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Re: stange results from AM and PM sun shots
From: Gary LaPook
Date: 2012 Sep 28, 00:19 -0700

It depends on how persnickety you want to be. As for your point that we call "sunrise" when the upper limb is on the visible horizon and the center of the sun is 50 minutes below the celestial horizon then the azimuth to the center of the upper limb will vary slightly from straight east and west but anywhere between 32° north and south the azimuth, rounded to a whole degree, will still be 090 and 270. Even at 60° north and south the azimuth will only be 1.4 degrees away from east and west, see attached.

With the Hc of zero the center of the sun is on the celestial horizon and you can use the amplitude table, table 27 in Bowditch, to determine the azimuth. This table gives the azimuth as 090° for all latitudes when the declination is zero. The sun's center is on the celestial horizon when the lower limb appears about 1/2 sun diameter above the visible horizon.

When the sun's declination is zero, 1446 Z on September 22nd this year, it is setting at some longitude and rising at a different longitude and the amplitudes will be 270 and 90 at all latitudes on those longitudes.

Due to refraction and the semidiameter of the sun, the day is always twelve hours and seven minutes long at the equator but the length of the day varies greatly at other latitudes so there is no flat "eight day" rule for determining the length of the day at all latitudes.

gl

--- On Thu, 9/27/12, bill <billyrem42@earthlink.net> wrote:

From: bill <billyrem42@earthlink.net>
Subject: [NavList] Re: stange results from AM and PM sun shots
To: NavList@fer3.com
Date: Thursday, September 27, 2012, 4:54 PM

On 9/27/2012 12:47 PM, Alan S wrote:
> I wonder is the above mentioned in just a peculiarity of late Sept. when
> Sun's declination is so small, in some instances, less than 1D South.
> Any ideas or light to be shed.

Alan

Keep in mind that what you see is not what you get. We are told with 0d declination the sun will rise and set due east and due west,and night and day will be equal lengths. This is not true for three reasons.

1. If the declination was 0d at LAN in September, the declination would be N at sunrise and S at sunset.

2.Even if declination was 0d at rise and set, that would be for the center of the sun, not the upper limb which we use to define rise and set.

3. Due to refraction, we are seeing a limb that is not there in a vacuum. The refraction correction would be -0d 33!8 for the center of the sun or upper limb on the horizon. Therefore it is quite possible to have an Hc north of 90d or 270d near the autumnal equinox. If memory serves there is about an 8 day lag past the equinox before light and dark are equal.

To make matters more confusing, 0d declination does not exactly define the equinox as I recall. Somewhere in my archives there is an explanation and diagram posted by a list member perhaps 7 years ago. I'll try to locate locate that.

In memory of George I'll mention that when I learned the above I was as disillusioned as a kid finding out his "sea monkeys" were nothing but brine shrimp. This caused George to rebuff me as he asserted younger and foreign list members had no idea what I was talking about. To get those not in the know up to speed:
http://en.wikipedia.org/wiki/Sea-Monkeys

Bill B

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