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1988-09-10
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Astropack Astronomical Tools:
Reference Manual
Version 2.0
(Aug. 1988)
2
Although this program and manual are copyrighted, they may be
freely copied and distributed provided that they are not modified in
any way and that the program is not distributed without this file.
(c) 1988 Graham Denham
3
Contents:
Licence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Registration Form . . . . . . . . . . . . . . . . . . . . . . . . 5
The program:
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 6
SETTING UP . . . . . . . . . . . . . . . . . . . . . . . . . 6
TIME CALCULATIONS . . . . . . . . . . . . . . . . . . . . . 7
POSITION CALCULATIONS . . . . . . . . . . . . . . . . . . . 8
CONVERSIONS . . . . . . . . . . . . . . . . . . . . . . . . 9
PRINTING . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A NOTE ON PRECISION . . . . . . . . . . . . . . . . . . . . 10
4
Licence:
This product is not free software. Instead, it is being
distributed as user-supported software. This means that if you decide
to use the program, you should register your copy with me by sending
your name, address, and $15 (1) to
Graham Denham
11611 50 Ave.
Edmonton, AB
Canada T6H 0J7
In return for your support, as a registered user you will receive
notice of future versions of Astropack and of the release of any
related products. The fee registers you to use this and any future
versions of the program. Additional copies and updates are available
to registered users at the cost of a diskette and postage. Once you
register, please feel free to send questions and suggestions. Also,
please contact me if you are interested in the C source code of this
program.
Remember that the future of user-supported software depends on
YOUR support.
-----------
1 Either U.S. or Canadian funds accepted.
Registration form: (Astropack, ver. 2.0 and up)
NAME: _____________________________________
ADDRESS: _________________________
_________________________
_________________________
POSTAL CODE / ZIP: _________
Do you want your name placed on the mailing list? Y N
6
INTRODUCTION:
Astropack is a general-purpose mathematical astronomy program. It
combines various time and position calculations that are often tedious
or difficult to do by hand. The program is intended for astronomy
hobbyists, students, or anyone else who might be curious about when
the sun will set next week, or how far it is from Stockholm to
Sacramento.
For users with serious requirements, Astropack provides
reasonably accurate values that will supplement those that an
astronomical almanac offers. It is especially valuable if an almanac
for the appropriate time period is not available. If you have a
standard printer, the program can also print tables of values for any
time periods.
Note: this manual can be printed on most printers from DOS by typing
COPY README.DOC LPT1:
SETTING UP:
To start the program, simply type ASTRO. The program will load
and you will see the main menu screen. To select an item from any of
the menus in the program, use the cursor keys to move the pointer, and
then press Enter. You can also escape from a menu to the previous one
by pressing Esc.
The first thing which you should do when you run Astropack is to
tell the program what timezone you are in. All of the times that you
will enter will be local times that depend on your timezone. It is
therefore important to set your timezone before doing any
calculations. The initial menu screen will display a flashing reminder
to this effect in the event that you forget.
To set the timezone, select the first item on the main menu. You
will then see the setup menu. Once again, take the first item on this
menu, and the program will prompt you for your timezone as the
separation in hours from Universal Time. The table provided will help
you if you are not certain about the number of hours.
Many calculations also require the observer's latitude and/or
longitude. If you know your latitude and longitude, this will present
no problem. If not, you can do one of two things. First, you could
determine your position from maps or charts. Alternately, Astropack
can supply your approximate latitude and longitude if you are located
in or near one of several major cities. If approximate values are
acceptable for your purposes, this latter method is easy and saves
time.
To use this built-in feature of the program, select the second
item on the setup menu: "Set Location". The program lists about 50
7
cities in Canada, the United States, and elsewhere. When you choose
one, the program will store its latitude and longitude as its default
values. This means that when you are prompted for your latitude or
longitude later, you can just press Enter to use the value for your
city.
(It should be noted, however, that these values are necessarily
somewhat imprecise. In large metropolitan areas, the coordinates
usually refer to the downtown area. In the Los Angeles area, for
example, the longitude varies by about 1/3 of a degree from one end to
the other. If you require more precise results, you should determine
your position from a map.)
TIME CALCULATIONS:
These include the determination of Greenwich Sidereal Time, Local
Sidereal Time, and Julian Date. You can get to the time calculations
menu by selecting the second item on the main menu. Once you select a
function on this menu, Astropack will prompt you for any necessary
values.
You will probably discover one of the program's useful features
at this point. When you have to enter a value for a calculation, the
program usually gives you a default value which you can use by just
pressing Enter. This default is either the value that you last used,
or the result from a previous calculation. The feature is designed to
make it unnecessary for you to keep re-entering the same value, or to
write down values that are needed only for further calculations.
