allbery@uunet.UU.NET (Brandon S. Allbery - comp.sources.misc) (11/28/89)
Posting-number: Volume 9, Issue 38
Submitted-by: ecd@umn-cs.cs.umn.edu@ncs-med.UUCP (Elwood C. Downey)
Archive-name: ephem2/part01
[Lousy shar. Sigh. ++bsa]
Please accept a new release of my interactive astronomical ephemeris, v4.8,
for posting in comp.sources.misc. I am mailing it to you as five shar files,
each under 64k bytes. thank you very much.
Elwood Downey
umn-cs!ncs-med!ecd
-----------------------------------------------------------------------------
# This is a "shell archive" file; run it with sh.
# This is file 1.
echo x Readme
cat > Readme << 'xXx'
1) There is a generic-printer-ready manual in Man.
2) for dos, here is a typical autoexec.bat
set TZ=CDT5
date
time
3) Note the change from ESC to 'q' from V4.6 of ephem.
If you have source:
1) Change the define in io.c depending on whether you wish to compile under
unix or dos turbo/lattice.
2) Try changing the define in time.c if you get link undefines related to time
functions.
3) The following files are pretty much just pure transliterations from BASIC
into C from machine-readable copies of the programs in Duffett-Smith's book.
They have nothing to do with the rest of ephem so they may be used for
completely different applications if so desired.
aa_hadec.c anomaly.c astro.h cal_mjd.c eq_ecl.c moon.c moonnf.c nutation.c
obliq.c parallax.c pelement.c plans.c precess.c refract.c riset.c
sex_dec.c sun.c utc_gst.c
4) I didn't make a Makefile because to make ephem just compile everthing
and link with -ltermcap and -lm, such as
cc -O -o ephem *.c -ltermcap -lm
xXx
echo x Man
cat > Man << 'xXx'
Ephem V4.8
by
Elwood Downey
Chaska, MN
Table of Contents
1. Introduction ................................................... 2
2. Sample Screens ................................................. 2
2.1. Planet Data Screen ........................................... 2
2.2. Rise/Set Info Screen ......................................... 3
2.3. Separation Screen ............................................ 4
3. Program Operation .............................................. 4
3.1. Command Line Format .......................................... 4
4. Screen Fields .................................................. 5
4.1. Top Screen Fields ............................................ 6
4.2. Data format columns .......................................... 7
4.3. RiseSet format columns ....................................... 7
4.4. Separation format fields ..................................... 8
5. Date and Time Formats .......................................... 8
6. Configuration File ............................................. 8
6.1. Configuration file fields .................................... 9
6.2. Example ephem.cfg ............................................ 9
7. Menu options ................................................... 10
7.1. Adaptive vs. Standard hzn .................................... 10
7.2. Geocentric vs. Topocentric ................................... 10
8. Plotting ....................................................... 11
8.1. Defining plot fields ......................................... 11
8.2. Displaying a plot file ....................................... 12
8.3. Cartesian or Polar coords .................................... 12
8.4. Begin Plotting ............................................... 12
8.5. Stopping Plotting ............................................ 12
9. Watching ....................................................... 12
9.1. Trails ....................................................... 12
9.2. Night sky .................................................... 13
9.3. Solar System ................................................. 13
10. Searching ..................................................... 13
10.1. Find extreme ................................................ 13
10.2. Find 0 ...................................................... 14
10.3. Binary ...................................................... 14
10.4. Define a New function ....................................... 14
10.4.1. Intrinsic functions ....................................... 14
10.4.2. Field Specifiers .......................................... 14
10.4.3. Constants ................................................. 15
10.4.4. Operators ................................................. 15
10.5. Specifying Search Accuracy .................................. 16
10.6. Stop ........................................................ 16
10.7. Example Searches ............................................ 16
10.8. Another Example ............................................. 17
10.9. Caution ..................................................... 17
11. Implementation Notes .......................................... 17
11.1. Program limits .............................................. 18
12. DOS Installation Procedure .................................... 18
12.1. Details ..................................................... 19
13. Wish List ..................................................... 20
- 2 -
1. Introduction
Ephem is a program that displays observing circumstances for all the
planets plus any one additional fixed object.
Information displayed about each object includes RA and Dec precessed to
any epoch, heliocentric coordinates, local azimuth and altitude, distance
from sun and earth, solar elongation, angular size, visual magnitude,
phase, local rise, transit and set times, length of time up, and
topocentric or geocentric angular separations between all combinations of
objects.
Observing circumstance information includes UTC and local date and time,
local sidereal time, times of astronomical twilight, length of day and
night, local temperature, pressure and height above sea level and a
monthly calendar.
RA/Dec calculations are geocentric and include the effects of precession,
nutation and aberration. Alt/az and rise/set/transit and, optionally,
angular separation calculations are topocentric and include effects of
refraction and parallax.
A running plot file of selected field values may be generated as the
program runs. Ephem includes a very crude quick-look capability to view
these plot files or they may be plotted by other programs. One may also
watch the night sky or the solar system with a simple screen-oriented
display.
Ephem may be asked to search for interesting conditions automatically,
using several algorithms. Most fields displayed on the screen may be used
as terms in an arbitrary arithmetic expression that can be solved for zero
or minimized or maximized, or the time of state change of any boolean
expression can be found.
The program is written in C for unix or DOS. It uses only a very simple
set of io routines and should be easily ported to any ASCII display. The
DOS version requires the ANSI.SYS screen driver.
The planetary data and correction algorithms are taken, with permission,
from "Astronomy With Your Personal Computer", by Peter Duffett-Smith,
Cambridge University Press, 1985.
