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.
At first, the turning of the celestial sphere perplexes everyone who
takes up skywatching. Sooner or later the picture snaps into place and
the whole setup becomes obvious. But 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 millenniums
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 incorporates 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 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
analyzed 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 universal 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 center represents the sky overhead -- much
like the all-sky constellation map in each month's issue of Sky &
Telescope.
In practice, several complications can throw beginners off. The worst
is that a planisphere's 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 star patterns appear much bigger in real
life than on the map.
Moving your eyes just a little way across the map 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 center corresponds
to craning your gaze 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 down. The correct
horizon on the map will now appear 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.
Then there's the distortion issue. On a planisphere designed for use
in the Northern Hemisphere, constellations in the southern part of the
sky are stretched sideways, taffy-like, making it hard to compare them
with real star patterns. This problem does not exist on a well-designed
map for fixed dates and times, such as the one in the center of each month's
Sky & Telescope. Some planisphere designers have come up with
a partial solution. David Chandler's planisphere The Night Sky
presents two maps, one on each side. One minimizes distortion north of
the celestial equator, the other south of it. Just flip it over for the
best view.
A further complication is that a planisphere works correctly for only
one latitude on Earth. Most today are made in several editions, each for
a particular latitude.
Then there's the matter of daylight saving time. When this is in effect
(from the first Sunday in April to the last Sunday in October in most
parts of the United States), 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 employ standard time either, but rather
local mean time. The difference, which depends on where you live in your
time zone, can amount to a half hour or more. Instructions for finding
your local mean time correction are included with the Skygazer's Almanac
in each January's issue of Sky & Telescope. 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.
Alan MacRobert is an associate editor of Sky & Telescope magazine
and an avid backyard astronomer.
Modern planispheres are direct descendants of the astrolabe, such as this
one made in Nuremberg, Germany, in 1532. The ornate scrollwork supports
27 points that form a rudimentary star map; each point is labeled with
the name of a star or constellation. On the plate under them are lines
marking the local horizon, altitude, and azimuth. The sky is portrayed
backward, right for left. Click on image for larger view.
David Chandler's Night
Sky planisphere features star maps on both sides, one minimizing distortion
in the north and the other in the south. This best-selling planisphere comes
in both plastic and cardboard and in both large and small sizes. You can
order The Night Sky at Sky & Telescope's online
store. Click on image for larger view.
David Kennedal's Precision
Planet and Star Locator includes settings to build in corrections for
daylight saving time and your longitude, as well as a marker on the sky
map that can be dialed to any celestial coordinates. Like David Chandler's
Night Sky planisphere, Kennedal's Precision Locator (available
at S&T's online
store) features an especially clear and realistic star map. Click on
image for larger view
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