Halley search; a program to help your find the comet. Harald Schenk; John J. Anderson.

As Halley's Comet passes through our solar system, many readers of Creative Computing may miss a chance to see it simply because they do not know where to look. Add two other factors--that the comet will not be as bright as it was in 1910 and "light pollution" (especially in urban areas) may obscure it--and you may be left out of the comet craze completely.

To help our readers who share our fascination with space, we present a Basic program, Halley Search, to guide you in locating the famous comet. In exchange for the current date, the program will reveal the following information:

* Location in terms of right ascension and declination.

* Distance from the Sun and Earth in astronomical units.

* Magnitude.

* Rising and setting times.

Because novice astronomers may be unfamiliar with some of the terms used in the output, we offer some brief definitions and take a look at how they relate to the program.

Right Ascension and Declination are coordinates for measuring the position of an object in the sky. In a way, they are like the X and Y coordinates used to locate a point on a graph. Right ascension provides the east-west position (the X coordinate) and declination provides the north-south position (the Y coordinate). Right ascension is measured in hours and declination is measured in degrees.

Universal Time (UT), a standard measurement of time for astronomers, is the time in Greenwich, England. Why Greenwich? Well, a large number of observations of star positions were made at the Royal Greenwich Observatory near London, and that place was selected arbitrarily as the starting point for measuring longitude.

Astronomical Units (AU) measure distances within our solar system. 1 AU equals the distance between the Earth and the Sun, approximately 93 million miles.

Magnitude refers to the brightness of the comet. The lower the number, the brighter the object. The higher the number, the greater the need to view the comet through binoculars or a telescope.

Rising and Setting Times tell you when the comet is visible above the horizon.

Running Halley Search

To run the program, all you have to do is specify whether you want the output in Daylight or Standard time and then enter the date. The program returns the necessary information.

Figure 1 lists various magnitudes and should give you an idea of what equipment you will need to see the comet. So grab your binoculars or telescope, plot the right ascension and declination on the sky map (Figure 2), step outside, and take a look. Under the right conditions and with the proper equipment, you too can view the famous comet.

Please allow the program time to make the calculations. The process can take several minutes, especially if you are running Halley Search on an Atari.

You may also enter historical dates. For example, Halley's Comet last appeared 1910. You might want to check the differences in location between appearances.

But Does It Play in Peoria?

The program is set for New York City latitude and longitude. Your latitude and longitude is probably different, so your rising and setting times will also be different.

To localize the program for your area, run the customization utility program in Listing 1, take the results, and make the substitutions in the main program (Listing 2).

Those of you who want to do the trigonometry instead of letting the customization program do it for you should perform the following steps:

Convert latitude degrees into radians by dividing your latitude by 57.2958. Take the SIN of this number, find line 720, and put it in place of 0.656059. Next, take the COS of the number, find line 720, and put it in place of 0.754710.

Take your longitude, subtract 75 if you are in the Eastern Standard Time xone, 90 if you are in the Central Standard Time zone, 105 if you are in the Mountain Standard Time zone, or 120 if you are in the Pacific Standard Time zone. Multiply this number by 0.066666, and substitute the result for--.066666 in lines 770 and 780.

If you are in the Central, Mountain, or Pacific time zone, remember to replace the word Eastern in lines 820 and 870 with your appropriate time zone.

Finally, replace the number --1 in line 1320 with 0 (if you are in the Central zone), 1 (if you are in the Mountain zone), or 2 (if you are in the Pacific zone).

You can find your latitude and longitude in any atlas. For those who do not need pinpoint accuracy, we include them for selected cities in Figure 3.

All three versions of Halley Search (in Atari, commodore, and microsoft Basic), as well as the customization program (runs with just about all dialects of Basic), are available on CompuServe in the Creative Computing SIG (go PCS-22). If you have a modem and CompuServe account, you can save some typing by downloading the programs from our SIG.

Space, the Final Frontier

We wish you good hunting in your search for Halley's Comet. We suggest you try to find an area as devoid of artificial light as possible. If you miss Halley's Comet this year, you can always try again later--about the year 2060.