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Name: Bill
Status: other
Grade: other
Location: NV
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Question:
I understand that if you connect the tips of a crescent moon with an imaginary line and then extend that line down to the earth's horizon, that will be pointing "south". Why is that so? Is it true for both hemispheres?


Replies:
This is one of those myths in astronomy. There are two reasons why this is wrong.

a) The Moon orbits the Earth at about 5 degrees to the Earth's equator, the position of "South" (or "North" in the southern hemisphere) would vary up to 5 degrees on this aspect.

b) The Earth's axis of rotation is 23.5 degrees away from the Earth's orbital plane. This means that the true "'south' point on the horizon" is a combination of these two factors. The true value does not exceed 28.5 degrees (23.5+5) in longitude, but can in azimuth.

In fact, it would be easier to find south (in the northern hemisphere) by simply finding Polaris and extending your arms in opposite directions, one toward Polaris (North) and therefore the other south.

Howard Barnes.

Astronomer.



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