• Menu files must be ASCII text files. The file can be any size up to the amount of memory remaining when the Electric Astrolabe is running.
• The first characters in the menu file MUST be EAMENU to identify the file as an Electric Astrolabe menu.
• The default menu file name is MENU.EAM. The name of the menu file can be changed on the program control page with the MENU option.
The following list of commands is provided as a convenient reference for the syntax of menu commands.
Format:
• @ = Alt shift key
• ^ = Ctrl shift key
• Parameters in braces ({}) are optional. Where one of a choice of parameters must be entered, the parameters are separated with |.
• Lower case letters are integers.
• t is a time in seconds (decimals are allowed).
• # is a null entry.
• Parameters can be separated by any number of spaces.
• With the exception of text blocks (“), commands must be on a single line.
^E t *Label ecliptic with zodiac symbols.
F n *Free running mode for n cycles.
Fn t *Function key n (set interval to function key n value).
G t *Gregorian/Julian calendar switch
I {“fn.GIF”} {/Cn} {/Dn} {/L} {/N} {/Sn} t *Display image. n = image index number.
J :label *Jump to :label
K c [:label | filepath] *Specify interactive key target as :label (this file) or menu file on key c
@K t *Wait for any key to be pressed for maximum of t seconds.
L “d {m}|#” “d {m}|#” “p|#” t *Specify latitude/longitude. d = degrees, m = minutes.
M t *Manual mode on/off
^M *End menu and return to normal Electric Astrolabe operation
N t *Now. Set to system date/time.
@N t *North/south projection switch.
O t *Display Messier objects on/off.
@O “m” t *Maximum object magnitude to display. m = magnitude, 0-12.
80 Menus
^O t *Display Messier object number on/off.
P t *Planet display on/off.
@P t *Precession on/off.
^P t *Label planets on/off.
^Q t *Quiet mode on/off.
R t *Rule display on/off.
R t *Refresh screen
S t *Star display on/off.
@S “hh{:mm}” t *Set sidereal time
^S t *Label constellations on/off.
T “h{:m}” t *Specify time. h = hour, m = minutes
@T t *Display time as 24hr/AM-PM.
^T t *Dynamical time on/off.
U “h{:m}” t *Specify universal time.
W t *Write screen. Refreshes screen to current settings.
X t *Turn shading on and off
@X t *Display planets as +, on and off.
^X t *High precision calculations on and off.
Y {/C c} /A|/Z d t *Display altitude or azimuth arc. c = color (default = 1), d = angle Z “z” t *Specify time zone. z = zone from Greenwich.
^Z *End submenu
^Z {:label} *Start submenu at :label in this menu file.
Hints
1. Most commands must have a delay. Forgetting to include a delay time is the most common error.
Commands with no delay time can produce unpredictable results. If you get a blank screen or other erratic behavior, this is the first thing to check.
2. The screen is not redrawn if the delay time is zero. If you need to redraw the screen, use the W command.
3. Don’t forget that commands that change the time put the Electric Astrolabe in manual mode which displays the date and time in the upper left hand corner. If you do not want the date and time displayed you need to use Quiet mode (^Q).
4. All astrolabe display elements are turned off when the menu or submenu is started. If you are confronted by a blank screen it is probably because you forgot to turn the astrolabe elements on.
5. If you specify /L for the last image in a list of images, you will get a blank screen. The last image in a list should generally not have the /L switch set.
6. Forgetting to put a closing “ at the end of an input parameter will cause unpredictable results.
7. When writing a menu it is a good idea to keep a log of the screen components that are on and off. It can get confusing and time consuming if screen elements do not appear when you expect.
Note: The commands in the menu file are processed as if they were entered from the keyboard but it is possible to specify commands in a menu file that would be impossible to enter from the keyboard. When this happens, the results are wildly unpredictable and the results can be range from being merely confusing to bringing the computer to a complete halt.
Exercises 81
EXERCISES
Following are some suggested exercises for getting familiar with the operation of the Electric Astrolabe.
Many other uses will occur to you as you gain familiarity with the program
SUNRISE / SUNSET Make sure shading is ON (X).
Set the interval to 1 hour (F5) and move the Sun image near the eastern horizon.
Set the interval to 1 minute (F1) and hold down the Big+ or big- until the Sun is very close to the horizon.
Sunrise is just before the shading turns from gray to blue.
If you want to be quite precise, go to the V page, press F1 to set the time interval to one minute and use the big+ or big- key to adjust the time until the altitude of the Sun is about -0.833°. This is the defined time of Sunrise.
Repeat for Sunset.
Now for a little more difficult exercise: Find the day of the year with the earliest Sunset; it is not the Winter Solstice. (Answer: about December 9). Why is this the earliest sunset?
THE SEASONS
Find the times of the equinoxes and solstices. That is, find the exact times when the right ascension of the Sun is 0h, 6h, 12h or 18h. Refer to the orrery to see the position of the Earth relative to the Sun. Notice the little white dot on the Earth's orbit. This is the point of perihelion, when the Earth is closest to the Sun.
