Taking Control

When you open the lid of the control box, this is what you will see (except of course for the time and place information, which should agree with your current time and place):

The controls consist of six buttons along the top of the display screen, six knobs below the display, and one further button (the one labelled Go) just below the knobs.

Here is what the buttons do:

• Shift: alters the functionality of some of the knobs. For example, to change the 'Latitude' setting, turn the 'Year' knob whilst holding the Shift button down.
• Calendar: changes the date from its representation in the Gregorian calendar (the one most of us use today) to its representation in the Julian calendar (the one brought in by Julius Caesar - abandoned sometime after 1582, the exact date differing from one country to another). Useful for putting in historical dates.
• Clock, Input: for changing between Almagest's two modes
  • Clock-mode: Almagest functions as a self-contained unit, showing the movements of the sun, moon, planets and the backdrop of stars as they actually happen - 'real-time'.
  • Input mode: Almagest functions as a reference instrument. You can put in a time and a place using the controls. Almagest will then show the position of the sun, moon, planets and stars as seen at that time and place.
• Data + and - : If you want detailed information about the position of the planets at a given time and place, press one or other of the 'Data' buttons. The data are displayed as a series of pages, one for each planet (roughly speaking). You move between the pages by pressing the 'Data +' and 'Data -' buttons repeatedly. For example, here are the data for the planet Mercury, at 10.20 GMT on the 11th of August, 1999, as observed from London (52N 0W).

  The data given are the

  • altitude (how far above the horizon)
  • azimuth (how far round from North)
  • ecliptic longitude (position in the Zodiac)
  • ecliptic latitude (how far above or below the arc of the ecliptic)

Here is what the knobs do:

The knobs are almost self-explanatory.

First get into input-mode, then use the knobs to set the day, the month, the year, the century (useful for historical dates), the hour and the minute.

Then use the 'Shift' button to set the latitude and longitude.

Finally use 'Shift' with the 'Hour' knob (and possibly the 'Minute' knob) to adjust the Greenwich Mean Time (GMT), also known as Universal Time (UT). The local time by itself is not enough to determine the configuration of the heavens at a given moment - for example, to tell the height of the sun above the horizon. This is determined by the GMT in conjunction with the latitude and longitude.

If you are uncertain as to how your local time is related to GMT, click here for the GMT at this moment and compare it to the time on your watch.

Finally, the Go button:

When you are satisfied that you have got the time and place information as you want it, press the Go button. The wheels of Almagest will begin to turn, and a few seconds later the planets will be in their correct places, and you will be able to read off their data if so desired.

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