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Name: Stuart
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: N/A 


Question:
I've noticed an apparent anomaly in all sunrise-sunset tables I've checked. As days get longer or shorter, the change is far from equally divided between morning and night. I.e., in a given week, sunrises may get 15 minutes earlier while sunsets only get 8 minutes later -- or vice versa. For some periods, sunset times change more; for other periods, sunrises change more; for others, they're fairly symmetrical. There seems to be no pattern or rhyme or reason, and the same holds for any latitude and/or longitude. Is it just the inaccuracy of the tables, or is there some "real" explanation?



Replies:
What you have found is what astronomers call the equation of time. The sun does not always fall on the meridian at exactly noon. This is bacuase we are in an elliptical (not circular) orbit about the sun. Sometimes, the sun is fast, and sometimes it is slow, depending on which part of the orbital ellipse the earth is occupying at that instant. On many globes, they have what appears to be a large figure 8. This is called the analemma. The analemma gives the declination and equation of time for each day of the year.

Nathan A. Unterman



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