2. Sample Screens
Here are typical ephem screens. They are generated using the sample
ephem.cfg file (listed in a later section).
2.1. Planet Data Screen
- 3 -
Move to another field, RETURN to change this field, ? for help, or q to run
CDT 8:10:20 10/31/1989 | LST 9:34:47 | Lat 44:50:37 | October 1989
UTC 13:10:20 10/31/1989 | | Long 93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2447831.04884 | Dawn 6:13 | Elev 800 ft | 1 2 3 4 5 6 7
| Dusk 19:43 | Temp 40 F | 8 9 10 11 12 13 FM
Watch | NiteLn 10:31 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search off | | TZ 5:00:00 | 22 23 24 25 26 27 28
Plot off | NStep 1 | Epoch (OfDate) | NM 30 31
Menu Planet Data | StpSz RT CLOCK | |
--------------------------------------------------------------------------------
Ob R.A. Dec Az Alt Helio Helio Ea Dst Sn Dst Elong Size VMag Phs
Hr:Mn.d Deg:Mn Deg E Deg Up Long Lat AU(mi) AU Deg E ArcS %
Su 14:22.9 -14:13 112:39 2:31 38:06 0:00 0.9927 0.0000 0.0 1933 -27
Mo 15:41.8 -24:46 106:59 -19:13 251296 0.9901 21.3 1773 -8 12
Me 13:59.4 -10:59 114:48 8:33 195:48 3:46 1.3809 0.4108 -6.6 4.9 -1.6 98
Ve 17:37.4 -26:43 89:49 -39:47 -8:53 -3:23 0.7287 0.7271 46.9 23.2 -5.1 53
Ma 13:43.1 -10:05 117:19 11:49 201:05 0:53 2.5823 1.6168 -10.6 3.6 1.6 100
Ju 6:47.2 22:46 251:12 49:26 90:57 -0:13 4.5980 5.1287 -117.2 42.8 -2.5 99
Sa 18:40.3 -22:44 72:49 -48:14 284:16 0:24 10.471 10.030 61.2 15.8 1.2 100
Ur 18:10.6 -23:41 80:20 -43:45 274:49 -0:17 19.931 19.369 54.3 3.3 5.7 100
Ne 18:43.5 -22:12 71:28 -48:24 281:43 0:53 30.666 30.212 62.0 2.0 8.0 100
Pl 15:06.9 -1:47 96:11 3:53 225:01 15:38 30.607 29.656 16.5 0.3 13.8 100
Or 6:00.0 0:00 242:37 24:51 -124.5
2.2. Rise/Set Info Screen
This was done using the Adaptive option.
Move to another field, RETURN to change this field, ? for help, or q to run
CDT 8:10:20 10/31/1989 | LST 9:34:47 | Lat 44:50:37 | October 1989
UTC 13:10:20 10/31/1989 | | Long 93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2447831.04884 | Dawn 6:13 | Elev 800 ft | 1 2 3 4 5 6 7
| Dusk 19:43 | Temp 40 F | 8 9 10 11 12 13 FM
Watch | NiteLn 10:31 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search off | | TZ 5:00:00 | 22 23 24 25 26 27 28
Plot off | NStep 1 | Epoch (OfDate) | NM 30 31
Menu Rise/Set Info | StpSz RT CLOCK | |
--------------------------------------------------------------------------------
Ob Rise Transit Set Hrs Up
Time Az Time Alt Time Az
Su 7:51 109:07 12:58 30:55 18:06 250:41 10:15
Mo 10:09 126:04 14:30 18:57 18:46 232:37 8:37
Me 7:15 104:44 12:35 34:04 17:55 254:50 10:40
Ve 12:08 128:21 16:13 18:28 20:19 231:37 8:11
Ma 6:55 103:28 12:18 35:03 17:41 256:23 10:47
Ju 21:37 55:59 5:23 67:55 13:06 304:01 15:29
Sa 12:48 122:05 17:14 22:28 21:40 237:55 8:52
Ur 12:24 123:34 16:45 21:31 21:06 236:27 8:42
Ne 12:49 121:16 17:18 23:00 21:46 238:44 8:57
Pl 7:45 91:43 13:42 43:24 19:38 268:16 11:53
Or 22:29 89:13 4:36 45:10 10:40 270:47 12:11
- 4 -
2.3. Separation Screen
This was done using the Geocentric option.
Move to another field, RETURN to change this field, ? for help, or q to run
CDT 8:10:20 10/31/1989 | LST 9:34:47 | Lat 44:50:37 | October 1989
UTC 13:10:20 10/31/1989 | | Long 93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2447831.04884 | Dawn 6:13 | Elev 800 ft | 1 2 3 4 5 6 7
| Dusk 19:43 | Temp 40 F | 8 9 10 11 12 13 FM
Watch | NiteLn 10:31 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search off | | TZ 5:00:00 | 22 23 24 25 26 27 28
Plot off | NStep 1 | Epoch (OfDate) | NM 30 31
Menu Separations | StpSz RT CLOCK | |
--------------------------------------------------------------------------------
Ob Sun Moon Merc Venus Mars Jup Saturn Uranus Nep Pluto
Su 21:21 6:34 46:57 10:34 117:14 61:11 54:20 61:58 16:31
Mo 21:21 27:54 26:03 31:47 137:45 40:43 33:50 41:32 24:29
Me 6:34 27:54 53:30 4:07 110:45 67:40 60:50 68:25 19:07
Ve 46:57 26:03 53:30 57:31 163:41 14:49 8:06 15:41 43:52
Ma 10:34 31:47 4:07 57:31 106:40 71:45 64:54 72:31 22:25
Ju 117:14 137:45 110:45 163:41 106:40 178:25 171:33 178:58 122:40
Sa 61:11 40:43 67:40 14:49 71:45 178:25 6:53 0:55 55:47
Ur 54:20 33:50 60:50 8:06 64:54 171:33 6:53 7:42 49:32
Ne 61:58 41:32 68:25 15:41 72:31 178:58 0:55 7:42 56:22
Pl 16:31 24:29 19:07 43:52 22:25 122:40 55:47 49:32 56:22
Or 124:29 138:25 119:15 152:44 115:20 25:30 155:15 156:10 155:25 136:42
3. Program Operation
3.1. Command Line Format
To run ephem, just type "ephem". You may also optionally specify an
alternate configuration file, and optionally specify values for several
screen fields. The command line syntax can be summarized as follows:
ephem [-c <configfile>] [field=value] ...