You will see that the Earth is quite a bit closer to the Sun in the northern hemisphere winter than in the summer.
LATITUDE
Use the city page (Alt+C) to display the sky in Stockholm (put the cursor on “Stockholm” and press Ctl+Enter). Notice how the horizon is nearly a circle. Find the times of Sunrise and Sunset for different times of the year. See how the astrolabe projection gives a graphic view of how an extreme northern latitude affects the length of the day during the year. Now, do the same for Singapore which is nearly on the equator and notice how the length of the day is nearly constant all year.
LONGITUDE
Go to the astrolabe display and press “0” to set the longitude to Greenwich. Notice that the distance of the Sun from the rule equals the equation of time. Use “=” to return to your location and see the combined effect of the equation of time and your difference in longitude from the center of your time zone to change the distance of the Sun from the rule. For an extreme example, select Madrid from the city page. The time zone in Madrid is European Continental time but Madrid’s longitude is actually west of Greenwich, resulting in over an hour difference between the Sun and the clock!
CIRCUMPOLAR CONSTELLATIONS
82 Exercises The circumpolar constellations never set. The circumpolar constellations depend on your latitude. Set the
interval to 10 min. (F3) and press F to go to free running mode. Notice that some of the constellations never go below the horizon. These are the circumpolar constellations. Now try the same for a fairly northern city such a Copenhagen and a fairly southern one such as Miami. You will see a significant difference in which constellations never set.
PLANNING
Find a night when Saturn is on your meridian at about 9:00 PM and there is no moon. As a search strategy, try setting the time to 21:00 (9 PM), set the interval to 1 day (F6) and go to free running mode. When Saturn gets near the meridian turn free running mode off and find a night without a moon using Big+/-.
Your sailing club (scout troop, family, ...) wants to have an evening sail (campout, picnic, ...) this summer.
In the interest of safety and esthetics it is desirable to have as full a moon as possible. Find the Saturday night this summer with the best conditions. Start with the first possible Saturday, set the time interval to 7 days on the Help screen and then find the Saturday with the best conditions.
SIDEREAL TIME, RIGHT ASCENSION AND DECLINATION
By definition, the sidereal time is the right ascension that is on the meridian. Using sidereal time, notice how you can find the right ascension of any object on the display by adjusting the display until the object of interest is on the meridian and then reading the sidereal time from the point that extends from Aries 0° on the ecliptic. Use the big dipper as a test case. The right ascension of the “pointer” stars is almost exactly 11 hours. Also note that the declination can be estimated by adjusting the time until the rule is over an object and reading the declination from the rule scale. You can set the rule over any object by using the long rule (Ctl+R) and rotating the rule with Ctl+ big+ or big-.
For circumpolar objects you can take an alternate approach. Set the latitude to 90°. At this latitude the horizon matches the equator so you can read declination directly from the altitude circles. Read right ascension as above; position the object on the meridian and read the sidereal time pointer.
THE CALENDAR
You are in London on September 10, 1752. What is the day of the week? First, go to the city page, put the cursor on London and press Ctl+Enter to make London the current location. Now, press “D” to get the date prompt and enter 9/10/1752. The day of the week shows on the upper left corner with the date. Right?
Wrong! There was no September 10, 1752 in England. See the Gregorian Calendar entry in the glossary.
RETROGRADE MOTION
Put the Electric Astrolabe on February 11, 1984. Remove everything from the display except the planets and stars. Set the time so Mars is near the meridian, set the interval to a sidereal day (F9), and enter Free Running Mode. Watch Mars go from posigrade to retrograde to posigrade motion. It is very close to Saturn during this period and you will see Mars and Saturn separate and than close back up as Mars goes back to posigrade motion. If you watch closely you will see that Mars, Jupiter and Saturn all exhibit retrograde motion at the same time. Go to the orrery at the same time and observe the conditions when planets exhibit retrograde motion.
DECLINATION OF THE SUN
Put the Sun on the meridian, remove everything from the display except the plate and planets. Go to free running mode with an interval of 1 day and watch the declination of the Sun change and trace out the analemma for the location. The analemma defines the “Equation of Time” which is the difference between the time shown by a Sundial and the time shown by a clock. The variation has two components. One component is due to the fact that the Earth travels at different speeds around its orbit and is called the
Exercises 83
“equation of the center” (equation in this context is used in the archaic sense as a value that is added to make both sides of an equality match). The other component is due to the fact that civil time is based on an artificial “Sun” that travels at a constant rate on the equator which is inclined to the true path of the Sun on the ecliptic and is called “the reduction to the equator”. The values are added to derive the equation of time. (Note: the earliest Sunset is not at the winter solstice because of the equation of time).
