What a Birth Chart Is

Think of the chart as a seating plan for that moment of sky: who sits in which section. When you read a chart, you find the seats one by one: first where the Sun sits, then the Moon, then where each of the other planets sits.

A chart records the direction of each body in the sky. This app works out which degree of the zodiac each body sits on — where it appears, seen from Earth. So when you read a chart, what you look at is which section each body sits in.

What the old texts say: in Book III ch. 2 of the Tetrabiblos, Ptolemy takes the degree rising on the eastern horizon at the moment of birth as the starting point of the whole chart; everything else is counted from there.

The things on a chart are easier to remember as three kinds: a planet is the who, a sign is the style, a house is which part of life. Take “Sun in Libra”: the Sun is the who, and Libra is its style — talking things out and smoothing things over. Read any row by asking three questions: who? in what style? in which part of life?

What sets them apart is whether they need a birth time. Planets and signs come from the date alone (on a day when one changes sign, the candidates are listed). Houses and the Ascendant depend on the Earth's rotation, so they need a birth time to be worked out. With a time, the chart gains the house layer; until a time is entered, that layer stays blank and the planets and signs can still be read.

What the old texts say: in Book I ch. 4 of the Tetrabiblos, Ptolemy sets out the nature of each of the seven planets one by one. William Lilly's Christian Astrology of 1647, in “Of the Twelve Houses,” names the first house the Ascendant, signifying life and the body.

Astrology is a long-lived symbolic language. It gives you words for describing yourself and angles for noticing things. You can hold those words up against your own life and see which fit you and which do not. Reading “Moon in Aries: under pressure, act first, think later,” think back to the last time someone rushed you: did you jump in, or stop and think?

As you read, the small tag beside each paragraph tells you what it is. Positions are really computed and can be used as they are; readings are handed down and are there to check against yourself. Look at the tag first, then decide how to use the paragraph.

What the old texts say: in Book I ch. 2 of the Tetrabiblos, Ptolemy argues that the motions of the heavens change the air and moisture around the Earth — seasons, tides, plants and animals shift with them — so inference is possible; ch. 3 then argues it is useful.