As might be expected, we have mapped the sky very Carefully, as we have mapped the surrface of our Earth the Moon and Mars. On Earth we invented a system which divided the circumference with 350 lines running north and south from pole to pole, which we call meridians of longitude. We started numbering these lines at Greenwich, England, and proceeded east a8o lines and west 180 lines. This gave us east longitude west longitude. Starting at the equator, we divided each hemisphere into 90 lines, all parallel to the equator. We called these parallels of latitude. Everything north of the equator is north latitude, and everything south is south latitude. When we want to locate a particular spot on the surface of the Earth, all we have to say is that it is located at so many degrecs north or south latitude and so many degrees east or west longitude, and everyone look it up on a map or a globe and locate the precise point.
For the sky we had to ivent a different system because the area is entirely different. We have a star that is fairly close to being directly over our North Pole. Although it is not always directly over the po1e, for convenience we assume that it is. Ii offers a fixed point that is substantially over the pole, and we call it tiie "Pole Star" or Polaris and base everythlng upon it.
Starting at the Pole Star, we erect an imaginary line to the zenith, which is the point directly overhead. From the zenith we continue the line to the horizon back of us, which. of course, will be due south of our position. We call this line the "meridian."
The next thing we do is to divide the heavens up into regions. The ancients lid al1 this for us, and they gave these regions names. But names carry no suggestion as to location, so some of us moderns gaye them numbers, as we do the streets and avenues in our modern cities. Forty-second Street in New York obviously is below Eighty-second Street, but no one knorvs whether Smith Street is above or below Jones Street.
These regions correspond to countries on our terrestrial maps. They needed subdividing. Our countries we divide into states or provinces. In our star maps we divide the regions into constellations. We had to have a map of the Northern Sky, which those of us in the Northern Hemisphere see, and one of the Southern Sky, which those of us in the Southern Hemisphere see. The latter are less fortunate than the northerners because there is no easily located star that happens to be close to or directly over the South Pole.
Every star will cross the meridian at the same place every night in the year and et the same star time, or "sidereal time," as star time is termed. The Dog Star, Sirius, is a conspicuous star, and it always. crosses the meridian at 6 hours and 40 minutes sidereal time. We say thet Sirius has a "right ascension" of 5 hours and 40 minutes. If we found that its angle in degrees from the pole Star was 160 degrees, we would be able to locate the exact spot where Sirius would be et 6 hours and 40 minutes sidereal time. A sidereal clock gives star time just as an ordinary clock gives Sun time. A sidereal time table gives sidereal time in terms of local or Sun time for every minute throughout the year. In the absence of a clock keeping sidereal time on may readily ascertain by consulting a sidereal time table the exact Sun time that Sirius crosses the meridian. By proper use of time tables and star maps we can locate in the heavens any given star at any given time.
The most conspicuous stars in the sky as viewed from our position in the galaxy would not be the most conspicuous from some other position in the galaxy. To gain a comprehension of the true status of the different stars, we must go by their brightness, their diameter, their weight, and their distance. If we go by their rate of radiation or brightness, it would appear that Rigel in Region 9 is the most important star that we know of in our galaxy. It is a sun that is 15,000 times as bright as our Sun. Its distance from us is of the order of 500 light years, and therefore it is only a star to us instead of sun. It should be borne in mind that our Sun is only 8 light minutes distant from us. Eight light minutes and 500 light years are very different distances. We might place Deneb second on the list, although we do not know much about him. He is in Region 12, and some authorities suggest that he might be 10,000 times as bright as our Sun His distance is equally doubtful, but something of the order of 600 light years has been suggested. Third we might name Antares in Region 18, who is better krown. He is about 4,000 times brighter than our Sun and is something like 380 light years away. Next we might name Beta Centauri in Region 18. He is 3,000 times brighter than our Sun and is 300 light years away. Next might come Alpha Crucis in Region 17. He is 1,600 times brighter than our Sun and is 230 light years away. Other typical bright stars in the order of luminosity are the following:
Star Region Times Brigher Distance in
than Our Sun Light Years
Spica 11 1500 230
Betelgeuse 9 1200 200
Achernar 14 200 70
Capella 3 185 52
Arcturus 11 100 41
Aldebaran 9 90 57
Regulus 10 70 56
Vega 7 50 26
Pollux 4 28 32
Sirius 9 26 8.6
Fomalhaut 14 13.5 24
Altair 12 9.2 16
Procyon 9 5.5 10.5
Alpha Centauri 18 1.3 4.3
There are thousands of millions of other stars in our galaxy that we can photograph with our present equipment. It is hard to imagine that we shall ever catalogue each of them and list their important characteristics. It would be a job of far greater magnitude than preparing a directory of all the men, women, children, dogs, cats and pet canary birds in the world, with their residence and business addresses and telephone numbers, if any.
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