When ephem starts, it first displays a disclaimer banner. Then, after any
key is pressed, it reads a configuration file to set the initial values of
several fields. The default filename is ephem.cfg or .ephemrc in the HOME
environment variable directory if available. The exact format of the file
is described below. Then it processes any additional command line
arguments exactly as it would if they too came from the configuration
file. It then draws all fields on the screen with their initial values.
The program then loops advancing time each step, by some amount you may
control, and updating all fields each loop.
There are two fields that control this looping behavior: NStep and StpSz.
These control the number of steps and the amount of time to add each step,
respectively. When the number of steps, NStep, goes to 0 or any key is
- 5 -
pressed, the looping stops and you enter a command mode.
Command mode allows you to modify most of the fields. The idea is that
you move to each field on the screen you wish to change and change it.
When you have changed everything you want to, type "q" to resume screen
updates.
To change a field:
1) move the cursor to the field (see below);
2) type RETURN;
3) type in the new value along the command line at the top according
to the format indicated in the prompt. To accept the new value
type RETURN, or to leave it unchanged after all type "q".
A few fields don't require you to type anything; just typing RETURN does
all the work. If you can't move to it, you can't change it.
The arrow keys on most systems move the cursor around. If these do not
function or function incorrectly, the h/j/k/l keys also move the cursor
left/down/up/right, respectively. You may also move the cursor
immediately to one of the planet rows by typing one of SMevmjsunp (to
avoid conflict with j, jupiter's row must actually be typed as control-j).
When you have changed a field that would invalidate any of the other
fields the message NEW CIRCUMSTANCES appears in the top center of the
screen. This will remain until you type "q" to allow at least one screen
update loop to occur. If you change any field that causes new
circumstances, the StpSz value is not added to the first loop. Note also
that after a series of loops, NStep is automatically reset to 1 so "q"
will do exactly one loop and return you to command mode.
To quit the program, type control-d from command mode. For a little more
help, type ?. The entire screen may be erased and redrawn with control-l.
4. Screen Fields
The screen is divided into two halves, top and bottom. The top fields are
always present. They define the general observing circumstances and
control features.
The planets and one additional object are displayed in a table in the
bottom portion of the screen. There is one object per row, and several
columns. There are three forms of this portion selected by picking the
Menu selection.
Some things may be turned off to reduce compute times. Calculations for
each planet may be turned on and off by selecting the planet name field.
Calculations for Dawn/Dusk/NiteLn may be turned off by selecting any of
these fields. Planet positions are only updated as often as necessary to
match the display precision of the screen unless plotting or searching is
on, in which case full precision is required at all times and so positions
are always fully recalculated at each iteration.
- 6 -
Follows is a list and description of each of the fields in each section.
Following each name, in parentheses, might be a "c" to mean the field may
be picked to be changed or a "p" to mean the field may be picked to be
included in a plot (see below).
4.1. Top Screen Fields
LT(c) the local timezone name, time and date. The name field may
be changed to any three-character mnemonic. Local time and
date may be changed as described in a later section. set to
"N" to set from computer clock.
UTC(cp) universally coordinated time and date. set to "N" to set
from computer clock.
JulianDat(cp) the current Julian date, to about 1-second accuracy.
LST(cp) the current local sidereal time. set to "N" to set from
computer clock.
TZ(cp) hours local time is behind utc, ie, >0 west, <0 east of
Greenwich. set to "N" to set from computer clock.
Lat(cp) location latitude, positive degrees north of equator.
Long(cp) location longitude, positive degrees west of Greenwich
meridian.
Temp(cp) local surface air temperature, in degrees F.
AtmPr(cp) local surface air pressure, in inches of mercury.
Epoch(c) the precession epoch, to nearest 0.1 years. This says
(OfDate) when coordinates are not precessed, ie, are in the
epoch of date.
NStep(c) The number of times the display with be updated (time
advanced by StpSz each step) before entering command mode.
StpSz(c) the amount of time UTC (and its derivatives) is incremented
each loop. set this to RTC to use real-time based on the
computer clock. you may also set it in terms of days by
appending a D (or d) after the number when you set it.
Elev(cp) local elevation of the ground above sea level, in feet.
Dawn(cp) local time when the sun is about 18 degrees below the
horizon before sunrise
Dusk(cp) local time when the sun is about 18 degrees below the
horizon after sunset
NiteLn(cp) length of astronomical night, ie, Dawn - Dusk.
Plot(c) controls plotting; see complete discussion below.
Watch selects the sky or solar system displays; see complete
discussion below.
Menu The three picks after this field select the format of the
lower half of the screen. See their complete discussion
below.
Search controls the automatic search feature of ephem. See the
complete discussion below.
Also in the upper right of the screen is a calendar for the current local
month. Dates of new and full moons are marked NM and FM, respectively.
