Sky and telescope magazine el paso10/4/2023 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. Moving your eyes just a little way across the map corresponds to swinging your gaze across a huge sweep of sky. So star patterns appear much bigger in real life than on the map. It compresses the entire celestial hemisphere above and around you into a little thing you hold in your hand. The worst is that a planisphere's map is necessarily small and distorted. In practice, several complications can throw beginners off. The part of the map at the oval's center represents the sky overhead - much like the all-sky map in Sky & Telescope each month, or the 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. 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. And that is how planispheres have been employed ever since. If you knew the time, you could use this kind of device to find stars. The invention of accurate clocks allowed the procedure to be reversed. By the end of the Middle Ages astrolabes were the universal trademark of astronomers and astrologers.Īstrolabes were commonly used to sight on the Sun and stars to tell time. "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. Some of these rnate "mathematical jewels" made their way to Europe, where they were prized as almost magical. Medieval Arabs and Persians refined the astrolabe to a peak of versatility and beauty. Its star map was a skeletal metal framework sliding over a solid plate engraved with the observer's horizon. a version known as the planispheric astrolabe was in use. The sky is portrayed backward, right for left.īy the 4th century A.D. On the plate under them are lines marking the local horizon, altitude, and azimuth. The ornate scrollwork supports 27 points that form a rudimentary star map each point is labeled with the name of a star or constellation. Modern planispheres are direct descendants of the astrolabe, such as this one made in Nuremberg, Germany, in 1532. Nearly two centuries later, Claudius PtolemyĪnalyzed the map projections used for such devices in his treatise A water clock turned the mask once a day to keep up with the sky. 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. The basic idea behind the planisphere was used in ancient Rome. Turning the map mimics the apparent daily motion of the sky, complete with risings and settings at the horizon edges. As the map revolves around the pivot, it slides under a mask that represents your horizon. It incorporates a map of the sky that pivots at the celestial pole. The word "planisphere" simply means flat sphere. Even the most experienced observers rely on them, especially at unfamiliar hours of the night. Untold numbers of these star finders have been designed and published in the last century. To bring the sky's motion down to Earth, astronomers for millennia have built little mechanisms that duplicate it.Ī working model not only illustrates how the sky turns but can help locate objects at any given time. And why do some stars move from west to east while most move east to west?Ĭan you explain why some constellations turn somersaults during the night while others just tilt from side to side? Sooner or later the picture snaps into place and the whole setup becomes obvious.īut 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. ![]() 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 amateurs today trying to point a new telescope at the Andromeda Galaxy.Īt first, the turning of the celestial sphere perplexes everyone who takes up skywatching.
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