., then 10:00 p.m. This shows how the stars progress across the sky in an evening. Now set it for tomorrow night at the same time to see how little difference there is in 24 hours. If you continue like this, you can see how the constellations change with the seasons (March-June-Sept-Dec). By the way, did you notice that there is no marking for the year?

Here’s another thing to do with your planisphere, to help you learn the sky’s mechanics. Measure the distance from Polaris to the northern horizon, then cut a circular hole with that radius in a piece of paper. Put this mask over the planisphere, centered on Polaris, and you will see that it shows only the circumpolar constellations. No matter how you turn the stars, no other constellations will enter the small circle, and none will leave it.

All these little learning tricks work well in classrooms. I made a planisphere for demonstrating on the overhead projector by photocopying both planisphere parts onto overhead transparencies. The pin that holds them together is nearly invisible – I aligned it with the handle of the Little Dipper.

The planisphere shows how the stars change hourly, nightly and seasonally, but does not tell why they change. The hourly changes are caused, of course, by the Earth’s rotation. A full circle in a day makes for 15 degrees in an hour, so an hour at night sees the stars move a full fifteen degrees across the sky – thirty times the diameter of the full Moon. In one day, though, the Earth also slips about one degree along its orbit around the Sun, and that causes the daily and seasonal changes in the stars’ positions. The difference from one night to the next is so slight you can hardly pick it out on the planisphere. That single degree – just two lunar diameters – is the equivalent of four minutes of time. In the course of a year the four minutes per day adds up to almost exactly 24 hours. The stars are where they were a year ago, and the planisphere needs no marking for the year.

If you have an extra planisphere star map, try circling the constellations I cover in Sky School. It will be good review, and you will get a better idea of how they are placed in relation to one another. You’ll also see which ones I have not done.

Sky School