Moon Wiggles

Please fix that bug !
I regularly get user emails asking me to fix the oscillations of the Moon's trajectory. The Moon can in no way have such a trajectory in the sky! Well ... that's not a bug. This is an effect called parallax. Let me explain...

First, if you want to see trajectories displayed on the map, go to the Settings in Starmap 2 (Standard version and above) and activate the Trajectories. Then, in the catalogs, select the Moon and touch the Search arrow. The Moon trajectory will be displayed on the map, with oscillations, depending from where you live on Earth.

The Moon's orbit is on the elliptic and of course does not oscillate. But seen from our spinning Earth, the orbit looks different. The stars on the celestial sphere are much more distant than the Moon and we use them as immutable references. The Moon's orbit is fixed relative to Earth's center. Since we live on the surface of Earth and not at its center, our relative position changes throughout the day. It changes most at the equator, and it does not change on the poles.

Try an experiment and change your location to plus or minus 90 degrees latitude (respectively the North and South poles) and search for the Moon from the catalogs. There will be no wiggles. Then set a zero latitude for your position, which is the equator latitude, and the wiggles will be most pronounced.

Measuring star distances
Actually, the same geometrical effect is used to measure relatively close stellar distances. Every six months Earth is at opposite "sides" of the Sun. When observing the precise position of stars relative to each other from different parts of Earth's orbit, close stars will appear to move (slightly) in front of distant stars. The relative displacement angle is linked to the distance of the star. You might be familiar with the term parsec if you are a fan of science fiction movies or have studied any astronomy.... a parsec is the distance equivalent to parallax effect of a second of arc... parallax per second ... par-sec.
This is not the only technique used to measure the distance of stars. You can learn more about these methods in the Starmap stories called The Ever-changing Sky and Measuring the Cosmos.

Well that said, no the wiggles are not a bug but just a reflection of the precision of Starmap!

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