Archive for March 7th, 2008

Crazy Eights

My memory was just jogged, bringing this to the fore. A series of cool analemma pictures taken in Greece, showing ruins in the foreground, with the additional feature of some being multiple exposures on a single frame of film.

The analemma — which you can often see on a globe — describes the variation in the position of the sun over the course of the year. Besides the pretty obvious vertical motion you get from the axial tilt of the earth, there is also the effect from the noncircular nature of the earth’s orbit. This causes the earth’s speed to change over the course of the year, so we do not sweep out exactly the same angle from day to day, if you were to assume that noon is when the sun reaches some position in the sky (like straight overhead, or on the vertical line that includes that point). In other words, the time you would read on a sundial will not agree with the time kept by a very good clock. The variation over the course of the year is on order of ± 15 minutes, or almost 4º of arc in each direction. The math that describes this is known as the equation of time.

More detail here or here or many other places on the web.

More Illusions

For a peek at a few more illusions, of the “interchanging 2-D and 3-D” variety, as well as some history & physics behind it all, head on over to Skulls in the Stars.

Archimedes is All Wet