Starship Asterisk*

Nodus-based sundial

[c]The shadow of the cross-shaped nodus moves along a hyperbola
showing the time of the year, indicated here by the zodiac figures.
It is 1:50pm on the 16th July, 25 days after the summer solstice.

A 1954 sundial, on St. Mary's Basilica, Kraków, Poland.
"Our days on the earth are as a shadow, and
there is none abiding.": 1 Chronicles 29:15[/c][/color][/b]

alphachap wrote:
Is it a sundial if it doesn't tell the time of day?

http://en.wikipedia.org/wiki/Sundial#Nodus-based_sundials wrote:

Nodus-based sundial

<<One type of sundial follows the motion of a single point of light or shadow, which may be called the nodus. For example, the sundial may follow the sharp tip of a gnomon's shadow, e.g., the shadow-tip of a vertical obelisk (e.g., the Solarium Augusti) or the tip of the horizontal marker in a shepherd's dial. Alternatively, sunlight may be allowed to pass through a small hole or reflected from a small (e.g., coin-sized) circular mirror, forming a small spot of light whose position may be followed. In such cases, the rays of light trace out a cone over the course of a day; when the rays fall on a surface, the path followed is the intersection of the cone with that surface. Most commonly, the receiving surface is a geometrical plane, so that the path of the shadow-tip or light-spot traces out a conic section such as a hyperbola or an ellipse. The collection of hyperbolae was called a pelekonon (axe) by the Greeks, because it resembles a double-bladed ax, narrow in the center (near the noonline) and flaring out at the ends (early morning and late evening hours).

Nodus-based sundials may use a small hole or mirror to isolate a single ray of light; the former are sometimes called aperture dials. The oldest example is perhaps the antiborean sundial (antiboreum), a spherical nodus-based sundial that faces true North; a ray of sunlight enters from the South through a small hole located at the sphere's pole and falls on the hour and date lines inscribed within the sphere, which resemble lines of longitude and latitude, respectively, on a globe.>>

orin stepanek wrote:I really never thought of the rings being like a sundial; but that does make sense!

mactavish wrote:The many images of Saturn relayed to us by Cassini are absolutely fascinating. If only Galileo could have seen them. We are soooooo fortunate!

nstahl wrote:Great image, great APOD.

Does anyone know what fraction of the sunlight the rings actually block from Saturn in, e.g. the season we're viewing? I'm thinking they could really complexify the atmospheric dynamics if there are those giant areas of real shadow that move slowly over the hemisphere. In the picture it looks like dense shadow but that could be a photographic effect. Or is Saturn so far from the sun that solar heating plays only a small part in its atmospheric dynamics? Or does anyone know yet?

zbvhs wrote:What exposure times are required to capture images such as this one? Is blurring of the rings caused in part by motion of ring material during long exposures?