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"Moonlight" clock:

Movements of the Sun and the Moon over the horizon







Mechanical clock showing the apparent movements of the Sun and the Moon


Working of the sun

At six AM, the Sun, engraved on the hand of hours, rises from the east, and the moving disc which covers half of the facade, until now night-blue, lets appear half a cycle (sky-blue) above the horizon. The sun moved in real time by the mechanism, will indicate the time the whole day, and will make a complete turn of the face in 24 hours and not 12 ( like for normal watches or clocks )

According to it's synodic cycle (29.53 days) the moon will also appear in the east, and turn with the same periodicity as the sun, 50' 28'' less. The moon will then take each day a 50'28'' cumulative delay.

At six PM, when the sun disappears in the West, the moving disc will make half a turn, and stop, it's blue-night starry half appearing then alone.

A slit forms a half cycle, in the hiding piece of the horizon (in the lowest part) ; it lets slightly see the sun, to indicate the hours of the night (from 6 PM to 6 AM , crossing midnight, new day) so the clock indicate the time for 24 hours.


Working of the moon 


1) The first day of a new lunar cycle begins at the new moon : it is the moment when the moon is exactly in the axle earth-sun, between them. The observer on the earth can't see the moon because it shows to the earth it's half which is in the shadow. On the clock the sun and the moon (black) appear at the same time in the eastern horizon and move together threw the 12 hours of this first day. They go down at 6 PM, nearly together, for in 12 hours the moon is already 25'14'' late. The next day morning (second day of the cycle) the sun rises 50'28'' sooner than the moon.


2) A quarter of moon, passing by the poles, will appear bigger and bigger with the passing of days, until after 7 days the moon, it's black half left, and it's white half right can be seen : it's the first quarter (it's croissant evokes the letter p). Thanks to the '' moonlight clock'', we can note that after one week the moon rises in the East while the sun is in it's zenith in the south (forming so a right angle) : the moon, having pivoted on itself while turning, shows it's first quarter, that is to say the quarter of a complete turn.


3) On the clock, we can check that the moon taking the same delay each day, will be (in the middle of it's cycle after 14.5 days ) again in the same alignment with the earth and the sun, but this time, it's the earth which will take place between the moon and the sun. The humans will see the moon completely enlightened, quite round at the full moon, but being seen only at night. In fact, being at the opposite of the sun, it will rise when the sun gets down and when the night comes and it gets down when the sun rise.

4) At the last quarter, the sun has made 3/4 of the cycle which will allow to catch up with the moon. The moon appears like as the first quarter, but reversed, that's to say half white on the left and half black on the right. (it's croissant evokes the letter d)

This white half will decrease with the time, taking the form of a croissant, the points of which always pass by the poles of the lunar disc, then it's going to disappear, taking the name of new moon.


Conclusion : the moon can be seen in daylight from it's last quarter to it's first quarter (except 2 or 3 days before and after the new moon) and at night from it's first quarter to it's last one ; that's what this clock permits to show.


More information about the moon:

Synodic cycle of the moon:

Definition: the synodic cycle in the value of time passing between two successive conjunctions of the earth, the moon and the sun, that is to say between two new moons.

The phases of the moon ( increasing from new moon to full moon and decreasing from full moon to new moon ) are only a game of shadow on the moon by the sun . The synodic cycle shows the quarters of the moon : shadow cast by the sun and varying according to the positions of the earth, the sun and the moon. This cycle last for 29 days, 12 hours, 44 ' and 2.8''.

The precision of this clock is -2.12'' for a lunar month. It's a mathematical distance which could be checked if the working of the clock was driven by a very precise mechanism electrically radio-controlled for instance.

The clockmaker however preferred to keep a mechanical working, with a pendulum, and suffer a little imprecision.


Hemispheres:

In the ''moonlight clock'', conceived for the northern hemisphere, the part which can be seen under the horizon line doesn't represent the southern hemisphere, but a time zone diametrically opposite to the one seen by the observer. In fact the sun lights a time zone from North to South, and when it's midday in London for instance, (meridian 0°), it is midday on the whole meridian 0 °, from North to South ; and it's midnight from North to South on the meridian 180° in New-Zealand (meridian of the line of changing date).

In the northern hemisphere, to see the sun in it's movement from East to West, it's necessary to look to the Equator, so to the south pole and to the plan of the Ecliptic in which all the planets and the sun are turning : the East is on the left and the West on the right. Whereas in the southern hemisphere, the sun can be seen when looking to the Equator, so to the north pole : the sun rises in the East on the right, and gets down in the West on the left.

The synodic cycle of the moon can only be seen by an observer on the earth, subject to the earthly moving, in front of the sun and the moon. It is the increasing moon and the decreasing moon.

Several other cycles of the moon exist, especially the ''sideral cycle'' : it is what could note an observer situated on the limit of the solar system, who could observe at the same moment the sun, the earth and the moon in their respective evolutions.

It's then called rising moon and getting down moon, compared with the plan of Ecliptic in which are moving the sun, the earth and the moon. The lunar or solar eclipses can take place only in this plan (see the other model of clock, called ''Monde'' -World-) . Another cycle called ''anomalistic'', shows the elliptic character of the lunar revolution, with the ''perigee'', when the moon is the closest to the earth, and the ''apogee '', when it's the most far away.



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