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S K Y P L O T
The Planetarium Simulator
Gerald M. Santoro
148 W. Hamilton Ave.
State College, PA 16801
USER DOCUMENTATION
Version 2.0/2.1 -- Copyright 1987
1. Preliminaries
-----------------
1.1 Introduction
-----------------
1.1.1 Quick Description
------------------------
SKYPLOT is a program that simulates many of the operations of a
Planetarium. In particular, when given a date and time (specified in
Universal Time and between the years 1975 and 2500), a position
(specified in longitude and latitude), and a direction (N-E-W-S), the
program will produce a simple plot of the sky.
This plot will contain the Sun, Moon and planets (assuming that these
are in that part of the sky at the specified time), and as many stars
as are in the star database SKYPLOT.DAT. These stars are taken from
the Yale Catalog of bright stars.
Once a plot is produced the user has a number of options. These
include changing the parameters (date, time, position, direction),
examining the Right Asencion and Declination of Solar System objects,
and dumping the data to an attached printer.
SKYPLOT has been developed on an IBM PC/XT with an 8088 processor.
The compiler utilized was Microsoft Quickbasic 3.0. Since SKYPLOT is
a graphics-based system your computer must be able at at least emulate
IBM CGA graphics. (However CGA-compatible composite-video displays
should work as well.)
SKYPLOTE is the EGA version of SKYPLOT and requires an EGA or EGA
compatible graphics emulation. Although the size and aspect of the
display are the same between CGA and EGA, the EGA resolution is much
finer and the display is in color. If you have any choice at all you
should opt for the EGA version.
1.1.2 Statement of Authorship
-----------------------------
SKYPLOT was originally written as a private endeavor of Gerald M.
Santoro during the Fall - 1983 on a TRS-80 Color Computer, and
converted to the IBM-PC series during March, 1985. Extensions to the
graphic symbols for solar system objects and to the year ephemeris
range were made by Robert Sheaffer. Along with a few other extensions
and changes this version was released as Version 2.0 in the fall of
1987.
SKYPLOT is distributed according to the principles of 'User-Supported
Software'.
These principles state that the value of a given software package can
be best determined by the end user. Therefore, in accordance with
these principles all persons are encouraged to copy, distribute and
use the program. A donation of $10.00 is requested by the author only
if the end user deems the program worthy and wishes to support the
creation of more software.
Please note that a donation is NOT neccessary to posess, use, copy or
distribute SKYPLOT. This condition is retroactive to all previous
versions of the program. On the other hand I will gladly accept all
contributions and will use them to support further enhancements to
SKYPLOT.
The only limitations imposed upon the distribution of the software are
that (1) the software not be distributed in modified form without the
expressed, written consent of the author, (2) that the copyright
notice, statement of authorship, and request for donation remain
intact in all versions of the software, and (3) that no charge be made
for distribution of the program other than that to cover reasonable
handling and mailing costs. If you have any questions regarding these
limitations, please feel free to write to me.
1.1.3 Obtaining a Copy of SKYPLOT
----------------------------------
As with other programs distributed under the 'User-Supported' software
concept, it is hoped that most of the distribution of SKYPLOT will
occur via computer clubs and other user's groups. However, if none of
these channels are available, a copy of the program may be obtained by
sending a 5.25" or 3.5" DS/DD floppy diskette, a preaddressed,
diskette mailer, and $10.00 (to cover handling and postage), to the
following address:
Gerald M. Santoro
148 W. Hamilton Ave.
State College, PA 16801
Please note that any diskettes recieved without a preaddresed,
diskette mailer and the $10.00 handling fee will be ignored. (Alas,
my time is really spread thin.)
Unlike previous versions of SKYPLOT, the BASIC source program will not
be distributed with the executable program and data files. If you
want to obtain a copy of the source program I ask that you write me
and send a preformatted disk as per the above. I will likely give you
the source but I want to protect the contributions and reputations of
all of the program contributors.
1.1.4 Statement of Purpose
---------------------------
I wrote SKYPLOT as a mental exercise stemming from my lifelong
interest in Astronomy. I am distributing it as a 'User-Supported'
program in the hope that it will stimulate a similar interest in
others.
As a point of historical interest, I wish to mention that the original
version of SKYPLOT premired during the 1983 Open House ceremonies at
Allegheny Observatory of the University of Pittsburgh. Indeed, as a
'would-be Astronomer turned Computer Scientist', it was one of the
proudest moments of my life to have this program presented to the
public by some of the best professional Astronomers of our time.