One object may also be added to the display by putting its name and
location in the bottom row of the screen. This is done by moving the
cursor to the bottom row (the row beneath Pluto) and selecting the field
under the Ob, R.A., and Dec columns.
- 7 -
4.2. Data format columns
Ob name of object.
R.A.(p) right ascension of object, precessed to given epoch, in
Hours, minutes and decimal minutes.
Dec(p) declination of object, precessed to given epoch, in degrees
and minutes.
Az(p) degrees eastward of true north for object.
Alt(p) degrees up from a horizontal plane Elev feet above sea
level.
Helio heliocentric longitude. the earth's is displayed on the
Sun's line.
Helio heliocentric latitude.
Ea Dst(p) distance from earth center to object center, in AU, except
distance to moon is in miles.
Sn Dst(p) distance from sun center to object center, in AU.
Elong(p) spherical angular separation between sun and given object,
calculated from the their geocentric ecliptic coordinates.
Note this is not just difference in ecliptic longitude. The
sign, however, is simply sign(obj's longitude - sun's
longitude), ie, degrees east. thus, a positive elongation
means the object rises after the sun.
Size(p) angular size of object, in arc seconds.
VMag(p) visual magnitude of object.
Phs(p) percent of visible surface in sunlight, ie, the phase. Note
the moon phase is calculated simplistically as just
abs(elongation)/180*100 which can be a few degrees off...
this means that because of how elongation is defined it
doesn't say 0 during new moon (or 100 during full) except
during close eclipses (maybe that's a "feature"?).
4.3. RiseSet format columns
Rise The local time and azimuth when the upper limb of the object
rises.
Transit The local time and altitude when the object crosses the
meridian, ie, when its azimuth is true south or, if no
precession, when the local sidereal time equals the object's
right ascension.
Set The local time and azimuth when the upper limb of the object
sets.
Hrs Up The number of hours the object is up on the local date.
Horizon displacement may be calculated in either of two ways; see the
horizon discussion in the Menu selection section.
Various oddball conditions are accounted for, such as an object that is up
sometime during the day but that doesn't rise, transit or set as such on
that day, an object that is circumpolar or that is never up or one that
rises twice on the same day. These are marked as "Never rises", "Never
transits", "Never sets", "Circumpolar", "Never up" or appended with a plus
"+" sign, respectively.
- 8 -
4.4. Separation format fields
This format is a table of angular separations between each pair of
objects. These angles are based on the local altitude/azimuth, and so in
general differ somewhat from the elongations reported for the sun in the
Data menu.
5. Date and Time Formats
Times are displayed and entered in h:m:s format. If you pick a time field
to change it any of the h, m, and s components that are not specified are
left unchanged from their current value. For example, 0:5:0 set hours to
0, minutes to 5, seconds to 0, whereas :5 sets minutes to 5 but leaves
hours and seconds unchanged. A negative time is indicated by a minus sign
(-) anywhere before the first digit.
Dates are displayed and entered in American m:d:y format. As with time,
components omitted when entering a new value remain the current value.
For example, if the current date is 10/20/1988 and you type 20/20 the new
date will become 20/20/1988. Note you must type the full year since the
program is accurate over several centuries either side of 1900.
As a matter of typing convenience, the program accepts any character other
than a digit or minus as the separator; you don't have to type a perfect
":" or "/".
6. Configuration File
The ephem.cfg configuration file allows you to set the initial values of
many of the screen fields. You can still change any field while the
program is running too; this file just sets the initial conditions. Note
that the order of entries in this file is important. They each take
effect immediately and so you should put them in the same order you wish
them to be processed, just as though you were changing the fields
interactively within ephem.
You can have several different configuration files if you wish. By
default, ephem looks for one named ephem.cfg. You can tell it to use an
alternate file by using the -c switch as follows:
ephem -c <filespec>
If your system supports the HOME environment variable then ephem also
looks for a configuration file there with the name
The format of the file uses the form KEYWORD=VALUE, where the possible
KEYWORDS and the types of VALUES for each are described below. Any
KEYWORDS not in the file will take on some sort of default. The separator
need not be an actual equals sign; any char will do because the VALUE is
assumed to start one character after the KEYWORD, regardless.
Note: because of the way unspecified time and date components are left
unchanged (see section on Date and Time Formats) always specify the
complete time and date for all entries in the configuration file. For
example, to initialize the longitude to zero degrees, say 0:0:0, not just
0.
- 9 -
6.1. Configuration file fields
UD initial UTC date, such as 10/20/1988, or "NOW" to use the
computer clock.
TZONE hours the local time is behind utc, such as 5:0:0. you need not
set this if you use "NOW" for UT or UD.
TZNAME name of the local time zone, such as CDT. 3 chars max. you need
not set this if you use "NOW" for UT or UD.
LONG longitude, in degrees west of greenwich, in the form d:m:s.
LAT latitude, in degrees north of the equator, in the form d:m:s.
HEIGHT height above sea level, in feet, such as 800
TEMP air temperature, in degrees F, such as 50
PRES air pressure, in inches of Mercury, such as 29
STPSZ the time increment between screen updates, such as "1" to give
one hour updates. this can be a specific amount or RTC to use
the system clock as a real-time source. You may also specify a
time in days, by appending a D (or d) after the number.
PROPTS this selects what you want included initially in the display.
since IBM-PC math is not very fast, you can reduce the time to
update the screen by only printing those fields of interest. the
VALUE is a collection of letters to turn on each item from the
following set:
T twilight (dawn-dusk)
S circumstances for the sun
M circumstances for the moon
e circumstances for mercury
v circumstances for venus
m circumstances for mars
j circumstances for jupiter
s circumstances for saturn
u circumstances for uranus
n circumstances for neptune
p circumstances for pluto
For example, to just track the sun and saturn, say PROPTS Ss
NSTEP number of times program will loop before entering command mode.
see the discussion under Program Operation.
