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Re: Peculiar definition of sun set/rise
From: Brad Morris
Date: 2019 Jun 21, 22:58 -0400
From: Brad Morris
Date: 2019 Jun 21, 22:58 -0400
Ha! All the letter writer need do is mark the center of the apparent sun's disk, so we can tell precisely when that spot is on the horizon.
Seriously, I can see the point of his argument. He neatly splits the sun in half, thus it doesn't belong to day or to night. His definition divides the illuminated from non-illuminated periods of the day, as if the sun was a point. The definition he complains of does tend to favor the length of day over the length of night. His definition favors neither. Utterly fair but completely impractical
B
On Fri, Jun 21, 2019, 10:33 PM Paul Hirose <NoReply_Hirose@fer3.com> wrote:
In a 1917 issue of The Observatory I saw this letter complaining about the definition of sunrise and sunset in the tables of an ephemeris. I must admit my inability to grasp the "parity of reason" in his argument, but it does show that "obvious" notions of rise and set are not universal. (The "Companion" mentioned in the letter apparently refers to a supplement to the magazine.) "There can be little doubt as to the usefulness of the tables of rising and setting of the Sun and Moon, lately published as a supplement to the American Ephemeris... From the figures in these tables it appears that sunset is defined as the instant when the upper limb is on the visible horizon. This, therefore, implies that sunset does not happen until the whole of the disc has disappeared, and it might be expected by parity of reason, that sunrise should not happen until the whole of the disc has reappeared, but in these tables, on the contrary, the times given are again those when the upper limb is on the visible horizon. By defining sunset or sunrise as the instant when the centre of the disc is on the visible horizon, as is done in your 'Companion,' any inconsistency of this kind is avoided." http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..345H&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES A later letter supports the "upper limb" definition: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..410O&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