Therefore please regard SKYPLOT as an educational tool for discovery
in observational Astronomy. The program puts the entire sky at your
disposal between the years 1975 thru 2500. Explore and enjoy!
1.1.5 An Important Note
-----------------------
It was pointed out to me, during the final testing for SKYPLOT, that
this program would be ideal for the generation of astrological charts
and other such nonsense. I wish to point out that the program is NOT
intended for that purpose, and anyone wanting to use it for that
purpose is better off reading Tarot Cards or looking for good
investment bridges.
1.1.6 Dedication
-----------------
This program is dedicated to the people at Allegheny Observatory who
made my brief tenure as a 'professional' astronomer so enjoyable and
to my wife Kathy, who has been very supportive of my crazy obsession
with Astronomy.
1.2 Universal Time
-------------------
When SKYPLOT requires a date and time they must be specified in
Universal Time. Universal Time is essentially Greenwich Mean Time, or
time as measured from 0 degrees longitude (which happens to run
through Greenwich, England.)
Therefore, you must convert your local mean time to Universal Time.
This is quite easy and simply requires that you add or subtract a
certain number of hours from local mean time, depending on your time
zone.
For example, if you are in the Eastern time zone of North America, you
would normally add 5 hours to local mean time to get Universal Time.
(During Daylight Savings Time you would add 4 hours.) Thus 12:00 noon
local time EDT corresponds to 17:00 UT.
Keep in mind that the date changes when you cross 24:00 (otherwise
known as midnight). Thus 21:00 on January 10 (9:00 pm) EDT
corresponds to 2:00 (am) on January 11 UT. A simple conversion chart
follows for the Western Hemisphere. (My apologies to persons in the
Eastern Hemisphere, but I did not have the data readily at hand.)
Conversion to Universal Time
(North America -- USA)
Time Zone Conversion
============================================================
Eastern Add 5 hours. (Add 4 hours for DST)
Central Add 6 hours. (Add 5 hours for DST)
Mountain Add 7 hours. (Add 6 hours for DST)
Pacific Add 8 hours. (Add 7 hours for DST)
1.3 Files on the SKYPLOT Disk
------------------------------
The following files should be contained on your copy of the SKYPLOT
disk:
SKYPLOT.EXE - The compiled program for CGA display.
or
SKYPLOTE.EXE - The compiled program for EGA display.
SKYPLOT.DAT - The file of stars to be plotted. (Initially
the same as STARS.450
SKYPLOT.DOC - This documentation file.
OBJECTS.DAT - Sample objects file.
STARS.250 - Star data to magnitude 2.50.
STARS.350 - Star data to magnitude 3.50.
STARS.400 - Star data to magnitude 4.00.
STARS.450 - Star data to magnitude 4.50.
STARS.500 - Star data to magnitude 5.00.
(This data came from the Yale catalog of bright stars>
1.4 The Algorithms
-------------------
The algorithms used in SKYPLOT were derived from the calculations
given in the book "Practical Astronomy With Your Calculator" by Peter
Duffett-Smith.
2. Running SKYPLOT
-------------------
2.1 Starting the program
-------------------------
To start SKYPLOT you should do the following:
1. Boot your system with DOS. (At least Version 2.0.)
2. Place the SKYPLOT disk in drive A: (or if you have
a two disk system you can put the SKYPLOT disk in
drive B: and change the default drive to B:).
3. Type
SKYPLOT <cr>
where <cr> is the RETURN key
You will then be shown an introductory screen that identifies the
program and author, presents a copyright notice, and specifies the
conditions under which the program may be copied and/or used.
You are instructed to press any key to proceed with the program. You
are then at the 'Enter Parameters' stage.
2.2 Exiting form SKYPLOT
-------------------------
To exit from SKYPLOT and return to DOS, you should press [Ctrl]-
[Break] a few times.
2.3 Parameters
---------------
The program parameters are information SKYPLOT needs to create a plot
of the sky. These parameters are grouped into the categories (1)
date/time, (2) location, (3) direction, and (4) options.
Please remember that the date/time must be entered in Universal Time.
Each of the parameters will be described in detail below. To
illustrate this better I will provide an example. If you have an
appropriate computer handy you should follow along with the example.
This example, as well as the description of the display following,
will make MUCH more sense if you run this on a computer as you are
reading it.
1. Year (1975-2500)
The year must be entered as a 4-digit number. Valid years are in the
range 1975-2500. This is because of the seed values for Siderial Time
and the planetary positions in the program.
