Measure of latitude with Polaris

 

Another method is to measure the height of Polaris above the horizon. A similar exercise as with the Sun shows that:

Latitude phi = H measured

 

The precession of the equinoxes shifts the celestial system by around 50 seconds of arc per year (or 28° in 2000 years). This variation is due to a slow conical movement of the rotation axis of the Earth (one full turn in 25 800 years). This means that the Earth’s axis does not always point to the same location in the sky. In other words, todays “polar star” has not always been on the Earth’s axis.

In fact, today’s polar star, Polaris, is at less than 1° of the Earth’s axis, but ancient astronomers had no bright polar star available.

Ancient seafarers looked for the Little Bear (Ursa Minor) to find the North at night (see Lucan, La Pharsale, Book8) and looked for the Sun at zenith for the South in daytime.

 

Track of the Earth’s rotation axis on the northern celestial sphere: its present position (in +2000 AD) is close to Polaris located in the Little Bear and called “α Ursae Minoris” or “α UMi”, that is “Cynosura” for the ancients.

 

Source :  Wikipedia

Measure of latitude with the Sun

References
Information accessible to non specialists in astronomy is available in textbooks on sundials (e.g. by Denis Savoie (2003), ed. Belin, France) and, of course, on Wikipedia. See also Journès & Georgelin (2000), “Pythéas, explorateur et astronome”, ed. Nerthes, Ollioules, France, for fascinating explanations on Pytheas’ astronomy.

2 thoughts on “Measure of latitude with Polaris”

  1. congratulation for your site.
    we are two fans of ancient navigation.
    please, could you tell us why you put “Traianus portus”in Punta Ala ?
    because whe think that Tolomeo is not accurate to define this location.
    thanks
    a.bani p.casini

    1. Yes, Ptolemaeus is not very accurate and I did not use his data except in the Red Sea (see also my section on Ptolemeaus).
      I placed Portus Trajanus at Punta Ala because that is mentioned in Hazlitt’s Classical Gazetteer of 1851 (p 283), but the more recent Barrington Atlas does not mention a port at this location. It just mentions a place called “Traianus” located near Centumcellae.
      Do you have any more information ?

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