NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sumner lines
From: Nigel Gardner
Date: 2001 Feb 18, 3:41 AM
From: Nigel Gardner
Date: 2001 Feb 18, 3:41 AM
The 'Sumner Line' was described by Captain Sumner in a book he published in 1843 in Boston. Bound from Charleston to Greenock in the South-west approaches (South of Ireland) on 17 Dec 1837, not having had a sight for some time and thus unsure of his latitude, he got a sight about 10am, worked out the longitude from the sight for three different latitudes. He noticed that the three different points lay in a straight line which ran through the Smalls Light. He concluded "that the observed altitude must have happened at all three points, at the Smalls light and at the ship at the same instant of time". His method (calculating the longitude for two different latitudes and running aposition line through them) was in use certainly into the beginning of the 20thC although by that time Lecky and others were advocating calculating one longitude from a DR latitude and drawing line at right angles to the sun's calculated azimuth. It might be well to remember the ethos of the age, as a previous contributor has commented, one has to get into the mind of the 18/19C mariner, "getting a fix" was not the be-all and end-all of things. A longitude by the Prime Vertical at breakfast and latitude by mer-pass at lunchtime served most of their needs. NG
