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index page 		Keith's Orrery -
a planetarium
style program
(a Java applet)
See below for comments on James E Morrison's Astrolabe Web Site

If you had a java-enabled browser, you would see:

a floating window containing
a representation of an astrolabe.

Keith's Automated
Astrolabe
I'm sorry it's so slow to download
and operate.

I recommend that you increase the size of the
astrolabe window to make it as large as possible.
There are menu options allowing you to select
the component parts of the astrolabe and to set
a precise size for window-grabbing and printing.
(Printing isn't supported in this applet.)

However, if you can run Java applications, the
contents of the zip file can be run as a Java
application, which allows printing if you are
using Java 1.1 or later.)

The program still needs a lot more work...

Click on the link to download the
25 applet class files and the html
file in .zip form (approx 75K):
astrol.zip.
This will allow you to run the
astrolabe program off-line.

Astrolabes were used in ancient times for many purposes including telling the time, measuring or predicting the angle of a star or the sun, determining one's latitude, ascertaining the time of sunrise and sunset and predicting the positions of stars throughout the year.

Ancient astrolabes essentially consisted of two circular metal plates, one rotating above the other. Large portions of the upper plate which were not essential to the function, technically or artistically, were cut away to allow you to see the engraved plate underneath. The engraving allowed the coordinates to be found of the stars indicated on the upper plate. Additionally, a pointer bar had sighting holes which enabled the angle of the Sun or a star to be measured. This allowed the astrolabe to be set so that other data could be obtained. Yet more data was engraved on the back, including charts.

The representation of an astrolabe provided here shows the two plates. (Menu options allow you to see the reverse side of the back plate, which contains useful scales, data and charts.)

Instead of a 'fretted' top plate (rete) the representation provided here shows just the data which would be on it, and this rotates as the time/date changes. The lower plate has scales on the rim, within which is a coloured area depicting much of the area of the sky the user can see, and beyond. The inside edge of the scales represents the tropic of capricorn. Further inwards are the circles of the equator and the tropic of cancer. The centre of the plates represents the north pole, the line from top to bottom showing the observer's meridian.

The curve at the bottom of the coloured area represents the observer's horizon. Several non-concentric circles - or parts of them - indicate the angle above the horizon to the observer's zenith at 90 degrees. Crossing these at right angles are azimuth curves showing the points of the compass.

The curves on the lower plate are all dependent on the latitude of the observer. A menu marked 'Observer' allows the observer's latitude to be set and this setting is shown as the Zenith angle at the top left of the window.

If you alter this angle, you will note that the horizon curve changes from being a complete circle at high latitudes to a saucer shape at lower ones. The position of the crossing points of the curves at the observer's zenith (the centre of the spider's web) also moves accordingly.

Further menu settings allow the number of elevation and azimuth lines to be altered, and the current settings are also shown at the top left of the window.

The upper 'disc' (rete) shows the stars and the position of the ecliptic circle (the circle showing the position of the sun among the stars throughout the year). These are positioned as if on a celestial sphere, as seen from above the sphere. In consequence, the familiar star patterns seem reversed, as if seen through a mirror. The names of the stars can be toggled on and off using a button.

However, a menu option and a button towards the top centre of the window allow the viewpoint to be toggled between the normal astrolabe view from the heavens to the view seen on 'planispheres' and star charts, which is from the earth.

The portion of the celestial sphere which is visible to an observer varies according to the time of year and the time of day. These times can be set either with the buttons or with menu settings. Alternatively, the angle can be set. The background colour of the three panels allowing selection of the current time, a selected time or the angle is yellow if the panel is active, and grey if inactive.

The ecliptic circle is divided into 12 portions representing the 12 zodiac divisions of the sky, and these have been sub-divided.

When you understand the above, it's use is intuitive (he says hopefully and not very convincingly).

Latitude

If you don't live at a latitude of 52.04 degrees North (the author's latitude), the first thing you should do is set your latitude. You can use the menu or the buttons to do this. Should you want to use this program regularly, you may want your current latitude to be set automatically whenever you load the program. If you are viewing this as an applet you can set a parameter. If you are viewing it as an application you can set an argument. Details are to be found in the Help menu.

The centre of the 'spider's web' of curves and circles is the point directly overhead.

One of the uses of the astrolabe was to determine your current latitude. The angle of the North Star was measured physically, using the pointer and the scale around the outside edge of the astrolabe. The angle from the horizon to the North Star is your present latitude.

Azimuth and Elevation settings

The 'spider's web' of curves shown above the horizon can have more or less lines according to the Azimuth and Elevation settings.

Points of the compass

Some of the lines of the 'spider's web' are coloured blue. At the ends of these are marked the points of the compass which they represent - N, NE, E, SE, S, SW, W and NW. Remember that the astrolabe shows a left-to-right mirror image of the sky unless you have toggled it otherwise.

Sunrise and sunset

If you have altered the latitude setting to show the latitude from which you are observing the sky, you will see the current position of the sun. If it is above the horizon line, it is day time. If it is below it, it is night time. If you alter the time settings until the sun is on the horizon, the time settings will roughly show the time of sunrise or sunset. However, this is an approximation. Astronomers use the equation of time to correct the setting. They also take refraction into account. For the most precise assessment, you need to set the centre of the sun to be about a degree (50') below the horizon. (You can measure the degrees above the horizon from the elevation scale, and can then guess where one degree under would be.)

A 'twilight' line is shown, representing the 18 degree point below the horizon where the sun will be at the start and end of twilight.

The times indicated are 'local times' and take no account of time zones or 'summer time' adjustments. You must make such adjustments yourself.

The curves between the 'Cancer' circle and the outer 'Capricorn' circle allow you to determine the time in unequal hours, which were used in medieval times.

Orion

You can find the position of the Orion constellation by searching for the three stars in a line which mark the belt. You can confirm this by clicking on the 'names+' button, when the names of most of the stars appear. (Click again on the button to hide the names.) Betelgeuse is one of the stars in the Orion constellation. You can then determine whether it is above or below the horizon at the present time. By altering the time and date settings either with the menu or the buttons, you can find when Orion will be visible. (The sun must be below the twilight line if you are to see the stars clearly.)

Planets

A menu option and a button allow you to see the position of the planets - and the moon. These are shown on the ecliptic circle, lines from the centre of the astrolabe highlighting them. When you select this option, a representation of the moon in its current phase is displayed at the top-right of the astrolabe. A table of the positions of these heavenly bodies is displayed below the panels.

Sidereal Time

[sid-ear-real] So what has Sid's real ear got to do with anything? It allows you to determine or set the rotational position of the celestial globe. The point of reference of this globe is known as the 'First point of Aries' (a name of historical interest only) which is the point on the celestial sphere where the sun is positioned on the first day of spring.

Printing

The Help:Description file gives advice on making a paper astrolabe.

Have fun!

Keith Powell.

The Astrolabe Web Site

The best site on the internet concerned with astrolabes is James E Morrison's web site at http://www.astrolabes.org/astrolab.htm.

There you will find pages on many aspects of astrolabes, including:
Collections,
History,
The Stereographic Projection,
The Parts of an Astrolabe,
Uses of the Astrolabe,
The Astrolabe Quadrant,
and the less common types of astrolabe.

There are also details of an astrolabe you can purchase, and another for use on PCs which you can download and use (DOS).

Links and References are at http://www.astrolabes.org/links.htm.

oops!
keith.powell@argonet.co.uk