Guenther Faltlhansl wrote: > The equation for y(0)= a. tan(phi) / cos(D) > (a= height of the gnomon, phi= latitude of location, D= declination of > wall). > > Would tan(phi) in the numerator also have to be exchanged for tan(delta) ? > (delta= declination of the sun)
First off, please accept my apologies for being unable to spell your name properly; my provincial keyboard won't permit letters with umlauts. The formula you quote for the y coordinate of a vertical dial's center is correct. The formula in question pertains to the y coordinate of a general point on a vertical dial, corresponding to a given hour angle and solar declination. Letting H represent solar hour angle, the following is correct: y = -a [ cos( phi ) * cos( H ) + sin( phi ) tan( delta ) ] / Q However, the 2nd edition of _Astronomical Algorithms_ mistakenly shows: y = -a [ cos( phi ) * cos( H ) + sin( phi ) tan( phi ) ] / Q ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Mark Gingrich [EMAIL PROTECTED] San Leandro, California You wrote... > I may be completely off the point with the following remark, as I do not > have the book you cited. So I am just guessing what "y" you are talking > about. > However I do possess the "Gnomonique Moderne" by Denis Savoie and I checked > the section on vertical dials there. The 1997 edition gives you on page 81 > the coordinates for the center of the dial. > > The equation for y(0)= a. tan(phi) / cos(D) > (a= height of the gnomon, phi= latitude of location, D= declination of > wall). > > Would tan(phi) in the numerator also have to be exchanged for tan(delta) ? > (delta= declination of the sun) > > I may try to verify in the next days. G¸nther Faltlhansl [EMAIL PROTECTED] ----- Original Message ----- From: "Mark Gingrich" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Monday, November 27, 2000 7:14 PM Subject: Sundial Formula Erratum in Meeus's Book > Dial folk, > > Those who rely on the sundial formulae in the second edition (1998) > of Jean Meeus's book _Astronomical Algorithms_ should be aware of > the following erratum: On page 406, in the chapter titled "Calculation > of a Planar Sundial", by R. Sagot and D. Savoie, in subsection (3) on > vertical sundials, the third equation, the one for "y", should have > a "tan delta" in the numerator, not a "tan phi" as is shown. > > This error does *not* occur in the first edition (1991). > > I've already brought this issue to Jean Meeus's attention. He replied > today with the following e-mail, which lists two other typographical > errors in the second edition. Owners of this later edition should make > the necessary corrections to their copies. > > > > Date: Mon, 27 Nov 2000 10:01:05 -0500 > > From: Jean Meeus > > Subject: Erratum in Astronomical Algorithms, 2nd Ed. > > To: Mark Gingrich <[EMAIL PROTECTED]> > > > > Thank you for mentioning that typographical error. It is my own fault : > > the error didn't appear in the first edition of the book. > > > > Here are two other typographical errors in the book : > > > > (1) Page 63 : If Z is equal to or larger than 2299161 (same constant as > > on the previous line), not 2291161. > > > > (2) Page 144, fifth line from the bottom : the double negative sign should > > be a single one. > > > > Thanks and best regards. > > > > Jean Meeus > > > ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ > Mark Gingrich [EMAIL PROTECTED] San Leandro, California >
