The nodes of an orbit are the points where the orbiting body pass through the ecliptic in either direcrion. The point where the object's latitude changes from minus to plus is the ascending node and the descending node is where the body goes from postive to negative latitude.
Some old clocks had a hand, called the "Dragon" hand that showed the approximate position of the lunar nodes. The moon must be near one of its nodes at full or new moon for an eclipse to occur. On the clock face, if the moon is new or full and the moon ball is near a node (shown by the dragon hand), an eclipse is possible. Note I say "possible" and not "will occur". The motion of the moon on old clocks used mean values and the true moon might be off by just enough for the ecllipse to not happen.
The time between passages of the moon through its nodes is called a draconitic month or nodical month (27.2122 days).
I do not believe either the Bern or Praugue clock have a dragon hand, but I may be wrong. The astrolabe wrist watch from Ulysse-Nardin does have a dragon hand. The Electric Astrolabe on my web site has a lunar node indicator.
Best regards,
Jim
James E. Morrison
[email protected]
Astrolabe web site at http://astrolabes.org
Mar 22, 2011 09:46:46 AM, [email protected] wrote:
Further, I want to know if those clocks, of Prague and Bern, can show when an eclipse occurs.
---------- Forwarded message ----------
From: Marcelo <[email protected]>
Date: 2011/3/22
Subject: Re: Prague Clock
To: Willy Leenders <[email protected]>
How is that lunar dial? Does it show the moon's phases and in what sign it is?
And I'm intrigued by that zodiac, why are the winter signs (Sagittarius, Capricorn, Aquarius) narrower than the summer signs, Gemini and Cancer?
2011/3/22 Willy Leenders <[email protected]>
More than the clock in Prague the clock in Bern has a calendar dial built in and an indication of the day of the week.The calendar in Prague is located separately under the clock.See http://upload.wikimedia.org/wikipedia/commons/5/5e/Zytglogge_astronomical_clock_with_labels.png
Willy LeendersHasselt in Flanders (Belgium)Visit my website about the sundials in the province of Limburg (Flanders) with a section 'worth knowing about sundials' (mostly in Dutch): http://www.wijzerweb.beOp 22-mrt-2011, om 11:26 heeft Willy Leenders het volgende geschreven:
Jan,---------------------------------------------------A better photo of the clock in Bern is at http://www.photographersdirect.com/buyers/stockphoto.asp?imageid=1511348This clock has at first sight a similar design as the clock in Prague.However, it has not a outer ring that moves to and fro to indicate Italian or Bohemian hours.An indication of sidereal time is also missing.
Willy LeendersHasselt in Flanders (Belgium)Visit my website about the sundials in the province of Limburg (Flanders) with a section 'worth knowing about sundials' (mostly in Dutch): http://www.wijzerweb.be
Op 22-mrt-2011, om 01:02 heeft Jan Bielawski het volgende geschreven:
Is the Prague clock of the same design as the one in Bern? See e.g.
http://starswebworx.lc-stars.com/swiss_photo/Bern.htm (scroll down a
bit).
--
Jan
On Mon, Feb 28, 2011 at 09:15, James E. Morrison
<[email protected]> wrote:
Perhaps some background would be useful to those on the list who are notfamiliar with astrolabes and astrolabe clocks.The Prague clock is based on an astrolabe with the projection origin at thenorth celestial pole. This orientation causes the horizon arc to arc in thereverse direction from a normal astrolabe, but it gives a more intuitiverepresentation of the Sun's position.The clock has three moving parts. A hand with a Sun figure rotates once ina mean solar day and the Sun's longitude is indicated by the position of theSun figure on the ecliptic (which is divided by the zodiac). The astrolaberete, which has only the projection of the ecliptic on the clock, rotatesonce in a sidereal day. The difference in the rate of the two componentskeeps the Sun figure on the (approximately) correct position on theecliptic. The third hand shows the position and phase of the Moon.The Prague clock, and all other devices based on the astrolabe, indicatesapparent time with the Sun hand. The only adjustment needed for it toindicate approximate civil time would be to simply move the Sun hand to atime. No changes in gearing are needed. But the Sun's position in theecliptic (zodiac) would no longer be correct and any events (sunrise/set,culmination, etc.) would no longer be valid. That is, the Sun figure nolonger represents the real Sun,It could be made to show apparent time again by simply moving the Sun hand.Best regards,JimJames E. Morrison[email protected]Astrolabe web site at http://astrolabes.org---------------------------------------------------https://lists.uni-koeln.de/mailman/listinfo/sundial---------------------------------------------------
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