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How to use a star map

September 18, 2009 at 03:01 PM

o get around, you need a good map. Here’s some steps on using a star map.

There is more than one kind. Some focus on the constellations, and others go deeper into the sky to allow you to locate celestial destinations with binoculars or telescopes.

Constellation maps are often included with general astronomy books, available at many public libraries, or within monthly magazines such as Sky and Telescope, and Astronomy. Be sure to use a map that is current for the season. The stars you see at night constantly shift through the year, and by the hour through the night. The map will have some notation saying when to use it.

Next you need to orient yourself with directions. If like many people you are not sure which way is north, it is really quite simple. At noon hour, the sun is due south. North is directly opposite from south. Facing north, west will be on your left and east on your right.

The entire sky seems to rotate through the night, around a point in the northern sky, conveniently next to the North Star. This is not the brightest star in the sky but it is +2nd magnitude, which is among the more conspicuous stars. 

The famous Big Dipper is your guidepost to the North Star. The seven stars marking the Big Dipper make an unmistakable impression of a dipper. During a September evening, you will find the Big Dipper low in the north-northwest, its handle on the left side pointing higher than the dipper’s bowl.

The two front stars of the bowl serve as a pointer to the North Star. Trace an imaginary line up and you will find it. Keep going an equal distance past it to find Cassiopeia, with five stars marking a big “W”—on its side at this time.

Your star map shows constellations in each direction, including overhead (which is known as the zenith). As our world turns, the constellations appear to rise in the east, circle around and set in the west. Looking north, a region of stars make this circle, but keep missing the flat horizon as the sky turns around the point by the North Star. The Big Dipper is among the star groups that never set.

This assumes you live in mid-northern latitudes, between +30 and +50 degrees above the equator. Most star maps, sold in the United States, are made for this region of the Earth.

Observers in the far north—such as northern Canada—will see the North Star very high in the sky, and many more constellations that never set. If you stargaze far south, in the tropics, few stars never set. The North Star is on the very horizon close to the equator. Down under, below the equator, a whole new section of southern sky comes into view that you will never see in the United States in our life span. There is no bright “South Star,” but there is a point on the southern sky that the constellations circle around.

A flat circular map of the whole visible sky always has difficulty with foreshortening, on the edge. Because the night sky appears as an inverted dome, the constellations along the horizon will appear stretched out. For the same reason, a flat rectangular map of the world shows Greenland as a huge mass of land, much bigger than it really is.

Some star maps take care of this by showing you sections at a time. There are also celestial globes, but these are not practical for learning constellations, since you are on the outside looking in, usually at a plastic ball representing the Earth in the middle of a clear globe marked with stars.

A very useful star map is the planisphere, which has a large circle plotted with constellations, the North Star being in the center. There is an outer cover over the star map, with a large oval cut out serving as a window. The star map is turned like a wheel, lining up the date on the rim with the hour of the night.

Star atlases help telescope users find star clusters, galaxies and much more in the deep sky. These atlases are plotted with a much deeper magnitude of faint stars, below the normal +6th magnitude naked eye limit, and shows thousands of stars you can only find with optical aid. These charts also are lined with a coordinate grid, similar to latitude and longitude on a terrestrial map. On a star map, however, they are referred to as declination and right ascension, respectively.

When using a star map outdoors, cover your flashlight lens with red cellophane or red paper, which will protect your dark-adapted eyes.

What you won’t find on star maps, unless it is in a monthly magazine, are positions of the moon and planets. That’s because their locations are always changing.

First-quarter moon is Sept. 26.

Peter Becker writes for the Wayne Independent in Honesdale, Pa. Contact him at .(JavaScript must be enabled to view this email address). Keep looking up!

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