Classic Computer Magazine Archive CREATIVE COMPUTING VOL. 9, NO. 6 / JUNE 1983 / PAGE 250

Star gazing. James A. Ingram.

Star Gazing

Whether you're a dyed-in-the-wool amateur astronomer or you can't tell a nebula from a noodle, the following programs (written in Microsoft Basic) should provide many hours of enjoyment. The first is designed to tell you what stars are visible on a given night and where they are located. The second is an interactive version to use with a telescope. I originally wrote them to help position my small field telescope, which doesn't have a clock drive mounting system for tracking stars. I discovered later that the programs make it super easy for the novice ot locate stars and constellations, too. My ninth grade students give them a thorough workout every fall!

Star position is best described by using right ascension and declination. These are the astronomer's grid system, similar to latitude and longitude. The trouble the average person has in making any sense of them is due to the fact that he is stationary with reference to the stars, while the earth spins on its axis and revolves around the sun. The effect of this is that we have a slightly different view of the sky every night. Each six months we see the opposite half of the sky.

What we need is a grid system that is fastened to the ground where we are standing. The system of compass bearing and altitude is probably the simplest and best known. North is a compass bearing of zero, while east, south, and west are 90~, 180~, and 270~ respectively. Full circle brings you to 360~, at the starting point of zero. Altitude measures the angular height straight up from the horizon in degrees; the horizon is zero and straight overhead is 90~ (called the zenith). An object at a compass bearing of 135~ and altitude of 45~ would be halfway between the horizon and overhead to the southeast.

The trick is to relate these two coordinate systems, one moving and one stationary, one tilted at an angle to the other. For the average mathematics professor, this is a piece of cake, but for the rest of us it poses a long and difficult problem. Enter the computer.

If we load the computer with a list of stars and their coordinates, we can let it decide what is visible on a given night and calculate the bearing and altitude. The first step is to freeze any motion, and for that reason the data is calculated only for a specific date and time of night. Next, the coordinates are mathematically rotated to your local horizon. Finally, the information is printed in a table. The calculations to perform all of this generate some other information useful to the astronomer and these data are included in the table.

Using The Program

In the first of the two Microsoft Basic programs, you are asked to enter the date and time. Enter both numbers as a fivecharacter string such as 01/22 or 18:30. Note that numbers must be two-digit and that the time is entered in 24-hour format. The program will accept any value to the nearest day or minute. Your entries are not checked to see if they are "real.'

Next, you are asked whether you are on standard time or daylight saving time (since the sky rotates 15~ every hour, this can make quite a difference in accuracy). Enter S for standard or D for daylight.

Last, you are asked for a lower limit of brightness to be included in the table. Astronomers call this magnitude; the larger the number, the dimmer the star. The brightest stars in the night sky are usually about zero or one, while the dimmest visible with the unaided eye are about five or six. For a table of bright stars only, enter a one or two; if you want everything, enter a magnitude of about 15.

After you enter the brightness limit, the computer takes over the work. First it prints a table heading with the date, the number of days that have elapsed from the first day of fall (the autumnal equinox), the time in three formats, your location in latitude and longitude, the brightness limit, and the position of due south in right ascension coordinates. It then begins searching through the data table, testing each item to see if it is above your horizon and if it is bright enough to include in the table. If it is, the program finishes the calculations and adds the item to the printed copy. The final table shows the name of the star, its brightness, its right ascension and declination, and its bearing and altitude.

With list in hand, head for the nearest open field at the appointed time. Establish where north lies (from the North Star, not by compass), then measure off the bearing and altitude of the first star on the list. You should be able to find it on the first try. To measure the angles you can make an instrument called an astrolabe for accuracy, or you can use a system of hand measures. Holding your hand at arm's length against the sky, the width of your little finger is about one degree; your pointer, middle, and ring fingers together measure about five degrees; your closed fist measures about 10 degrees, and the distance from your pointer to your little finger when spread apart (until it hurts!) is about 15 degrees. Use both hands, and you can work your way around the horizon and up into the sky to the correct location. Just remember, the sky will be turning slowly while you gaze--after after an hour or so, stars in the south and overhead will be visibly farther west than your table coordinates show.

The program is well documented with remark statements; by deleting them you may be able to pare down memory requirements. The data statements are arranged with name (string), magnitude (numeric), right ascension (string), and declination (numeric). You can add more, or shorten the table as long as you remember to change statement 265 to the new data table length. Be sure to enter your own latitude and longitude at lines 160 and 165. The value at line 170 corrects your local zone time to Greenwich time; the correct values for the continental United States are as follows: EST, 19; CST, 18; MST, 17; PST, 16.

I have included stars, nebulae, clusters, and other objects of interest in the data table, but it may not suit your interests. You may want to enter the coordinates of the centers of major constellations, for instance. In any case, such information should be available at your public library in the astronomy section.

The second program is simply a bearing and altitude calculator. To use it, enter the date and time as before. To locate a given star, look up its right ascension and declination in a star atlas and enter the values into the computer. The CRT display will show some useful values and the bearing and altitude of the star in question.

You may enter data for several stars without having to re-enter the date each time; just respond Y to the question ANOTHER STAR (Y/N)? Typing an N stops execution.

The next big project will be to develop a program to locate the moon and planets on any given night. Any suggestions? What ever happens, the results should be far out.

Table: Sample run of STARS.

Table: Listing 1.

Table: Listing 2.

Photo: Flowchart for STARS.