EPOCH this sets the desired ra/dec precession epoch. you can put any
date here or EOD to use the current instant ("Epoch of Date").
OBJN name of the extra object to track.
OBJRA right ascension of the extra object to track.
OBJDEC declination of the extra object to track.
6.2. Example ephem.cfg
This is the ephem.cfg file that was in effect when the sample screens (in
an earlier section) were generated. You might run ephem with this
configuration file and compare with the samples as a check.
- 10 -
TZONE 5
TZNAME CDT
UT 13:10:20
UD 10/31/1989
LONG 93:42:8
LAT 44:50:37
HEIGHT 800
TEMP 40
PRES 29.5
STPSZ RTC
PROPTS TSMevmjsunp
EPOCH EOD
NSTEP 1
OBJN Or
OBJRA 6:0:0
OBJDEC 0:0:0
As another common example, this ephem.cfg creates an essentially free-
running real-time screen based on the computer clock:
UT NOW
LONG 90:10:8
LAT 40:50:20
HEIGHT 800
TEMP 50
PRES 29
STPSZ RTC
PROPTS TSMevmjsunp
NSTEP 9999999
EPOCH EOD
7. Menu options
When you select "Menu" you can change among the three styles of bottom
screens. There are also two options that can be set from the Menu quick-
choice menu. These options toggle when picked and retain their values so
they need only be changed when desired.
7.1. Adaptive vs. Standard hzn
This selects the horizon refraction displacement algorithm used by the
Rise/Set menu. "Adaptive" uses the local conditions known to ephem and
matches the Planet Info times very nicely. "Standard" uses the "accepted
nominal" refraction value of 32 arc minutes, and agrees to a minute or two
with published tables.
7.2. Geocentric vs. Topocentric
This selects the vantage point for the Separation menu. "Geocentric"
ignores local conditions and gives the separation as seen from Earth
center. "Topocentric" uses the local conditions known to ephem. They are
particularly critical for lunar occultations, but the effect can be
- 11 -
significant for the planets.
Note that searching over a period that will include the rise or set times
of either object is generally better performed from the geocentric
viewpoint. The refraction effect of the topocentric viewpoint causes many
arcminutes of rapid whiplash displacement as the objects rise and set that
overlays the smooth celestial motion of the objects. This rapid position
variation can confuse the solver algorithms that expect fairly smooth
functions.
8. Plotting
Each time a field is drawn on the screen its full-precision value may be
written to a file. This implies you may not plot a field from other than
the current menu at the time plotting is on.
Each line in the file consists of a tag character followed by two or three
floating point variables, all separated by commas. If there are two
values, they should be interpreted to be x and y (or perhaps r and theta).
If there is a third, it is a z or trace value.
For efficiency on systems that can compute a screenfull faster than they
can display it, screen updates are suppressed while plotting is on and
NStep is greater than 1. This can greatly reduce the time to generate a
long plot file. Fields are still logged for plotting; they just are not
drawn on the screen.
The Plot field controls plotting. Whether plotting is currently active is
indicated by "on" or "off" immediately to its right.
Picking "Plot" brings up a quick-choice menu, as follows:
Select: Select fields, Display a plot file, Cartesian coords, Begin plotting
8.1. Defining plot fields
Select the "Select fields" option. You will be asked to move the cursor
to the field you want to use as the x coordinate (abscissa), then asked to
choose the y coordinate (ordinate), then asked to choose an optional z
trace variable and finally a tag character. (X and Y may be for other
coordinate systems too but ephem's quicky plotter can only plot in
Cartesian coordinates.) If you type q for either x or y then no more
fields will be defined. If you type q for the z field there will be no z
field. You can not label a plot line with the letter "q" at this time.
This then repeats so you may choose up to ten of these sets for any given
plot run. Each set defines what will become a line on the final plot.
Note that you may select the "Search" field to indicate use of the current
search function; that function must be defined by the time plotting is
turned on.
If you turn plotting off and back on the fields selected for plotting are
reactivated the same as they were last time. You may change them if
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desired, of course, but there is no need to redefine them if you do not
wish to change them.
8.2. Displaying a plot file
Select the "Display a plot file" option to make a crude plot of an
existing plot file previously created by ephem. The entries in the file
will be drawn on the screen using their tag characters; the plot remains
on the screen until you type any character.
The plot may be made in polar or Cartesian coordinates, depending on the
setting of the plotting mode in the quick-choice (see next section).
8.3. Cartesian or Polar coords
This toggles the plotting mode coordinate system. The mode remains until
changed. Polar coordinates assume the first numeric field in the plot
file is the radius, and the second is the angle counterclockwise from
right, in degrees.
8.4. Begin Plotting
If there are defined plot field lines then the third option, "Begin
plotting" will be available. You will be asked for the name of the file
to use and, if it already exists, whether to overwrite it or append to it.
Once you have chosen a file, plotting is on and the top menu plotting
status field changes to "on". The default plot filename is ephem.plt.
The values are written to the plot file each time they are updated on the
screen until you select "Plot" again and select the "Stop" option to turn
plotting back off.
8.5. Stopping Plotting
If plotting is on, then selecting the Plot field in the top section will
turn plotting off. You may pick Plot again and resume with the same fields
by selecting "Begin plotting" again.
Note that due to internal buffering the plot file will not be completely
written to disk until plotting is turned off.
