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Re: Shadow sights for longitude
From: Brad Morris
Date: 2018 Jan 13, 15:05 -0500

Hello Geoffrey

I found your description quite compelling and read it several times over to be assured that I properly understood it. Would you mind confirming my understanding?

1) Two stars have the same celestial longitude.

2) we have a vertical line and a vertical slit, separated by a few meters

3) when the stars are observed and they are not on our meridian, only one or the other will be blocked by our equipment referenced in (2). Why? The azimuth of the objects are dissimilar when not on our meridian.

4) as the earth rotates, the vertical slit is moved, keeping one of the two stars occluded. The delta azimuth diminishes as the meridian transit nears.

5) eventually, both stars are on our meridian and the delta azimuth becomes momentarily zero. We can then use that as our reference to project other lines on the ground.

Do I have that right?

Questions:

a) two points define a line, consequently, there will be pairs of stars which coincidentally will align vertically when not on the prime meridian. How did the Egyptians know which two stars had the the same SHA and therefore which pair to use?

b) You mentioned that the referenced stars are circumpolar. This would mean that they would align twice a day, unlike other random pairings. Is this how the star pair was selected?

c) How did you keep the vertical slit vertical? Or from rotating in the slightest of breezes?

Thanks in advance!

Brad

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