By Alan MacRobert
The movements of the stars have taxed the human intellect throughout the ages — from ancient Babylonians seeking to predict sky events, to Greek philosophers wrestling with the structure of the universe, to beginning skywatchers today trying to figure out where's the Orion Nebula.
The turning of the celestial sphere perplexes everyone who takes up skywatching, but sooner or later the picture snaps into place and the whole setup becomes obvious. However, those who think the sky's motion is inherently simple should try explaining to a beginner why every star follows a different curved path across the sky at a different speed. And why do some stars move from west to east while most move east to west? Can you explain why some constellations turn somersaults during the night while others just tilt from side to side?
To bring the sky's motion down to Earth, astronomers for millennia have built little mechanisms that duplicate it. A working model not only illustrates how the sky turns but can help locate objects at any given time. The simplest sky model is a planisphere.
Untold numbers of these star finders have been designed and published in the last century. Even the most experienced observers rely on them, especially at unfamiliar hours of the night. The word "planisphere" simply means flat sphere. It has a map of the sky that pivots at the celestial pole. As the map revolves around the pivot, it slides under a mask that represents your horizon. Turning the map mimics the apparent daily motion of the sky, complete with risings and settings at the horizon edges.
The basic idea behind the planisphere was used in ancient Rome. The architect and engineer Vitruvius, writing around 27 B.C., described a star map engraved on a solid plate and a horizon mask that rotated over it to show the risings and settings of celestial bodies. A water clock turned the mask once a day to keep up with the sky. Nearly two centuries later, Claudius Ptolemy analysed the map projections used for such devices in his treatise Planisphaerium.
By the 4th century A.D. a version known as the planispheric astrolabe was in use. Its star map was a skeletal metal framework sliding over a solid plate engraved with the observer's horizon. Medieval Arabs and Persians refined the astrolabe to a peak of versatility and beauty. Some of these ornate "mathematical jewels" made their way to Europe, where they were prized as almost magical. "All the conclusions that have been found, or might be found in so noble an instrument as an astrolabe, are not known perfectly to any mortal man in this region," wrote Geoffrey Chaucer in 1391. By the end of the Middle Ages, astrolabes were the universally recognised trademark of astronomers and astrologers.
Astrolabes were commonly used to sight on the Sun and stars to tell time. The invention of accurate clocks allowed the procedure to be reversed. If you knew the time, you could use this kind of device to find stars. And that is how planispheres have been employed ever since.
Using a Planisphere
In principle nothing could be simpler. You turn a wheel to put your time next to your date, and presto, there's a custom-made map of the stars that are above your horizon for that moment. The edge of the oval star map represents the horizon all around you, as you would see if you were standing in an open field and turned around in a complete circle. The part of the map at the oval's centre represents the sky straight overhead — pretty much like the all-sky map in Australian Sky & Telescope each month.
In practice, several complications can throw beginners off.
First, sadly, many planispheres on the market are so poorly designed that they would frustrate an expert. The worst problem is carelessly drawn star maps that make all stars, whether very bright or very faint, look too much alike. Planispheres with gimmicky glow-in-the-dark stars are the worst offenders in this regard; the glow paint can't be printed very accurately, so all stars come out looking more or less alike. The result is star patterns that look almost nothing like what you'll see in the sky.
Other issues are inherent to even well-designed planispheres. One is that the map is necessarily small and distorted. It compresses the entire celestial hemisphere above and around you into a little thing you hold in your hand. So be aware that star patterns will appear much bigger in real life than on the map.
Because the map is so small, moving your eyes just a little way across it corresponds to swinging your gaze across a huge sweep of sky. The east and west horizons may look close together on a planisphere, but of course when east is in front of you, west is behind your back! Glancing from the map's edge to centre corresponds to craning your gaze all the way from horizontal to straight up.
There's only one way to get to know a map like this. Hold it out in front of you as you face the horizon. Twist it around so the map edge labeled with the direction you're facing is at the bottom. The correct horizon on the map will now be horizontal and match the horizon in front of you. Now you can compare stars above the horizon on the map with those you're facing in the sky. Ignore all the parts of the map above horizons you're not facing.
Then there's the distortion issue. On a planisphere designed for use in the Southern Hemisphere, constellations in the northern part of the sky are stretched sideways, taffy-like, making it hard to compare them with real star patterns. This problem doesn't exist on a well-designed map drawn for fixed dates and times — such as the one in the centre of each month's Australian Sky & Telescope. Some planisphere designers have come up with a partial solution. The Sky & Telescope Star Wheel presents two maps, one on each side. One side minimises distortion south of the celestial equator, the other north of it. Just flip it over to choose the best view.
A further complication is that a planisphere works correctly for only one narrow range of latitudes on Earth. Fortunately, many models are made in several editions, each for a particular latitude range.
Then there's the matter of daylight saving time. When this is in effect (from the last Sunday in October to the last Sunday in March for much of Australia), remember to "fall back" to standard time by subtracting an hour from what your clock says before you set the planisphere's dial.
Actually, planispheres don't work in standard time either, but rather in something called local mean time. The difference between the two, which depends on where you live in your time zone, can amount to a half hour or more. It results from the fact that the Earth is round — and that the curvature of the Earth's surface is noticeable even when you go a few kilometres east or west.
Fortunately, even a half hour one way or the other doesn't really matter for most star finding. In fact, if you just want to know which constellations are up and where they are, a planisphere's limitations can largely be overlooked. It's remarkable that such a simple working model of the sky can work so well.