SKYSCAPE
(COMPUTE! Magazine November 1985 by Robert M. Simons)

For thousands of years the sun, moon, and planets in our solar
system have excited human imagination. In ancient times they were
regarded as gods whose distant motions influenced the course of
earthly events. Though we now understand more about the true nature
of celestial objects, many facts remain unknown, and a brilliant
nighttime sky still presents an inspiring spectacle.

Whether you're seriously interested in the sky or just casually
curious, "Skyscape" is a convenient tool for extending your knowledge.
It opens a movable window on the heavens, displaying the position of
our sun, moon, and neighboring planets from almost any location on
Earth, at any point in time from 1977 into the distant future. Since
it performs all the necessary calculations, you can enjoy and learn
from this program even if you're not an expert in astronomy. In
addition to providing data about the position of celestial objects,
it draws a sky map on the screen, showing each object as it would
appear to you at the chosen location and time.

Skyscape begins by asking you to answer several questions. Enter
the year, choosing any year from 1977 forward. In some ways this is
the most important input of all, since objects in our solar system
move significantly from one year to the next. After you choose the
year, Skyscape allows you to enter the month and day.

Next you must enter the latitude (north/south position on Earth)
from which you wish to view the sky. Latitude 0 places you, the
observer, at the equator. Latitudes 1-90 place you in the northern
hemisphere. To choose a southern latitude, enter a negative number
from -1 to -90. Skyscape generally represents southerly locations
with negative values.

Whenever Skyscape asks for information, it checks your entry to
make sure it's in the acceptable range. If you enter an illegal
value, the program displays an error message and gives you another
chance.

Though very different in size and composition, the sun and moon
are alike in being the largest celestial objects visible from Earth.
After you enter the date and latitude, Skyscape displays a table of
data for the sun and moon. In addition to the date, day of the year,
and latitude north or south, you'll see the following information:

- Sun's geocentric angle. This figure represents the sun's
position as a number of degrees relative to the vernal equinox. The
vernal equinox is where the sun is located when spring begins in the
northern hemisphere (the same time that autumn beings in the southern
hemisphere).

- Sun's declination. The number of degrees north or south
of the equator. Negative values indicate a southerly location.


- Sun's altitude at noon. The location of the sun in
degrees from the northern or southern horizon at noon.

- Sun's right ascension. Just as longitude and latitude
indicate locations on the Earth, right ascension and declination are
used to pinpoint locations in the sky. For this purpose the sky is
visualized as a gigantic sphere surrounding the Earth. Declination
locates a point vertically in the celestial sphere and right
ascension locates it horizontally. Right ascension values are given
in hours and minutes in the range 0:00-23:59. Right ascension 0:00
is exactly at the vernal equinox. Larger right ascension values lie
to the east of smaller ones.

- Right ascension at 9 p.m. The right ascension which would
be on the meridian at 9 p.m. This coordinate system would be found on
star charts. By comparing this number with those charts, you can tell
what stars and constellations would be visible at that time.

- Moon's age. The number of days since the last new moon.

- Moon's elongation. The location of the moon in degrees
east or west of the sun.

- Moon's phase. The phase of the moon on this particular
day.

After viewing the sun and moon display, press P to continue to
the next display screen, which contains the planet table. (Press D
if you wish to enter a new date.) The planet table shows vital
information about the visible planets (through Uranus, which is at
the limit of our visibility). The table shows the position of each
planet in right ascension and degrees east or west of the sun. It
also shows the distance of each planet from Earth in millions of miles.
If you'd rather see the distance in kilometers, change the value in
line 130 of SKYSCAPE.BAS from ES = 93 to ES = 149.6.

Some planets have an asterisk to the left of the right ascension
figure. This signifies that they are visible at 9 o'clock this
evening. For reference, the planet table also includes the sun's
present right ascension and its right ascension at 9 p.m. Press D to
input a new date or S to view a graphics display of the sky at any
time in the current day.

After selecting the sky display, you must enter the hour when you
wish to view the sky. The hour value should be a whole number from 0
to 23 (enter 22 for 10 p.m., etc.). You'll also need to enter the
minutes (0-59). Skyscape then displays the time and offers you a
chance to enter different values. Press Enter when you're satisfied
with the time.






Skyscape now displays the sky as it would appear at the chosen
latitude, date, and time. Since the sky looks different from different
places on Earth, the latitude affects the display considerably. If
your latitude is in the range 24-90 degrees north or south, the sky
shows a dashed line representing the position of the celestial
equator, along with symbols representing the sun, moon, and planets
visible at that time. If your latitude is in the tropical region --
from 23.5 degrees north to 23.5 degrees south -- the dashed line
indicates a position directly overhead.

If you're viewing in the northern hemisphere, north is above the
dashed line and south is below it. In the southern hemisphere these
directions are reversed. Below the sky display is a key that
interprets the symbols used to represent celestial objects. If more
that one object is positioned at the same spot, the symbols are
displayed above each other.

At the bottom of the sky you may see two-letter abbreviations.
These represent zodiac constellations that would be visible from your
chosen vantage point. Skyscape uses the abbreviations AR (Aries),
PI (Pisces), AQ (Aquarius), CP (Capricorn), SA (Sagittarius),
SC (Scorpio), LI (Libra), VI (Virgo), LE (Leo), CA (Cancer),
GE (Gemini), and TA (Taurus). Each constellation is located above
the spot where its abbreviation appears. In northern latitudes, the
border of each constellation's zone begins at its abbreviation and
extends left. In southern latitudes, the constellation extends right
from the position of its abbreviation.

Daytime skies are shown in blue and nighttime skies in black.
Skyscape does not calculate the actual rising or setting time of the
sun. Average rising and setting times of 6 a.m. and 6 p.m. are used
in every case. You may obtain exact rising and setting times from
local newspapers. However, keep in mind that there is usually about
an hour of twilight before sunrise and after sunset.

In addition to permanent objects, Skyscape's graphics display
includes Halley's Comet, which should be visible during late 1985 and
early 1986. If you choose a date from November 1, 1985 to May 29,
1986, Skyscape calculates the position of Halley's Comet and includes
it in the graphics display (if it would be visible at the place and
time you select). The comet's position is based on the best
predictions available as of the summer 1985. These positions may
differ slightly from the comet's actual position when it finally
makes its appearance.

While Skyscape is generally accurate, it bases most position
calculations on circular orbits. This introduces a certain element of
error, since no object in our solar system has a perfectly circular
orbit. The position error is most pronounced for Mercury and Mars
(whose orbits are quite elliptical), but does not significantly affect
other objects. Skyscape is still accurate enough for planning
astronomy lessons and planetarium displays.



The following table contains latitudes of some of the major
cities in the United States which may be used in Skyscape.

City Latitude
---- --------
Albuquerque, NM 35
Boston, MA 42
Buffalo, NY 43
Chicago, IL 42
Dallas, TX 33
Denver, CO 40
Los Angeles, CA 34
Miami, FL 26
Minneapolis, MN 45
New York, NY 41
Philadelphia, PA 40
Phoenix, AZ 33
St. Louis, MO 39
Salt Lake City, UT 41
San Francisco, CA 38
Seattle, WA 48
Washington, DC 39