Now imagine you had fayled the fame yeare through the ftreights of Magellane...

Edward Wright in his 1599 printing of Certaine Errors in Navigation tells us that he has observed that seamen "do take the funnes declination out of their regiments without any aequation, by addition or fubtraction of the part proportional agreeable to the difference of longitude of the place where they are..."

He says that when determining latitude this may cause them to "be deceiued fometimes 10 or 12 min (or more in a long voyage) in taking the funnes declination."

He goes on with verbal instruction to make the interpolation, and works through several examples, pointing out that the longitude need not be known with precision, that "though you miffe halfe a dozen or halfe a score degrees herein, it cannot in this case breed fensfible error.

"Now imagine that you had fayled the fame yeare through the ftreights of Magellan, and hauing paffed over the fouth sea, were 13 of September come neare the Philippinas, differing in longitude from London weftwards about 210 deg. In this example, because the funne is near the equinoctial poynt (altering his declination 24 min. in 24 houres, that is, for euerie houre one minute) therefore diuide 210 (the difference of longitude) by 15 (the number of degrees contained in one houre) the quotient will be 14 (the difference of declination anfwerable to that difference of longitude.) The declination found in the table for that day is 4 minutes northerly: which (declination) becaufe it decreaseth (the funne not yet yet come to the equinoctiall) must be fubtracted out of 14, and there fhall remaine 10 min. the declination of the funne that day at noone for the place. But that declination is foutherly, becaufe the quotient is greater then 4, the declination found in the table"