EXPLORING THE ECLIPTIC
Note that at a latitude of 90° minus the obliquity of the ecliptic (about 66° 33′ North) the horizon is the same as the ecliptic. If you set the latitude to this value and, using manual mode, position the ecliptic to 18 hours sidereal time, the ecliptic and horizon circles will match. At this latitude the zenith is the ecliptic pole and the altitude and azimuth lines correspond to geocentric latitude and longitude. Therefore, you can read geocentric latitude and longitude of stars and planets directly from the plate (you can always read the geocentric longitude of the planets directly from the ecliptic divisions since that is how the ecliptic is divided).
THE ORRERY
Interpreting the orrery completely takes a little practice. Here are a few pointers. Orbital longitude is measured from the vernal equinox (an imaginary horizontal line from the Sun to the right edge of the screen) to the planet. This value is tabulated on the page of orbit calculations as True Longitude. True Anomaly is measured from perihelion to the planet location. Perihelion is shown as a white dot on the orbit. You should be able to relate the tabulated values to the screen positions.
All of the planet orbits are inclined a few degrees to the ecliptic. The point where the orbit crosses the ecliptic plane from south to north is called the ascending node. The ascending node is shown as a red dot on the orbits and the longitude of the ascending node is tabulated as “Ascending Node”. The descending node (the point where the orbit crosses the ecliptic from north to south is directly opposite the ascending node). All of the planets move counterclockwise around the Sun when viewed from the north. When a planet passes its ascending node it will be above the ecliptic and the tabulated Ecliptic Latitude will be positive. Similarly, when it passes the descending node it will be negative. A glance at the orrery shows whether the ecliptic latitude is positive or negative.
ECLIPSES
An eclipse can occur whenever the moon is near one of its nodes (i.e. is close to the ecliptic) and is either full or new. Specific eclipse conditions are rather more complicated but the Electric Astrolabe can be used as a first order approximation of potential eclipses.
Some dates of lunar eclipses are: June 15, 1973, March 2, 1961, and April 24, 1967. Solar eclipses visible in the United States were on July 9, 1945, June 30, 1954, March 7, 1970, October 2, 1978, and May 30, 1984. Set the Electric Astrolabe to these dates and see if you can determine the conditions of the eclipse.
Remember that the short line at the center of the astrolabe shows the line connecting the nodes of the lunar orbit.
See Gingerich, Owen, “The Making of a Prize Eclipse”, Sky and Telescope, (July, 1991), pp. 15-17, for a table of eclipse conditions between 1990 and 2010.
CONJUNCTIONS AND OPPOSITIONS
A planet is in conjunction with the Sun when the planet and the Sun have the same longitude (i.e. there is a straight line from the Earth to the Sun through the planet). If the planet is on the same side of the Sun as the Earth the conjunction is called Inferior Conjunction and if the planet is on the opposite side of the Sun it is called Superior Conjunction. Only the inner planets can have Inferior Conjunctions. If the longitude
84 Exercises of a planet differs from the longitude of the Sun by 180° the planet is said to be in Opposition. Only the
outer planets can be in opposition with the Sun. Here is a list of some dates of conjunctions and oppositions. See the table below. Set the Electric Astrolabe to these dates and see the results.
Superior Inferior
Conjunction Conjunction Opposition Venus Jun. 22, 1960 Apr. 10, 1961
Jan. 27, 1962 Nov. 12, 1962 Nov 9, 1966 Aug. 30, 1967 Jan. 22, 1978 Nov. 7, 1978
Mars Sep. 10, 1956 Nov. 16, 1958 Aug. 10, 1971 Jan. 22, 1978 Jupiter Jun. 20, 1960 Dec. 18, 1965 Jul. 10, 1978
Saturn Jul. 19, 1961 Aug. 24, 1964 Aug. 27, 1978 Feb. 16, 1978
When the conditions for conjunction and opposition are firmly fixed in your mind it will be easy to predict future ones.
TRANSITS OF VENUS
A transit of Venus (or Mercury) occurs when the planet goes across the Sun’s disk. The conditions are very similar to a solar eclipse but occur much less often: a transit will occur when the inner planet is at inferior conjunctions and near a node in its orbit. Transits of Venus occurred in 1639, 1761 and 1769.
Find the exact dates and conditions of the transits. The fastest way is to use the Orrery to find when Venus is near a node and between the Earth and the Sun. Then use the Astrolabe and the text pages to fix the exact dates and times. Note that the diameter of the Sun is about a half degree.
THE MOON
The motion of the moon is quite complicated but there are some exercises that may add understanding.
One interesting thought exercise is to think through why the winter full moon is so much higher in the sky than the full moon in summer.
Note that the moon’s orbit is inclined 5° to the ecliptic. Find a time when the moon’s ascending node is aligned with the vernal equinox (use the calculated values to find a time when the longitude of the moon’s ascending node is nearly zero) and note the effect on the moon’s declination and altitude when it is full.
Do the same for when the descending node is aligned with the autumnal equinox. These conditions result in the minimum and maximum declination of the moon (23½° ± 5°).
Exercises 85