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Re: Measuring Dip in the 18th Century
From: Gary LaPook
Date: 2013 Dec 21, 00:31 -0800
From: Gary LaPook
Date: 2013 Dec 21, 00:31 -0800
I'm not so sure how tricky it was to make those measurements because they sure got accurate results. I am absolutely blown away by the accuracy of the refraction and the dip tables in the 1799 edition of the New Practical Navigator, edited by Bowditch. I compared a sample of the values in the 1799 tables with the modern tables and only rarely did the discrepancy exceed six seconds of arc, 0.1'. See attachments.
gl
From: Marcel Tschudin <marcel.e.tschudin@gmail.com>
To: garylapook@pacbell.net
Sent: Friday, December 20, 2013 1:24 AM
Subject: [NavList] Measuring Dip in the 18th Century
gl
From: Marcel Tschudin <marcel.e.tschudin@gmail.com>
To: garylapook@pacbell.net
Sent: Friday, December 20, 2013 1:24 AM
Subject: [NavList] Measuring Dip in the 18th Century
Andy Young draw my attention to the following publication:
Huddart Joseph (1797): Observations on Horizontal Refractions Which Affect the Appearance of Terrestrial Objects, and the Dip, or Depression of the Horizon of the Sea. Philosophical Transactions of the Royal Society of London, Vol. 87, (1797), pp. 29-42
a pdf copy of it can be obtained from JSTOR http://www.jstor.org/betasearch?Query=Huddart+joseph&fq=py:[1796+TO+1798]
Andy wrote: "Huddart managed to measure the altitudes of the Sun's limbs above both the northern and southern horizons around noon, interpolating to find the apparent altitudes exactly at culmination. This was evidently a difficult and tricky measurement to carry out, especially with the rather primitive instrumentation available to him; he comments that the instrumental limitations prevent it from being done except in a restricted zone of latitude, and that it can't be done near the equator because of the rapid change in azimuth as the Sun passes near the zenith. Nevertheless, he apparently was able to get useful information from this work."
He thought that this publication may eventually also be of interest to some members of NavList.
Marcel
Huddart Joseph (1797): Observations on Horizontal Refractions Which Affect the Appearance of Terrestrial Objects, and the Dip, or Depression of the Horizon of the Sea. Philosophical Transactions of the Royal Society of London, Vol. 87, (1797), pp. 29-42
a pdf copy of it can be obtained from JSTOR http://www.jstor.org/betasearch?Query=Huddart+joseph&fq=py:[1796+TO+1798]
Andy wrote: "Huddart managed to measure the altitudes of the Sun's limbs above both the northern and southern horizons around noon, interpolating to find the apparent altitudes exactly at culmination. This was evidently a difficult and tricky measurement to carry out, especially with the rather primitive instrumentation available to him; he comments that the instrumental limitations prevent it from being done except in a restricted zone of latitude, and that it can't be done near the equator because of the rapid change in azimuth as the Sun passes near the zenith. Nevertheless, he apparently was able to get useful information from this work."
He thought that this publication may eventually also be of interest to some members of NavList.
Marcel
: http://fer3.com/arc/m2.aspx?i=125858
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