NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Using calculated altitudes
From: Bruce Stark
Date: 2002 Mar 25, 16:50 EST
From: Bruce Stark
Date: 2002 Mar 25, 16:50 EST
In a message dated 3/24/02 4:30:58 PM, george@HUXTABLE.U-NET.COM writes: << Well, I don't see that the change from Right-ascension to GHA made any difference to the difficulty of working lunars from calculated (as opposed to measured) altitudes. The altitude of the Moon (and therefore its contribution to lunar parallax) changed just as fast, no matter how it was tabulated in the almanac. Perhaps Bruce will explain this "old system" for us. >> Response from Bruce Where the change to GHA makes a difference is in the calculated altitude. Whichever system is used, you need the local hour angle of the moon to calculate her altitude. Using our present system, an hour off in Greenwich time throws that local hour angle off about fourteen and a half degrees. That can have quite an effect on the altitude you get. Using the old nautical astronomy, an hour off in Greenwich time throws the local hour angle off about half a degree. That's only about one-thirtieth as much as with the GHA system, but still quite a lot. It's one of the reasons the old navigators preferred to measure their altitudes (at least the altitude of the moon) if it was at all convenient to do so. The other reason was that the Almanac gave the moon's RA and declination only for noon and midnight. Proportioning was no fun. In the old system the word "time" meant local apparent time. Navigators "regulated" their watches by time sight, then allowed for change of longitude since the last regulation. To get Greenwich time to enter the Almanac they converted dead reckoning longitude to time and applied that to local time. Since the time their watches kept was the local hour angle of the sun, a fifteen degree error in dead reckoning longitude (and thus an hour in the supposed Greenwich time) would have zero effect on the hour angle they used to calculate the sun's altitude. The effect on declination could be no more than 1'. With our present system the error in declination could still be no more than 1'. But the error in local hour angle would not be zero. It would be fifteen degrees. As for Lewis and Clark, the captains had three different arificial horizons, and made good use of them. But I don't believe they ever used them when observing a lunar distance. In working their lunars I've nearly always started with a longitude (and thus a Greenwich time) pretty near the truth. The exception was the lunar they took at Bald-pated Prairie. I calculated the altitudes with a Greenwich time that was wrong by about half an hour. A repeat changed the moon's altitude slightly, but not the cleared distance. Generally, though, it was thought wise to repeat the calculation if the Greenwich time found differed more than four of five minutes from that used in the first calculation. Bruce