← the temple the twelve signs · Babylon, about 500 BC; the decans, Egypt, older
The zodiac, drawn the way the astrologers draw a chart: the ascendant on the left, the signs going round against the clock, the planets where the ephemeris puts them at this minute, the Moon in the middle lit as it is tonight, the horizon and the meridian across it. On the rim the thirty-six decans are ticked, ten degrees each, and each is numbered with its card, since the Golden Dawn laid the small cards of the tarot on them; the reading under the wheel says where everything stands and which card that is. Set the moment to any date and hour and it shows that sky instead, which is as near a birth chart as a page without your birthplace can come.
each with its element, quality and ruler, its letter and trump in the Golden Dawn's order, and its three decans with their planets and cards; the dates are this year's, from the ephemeris
The zodiac is the band of sky the Sun walks through in a year, cut into twelve equal signs of thirty degrees from the point where the Sun crosses the equator in spring. The Babylonians cut it so about 500 BC, naming the signs for the constellations that stood behind them then; the signs have since slid a whole sign away from those stars, because the equinox moves, and the wheel here is the signs, not the stars, as Western astrology has always used them. The decans are older and Egyptian, thirty-six stars or groups whose risings marked the hours of the night, ten days to each; the Greeks laid them under the signs, three to a sign, and gave each a planet, going down through the seven from Mars at the first of Aries in the old order of their speed. The planets have their houses, two each except the lights, and their exaltations, and the Golden Dawn laid a Hebrew letter and a trump on each sign and each planet, which is how the tarot and the wheel know one another.
How the sky is computed. The positions are Paul Schlyter's low-precision method: each planet's mean orbital elements for the date, Kepler's equation solved for where it is in its orbit, the main perturbations for the Moon, Jupiter and Saturn added, and the Earth's own position subtracted to give the longitude as seen from here. That is good to a degree or so for the planets and a fraction of one for the Sun, which is what a wheel in signs needs, and nothing like what a navigator would want. The ascendant is the degree of the zodiac rising on the eastern horizon at Portland at that moment, from the sidereal time and the latitude; it changes a degree every four minutes, which is why the astrologers wanted the hour of a birth. Give the page another moment and the horizon stays Portland's; the planets do not care where you stand. Nothing is stored or sent.
The ephemeris follows Paul Schlyter's How to compute planetary positions, after Meeus; the decans and their cards are the Golden Dawn's Book T, 1888, and the cards themselves are drawn here. The planetary hour is from the same clock the paper prints. The page is this site's own. One script, and the ephemeris.