The Moon

The way the observer on earth sees the motion of the moon is quite complicated. The moon rises in the east and sets in the west with a cycle similar to that of the sun over the course of a year, that is, with one extreme located north of east and the other, symmetrical, south of east, a cycle that only lasts 27.2 days, though. So we talk about northern and southern lunar standstills, similar to the two solstices. The lunar phases depend on the fact that an observer on the earth only sees the part of the visible face of the moon that is illuminated by the sun. The cycle of the phases is completed in 29/ days. The ideal plane that contains the earth and the orbit of the moon is inclined by about 5 degrees (5 degrees and 9 arc minutes) compared to the plane of the ecliptic (the two intersection points of the orbit of the moon with the ecliptic are called nodes). This fact has extremely important consequences. Indeed, there would be many eclipses, periods of time when the moon is located between the earth and the sun, obscuring the latter, or the earth is between the sun and the moon, obscuring the latter. On the contrary, since eclipses occur when the three points are aligned, they can

Figure A1.3: The rising-setting azimuths of the moon at the two standstills (heavy solid lines) compared with those of the sun.

Figure A1.3: The rising-setting azimuths of the moon at the two standstills (heavy solid lines) compared with those of the sun.

only take place in the vicinity of one of the nodes. Eclipses thus follow cycles that can be studied, enabling future predictions to be made (see Aveni 2001 for a complete discussion of the subject).

As a result of various factors that would take too long to go into, the node line of the moon revolves clockwise in relation to the moon, completing a cycle every 18.61 years. Thanks to this phenomenon, lunar standstills, unlike their solar counterparts, are subject to an oscillation, whose complete cycle lasts 18.61 years. During this cycle the azimuths of the northern lunar standstills fluctuate between a maximum that is further north than the summer solstice and a minimum that is further south than this, and a symmetrical situation occurs for the southern lunar standstills. Accordingly, we talk about north and south major and minor lunar standstills. A slight effect called wobbling, with a cycle of 173 days, also causes the position of the lunar standstills to oscillate slightly by approximately 9 arc minutes.

The motion of the planets is simple: they move along ellipses of which the sun occupies one of the focuses. This, however, is what an observer from outside the solar system would see. We are on one of the planets, and so what we see of the motion of the others depends on this limited position of ours. The inner planets, that is, those between us and the sun, Mercury and Venus, seem to swing back and forth in the vicinity of the sun. Venus is by far the most brilliant body in the sky after the sun and the moon. The time it

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