The Planetarium or Orrery
In the back room of a simple dwelling facing the Town Hall is a
technical work of art and craftmanship, visited every year by thousands
of people from all over of the world. This is the Planetarium or Orrery,
of Eise Eisinga.
A stone on the front of the house shows the year in which it was
built, 1768. Originally the house was called 'De Ooyevaar' (the Stork)
after a stork chiselled on the gable. Between the windows of the first
floor, if the sun hap- pens to be shining, a sundial tells the local
time.
Over the little gate between the Orrery and the adjacent house to the
right appears the following inscription:
"Voersint eer Ghy begint"
(Look before you leap). Eise Eisinga must have had this motto
in mind when he designed the Orrery.
The Planetary System
On the ceiling of the living-room Eisinga made a true-to-nature
representation of our planetary system. Photo 1 shows the central part
of it. In the middle the |
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sun is represented by a star. All round this
star a number of circular grooves are cut in the ceiling.
Through each groove passes a thin metal pin, to which a small ball is
attached, representing a planet. The pin is attached to a cogwheel above
the ceiling which takes about the same time to make one rotation as it
takes the respective planet to perform its rotation round the sun.
Each orbit carries the symbols of the signs of the zodiac and
each sign is graduated. Consequently visitors can read off the exact
position of every planet at any time.
The inclination of the orbit of a planet in relation to the plane of
the orbit of the earth (the ecliptic plane) is indicated by means of a
white circle, drawn partly inside and partly outside the planet orbit.
Where this white circle is drawn inside the planet orbit, the planet
is in the northern latitude, and outside in the southern. Figures in the
circle indicate the amount of inclination (given in degrees and
arch-minutes), whilst the intersections are marked by symbolic signs.
Thus one can at any |
time read off the degree of inclination of a
planet north or south of the ecliptic. As the planets really move in
ellipse round the sun, which establishes itself as one of the focal
points, Eisinga placed the planet orbits on the ceiling eccentrically in
relation to the sun. The place where the planet comes nearest to the sun
he marked N.P. (nearest point), and the opposite spot he marked Y.P. (verste
punt, farthest point).
The little balls are painted half gilt, half black (day and night
sides): the gilt half is always to- wards the sun. The distances between
the planets and the sun are exactly to - scale (1 / 1 000 metre - one
million kilometres). On that scale it was impossible to repro- duce the
diameter of the planets, since that of the earth, for in- stance, would
have to have been only 1/100 of a millimetre.
The reproduction shows the orbits of the planets Mercury, Venus,
Earth and Mars, which travel round the sun in 88, 225, 365 and 687 days
respectively. The little ball near the earth is the moon, which travels
round the earth once a month, accompany- ing the latter in her orbit
round the sun, gilt side always turned to the sun. |
