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
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.