The part of the chart seen through the opening represents the stars and constellations visible that moment above the horizon at Franeker. Also on the chart are the celestial equator, both tropics and the arctic circle. The ecliptic is represented by a circular groove cut in the celestial chart. This groove is eccentric to the middle of the chart (the astronomical pole) and touches the northern tropic at Cancer and the southern at Capricorn. 

A cogwheel behind the celestial chart and consequently invisible inside the room moves round on- ce every 24 hours. This wheel pulls a pin which traverses the groove and to which, just in front of the chart and easily visible from the room, a little metal disc is fixed. 

The disc represents the sun and rotates with the wheel once every 24 hours. The sun therefore lags behind the rotation of the celestial chart by 3 minutes 56 seconds

ter establishes herself as one of the focal points. Consequently the distance from the moon to the earth is not always the same.

The point where the distance is greatest is, as we have seen, called the apogee or farthest point; that where the distance is smallest, the perigee, or nearest point. The apogee and perigee are not stationary, but move round once every 8 years and 311 days. The pointers on the dial on the pilaster indicate the distance of the moon from the apogee.

This distance, like that from the northern node, is expressed in zodiacal signs and degrees. There is still another dial on the ceiling: this shows the length of the route traveled by the moon on the ecliptica.

Here one can read off in which sign of the zodiac the moon stands, and at what degree. Finally, over the press there is another dial which tells the time of the rising of the moon.

These four moon dials move irregularly, especially those indicating the time of the rising and setting of the moon. On an average they advance about fifty minutes a day, occasionally as much as eighty-five, but sometimes not more than eighteen minutes.

Astronomical Chart or Planisphaerium

Just as a Planetarium shows the movement of the planets round the sun, so does an astronomical chart or Planisphaerium indicate the apparent movement of the sun and stationary stars.

The rotation of the earth makes the starry sky appear to move around us in 23 hours, 56 minutes and 4 seconds. During each rotation the earth also covers part of her yearly course round the sun: consequently she has still to rotate an average, of 3 minutes 56 seconds before any given point faces the sun again. That is why the sun apparently rotates round the earth in approximately 24 hours (a solar day) and the starry sky in 23 hours, 56 minutes and 4 seconds (a sidereal day). 

In the panel over the cupboard- bed (photo 7) Eisinga cut an almost circular opening with a diameter of 50 cm. The border of this opening represents the horizon of Franeker and is divided into hours. Behind this opening a celestial chart with a diameter of 70 cm. rotates once a day.