9. Watching
You may make a simple drawing on the screen of the night sky or the solar
system by selecting "Watch". It will bring up a quick-choice menu as
follows:
Select: Night sky, Solar system, No trails
9.1. Trails
You may either erase after each iteration or leave the tags up, referred
to as "trails". Picking the right-most choice will toggle between "No
trails" and "Leave trails"; set it accordingly before you select the style
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of sky plot you wish. Ephem will remember your selection.
9.2. Night sky
This selection draws the currently active planets as they would appear in
the sky at the current time and date. The coordinate system is such that
0 degrees azimuth (north) through 360 degrees (north, once around) is
mapped to the horizontal screen dimension, and 0 degrees altitude (level)
through 90 degrees (the zenith) is mapped to the vertical dimension. Thus,
the bottom row is the horizon and all across the top is the zenith.
9.3. Solar System
This selection draws the currently active planets as they would appear
looking "down from the top" of the sun.
In either style of display, pressing RETURN advances the time by whatever
amount StpSz is set to. Pressing h advances it by one hour and d advances
it by one day. Pressing "q" returns to the tabular main screen. Pressing
any other key starts an automatic loop with each step advancing by StpSz;
pressing any key stops the looping.
10. Searching
Ephem can search for arbitrary conditions to exist among most displayed
fields. You first enter a function, then select from among three forms of
equation solvers to iteratively solve for the next time when the function
meets the requirements of the solver. The solver selects the next time for
which it wants the function evaluated and sets StpSz so that the next
iteration will occur at that time. The solvers continue to iterate until
either they achieve their goal or NStep reaches 0.
You may set NStep quite large and let ephem search unattended or set it to
1 and watch it converge one step at a time. You may also plot at the same
time as search to record the exact steps ephem took to converge. (But
recall that screen updates are suppressed if plotting is also on).
The "Search" selection in the top half of the screen controls all
searching. Picking it brings up a quick-choice menu as follows:
Select: Find extreme, Find 0, Binary, New function, Accuracy
10.1. Find extreme
This search algorithm searches for a local maximum or a minimum in the
search function, whichever it finds first. It begins by evaluating the
search function at the current time then for two more times each separated
by StpSz. It then fits these three points to a parabola and solves it for
the time of its maximum (or minimum). StpSz is set so that the next
iteration will evaluate at this point. This parabolic fit solution keeps
repeating until StpSz changes by less than the desired accuracy or until
the curve becomes so flat that an extrema appears too broad to find.
- 14 -
10.2. Find 0
This search algorithm uses the secant method to solve for the time at
which the search function is zero. The function is evaluated at the
current time and then again StpSz later to establish a slope for which the
x-intercept is found as the next zero guess. This is used to set StpSz for
the next desired time value and the slope hunting process repeats until
StpSz changes by less than the desired accuracy.
10.3. Binary
This search algorithm must be used with a search function that yields a
boolean result, ie, a true or false value. The idea is that the function
is assumed to be one truth value when evaluated at the present time, and
the opposite truth value when it is evaluated StpSz later. The algorithm
will then do a binary search for the time at which the truth value
changes.
The binary algorithm does not begin until the state change is bounded in
time. Initially, as long as the truth value at StpSz is the same as the
previous value the algorithm will just keep moving in time by StpSz
looking for when the state changes. That is, a linear search is initiated
to bound the state change, then the binary search proceeds.
10.4. Define a New function
Select "New function" to display the current search function. If you type
"q" it will be left unchanged. If you type RETURN it will be erased. If
you type anything else it will be compiled and, if there are no errors, it
will become the new search function. Once a valid function has been
stored, it will remain unless changed. If a search function is selected
and there is as yet no valid search function defined, you will
automatically be asked to enter one as though you had selected "New
function."
A search function consists of intrinsic functions, field-specifiers,
constants and operators, and precedence may be overridden with
parentheses.
10.4.1. Intrinsic functions
In this release, the only intrinsic function available is abs(), which
returns the absolute value of its argument.
10.4.2. Field Specifiers
A field in the bottom half of the menu is specified in the form of
"object_name.column_name". The object_name is enough of the planet name to
be unique, or the exact name of the optional object. The column_name is
from the following table, depending on which menu is up. In all cases
additional characters may be entered but are ignored.
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Planet Data Menu Rise/Set Menu Separation Menu
------------------ -------------------- ---------------
al Alt hr Hrs Up, or j Jup
az Az hu Hrs Up ma Mars
d Dec raz Rise Az me Merc
ed Ea Dst rt Rise Time mo Moon
el Elong saz Set Az n Nep
hla Helio Lat st Set Time pl Pluto
hlo Helio Long ta Transit Alt sa Saturn
ph Phs tt Transit Time su Sun
ra R.A. u Uranus
sd Sn Dst ve Venus
si Size
vm VMag
In addition, the following top-half fields may be used:
da Dawn
du Dusk
n NiteLn
Note that while you may include any field in a search function, it will
only be calculated if it is actually being displayed. This implies it is
useless to define a search that uses fields from other than the one menu,
namely, the current menu at the time the search is running.
10.4.3. Constants
Constants may be integers or floating point numbers. The latter may be
expressed in scientific notation if desired. Examples include 100, .9,
1.234, 1e10 and 1.2e-4. Any number may be preceded by - to make it
negative.
10.4.4. Operators
The collection of arithmetic, relational and boolean operators provided
mimics those of C language as listed in the following table, in decreasing
order of precedence. Operators grouped together have the same precedence
and all have left-to-right associativity. Parentheses may be used as
desired.
