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USIN A NAKED-EYE STAR MAP

By Alan MacRobert
Adapted from Sky & Telescope

LOTS OF PEOPLE buy a telescope only to discover that they can't find much of anything with it in the sky. Their problem? They haven't learned their way around the stars as seen with the naked eye, and they try to use inadequate maps.

Here are some simple tricks for finding your way that should save you a lot of grief.

First things first. You need maps. To start with, you need a simple all-sky map, for use with the naked eye, that shows where to find the brightest stars and constellations as seen at your particular time, date, and latitude on Earth.

A simple planisphere or "star wheel" can do the trick. (See "Star-Finding with a Planisphere.") You turn a plastic or cardboard dial to set your time and date and get a rough map of your whole sky. The map's edges represent the horizon all around you, as if you were standing in an open field and turning around in a complete circle. Compass directions should be printed around the horizon/edge.

The center of the map represents the part of the sky directly overhead. A star that's plotted on the map halfway from the edge to the center, therefore, can be found about halfway up the sky -- halfway from horizontal to overhead.

That's really all there is to it!

Many planispheres are offered for sale, all too many of them poorly designed. Look for one with small, fine, carefully drafted star dots and patterns. These will be easier to match to real stars in the sky. Avoid glow-in-the-dark star maps; the glow paint can't be printed very accurately, so the result is usually a map that looks confusingly different from what it's supposed to represent.

An excellent all-sky map appears near the center of every month's issue of Sky & Telescope magazine. Less detailed maps for an entire year appear in SkyWatch. They all work the same way: the big round edge is the horizon all around you (with compass directions labeled), and the center is the point overhead.

The maps in Sky & Telescope and SkyWatch are drafted for specific times and dates (printed in the upper right corner). This avoids the distortion of the southern sky that a planisphere has to be drawn with in order to work for all times and dates.

Many planetarium programs for computers can display and print a customized all-sky map for whatever time, date, latitude, and longitude you specify.

Into the Night

To read the map outdoors, bring along a dim flashlight. The best flashlight for astronomy is red, not white; red light affects your night vision less. You can rubber-band a piece of red paper or plastic over the front of the flashlight. This both dims and reddens the light.

Outdoors with your map, start by looking for only the brightest stars plotted on it. The difference between bright and faint stars in the sky is much greater than is represented on paper. In fact, if you live in a populated area where there is much light pollution (artificial skyglow), the faint stars will be completely invisible.

Also, be aware than the constellations on an all-sky map appear much smaller than they do in real life. The star patterns you're hunting in the night are mostly big!

Go out often with your map, and use it to learn all the constellations you can. You are establishing the familiar, major landmarks that you'll need when you start using a more detailed map with binoculars or a telescope.

Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.

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