A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2013 Apr 24, 01:15 -0700
|This is another illustration of Mr. van Asten's lack of understanding of celestial navigation. Two years ago I went around with him on the TIGHAR website regarding his mistaken idea about what constitutes "sunrise" and "sunset." Mr. van Asten believes these occur when the sun is coincident with the "sensible horizon" (the local horizontal, parallel with the "celestial horizon" which runs through the center of the earth) and not, as everybody else believes, when its upper limb is coincident with the visible horizon. I called this "van Asten sunrise." Here are links to that discussion.|
Now van Asten expands his misunderstanding of rising and setting phenomena to the moon but now wants to use the celestial horizon for the reference instead of the sensible horizon. However, everybody else continues to use the visible horizon as the reference line and van
Asten does this by misunderstanding the "parallax in altitude" correction. When the upper limb of the moon is coincident with the visible horizon it is actually below the sensible horizon due to refraction and dip just like with sunrise, nothing different here. But then he applies the parallax in altitude "correction" and determines that, since the moon is already above the celestial horizon (as measured at the center of the earth, very few observers are actually located there) that an observer can't observe moonrise! Let's call this "van Asten moonrise." But, as I have already shown, a real observer located on the surface of the earth sees the moonrise just like seeing a sunrise, as the upper limb climbs through the visible horizon.
This brings up an interesting point, the "parallax" (parallax in altitude) "correction" is not really a correction at all since there is nothing erroneous that needs to be "corrected," the measured altitude of the moon is just as accurate as any other measured altitude before the parallax "correction" is applied. In fact, it shouldn't be called a "correction" since it is actually a conversion factor used to allow the measurement to be referenced to a different reference frame, that of the celestial horizon for an imaginary observer located at the center of the earth. So I suppose Mr. van Asten might be right but to be so he would have to dig a hole down to the center of the earth where he could then sit and he would then be able to see the moon (at the point of moonrise for the surface observer) above his celestial horizon. But for the rest of us surface dwellers, we don't have any problem with observing moonrise if the air is clear enough.
--- On Sun, 4/21/13, h.a.c. van Asten <firstname.lastname@example.org> wrote: