Today's Image

# Analemma shows the sun’s path over a year

Rick Williams of Woodland, California, caught this image on the day of the solstice, June 20, 2021. It’s what’s called an analemma, a figure-8 curve showing the position of the sun at a chosen time (often noon) over a year. Rick wrote of this image that he captured it …:

… a few seconds before noon PST (=1 PM PDT) on the solstice … A small stationary mirror reflects sunlight onto the wall. I marked the center of the reflected light each day over the course of 2 years at exactly noon PST. When connected, the dots form an analemma mapping out the sun’s annual course.

Rick wrote:

At the solstice (sol=sun, sistere= to stand still), the declination of the sun seems to “pause” and reverses its course. On any given day, a horizontal line through the sun’s position shows the latitude at which the sun is directly overhead at solar noon, in this case, 23.5 degrees North.

A vertical line through the position intersects the Equation of Time correction scale (just below the 10 degree North line). This shows the difference between solar time and standard time for that day; in this case, about + 8-10 minutes, corrected for longitude (today exactly 8 m 36 s). Solar/sun time is on the slow side, so 8.5 minutes can be added to obtain standard time; if the minute number is negative, solar time is faster, and that amount is subtracted to compute standard time. Standard time is the average of these times; solar time is the same as standard time on only four days of the year. This is the explanation for analemmas in the Pacific Ocean on many globes, which also have a time scale reflecting the Equation of Time at the Prime Meridian. This scale is corrected for my longitude, 121.77 degrees West.

Bottom line: An analemma is a figure-8 curve showing the position of the sun at a chosen time of day (often, noon) over a year. Here’s one on the side of a house.

Posted
June 22, 2021
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Today's Image