Coordinates, Time Zones, and Birthplace: Putting Birth Data on One Scale

By now you know how the Sun and Moon move through the sky (the ecliptic and Moon phases), and how the horizon and meridian define the Ascendant and Midheaven. These angles share one premise: they describe the sky seen by someone, somewhere, at a particular instant. Chart software must translate your birth details into numbers it can calculate with. This lesson explains that translation.

People often wonder why a chart form requires a birth city. Earlier lessons already supplied the answer: the ecliptic degrees of the Sun, Moon, and planets depend almost entirely on time and are similar worldwide at the same instant. But the Ascendant and MC come from the horizon and meridian at your location. Two births in Taipei and London at the same second can have very different rising signs. Location is essential for the angles and houses.

Looking down on Earth from the North Pole makes this clear: Taipei and London occupy different positions, so their eastern horizons point toward different parts of the sky and different signs rise.