Page:Philosophical Transactions - Volume 004.djvu/60

 at the place where you are must be known (as hath been said above) you must have a care to observe that time as precisely as is possible. For every minute of time, that you misreckon, makes a 4th part of a degree in longitude, which amounts, near the Æquator, to above 15. English miles, but less elsewhere. Wherefore to find the time of the clay with certainty, you are not to trust to the Observation of the Suns greatest Altitude, thence to conclude that 'tis just Noon, or that the Sun is in the South, unless, being betwixt the Tropicks, you have it just in the Zenith. For else the Sun being near the Meridian, remains for some time without any sensible alteration of its Altitude. Wherefore, though the Meridian Altitude may serve well enough for knowing the Latitude or the Hight of the Pole upon occasion; yet it will not serve for finding precisely the Longitude of that place. Much less are you to rely upon the Sea-compasses, thereby to End the precise time of Noon. Neither are the Astronomical Rings or other sorts of Sundyals sure enough for shewing the time to minutes and seconds. But it is better to observe the Suns Altitude, when 'tis in the East or West, (the nearer, the better:) for being there, its Altitude changes in a short time more sensibly than before or after; and thus from the Hight of the Pale, and the Declination of the Sun the Hour may be calculated; the manner whereof is sufficiently taught by others; yet by reason that this Calculation is somewhat troublesome, and that also there may be some Errors in the taking of the Suns Altitude, here follows an easier way.

This way doth neither require the Knowledg of the Hight of the Pole, nor of the Declination of the Sun, nor the Use of any Astronomical Instruments: Neither can the Refractions of the Sun or Stars cause any considerable Error; the refraction of the Morning differing but little or nothing from that of the Evening of one and the same day, especially at Sea. Thus then you are to proceed; Rh