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Are these statements correct?

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- Thread starter bigislander72
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- #1

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Are these statements correct?

- #2

russ_watters

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Not the main claim, no.

- #3

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I am still confused as to why. It seems that on that day ( approx. may 25) the sun is directley over the latitude for which Hawaii lies and on the summer solstice, the sun makes an arc that is slightly to the north(shadow points due south at solar noon). So I don't understand why the day would be longer. Is this true anywhere in the norther hemisphere?Please explain.

- #4

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cos(LD/2) = - tan(declination) tan(latitude)

LD is in degrees that can be converted to hours, min, sec

For locations north of EQUATOR the longest day of the year coincides with the summer solstice.

- #5

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For locations north of EQUATOR the longest day of the year coincides with the summer solstice.

If this is true and if it is also true that the longest day of the year south of the equator is the summer solstice, then it must be that all days along the equator are always 12 hours long. Is this true?

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Yes, this is true

- #7

jim mcnamara

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Not strictly true.

For an arbitrary position on Earth at the Equator given as:

0 0 N 106 35 W - for 2009

May 29 2009 sunrise 06:00:11 sunset 18:07:21 12hrs 7min 10sec

Solstice Jun 20 sunrise 06:04:37 sunset 18:11:53 12hrs 7min 16sec

Solstice Dec 21 sunrise 06:01:21 sunset 18:08:36 12hrs 7min 15sec

The days are very slighty varying in length but the mean time between official sunrise and official sunset are clearly 12 hours 7 minutes and few seconds. Due to the arc subtended by the disc of the sun it is not 12 hours even - sunset occurs when the top of the disc appears, sunset at the time when the "other side" top of solar the disc disappears. The difference is the transit time for the angle of solar disc.

Works out to circa 7 minutes.

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- #9

jim mcnamara

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- #10

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This is an interesting question - I've had to think about it for a while myself. Here's a suggestion for another way to look at it, which might help:

I am still confused as to why. It seems that on that day ( approx. may 25) the sun is directley over the latitude for which Hawaii lies and on the summer solstice, the sun makes an arc that is slightly to the north(shadow points due south at solar noon). So I don't understand why the day would be longer. Is this true anywhere in the norther hemisphere?Please explain.

Instead of thinking about the arc traversed by the Sun as it "moves" across the sky (I admit that I haven't yet found a way to explain the answer in those terms), try thinking about the arc you traverse as your location on the Earth's surface moves throughout the day. If you look at portion of this path that is in the sunlit half of the Earth's sphere, it's clearly half at equinox, and pretty clearly its longest at Summer Solstice, when the path extends the farthest distance past the noon +/- 6 hour angle points to the terminator line. It's a little harder to see what's going on at the days you reference in your OP, but maybe you can convince yourself that it must be shorter than on Solstice.

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Well, that would account for two of the seven minutes - one in the morning and one in the evening.

- #12

tony873004

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Or slightly longer than 2 if the sun is setting at an angle. I think the remainder of the difference is refraction. Although this varies from day to day, it is not predictable long-term. So the predicted sunset / sunrise times probably use an average which is about 3-4 extra minutes before sunrise and after sunset.Well, that would account for two of the seven minutes - one in the morning and one in the evening.

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