Contents of the PLANET.DOC file
Man has always been fascinated by the stars, and the study of the stars
is the oldest of the sciences. PLANETARIUM brings a little bit of this
wonder to you via your computer. Whether you are a novice or a seasoned
astronomer, you can find it useful for displaying the positions of the sun,
moon, and visible planets for any date since 1977. You can also plot a star
chart that shows up to 1600 stars.
With one exception (the F1 key), all options are displayed on the menu
bar. The initial screen gives a brief summary of the options, which are:
(D)ate/Time/Latitude - This option requests data from the user. The
exact positions of the sun, moon, and planets are dependent on the
date. The portion of the sky you can see is dependent on both your
latitude and the time of day. You must enter this information before
the program can do the necessary calculations for the remainder of
the options concerning the solar system. Select this option to either
enter the initial data or to change the data once it is entered. If
the data have already been entered, the prompts will display the
current values. Either enter a new value or just press RETURN to
accept the current value. The allowable ranges of the data are shown
and entering values outside these ranges will generate an error
message. Once all the data have been entered, you will be given a
chance to make any corrections. When the new data have been entered,
the program will make all the necessary calculations. How long this
takes depends on your computer (less than 0.5 second for most).
(S)un/Moon Data - This option displays a table with all the information
on the position of the sun and the moon for the date entered. See
the next section if you are not familiar with astronomical terms.
(P)lanet Data - This option displays a table with all the information on
the positions of the visible planets.
(Z)odiac - This option displays a sky "window" showing the positions of
the sun, moon, and planets for the date, time, and latitude that have
been entered. Each heavenly body is represented by a graphics symbol
which appears in a legend. A strip just below the window shows the
approximate positions of the zodiac constellations and a strip below
that shows the directions in which you should be looking. The sky is
shown as blue for daytime hours and as black for nighttime hours
(arbitrarily set at 6 AM and 6 PM respectively).
(N)ight Sky - This option enters the star chart portion of the program.
Since the relative positions of the stars do not change with time
(except over periods of thousands of years), no date and time infor-
ation is required; however, you must enter the coordinates (expressed
in terms of Right Ascension and Declination) for the center of the
star field you wish to view as well as the width of the field. If
you have already entered a date and latitude, the coordinates of the
star field directly above at midnight are presented as default
values, otherwise coordinates 0, 0 are given. Although the width
of the field can vary from 10 to 180 degrees, the default value of 60
gives the best proportions. The first time you enter this portion,
the program will also request your monitor type (enter CGA if you
have a monochrome monitor.) Once the computational data have been
entered, the data file for the star fields will be loaded. This will
take a minute or so. You will then be told that once the plotting
begins you can press "Q" to exit back to the main menu or press "N"
to go back and enter new data for another star field. In order to
make the most efficient use of memory, the star data will be erased
from memory whenever you exit Night Sky. (This just means that if
you reenter Night Sky, the data must be loaded in again.)
F1 key - At certain times the program will beep to get your attention,
such as when you enter a wrong value. The F1 key will toggle the
beep. (If it is on, pressing F1 will turn it off; if it is off,
pressing F1 will turn it on.) Of course, if your computer does not
have a speaker then you can ignore this altogether.
The representation of the positions of the sun, moon, planets, and stars
is based on a sphere that has the earth as its center. The celestial
equator is that point where the earth's equator intersects the surface of the
sphere. The vertical position (called declination) of all heavenly bodies is
measured in degrees above or below the celestial equator. A star with a
declination of 0 will be directly above the earth's equator, one with a dec-
lination of 90 will be directly above the north pole while one with -90 will
be directly above the south pole. (South latitudes are normally expressed as
Since the earth is tilted on its axis (thereby giving us the seasons) the
plane of the equator is different than the solar plane. Because of this, the
"path" of the sun (called the ecliptic) varies above and below the equator.
The two points where the ecliptic crosses the equator are called the
Equinox. The starting point for measuring celestial horizontal positions
(called the Right Ascension) is the Vernal (Spring) Equinox. The value is
given in terms of a 24 hour clock. So R.A. 0:00 is the Vernal Equinox, R.A.
12:00 is the opposite (Autumnal) Equinox, and R.A.s 6:00 and 18:00 are half
way between the two (called the Summer and Winter Solstice). As a little
math will tell you, one hour of R.A. is 15 degrees. In fact, expressing a
position in terms of degrees is called the geocentric angle.
(NOTE: In the Night Sky option, R.A. is entered as whole hours only.)
Altitude at noon - location in degrees from the northern or southern
horizon at noon.
Distance - distance in millions of miles from the earth.
Elongation - location of other bodies of the solar system in relation to
the sun. Expressed in terms of degrees east or west of the sun's
Moon's age - number of days since the last new moon.
Right Ascension at 9 P.M. - The Right Ascension which would be directly
above at 9 P.M. This is the figure given in most star charts since
9 P.M. is considered to be the best time to start star gazing.