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Botjes' description of sun and moon motion

Academics / Departments & Programs / Physics & Astronomy / Wildrik Botjes Planetarium / Motions of the Sun and Moon / Botjes' description of sun and moon motion

Indication of the position of the sun and the moon with respect to the zodiacal constellations; and of the relation of the sun and moon to the nodes of the moon's orbit, and the perigee and apogee.

I. The upper [gold] ball represents the sun, and rotates to the left in 8765.816 hours, passing each day along the ecliptic 59 minutes and 8 seconds of arc; at the same time it indicates, with the point, the current month and date, and the celestial sign in which it is located; the circle which it describes is the ecliptic.

II. The lower [silver] ball, the moon, rotates left in 27 days, 7 hours, 42 minutes, and 52 seconds, and thus rotates 360 degrees, each day 12 degrees, 12 1/5 minutes of arc, calculated average. When it is under the sun, it is new moon; when it is off by 90 degrees first quarter, 180 degrees full moon, and 270 degrees last quarter. When it is at its farthest point [apogee], it passes in a 24 hour period only about 11 degrees, at the nearest point [perigee] about 13 1/2 degrees. Because the sun meanwhile also has moved ahead, the moon does not reach the sun again in 27 days, 7 hours, etc., but after 29 days, 12 hours, 44 minutes, and 3 seconds, calculated average, because in the summer the moonlight is about 6 hours shorter, and in the winter that much longer.

III. The upper hand, which indicates the lunar nodes, when the moon passes throught the ecliptic, during the proximity of which eclipses may occur, rotates to the right in 18 years, 229 days, and passes each month 1.614 degrees. When the moon passes the northern [ascending] node it moves above the ecliptic plane, and the southern [descending] node it moves below the ecliptic.

IV. The lower hand indicates the places where the moon is in its farthest position [apogee] and nearest position [perigee] from the earth; in the region of the former the quarter is longer than average, about 8 days and 4 hours; in the region of the latter, the nearest position, only about 6 days 15 hours, because it then moves the fastest.

All these irregular motions are generated by the gear system itself.

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