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General Astronomy >> Beginners Forum

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Tony Flanders

Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: How to use moon map? [Re: Billytk]
      #5631086 - 01/19/13 06:18 AM



I see so even though the same face always faces us, it's not always rotated the same way? That might explain it....I thought the bottom would always be S and the top always N.

This is a GREAT video of the Moon as we see it over the course of a year. It wabbles alot.

OK, we're confusing two different things.

The wobble shown on that video is rotation with respect to celestial north. Moon-north isn't quite the same as celestial north due to the fact that the planes of Earth's orbit, the Moon's orbit, and Earth's spin aren't quite the same. This effect is particular to the Moon, but the rotation involved is quite minor.

I'm talking about a completely different kind of rotation -- of celestial north with respect to up and down. This affects everything in the sky, often dramatically. In an extreme case like the Big Dipper, which makes a full rotation around Polaris without ever setting, celestial north makes a full, 360-degree rotation -- equal to up at one time of night and/or year and directly down at another time of night and/or year.

This is why you need to get used to tilting any kind of celestial chart to match the object, whether it's a chart of the Moon's features, Mars's features, or the constellations.

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