First, the geometry

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No other object in the sky stimulates us humans in the positive as well as in the negative sense, like the moon. Perhaps this is due to the size and luminosity of the full moon, or perhaps it is due to the fact that the moon was a part of our earth ages ago, blasted out by the impact of a huge meteorite. But whatever influence it may have on us and on nature, emotionally and mythologically, it is a creature of the night, the perfect embodiment of the dark hours of the day. And therein, in part, probably lies the appeal of lunar photography: One can actively experience the night and our satellite in a way that is otherwise unfamiliar. But as much as tradition may fix the moon to the night, it does not always stand up to reality. Our neighbor does not always appear in the sky with the sunset and departs from this stage with the sunrise, but it can still be seen during the day. But slowly. Let’s deal with the basic facts first.

Because of the earth, sun, and moon’s gravitational forces and interactions, the moon’s orbit around earth is very complicated. Simplifying, we can say that the moon moves on a slightly elliptical orbit around the earth, inclined about 5°9′ against the ecliptic, on which it is 356,410 km away from the earth at the closest point (perigee) and 406,740 km far at the farthest point (apogee). On this orbit, the moon moves in relation to the earth in a counterclockwise direction, essentially from south to north and back to south. The points where the moon’s orbit intersects the ecliptic are called ascending node (for the section from south to north) and descending node (for the section from north to south). These nodes are subject to their own so-called retrograde rotational motion, on which they move a good 20° counterclockwise around the earth per year, thus circling it once in about 18.6 years. Viewed from space, the moon’s orbit therefore resembles a wobbling motion (the northernmost point of the orbit points once to the right and once to the left), as described by a spinning top losing speed and threatening to fall. Only this retrograde movement of the moon’s orbit can lead to lunar and solar eclipses, as the point where the moon’s orbit intersects the ecliptic (one of the nodes) must precisely align with the sun-earth axis. Because if the moon’s orbit always remained in one position, the moon would miss the sun and the earth’s shadow at every revolution around the earth.

Further disturbances of the moon’s orbit caused by the sun’s gravity include periodic changes in its eccentricity (evection), variations in the orbit’s inclination between 4°58′ and 5°19′, and the orbit of the point closest to the earth (perigee), which rotates around the earth in 8.85 years. Thus the complete calculation of the moon’s orbit, as already indicated above, becomes one of the most difficult problems of astronomy.

The ecliptic is the plane of the earth’s orbit around the sun, or the apparent motion of the sun in our sky as seen from earth.

Strictly speaking, however, the moon does not orbit the earth, but both orbit their common center of mass (barycenter), which lies about 4,800 km outside the center of the earth, i.e., still inside the earth, at a depth of about 1,600 meters. The time the moon needs for such an orbit is divided by astronomers into different months, of which the sidereal and the synodic months are the best known. The sidereal month defines the time it takes the moon to orbit the earth and return to the same position relative to the stars. It lasts 27 days, 7 hours, 43 minutes and 11.6 seconds. The synodic month, on the other hand, refers to the amount of time it takes the moon to return to the same position relative to the sun. More tangibly, the time between two similar lunar phases, for example, from new moon to new moon. It lasts 29 days, 12 hours, 44 minutes and 2.9 seconds. This longer time results from the fact that after a sidereal month, the sun has moved about 28° further along the ecliptic, and the moon needs an extra 2 days to catch up with it and get to the next new moon phase.

Now, what position relative to the earth the moon takes at a given time depends on its position on the orbit, the location (latitude) of the observer on the earth’s surface, and the position of the earth in space (tilt of the earth’s axis) in the seasons. – Let’s see if we can put all the factors into a meaningful context to explain what we see in the sky.

Next High in the sky it shall stand …

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