National Geographic : 1943 Jul
How to Use the Star Charts THE monthly sky maps presented on pages 117 through 128 were especially designed by the author to show the appearance of the sky over a range of latitudes. They represent the interior of the "bowl" of the sky over head, as if it had been sawed through on an east-west line and the southern half folded back upon the northern. The narrow part of the hourglass figures is the zenith, directly overhead. One half of the hourglass repre sents the northern half of the sky, the other the southern half. If you were to fold one of the maps over on itself, drawing the corners together, it would form a rough bowl shape, like the real sky. The curved edges of the upper and lower halves of the hourglass would come to gether. Imagining this procedure may help you to understand the maps better. The straight top and bottom edges of the hourglass figures form the northern and south ern horizons. To locate stars in the north ern half of the sky, face north, and hold the map so that the horizon marked "north" is at the bottom. To locate stars in the southern sky, face south, and hold the map with the horizon marked "south" at the bottom. Be sure to note the positions of the bright planets, for they will help you to identify the zodiacal constellations (page 101). When you use the charts for the morning skies, remember that any planets showing will have moved, though the star positions are the same. Latitude Where You Observe Determines What Stars You Can See The stars visible to you depend upon your latitude. If you were to sail south, the stars in the north would sink gradually below the horizon behind you, and southern stars would rise over the curved surface of the earth in front. If you were to go north, the opposite would take place. Find your own approximate latitude from the map on pages 98 and 99. Then draw straight lines across the top and bottom of the sky maps corresponding to this latitude. For example, if you live in Philadelphia or Salt Lake City your latitude is about 40 de grees. Draw lines connecting the 40-degree marks at the top and bottom of the sky map. The stars between these two lines will be visible to you; all others will be below the horizon. The crosses near the "stem" of the hourglass indicate the zeniths for various lati tudes. Look at the dates and times shown on the left-hand side of each map, and select the one nearest the day and hour when you plan to observe. The time given on the maps is standard time, one hour later than your local "war time." For example, 9:30 p. m. on the map is 10:30 p. m. by war time. Find a place where you have a clear view of the sky, if possible away from city lights, and on a moonless night, and compare your chart with the actual stars in the heavens. Use a flashlight, preferably dimmed, to help you read the chart. The different sizes of the circles represent ing the stars on the charts show how they differ in brightness, or magnitude, as the astronomers call it. On the right of each chart is a table showing the sizes of circles for the various magnitudes, ranging downward from Sirius, brightest star in the sky, whose magnitude is minus 1.6. (For Sirius see Charts, pp. 121-6). Planets Are Not Stars Magnitudes go in reverse; that is, the brightest stars we can see are of zero or first magnitude, or minus 1.6 in the case of Sirius. Some planets are even brighter than Sirius, but they are not stars. The planets, revolving around the sun, shine only by its reflected light, and are con spicuous only because they are so much nearer the earth. Sixth-magnitude stars are barely visible to the naked eye under the best conditions. A star of fifth magnitude is about two and one half times brighter than one of sixth magni tude; a star of fourth magnitude is two and one-half times brighter than one of fifth magnitude; etc. Thus a star of first magni tude is 100 times as bright as one of sixth magnitude. On the charts, a cross superimposed on a star circle indicates a variable star, one that fluctuates in brightness. Some stars vary so much in brightness from time to time that you would be confused in locating some con stellations if you did not keep this in mind. You will find it interesting to follow the fluctuations in brightness of some variable stars, and the most significant are mentioned in the constellation descriptions. The monthly sky maps show all the stars down to magnitude four and one half (4.5), although a few slightly fainter stars, essential to the recognition of the constellations, have been included. The diagrams of the constellations have been drawn in greater detail and are complete to magnitude 5.0, although they also include a few fainter stars. Lines connecting the indi vidual stars in the constellations are put in to assist in recognizing the patterns.