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Sunrise-Sunset Tables
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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Update: June 2012
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