Help me understand sunrise/sunset & moonrise/moonset tables
October 20, 2011 2:59 PM   Subscribe

Please help me understand these sunrise/sunset & moonrise/moonset tables.

Can't link to result pages, it's year 2012, W 46º38', S23º32', 3h west of Greenwich in Form B.

1) I'm in the southern hemisphere, so the winter solstice will be on June 20, 2012; and the summer one on December 21, 2012. But the latest sunrises occur between June 25 and July 12, and the earliest sunsets between June 2 - 12. The earliest sunrises are between November 25 and December 2, latest sunsets January 12-16. I thought that the latest sunrise and earliest sunset coincided on the winter solstice, and earliest sunrise and latest sunset on the summer solstice, but that doesn't seem to be the case. Why?

2) Blank spaces in the moonrise/moonset table. According to the disclaimer, "blank spaces in the table indicate that a rising or a setting did not occur during that 24 hr interval". OK, when did it occur then? Take January 15 for example. We have a moonset at 11:33, on the 16th there's no moonrise, but there's a moonset at 12:34. How come it set again if it didn't rise?!?

posted by Tom-B to Science & Nature (4 answers total) 2 users marked this as a favorite
Best answer: Look here.
posted by doctord at 3:32 PM on October 20, 2011 [1 favorite]

Best answer: The moon will rise at 2320 on the 15th, set at 1234 on the 16th, and rise again at 0005 on the 17th. Yes, it's confusing when things in the righthand column happen before the things in the lefthand column, but at least for the second half of January they're the way round that makes immediate sense....
posted by Lebannen at 3:35 PM on October 20, 2011 [1 favorite]

Best answer: The reason the earliest sunsets are not on the solstice is because the earth does not traverse a circle when orbiting the sun. In fact, the path is an ellipse; the area of the wedge of ellipse is constant for any amount of time. When the Earth is farther away from the sun, it moves more slowly; when it's closer it's faster. The net result is that the planet is very rarely sweeping out 1/365.24th of the way around the sun in a particular 24 hour period.

However, the Earth rotates about its axis perfectly periodically. What happens is, the earth rotates around once in 24 hours (well, it rotates (1+1/365.24)*360 degrees), but does not sweep the same amount.

See "analemma" for mo better explanation.
posted by notsnot at 3:37 PM on October 20, 2011 [2 favorites]

Response by poster: It all makes sense now, thanks everyone!
posted by Tom-B at 3:51 PM on October 20, 2011

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