Example -- 1985
2. Month (1-12)
The month must be entered as a 1 or 2 digit number in the range 1-12,
where 1 corresponds to January and 12 corresponds to December.
Example -- 3
3. Day (1-31)
The day must be entered as a 1 or 2 digit number in the range 1-31.
The program does not care about months with fewer days than 31. If
you ask for February 31, 1985 you will actually get the sky for March
2, 1985 (of course the display will lie and say February 31, 1985).
Example -- 16
4. Hour (0-24)
The hour must be a number in the range 0-24. Decimal fractions of
hours are permitted. (Again remember that Universal Time is required,
and that it may have an effect on the value for Day.)
Example -- 22
5. West Longitude (0-360)
The Longitude of the observer must be entered in degrees west from the
Prime Meridian, (0 degrees).
Example -- 80 (the rough West Longitude of Pittsburgh)
6. Latitude (-90 to 90)
The Latitude of the observer must be entered in degrees. For southern
latitudes preceed the value with a minus sign (-), for northern
latitudes just the value will suffice.
Example -- 40 (the rough Latitude of Pittsburgh)
7. Horizon (N-E-S-W)
This is the compass direction of the horizon upon which the plot is to
be centered. Either upper or lower case may be used.
Example -- W
8. Option
1 = Solar System Only
2 = Solar System and Stars (naked-eye planets only)
2 = Solar System and Stars (telescopic planets also)
3 = Also plot objects from user-supplied file
This selects the plotting option for the plot. Option 1 will cause
only the Solar System objects to be plotted. This is very fast and is
nice if all you are concerned with is the planetary groupings with
respect to the horizon.
Option 2 will plot the Sun, Moon and all naked-eye planets, as well as
the stars found in the file SKYPLOT.DAT. You can control the limiting
magnitude of the stars plotted by copying the sppropriate STARS.* file
to be SKYPLOT.DAT. (Copying is preferred over renaming so you don't
lose the original data.)
For example, to plot stars to a limiting magnitude of 4.00 (which I
have found to show constellations best) you would issue the dos
command:
COPY STARS.400 SKYPLOT.DAT
Bear in mind that there is a tradeoff between magnitude and plotting
speed. In other words the files with brighter limiting magnitudes
will plot faster because there are fewer stars to plot.
Option 3 will do the same as option 2 but it will also plot the
telescopic planets Uranus, Neptune and Pluto.
Option 4 will do the same as option 3, however it will also open a
user-supplied file and plot the objects whose positions are found
there. A description of this process may be found later in this
document. If you choose this option, SKYPLOT will ask you for a file
name.
For the sake of this example, enter 2.
2.4 The Plot of the Sky
------------------------
Upon entering the last parameter, SKYPLOT will generate a plot of the
sky at the date/time specified, as seen from the longitude and
latitude specified, centered on the compass direction of the horizon
specified.
If you have followed the example you should see a plot of the western
sky as seen from Pittsburgh Pennsylvania on March 16, 1985. This
happens to be the evening of one of the best eastern elongations of
the planet Mercury.
The plot area itself is the dome-shaped area on the display. It is
dome-shaped to reduce the distortion generated in trying to plot a
curved surface onto a flat surface. (This is apparant in Mercator
projection maps where the island of Greenland appears as large as
North America.
This distortion, if not corrected, would make star groupings and
constellations at the top of the plot unrecognizable. As it is the
correction method is not optimal and some distortion will still be
noticed. (I am working on the problem and version 3.0 should have it
corrected!)
One way to think of the plot area is in terms of a planetarium dome.
If you were to project the sky on the inside of a planetarium dome,
then split the dome in half along a line perpendicular to the
direction you are looking, and you eliminated the half closest to you,
you would roughly see what is displayed in the plot. (If you are not
looking at a plot right now this probably will not make much sense.)
The vertical tic marks along the sides of the plot indicate 10 degree
increments of altitude, with zero degrees along the bottom line and 90
degrees at the top of the 'dome'.
The horizontal tic marks along the horizon indicate increments of 30
degrees in azimuth. A ststus line just below the tic marks indicates
the compass headings for each tic mark.
The middle status line displays the option selected, the latitude,
longitude, year, month, day, universal time, and local siderial time
for the plot.
The Sun is indicated by a round circle towards the lower central part
of the display. Planets are indicated by little circles, with an
identifying number printed directly below the circle at the bottom of
the screen. This number identifies the planet by its order outwards
from the Sun. Thus Mercury is a 1, Venus a 2, Saturn a 6, and so on.
