NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sun squash- was Green Flash and Longitude
From: Ken Gebhart
Date: 2006 Jan 14, 20:58 -0600
Re: Sun squash- was Green Flash and Longitude
Gentlemen,
The comments on green flash and longitude reminded me of a project I once undertook to establish longitude from estimating the percent squash of the sun due to refraction after rising, or before setting. It all started during a flight I was making in a single-engine Cessna airplane from Honolulu to Wake Is. In 1973 we had no navigation equipment except a radio direction finder which told which way to go, but not when you would get there. Also it could fail. Thus, I always used an aircraft sextant on such trips. Around noon I actually crossed the subpoint of the sun. Sextant read 90 deg. no matter which way I looked. This was handy since I could immediately put a fix on my chart without using the 249 tables. It otherwise took a long time to work out a sight while flying with my knees, not having an autopilot. Later as the sun was beginning to set (this was a 15 hour flight), and I was still a few hundred miles out, I planned to grab a longitude by observing the sunset. All was fine until it did set... on a cloud deck! So without a real horizon I couldn’t use the sight. But I remembered that the sun began to show a squash as it approached the clouds. So I began to think about correlating the percent squash with the actual altitude, so I could get around this cloud problem. And I thought it may be of use to people on land too, who had no accurate horizon.
For the next several months I took hundreds of photos of the sun as it was rising or setting, noting the time and known geographic position. I projected each photo on the wall of a room, and measured the height and width of the disk, thus getting the amount of squash. Working backwards from the almanac refraction tables, I was able to correlate the percent squash not with just Hs, but directly with Ho. I even developed correction curves for temperature and pressure (including altitude). As it turned out, maximum squash ever observed was only about 17%. I tested this method with my co-workers who while driving to work in the morning would note the squash and the time. We would work the sight out, and to our surprise, they were never more than 3 or 4 miles off. For a long time after that, I would note sun squash while driving cross country, and work the sight upon return, with the same apparent accuracy.
I had in mind to publish a book on celestial navigation anyway, and the sun squash chapter could be “the hook”, that is, some information that had never been published before to give it intrinsic value. There would be an insert with ellipses of 5, 10, and 15 % printed on sun shade material, that could be held up to compare with the actual sun. Trouble was, I could not produce such ellipses of suitable quality for publication. I needed a PC with a desktop publishing program which had not yet been invented. So, the whole project languished in a file cabinet all these years. Now the issue is moot except for the interest some list members may have in knowing about it.
Ken
From: Ken Gebhart
Date: 2006 Jan 14, 20:58 -0600
The comments on green flash and longitude reminded me of a project I once undertook to establish longitude from estimating the percent squash of the sun due to refraction after rising, or before setting. It all started during a flight I was making in a single-engine Cessna airplane from Honolulu to Wake Is. In 1973 we had no navigation equipment except a radio direction finder which told which way to go, but not when you would get there. Also it could fail. Thus, I always used an aircraft sextant on such trips. Around noon I actually crossed the subpoint of the sun. Sextant read 90 deg. no matter which way I looked. This was handy since I could immediately put a fix on my chart without using the 249 tables. It otherwise took a long time to work out a sight while flying with my knees, not having an autopilot. Later as the sun was beginning to set (this was a 15 hour flight), and I was still a few hundred miles out, I planned to grab a longitude by observing the sunset. All was fine until it did set... on a cloud deck! So without a real horizon I couldn’t use the sight. But I remembered that the sun began to show a squash as it approached the clouds. So I began to think about correlating the percent squash with the actual altitude, so I could get around this cloud problem. And I thought it may be of use to people on land too, who had no accurate horizon.
For the next several months I took hundreds of photos of the sun as it was rising or setting, noting the time and known geographic position. I projected each photo on the wall of a room, and measured the height and width of the disk, thus getting the amount of squash. Working backwards from the almanac refraction tables, I was able to correlate the percent squash not with just Hs, but directly with Ho. I even developed correction curves for temperature and pressure (including altitude). As it turned out, maximum squash ever observed was only about 17%. I tested this method with my co-workers who while driving to work in the morning would note the squash and the time. We would work the sight out, and to our surprise, they were never more than 3 or 4 miles off. For a long time after that, I would note sun squash while driving cross country, and work the sight upon return, with the same apparent accuracy.
I had in mind to publish a book on celestial navigation anyway, and the sun squash chapter could be “the hook”, that is, some information that had never been published before to give it intrinsic value. There would be an insert with ellipses of 5, 10, and 15 % printed on sun shade material, that could be held up to compare with the actual sun. Trouble was, I could not produce such ellipses of suitable quality for publication. I needed a PC with a desktop publishing program which had not yet been invented. So, the whole project languished in a file cabinet all these years. Now the issue is moot except for the interest some list members may have in knowing about it.
Ken
