What is the Poles Altitude, and how it may be known?

• Altitude is the distance, height, or mounting of one thing above another, so that the Altitude of the Pole, is the distance, height, or mounting of the Pole from the Horizon, and is defined to be that position of the Meridian which is contained between the Pole and the Horizon, which Altitude or Elevation is to be found either by the Sun, or by the sited Stars, with the help of your Cross-Staff, Quadrant, or Astrolaby, but the Cross-Staff is the only best Instrument for the Seamans use.

And in the observation of this Altitude there are five things especially to be regarded:

• the first is, that you know your Meridian distance between your Zenith and the Sun or Stars, which by your Cross-Staff or Astrolaby is given:
• the second, that the declination be truly known at the time of your observation.
• And the other three are, that you consider whether your Zenith be between the the Equinoctial and the Sun or Stars, or whether they be between your Zenith and the Equator, for there is a several order of working upon each of these differences.

Latitude you must also know, that so much as the Pole is above the Horizon, so much is the Zenith from the Equinoctal, and this distance between the Zenith and the Equator is called Latitude or wideness, and is that position of the Meridian which is included between your Zenith and the Equator:

For it is a general Rule for ever, that so much as the Pole is above the Horizon, so much the Zenith is from the Equinoctal: so that in this sense, Altitude and Latitude is all one thing, the one having relation to that part of the meridian, contained between the Pole and the Horizon: and the other to that part of the meridian which is contained between the Zenith and the Equinoctal. (Readers in the 21st Century are reminded that the current pole star Polaris circled the celestial pole at a larger distance in the 16th Century than it does now, due to the precession of the equinoxes. At the time of writing, Polaris was approximately 3 degrees away from the celestial pole, giving a potential error in latitude of 180 nautical miles. The celestial pole describes a circle with a diameter of 47 degrees returning to the same place every 25,920 years.)

You must further understand, that between the Zenith and Horizon, it is a quarter of a great Circle containing 90 degrees so that knowing how much the Sun or any Star is from the Horizon, if you take that distance from 90, the remainder is the distance between the said Body, and the Zenith.

• As for Example: If the Sun be 40 degrees 37 minutes from the horizon, I subtract 40 degrees 37 minutes from 90, and there remaineth 49 degrees 23 minutes, which is the differance between my Zenith and the Sun. Those Instruments that begin to accompt of their degrees at the Zenith, concluding 90 in the Horizon, are most ease for the finding of the Latitude by the Sun or fixed Stars, because they give the difference between the Zenith and the Body observed without further trouble, and that is the number which you must have, and for which you do search in your Observation:
• All which things considered, you must in this sort proceed for the finding of the Poles height or Altitude.