Denis Savoie

> Babelio : https://www.babelio.com/livres/Savoie-Cosmographie/301863

> LA COSMOGRAPHIE, de Denis Savoie, Belin Sciences, 2006, 128 pages. — Redécouvrir le plaisir de regarder le ciel à l'oeil nu et pouvoir enfin répondre de façon simple aux questions embarrassantes de son petit neveu lycéen concernant les saisons sur la terre, les phases de la lune, la visibilité des planètes …
L'auteur prend l'option de décrire l'univers à partir des yeux du sujet, comme le faisaient nos ancêtres. Des schémas et des explications claires conduisent à la compréhension des fondements essentiels de l'astronomie. (Jacques MARMEY)
—Carnets du Yoga, (253), Janvier 2007, (p. 19)

> « - Je vous apporte
mes voeux.
- Merci. Je tâcherai
d'en faire quelque
chose. »
—Jules Renard, Journal… (más)

Contemporary, mathematical, many excellent diagrams. All by a French professor of astronomy, and translated from the original French.

Chapter 1: A Little Astronomy
1.1 The Moving Earth
Asserts that the precession of the eclipses can be entirely ignored when discussing sundials. I can see how that might be possible, as our year is really the tropical year, determined by the timing of the equinoxes.
1.2 Geographical Coordinates
Parallels and Meridians
Tropics of Cancer and Capricorn, Arctic and Antarctic Circles
Latitude and Longitude
1.3 The Celestial Sphere and the Local Celestial Sphere
The angle of the north celestial pole w/ the horizon is equal to the latitude. This can be proved with similar triangles. When the observer is at the north pole, their horizon and zenith coincide with the celestial equator and the north celestial pole.
1.4 The Sun's Annual Motion
The Ecliptic
At this point, I start to imagine the celestial sphere as simply a three-dimensional map, dotted with stars, on which the paths of the sun and other things can be marked. The ecliptic is, of course, the course the sun traces out in a given year. But, due to precession of the equinoxes, this course changes slightly every year. In any case, the obliquity of the ecliptic, epsilon, is 23 deg. 26 min. and is decreasing a bit. Maybe this is due to precession, actually? I wonder, because the obliquity of the ecliptic decreasing is equivalent to the axis of the earth's rotation coming closer to the perpendicular wrt. its orbital plane. At the June solstice the sun is directly above the Tropic of Cancer, at the December, the Tropic of Capricorn. On the equinoxes, it is directly above the equator. Solar longitude, l, is measured in degrees along the ecliptic from the March equinox. The sun spends longer between the March and September equinoxes than between September and March, by about 10 days. Presumably, this is due to the eliptical orbit of the earth, and the earth is actually further from the sun between March and September? The sun's declination, delta, is its angular distance from the equator. Its right ascension, alpha, is the angle between the March nodal point and the projection of the sun's position on the celestial equator.

tan alpha = cos epsilon tan l
sin delta = sin epsilon sin l

This is all obvious from trigonometry and cancellations.

1.5 The Sun's Diurnal Motion
Azimuth
The sun's azimuth is the angle between due south and the projection of the sun's direction onto the observer's plane. The azimuth at sunrise and sunsets obeys the following equation:

cos A = -sin delta / cos phi (phi is the observer's latitude)

This checks out with the assertion that, at the equinoxes, the sun rises due east and sets due west. At the north and south poles cos phi = 0 and therefore cos A is undefined. This is fine, because at the poles the azimuth has no meaning because south is in every direction.… (más)