The Moon is indicated by a symbol that roughly also indicates its
phase.
Since the resolution of the plotting area is limited by the overall
640 by 200 resolution of GW-BASIC, you must keep in mind that the
symbol indicating the solar system object is only approximating the
real position.
The stars are indicated by three levels of star marker. This is to
give some indication of the relative brightness of the stars. This
has been fudged slightly in the SKYPLOT.DAT star database to allow
familiar constellation outlines to be more easily apparant.
Upon completion of the plot the program will beep briefly to indicate
that the plot is finished.
2.3 SKYPLOT Options Menu
-------------------------
If you now press any key, the plot will be erased and the SKYPLOT
Options Menu will be displayed. This menu will allow you to alter the
parameters and replot the sky. Thus allowing you to 'play' with the
sky.
In the next section, a number of examples will demonstrate the use of
these options, however a brief explaination of the options is provided
immediately below.
1. Change Year
This option changes the year of the plot.
2. Change Month
This option changes the month of the plot.
3. Change Day
This option changes the day of the plot.
4. Change Hour
This option changes the hour of the plot. (Remember that the
hour must be in Universal Time.
5. Change Longitude
This option changes the west longitude of the plot.
6. Change Latitude
This option changes the latitude of the plot.
7. Change Horizon
This option changes the horizon on which the plot is centered.
8. Change Plot Option
This option allows the user to change the plot option.
9. Examine Solar System Data
This option may be used to produce a view on the display, or
print to an attached printer the Right Asencion and Declination
for the Sun, Moon and Planets. These values may be used to
verify the accuracy of the projected positions against a
reference source such as the American Ephemeris and Nautical
Almanac.
The accuracy of the positions calculated by the program is rough,
and will only be within about a degree of their true positions.
This degree of accuracy is adequate because (1) the pixel
resolution of the plot is only that good, and (2) to achieve
greater accuracy (even though you could not see it) would slow
the program down.
10. Plot Sky
This option is used to replot the sky after one or more of the
parameters has been changed via the Options menu.
11. Enter All New Parameters
This option may be used if you know that you are going to change
most or all of the parameters, and do not want to do it one
parameter at a time with the Options Menu.
12. Exit to DOS
This option allows the user to exit SKYPLOT and return to DOS.
The users DOS disk with the COMMAND.COM file should be on a disk
in the default system drive.
2.5 Examples with the Options
------------------------------
These examples will assume that you are in front of a computer and
have run the example described under the Parameters section above. If
you have not done this you should now enter the following values:
Year - 1985
Month - 3
Day - 16
Hour - 22
West Longitude - 80
Latitude - 40
Horizon - W
Option - 2
This set of parameters will display the western sky just before sunset
at the approximate time of greatest eastern elongation of Mercury from
the Sun. In addition, the presence of Mars and a very brilliant Venus
made this a particularly beautiful planetary grouping.
Having observed and photographed this grouping I can testify that the
display produced by SKYPLOT does not truly do justice to the real
thing seen in person!
Now lets try some of the options for a demonstration of their use.
Press the spacebar to go to the Options Menu. Select option 8 to
change the plot option, then specify option 1 to plot only the Solar
System objects. SKYPLOT will acknowledge the option by clearing the
screen and redisplaying the Options Menu. Now you may enter other
options, however for the sake of this example enter option 10 to
replot the sky. Plotting only the Solar System objects causes the
plot to be completed very quickly.
Now press the spacebar again and select option 4 to change the hour.
Enter 23 for the hour and select option 10 to replot the sky. Notice
that the Sun has set. In fact, it has just set, but the program does
not account for twilight, or for the fact that the Sun is large enough
to be in a condition of partial rising or setting. To SKYPLOT an
object is essentially a point on the sky and is either visible or not.
Now press the spacebar and select option 3 to change the date. Enter
20 as the date, then select option 10 to redraw the plot. You should
see the Sun again because its motion along the ecliptic is taking it
northward, and it is now above the theoretical horizon. You should
also now see the Moon, its daily motions having taken it past new
phase to emerge in the evening sky.
You may want to advance another day or two to watch the Moon's
motions. The process should be fairly obvious by now - pick one or
more options and replot the sky. You will also notice that Venus is
moving towards Mercury, and both are rushing towards inferior
conjunction with the Sun.