- 16 -
Symbol Meaning Resulting type
------ -------------------- --------------
* multiply arithmetic
/ divide arithmetic
+ add arithmetic
- subtract arithmetic
> greater than boolean
>= greater than or equal boolean
< less than boolean
<= less than or equal boolean
== equality boolean
!= inequality boolean
&& logical and boolean
|| logical or boolean
10.5. Specifying Search Accuracy
Selecting "Accuracy" allows you to specify when the search will stop. The
search algorithms will stop when StpSz becomes equal to or less than this
value. The default is one minute. If ephem has not yet converged to the
specified accuracy but NStep has decremented to 1, the searching will stop
but the search status field will still indicate which search procedure is
in effect. To try more iterations you may increase NStep and resume
searching. If the accuracy was achieved, the search status field will
switch to "off" with the number of "unused" steps remaining in NStep and
the last step size in the StpSz fields.
10.6. Stop
If searching is on, this option will also appear on the quick-choice menu
and may be selected to turn off the search.
10.7. Example Searches
As an example, let's find when Pluto again becomes the furthest planet
from Sol. You may find when the difference in their sun distance is zero,
or you might use a binary search on the condition that Pluto's sun
distance is larger then Neptune's.
To try the former approach select Search, select "Find 0", specify the
search function to be:
pl.sd - nep.sd
set StpSz to something large like 10d, NStep to allow several iterations
like 20, and then type "q" to start the search and watch ephem do the
hunt. Ephem will settle on about 21:02 1/10/1999 UT.
- 17 -
To try a binary search, you first need to have some idea of when the event
will occur so you can eliminate the initial linear search for the state
change. We can start at, say, 1/1/1999, set StpSz to 30d, select Binary
search, specify the search function to be:
pl.sd > nep.sd
and go. Once it brackets the state change note how StpSz keeps being cut
in half but can go in either direction (sign) as it divides each interval
in half. Ephem will converge on the same answer.
10.8. Another Example
To find the time of last quarter moon during December, 1989, use the "Find
0" search algorithm to solve "moon.el + 90". (At last quarter, the moon
is 90 degrees west of the sun, or -90 east in ephem's elongation display.)
Set the initial time to mid-month, 12/15/1989, StpSz to 1 day and NSteps
to 10. Ephem takes only a few iterations to settle on 23:57 12/19 UT.
10.9. Caution
Beware that most celestial phenomena are generally pseudo-periodic in
nature. In early search steps ephem can easily skip over a local maxima
and find a later one, which, while correct, may not be what was desired.
In general, the closer you can be when you start the search the better
ephem can refine it; it is not as good with very broad searches that can
go "wild". Set StpSz large enough to offer significant change in the
function value, but small enough not to skip too far.
For example, Saturn and Neptune had three close approaches during 1989.
If you did not know this then just asking ephem to find a minimum would
have produced different results depending on the starting conditions.
When starting a search for a certain class of event it is a good idea to
first use the plotting or watching facility of ephem to get a broad
picture of the general circumstances then use ephem's search facility to
refine a given region (or create and inspect a plot file and do your own
interpolation directly from it separately).
Similarly, ephem's searching techniques are not good for eclipses because
the moon and sun are close every month; the trick is sorting through the
frequent conjunctions for ones that are particularly close. One needs a
way of establishing an envelope fit to the local extrema of a cyclic
function in order to find a more global extreme.
11. Implementation Notes
Remember that everything is for the current local time. So, for example,
the calendar marks moon events in local time; commercial calendars usually
mark the UT date. Similarly, the rise/set times are for the current local
day.
The program uses a horizontal plane tangent to the earth as the horizon
for all altitude calculations, rise/set events, etc. This is not the same
as the angle up from the local horizon unless the observer is directly on
- 18 -
the ground due to earth's curvature. The effect can be found from:
sin(a)**2 = (h**2 + 2Rh) / (R+h)**2
where:
R = radius of earth
h = height above ground (same units as R)
a = increase in altitude
For example, the effect is more than two arc minutes at a height of 5
feet.
Visual magnitudes are not very accurate at all... I haven't bother to fix.
The accuracy of ephem can not be specifically stated since the Duffett-
Smith book does not warrant its planet position polynomials to any given
degree. I know for sure that better accuracy could be achieved if ephem
used ET but I have not yet decided on a suitable UT-ET algorithm.
The program uses double precision throughout. While this precision might
seem a little ridiculous, it is actually more efficient for most
traditional K&R C compilers, the search functions are far more stable, it
improves small angles (conjunctions) calculated using acos(), etc.
Searching and plotting always use full precision but if neither of these
are turned on pure display and watching only recompute a given planets new
location if the time has changed enough to effect the required display
precision, based on the planets mean apparent orbital motion.
A "negative 0" is displayed when a value is negative but less than half
the display precision.
The sun-moon distance is the solution for the third side of a planar
triangle whose two other sides are the earth-moon distance and earth-sun
distance separated by the angle of elongation.
11.1. Program limits
The search function is limited to a maximum of 32 instructions (each
constant, field spec, and operation is one instruction), with no more than
a total of 16 constants and fields specs. At run time, the function can
not require more than 16 stacked values (due to operator precedence or
explicit parenthetical expressions) to evaluate.
No more than 32 fields can be tracked simultaneously for plotting and/or
searching.
No more than 10 lines may be plotted at once.
The maximum file name length is 14 characters.
12. DOS Installation Procedure
Summary:
- 19 -
You must be running DOS V2.0 or later, though somewhere between V2.0 and
V3.21 the behavior of control-c to terminate the program was fixed. A
8087 floating point chip will be used if present.
The distribution floppy contains two files, ephem.exe and ephem.cfg.
