… and rise and set on time

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The far larger and more confusing irregularity, which probably everyone of us in Central Europe already noticed once, is however the time of the moonrise, which appears unpredictable at first sight. But everything could be so simple, because to circle the earth once, the moon needs 27.3 days, as seen above. If we divide the full circle by the number of days (360°/27.3=13.2°), then we get the measure by which the moon moves away on average per day to the east from the sun and, in a further step ({[24h*60 min]/360°}*13.2=52.8 min), the time by which the moon would have to rise consequently each day later.

A visit to the US Naval Observatory’s website (http://aa.usno.navy.mil) and an evaluation of the moonrise and moonset data for a position in the temperate latitudes reveal, however, that the moonrises on successive days deviate from each other by different values of between around 10 and 90 minutes, but on average over the year add up to approximately the calculated value of 52.8 minutes. We call this time difference between one moonrise and the next „relaxation.“

To understand this, we must remember the section „Earth and sun – The different lengths of day and night„. There we found out that the sun over the northern hemisphere rises earlier and sets later in summer than in winter, because then a larger part of its daily orbit lies above the horizon due to the geometrical relations between earth and sun. Well, it’s similar with the moon. Only the positions of earth and sun change over the year. The moon, on the other hand, moves in only 27 days around the earth and goes through all the seasonal peculiarities of the sun, like rising in the northeast, culmination at 75°, setting in the northwest and rising in the southeast, culmination at 15°, setting in the southwest, which are also responsible for the shifting of the rising and setting times with it, in this short time and strike us so much with it.

And it is not completely unpredictable. As long as the moon maintains a higher orbit across the sky from day to day, shifting its rising and setting points to the north, the time difference between successive moonrises is less than the average of 52.8 minutes, indicating that the moon rises earlier than average. In the northern hemisphere, we record the minimum time difference when the moon passes the equator on its journey from south to north, with its northward movement being the greatest from rise to rise. This effect becomes more pronounced with increasing latitude. At 62° north or south (an observer in the southern hemisphere experiences the entire situation with a seasonal shift), the moon rises at the same time during this period and even earlier from day to day beyond that latitude.

This constellation can be reached in every moon phase. In winter the moon is in this position approximately in the first quarter, in spring in the new moon, in summer in the last quarter, and in autumn in the full moon. This autumnal phenomenon of the full moon rising a few days at about the same time is known as the Harvest moon (by definition, the full moon closest to the autumnal equinox on September 23) because it gives farmers some extra light at the right time just after sunset to bring in crops. Those Harvest moons that are particularly close to this equinox have the added advantage of rising almost exactly in the east and setting in the west. So it’s worth looking at the calendar and taking advantage of this symmetry for a wonderful image with the setting sun in the west and the rising full moon in the east.

If, on the other hand, the moon describes a lower orbit from day to day, on which its rising and setting points shift southward, the time interval between successive moonrises is also greater than the average amount of 52.8 minutes, and it rises later than average. In contrast to the first case, we now record the greatest time delay for the northern hemisphere when the moon passes the equator on its way from north to south and its southward movement is greatest from rise to rise. This constellation occurs approximately during the last quarter of winter, the full moon in spring, the first quarter in summer, and the new moon in autumn.

The exact length of the time differences between the moon rises always depends on the geographical latitude of the observer. As with the sun, only the latitudes around the equator are excluded from this game due to their approximately constant position throughout the year. These latitudes also experience a moon whose rises are consistently delayed from day to day by about 50 minutes.

But, I admit it, this is a lot of theory. The most helpful thing to do is to watch the moon carefully over several cycles, from new moon to new moon, making notes every few days about where and when it rises and how high it climbs in the sky and then comparing these observations with what you have read.

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