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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Corrections for latitude when taking sights
From: Brad Morris
Date: 2019 Mar 12, 22:19 -0400
From: Brad Morris
Date: 2019 Mar 12, 22:19 -0400
David
The sentence
The watch time of sunrise/sunset, at a place 90° different in latitude from your own, gives the time when the Sun is due east or west of you at your own latitude
It seems like it should read
The watch time of sunrise/sunset, at a place 90° different in longitude from your own, gives the time when the Sun is due east or west of you at your own latitude
That makes alot more sense. I think it correct but we can permit Brian to verify.
It still suffers, however, from a misunderstanding. Unless the Sun's declination is equal to your latitude, the sun can never be "due east or west" of you at sunset. Consider, as you say, N50° when the Sun's declination is S23°. The sun won't rise due east of you. It won't set due west of you. It will be distinctly to the SE and SW of you. (Note: not calculated, others may wish to determine the azimuth). The period of the day is far less than 12 hrs, meaning we cannot have 90° of rotation (half 12 is 6 hrs * 15 deg/hr =90°) for sunrise or set from the meridian.
Brian is correct for time sights, we want the sun far from the meridian. We just don't get the values he asserts.
Brad
On Tue, Mar 12, 2019, 10:02 PM David Pike <NoReply_DavidPike@fer3.com> wrote:
The watch time of sunrise/sunset, at a place 90° different in latitude from your own, gives the time when the Sun is due east or west of you at your own latitude. Prime Vertical, the best time to do time sights for longitude by chronometer, and reduce and simplify the wretched mathematics involved. Brian Walton
That sounds a cunning trick Brian, but I’m not sure I follow all of it.I can draw a diagram to show that the plane of the horizon of someone at 50N would be parallel to the plane of the prime vertical of someone at the same longitude and 40S and vice versa, but I can’t see where to go from there. Please explain further. DaveP