Ephem.exe is the executable program; ephem.cfg is a sample configuration
file. To run the program, make working copies of these two files in a
directory and run "ephem" from that directory. The program uses the
ANSI.SYS terminal driver for screen control. It also uses an environment
variable, TZ, to establish the local timezone.
12.1. Details
1) The ANSI.SYS screen driver is required for this program. Edit the
CONFIG.SYS file, if necessary, so it contains the following line:
device=ANSI.SYS
If it wasn't already there and you had to add it, note it will not
take effect until you reboot DOS.
2) Set a DOS environment variable, TZ, in the following form:
set TZ=SSSnDDD
"SSS" is the 3-letter abbreviation for the local standard timezone;
"n" is a number between -23 to 24 indicating the number of hours
that are subtracted from GMT to obtain local standard time;
"DDD" is an optional 3-letter abbreviation for the local daylight
savings time zone name. Leave it off if you do not have savings
time in your area or it is not currently in effect. If the
changeover dates differ from the internal algorithm, just use
SSS and n directly.
For example, in the midwestern United States with savings times:
set TZ=CST6CDT
If for some reason your system does not change to savings time at the
right time, then omit the DDD parameter and just set the SSS and n
to exactly what you want.
You can put this in your AUTOEXEC.BAT file so it gets set each time
you boot DOS.
This environment variable is used to establish the timezone name and
hours offset whenever the "NOW" shorthand is used from ephem, either
from the configuration startup file or whenever any time field is
changed manually.
- 20 -
3) place the distribution floppy into drive a:.
4) copy the ephem.* files to a working diskette:
copy ephem.* b:*.*/v
or hard disk:
copy ephem.* c:*.*/v
5) run using the sample configuration file by just running
ephem
To run with a different configuration file, use the -c switch:
ephem -c <filespec>
13. Wish List
allow specifying the extra object with orbital elements so it could be
used for earth satellites, comets, etc.
incorporate ephemeris time, not just UT.
xXx
echo x astro.h
cat > astro.h << 'xXx'
#ifndef PI
#define PI 3.141592653589793
#endif
/* conversions among hours (of ra), degrees and radians. */
#define degrad(x) ((x)*PI/180.)
#define raddeg(x) ((x)*180./PI)
#define hrdeg(x) ((x)*15.)
#define deghr(x) ((x)/15.)
#define hrrad(x) degrad(hrdeg(x))
#define radhr(x) deghr(raddeg(x))
/* ratio of from synodic (solar) to sidereal (stellar) rate */
#define SIDRATE .9972695677
/* manifest names for planets.
* N.B. must cooincide with usage in pelement.c and plans.c.
*/
#define MERCURY 0
#define VENUS 1
#define MARS 2
#define JUPITER 3
#define SATURN 4
#define URANUS 5
#define NEPTUNE 6
#define PLUTO 7
xXx
echo x circum.h
cat > circum.h << 'xXx'
#define SPD (24.0*3600.0) /* seconds per day */
#define EOD (-9876) /* special epoch flag: use epoch of date */
#define RTC (-1234) /* special tminc flag: use rt clock */
#define STDHZN 0 /* rise/set times based on nominal conditions */
#define ADPHZN 1 /* rise/set times based on exact current " */
#define TWILIGHT 2 /* rise/set times for sun 18 degs below hor */
/* info about our local observing circumstances */
typedef struct {
double n_mjd; /* modified Julian date, ie, days since
* Jan 0.5 1900 (== 12 noon, Dec 30, 1899), utc.
* enough precision to get well better than 1 second.
*/
double n_lat; /* latitude, >0 north, rads */
double n_lng; /* longitude, >0 east, rads */
double n_tz; /* time zone, hrs behind UTC */
double n_temp; /* atmospheric temp, degrees C */
double n_pressure; /* atmospheric pressure, mBar */
double n_height; /* height above sea level, earth radii */
double n_epoch; /* desired precession display epoch as an mjd, or EOD */
char n_tznm[4]; /* time zone name; 3 chars or less, always 0 at end */
} Now;
extern double mjd_day(), mjd_hr();
/* info about where and how we see something in the sky */
typedef struct {
double s_ra; /* ra, rads (precessed to n_epoch) */
double s_dec; /* dec, rads (precessed to n_epoch) */
double s_az; /* azimuth, >0 e of n, rads */
double s_alt; /* altitude above topocentric horizon, rads */
double s_sdist; /* dist from object to sun, au */
double s_edist; /* dist from object to earth, au */
double s_elong; /* angular sep between object and sun, >0 if east */
double s_hlong; /* heliocentric longitude, rads */
double s_hlat; /* heliocentric latitude, rads */
double s_size; /* angular size, arc secs */
double s_phase; /* phase, % */
double s_mag; /* visual magnitude */
} Sky;
/* flags for riset_cir() status */
#define RS_NORISE 0x001 /* object does not rise as such today */
#define RS_2RISES 0x002 /* object rises more than once today */
#define RS_NOSET 0x004 /* object does not set as such today */
#define RS_2SETS 0x008 /* object sets more than once today */
#define RS_CIRCUMPOLAR 0x010 /* object stays up all day today */
#define RS_2TRANS 0x020 /* transits twice in one day */
#define RS_NEVERUP 0x040 /* object never rises today */
#define RS_NOTRANS 0x080 /* doesn't transit today */
#define RS_ERROR 0x100 /* can't figure out times... */
/* shorthands for fields a Now pointer, np */
#define mjd np->n_mjd
#define lat np->n_lat
#define lng np->n_lng
#define tz np->n_tz
#define temp np->n_temp
#define pressure np->n_pressure
#define height np->n_height
#define epoch np->n_epoch
#define tznm np->n_tznm
xXx