Free birth chart calculator: your natal chart, explained

A birth chart, also called a natal chart, is a record of the sky at the moment and place you were born. It's drawn as a wheel: the twelve zodiac signs run around the outside, the twelve houses divide the inside, and between them sit the Sun, the Moon, the planets from Mercury to Pluto and the lunar node.

Your Sun sign, the one you know from horoscope columns, is only one piece of this map. A birth chart also shows where the Moon was, which sign was rising on the eastern horizon (your ascendant, or rising sign), which areas of life the planets fall in and what angles they form with each other. That's why two people born on the same day but at different times have noticeably different charts.

Three things: your date, time and place of birth. The date gives the signs of the planets. The time and place determine the ascendant, the Midheaven and the houses, the most personal part of a chart.

Enter the time exactly as it was recorded: local time at your birthplace. Don't convert it to standard time or to UTC. We look up the time zone and the daylight saving rules in force there on that date, from War Time during World War II to Arizona and Hawaii, which skip daylight saving time. If you don't know your birth time, ask your parents or check your long-form birth certificate. The short version often leaves the time out, but hospital records or a baby book may have it.

The Earth turns once a day, so in 24 hours the whole zodiac passes over the eastern horizon. The ascendant changes sign about every two hours on average, and the houses turn with it. A 15-minute difference moves the ascendant by several degrees, and by up to about seven in the fast-rising signs.

The Moon crosses a sign in about two and a half days, moving 12–15° a day. If it changed signs on the day you were born, there's no telling which one it was in without the time. The Sun and planets move more slowly: the Sun about one degree a day, and Mercury, the fastest planet, a little over two. Your birth time only changes their sign if they sat right on the border between two signs. The calculator checks this for you and tells you when it happened.

Read a birth chart like a sentence: the planet tells you which part of you is at work, the sign tells you how, and the house tells you where in life. The Sun in Cancer in the 9th house is your identity (the Sun) expressed through care and sensitivity (Cancer) in matters of beliefs, learning and travel (the 9th house).

The most important part is the big three: the Sun, who you are; the Moon, what you need; and the ascendant, how you meet the world. Every astrologer starts there. Here's what each planet stands for:

The houses divide the sky into twelve parts, counted counterclockwise from the ascendant. Each house is one area of life, and a planet in a house shows where its energy works most strongly.

We calculate the houses with the Placidus system, one of the most widely used in Western astrology. It splits into three equal parts the time each point of the zodiac takes to travel from the horizon to the meridian, which is why the houses in one chart come in different sizes. Above 66° latitude, beyond the Arctic and Antarctic Circles, Placidus can't be calculated, so there we use the Porphyry system. A different house system is the most common reason two calculators put a planet in different houses.

An aspect is a specific angle between two planets, measured along the zodiac. We use the five major aspects, with an allowance (the orb) of up to 8° for conjunctions and oppositions, 7° for trines and squares, and 5° for sextiles.

We calculate the positions of the Sun, Moon and planets from astronomical ephemerides, in the tropical zodiac that Western astrology uses. Your birth time is converted to Universal Time using historical time zones, and the lunar node is given as the mean node. If another calculator shows different houses, or a node a degree or two away, it's usually using another house system or the true node.

The calculations are astronomically accurate, but astrology is a tradition, not a science. Treat the meanings of planets, signs and houses as a starting point for thinking about yourself, not as a verdict.