Now press the space bar and select first option 7 to change the
horizon of the plot. Specify S for the southern horizon. Then select
option 9 to plot both Solar System objects and stars. Select option 10
to replot the sky. You should be faced with a winter sky dominated by
Orion and Canis Major.
Press the spacebar to bring up the Options Menu and select option 5 to
change the longitude for the plot. Since you are currently at
longitude 80 West (All longitudes must be in degrees West from the
Greenwich meridian) entering a value greater than 80 (such as 100 or
120) and replotting should show the constellations shift to the least.
Liklewise, entry of a smaller number (such as 50 or 60) and replotting
should produce a shift to the west.
As mentioned before, this is the reason why Universal Time is used
rather than local time. Accounting for local time zone changes would
be too complicated for most user to deal with.
After playing with the longitude, select option 6 to change the
latitude for the plot. Since you are currently at latitude 40 degrees
north, entering a value lower than +40 will result in the
constellations shifting north in the plot. Likewise entering a value
greater than +40 will result in the constellations shifting south.
If you have a printer attached to LPT1: on your PC, you can select
option 9 and get a printout of the Right Ascention and Declination for
the Solar System objects as calculated by SKYPLOT. At very least you
should pick option 9 to examine their positions on the display. This
may help to determine how close the program would place two nearby
objects. As mentioned earlier, these values should be within about 1
degree of the actual position of the object. A good book to verify
this is the American Ephemeris and Nautical Almanac, which is
available in the reference section of many University and public
libraries.
Option 11 may be selected to enter all new parameters, just in case
you find this less of a hassle than doing it option by option.
Option 12 may be used to exit from the program to DOS. (Your PC will
likely expect to see COMMAND.COM when it exits from SKYPLOT.)
2.6 Plotting Objects from a User-Supplied File
-----------------------------------------------
As as extra feature, SKYPLOT allows the user to plot other objects
along with the Solar System and stars. To do this the user must know
the Right Asencion and Declination of the object. Of course, like
most objects in the sky, this information will only be relevant for a
particular date -- so bear this in mind when creating the plot.
The data is entered into a file with any simple ASCII text editor
(EDLIN will suffice). It is in the form of 5 parameters. These are
(1) the hour of Right Asencion, (2) the minutes of Right Asencion, (3)
the degrees (positive or negative) of Declination, (4) the minutes of
Declination, and a marker symbol indicator.
The marker symbol indicatior is used to specify a marker which is used
to plot the object on the sky. Four marker symbols are available,
they are:
Marker Marker Symbol Indicator
-------------------- -----------------------------------
alpha 6
beta 7
gamma 8
delta 9
The Marker is one of the first four letters of the Greek alphabet. A
given marker may be used more than once. This could be useful if you
want a rough track of an object (seen against the stars) over a range
of dates (you could use the same symbol with different positions).
Again, note that the program dosen't care if the data you provide is
false. If you create a file, say Z.Z, and put in the position of a
comet for May 1, 1975, then you plot the sky for August 20, 1990 and
choose option 3 with the file Z.Z, SKYPLOT will proceed to plot the
object just where you said it would be on May 1, 1975.
As an example, the file OBJECTS.DAT is provided. This file will serve
to demonstrate the format you must observe for the object file. Note
that the last line must contain the value 999. This file will plot
the alpha, beta, gamma and delta symbols in northwest Orion. Pick an
appropriate winter sky and choose option 3 with the file OBJECTS.DAT
and you will observe this.
This technique was successfully used to plan for the 1986 apparition
of Comet P/Halley. It enabled observing expiditions to be organized
and carried out well in advance of the comets arrival.
2.7 SKYPLOT.DAT - The Stars Database
-------------------------------------
The stars database should be a copy of one of the STARS.* files
according to your desire for limiting magnitude. I find STARS.400 to
be the best one overall for constellation identification. Given
normal CGA resolution it is probably unwise to add many more stars
than this. When I have built in EGA graphics this problem may be
resolved.
The format of this file is the same as that for the objects file. If
you wish to add stars to it you can use any vanilla ASCII text editor.
(EDLIN works fine although it is a horrible editor!)
3.0 Concluding Statement
------------------------
I hope you enjoy the program as much as I've enjoyed creating it. I
know that there are other similar, and in some cases much better,
planetarium simulators on the market. However this one should add a
simple but different twist to the genre.
In conclusion, please explore and enjoy, and try to take some time out
to experience the Real Thing!
Gerry Santoro
State College, PA -- (Happy Valley)
September 13, 1987