It is often desirable for you to be able to enter values directly
as degrees, minutes, and seconds, or as hours, minutes, and seconds,
rather than having to convert them to decimal. Accordingly, the
program usually requests that you enter numbers in the form
DDD.MMSSss or HH.MMSSss. This means that you enter the degrees (or
hours) first followed by the decimal, and then the minutes, seconds,
and any fractions of seconds. For example, one would enter 30 deg.,
21 min., 15.3 sec. as 30.21153. Similarly, one would enter 1 h, 3
min., 45 sec. as 1.0345. Notice the zero before the three in this
example: it is necessary to preserve the place value of the decimals.
If you select "Ephemeris" from the Time menu, Astropack will
print or display a table of sidereal times. First, you are asked
whether you want the table to be produced on the screen or on the
printer. Hit 's' for screen or 'p' for printer. (Before you print for
the first time, please see the section on PRINTING, p. 9) Next, the
program asks you for the start and end dates, as well as the number of
hours between each entry on the table.
8
POSITION CALCULATIONS:
This category actually includes rise, set, and transit
calculations as well as position. An unrelated but interesting
function is also grouped here for convenience - the determination of
the great-circle distance between any two points on the Earth.
The first item on the Position menu allows you to determine the
position of the sun for any date and time. It gives the apparent
rectangular coordinates in addition to the right ascension and
declination.
The second and third items let you calculate the times at which
the sun or some other object rises, transits, and sets. The
calculation for the sun automatically finds the sun's position first,
whereas you have to specify the position in the other calculation. For
example, to find the approximate rise, set, and transit of Mercury
given its position, you would select "Any Times" from the Position
menu, enter the date, your position, and its position. (The program
asks for an object's position in equatorial coordinates. If you have
its ecliptic coordinates instead, see the section on conversions,
p. 9).
Once again, please remember to set your timezone. If you do not,
the results from these calculations will not be in your local time.
All of the "rise, set, transit" calculations take atmospheric
refraction into account. The sunrise time is for the ascending limb
of the sun, and the sunset time is for the descending limb. These
calculations use only one position value, although they are generally
accurate to within five minutes of the actual, observed rise, set, or
transit time. Note that if the position of the object is changing
rapidly, then the error could be somewhat greater.
You can also produce a table of sunrise, transit, and sunset
times using "Ephemeris". This function works in essentially the same
manner as "Ephemeris" on the Time menu (see p. 7). It asks you whether
you want the table sent to the printer or the screen, and then it asks
for the time period of the table.
The second-last item on this menu, "Distance", lets you calculate
the distance between any two points on the Earth and the heading from
the first point to the second. It gives you the great-circle distance
(the shortest distance along the surface of the Earth) rather than
straight-line distance (i.e., through the Earth). The program will
prompt you for the first location, for which you can use your default
latitude and longitude. It then requires the coordinates of the second
location, which you can find on a map or in the table of cities within
the program (for this, see pp. 6 - 7) The heading which the function
returns is measured in degrees, clockwise from north.
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CONVERSIONS:
Astropack can convert between ecliptic and equatorial
coordinates. To make a conversion, select "Conversions" on the main
menu.
Choose the first item on the Conversions menu if you wish to
convert a pair of equatorial coordinates to ecliptic coordinates. The
function will let you use the default equatorial coordinates if you
wish, which is useful if another calculation has given you equatorial
coordinates and you would like to convert directly.
The second menu item lets you do the complementary conversion:
ecliptic to equatorial coordinates. You must enter the ecliptic
latitude and longitude, and then the program will return the
equatorial coordinates. As usual, it also stores them as the default
values for your next calculation.
Finally, you can calculate the altitude and azimuth of an object
from its coordinates with the third item on the menu. When you use
this feature, the program asks for a date, time, and position, as well
as the equatorial coordinates of the object. It returns the altitude
and azimuth in degrees.
PRINTING:
Before you can print any tables, it is necessary to configure the
printer. This can be done by selecting "Printer Configuration" on the
Setup menu. Astropack supports three types of printer interface:
LPT1: - this is the standard, parallel connection used by most
printers.
COM1: - serial communication. If you use this, set the
protocol from DOS using the MODE command. See the DOS
manual for details.
Disk - the program copies the table into a disk file. This is
useful in a variety of situations. If your printer
does not work with either of the previous interfaces,
try using this and then printing the disk file later
from MSDOS. You might also use this option if you do
not actually want to print the table, but would like a
stored copy on disk.
When you select the third option from the Printer menu, the
program will ask you for a filename. Be sure to give the name of a
file that does not exist or is not valuable; any existing file would
be overwritten.
10
A NOTE ON PRECISION:
The sidereal time values that the program generates are accurate
to within 1/10 of a second between the years 1930 to 2100. (Any values
outside of these years from any function will probably be somewhat
less accurate.)
The results of the other functions should generally be treated as
having three significant figures, even if the program gives more
digits. Although some of the calculations are perhaps more precise, I
prefer to err on the side of caution here.
