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Appendix: Observing sequences of moonrises along the horizon - a footnote by Clive Ruggles

Given an observer viewing moonrise on successive nights, and supposing a sequence of clear nights, then the moon rises a little under an hour later each night, and during the waxing part of the phase cycle is not seen to rise as this happens during the day. The first time moonrise can be seen is around the time of full moon, at around the time of sunset, and then successive moonrises are seen later in the night until the last crescent, which rises just before the sun. Thus moonrises can be monitored in the run-up to full moon and the start of a new month.

The position of this sequence of moonrises along the horizon depends mainly upon the time of year but there is also an effect due to the timing within the 18.6-year node cycle. What is seen at sunrise and sunset is also affected by the time of year because of the varying length of night.

Around June solstice, the full moon will be seen to rise close to its most southerly position (around the December solstice sunrise position), with successive moonrises appearing progressively further northwards until the last crescent is seen close to the northernmost limit (June sunrise position).

Around December solstice, the opposite is the case. The full moon will be seen to rise close to its most northerly position (around the June solstice sunrise position), with successive moonrises appearing progressively further northwards until the last crescent is seen close to the southernmost limit (December sunrise position).

At intermediate times, the full moon will first appear at a position somewhere between the northerly and southerly limits. In particular, close to the equinoxes the full moon will appear approximately due east. Around the autumn equinox successive moons will appear progressively further north until around the time of last quarter, whereupon they will start southwards again. Around the spring equinox successive moons will appear progressively further south until around the time of last quarter, whereupon they will start northwards again.

It must also be borne in mind that in any given year, the closest full moon to a given time of year, such as a solstice or equinox, may actually be up to two weeks either side of it.


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