Keith's astrolabes: setting the time and date used by my Java applet
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Keith's Astrolabes
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Time and Date

NOTE: The Menu selections and
the Time and Date buttons
do not affect the settings
of your computer clock.

Initially, the traditional and equinoctial astrolabes displayed with this program show their retes and the pointer in positions appropriate for the present time and date. The display then shows the Sun and/or stars currently visible in the sky over a grid of the coordinates applicable to the selected latitude. As the time shown by your computer clock changes, the retes and pointers on these astrolabes automatically move to show the current positions of the objects in the sky.

To see how the sky will appear at a different time/date, click on the buttons in the time/date panel, or select a setting from the menu:

Menu: Date/Time /...
To revert to the mode where the display continuously updates to show the present position of the Sun and stars, click on the 'Now' button or select:
Menu: Date/Time /Now

The 'Inst' button sets the date and time to the date and time at the instant when you click on the button but this setting then doesn't change. Click on the 'now' button if you want the display to reflect the changing time as shown by your computer clock.

If you click on the +++ or --- buttons which are next to the display of the year, the year will change by 100 years at a time. The effect of this is to show precession, whereby the stars appear to rotate about the ecliptic pole by about 360 degrees in 25800 years. If you click and hold down the +++ or --- button, the centuries change at a rate of up to three times a second (depending upon the speed of your computer) and it is interesting to see how the Pole star moves away from the North Pole, and returns to its present position after 25800 years.

The buttons marked Jan 1st, and se, ss, ae and ws set the astrolabe to interesting times and dates throughout the year. Thus:

Jan 1st = 00.00am January 1st,
se = midday on the day of the Spring Equinox
(about March 22nd),
ss = midday on the day of the Summer Solstice
(about June 21st),
ae = midday on the day of the Autumn Equinox
(about September 20th), and
ws = midday on the day of the Winter Solstice
(about December 21st).
With any of these selections, you may want to adjust the time setting by a minute or two so that the ecliptic circle is rotated precisely to a 90 degree point.

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Copyright Keith Powell 1999-2002