What is a birth chart? How the chart is calculated

The three inputs, and what each one does

This is why a chart without a birth time is a partial chart instead of a wrong one. The planetary positions survive; the houses and the rising sign don't. There are straight ways to work around it, covered in finding your rising sign without a birth time.

What the wheel is showing

The circle is the ecliptic, the path the Sun appears to trace through the year, which is really the plane of Earth's orbit seen from the inside. Everything in a chart is placed on that circle.

The wheel is then cut two ways at once, and confusing the two cuts is the most common beginner error.

Signs divide the circle into twelve 30-degree segments measured from the spring equinox. They are fixed, shared by everyone born that day, and they say *what quality* something has.

Houses divide the circle into twelve segments measured from your local horizon at your birth minute. They're personal, they rotate through a full cycle every day, and they say *what area of life* something happens in.

So a planet is always in both: a sign and a house. Mars in Aries in the seventh house is Mars behaving in an Aries way, in the area of partnership.

My rule: if a reading never mentions the birth time, walk away. Half the wheel depends on it.

The four angles

Two lines cross the wheel and generate four points that most traditions treat as the most sensitive in a chart.

The Ascendant or rising sign is the point of the zodiac coming up over the eastern horizon at your birth minute, and it begins the first house. The Descendant opposite it begins the seventh. The Midheaven, from the Latin *medium coeli*, is the highest point the ecliptic reaches (roughly overhead) and usually begins the tenth house, with the Imum Coeli opposite beginning the fourth.

These four are entirely dependent on birth time and place, which is the practical reason a rounded or remembered time weakens a chart more than people expect. An hour of error can move the Ascendant into a different sign and rotate every house with it.

How the positions are computed

This part is ordinary astronomy, the half of the practice that's precise.

Your birth date, time and place are converted to Universal Time, which means handling the time zone that applied at that location on that date, including the daylight-saving rules of the year in question, which have changed more often than people assume and are a common source of wrong charts.

Planetary positions are then looked up in an ephemeris, a table of where each body was at a given instant. Modern software uses numerically integrated data derived from the same JPL solar-system models that navigate spacecraft. Positions are given in ecliptic longitude, from 0 to 360 degrees around the circle, then expressed as a sign plus degrees within it.

Finally the houses are calculated from your local sidereal time and latitude, using whichever house system has been chosen: a choice that changes the chart, and one nobody agrees on.

What is measurement and what is tradition

Everything in the left column can be independently verified and will match across any competent software. Nothing in the right column has been demonstrated to predict anything, and where studies have tested chart interpretation against outcomes they haven't found an effect.

That isn't an argument for ignoring the right column. It's an argument for holding it as a language for reflection rather than as a set of findings, which is the position taken throughout this site.

Reading one line of a chart

A chart listing gives each body on a line, and a line like Mars 17°42' Aries, 7th house, retrograde contains four separate facts.

Seventeen degrees and forty-two minutes of Aries is the ecliptic longitude, expressed as a sign plus a position within it: 17.7 degrees past the start of Aries, or 17.7 degrees round the circle from the spring equinox, since Aries is the first sign. Seventh house is where that longitude fell relative to your horizon, which depends on your time, place and house system. Retrograde means Mars appeared to be moving backwards through the zodiac on that date, as explained in what retrograde means. The minutes matter for aspects, since an orb of one degree is sixty of them, and they are the reason a chart from a rounded time is different from a chart from a recorded one.

Time zones: the most common way a chart is wrong

The planets are never the problem. The conversion from local clock time to Universal Time is, because the rules that governed the clock at your birthplace on your birth date may not be the rules that apply now.

Britain ran double summer time during the Second World War, two hours ahead of Greenwich in summer and one ahead all winter. India used Calcutta and Bombay time before adopting a single standard, and cities kept local time later than the law suggests. Singapore changed its offset several times in the twentieth century and did not settle on UTC+8 until 1982. Parts of the United States observed daylight saving locally before it was federal, and Indiana did its own thing for decades. A chart program with a good historical time-zone table handles all of this. A form that asks you for 'your time zone' and applies today's rule to 1974 does not, and the error is a full hour, which moves the Ascendant by up to a sign.

The other silent choice: tropical or sidereal

Western charts measure the signs from the spring equinox. Vedic charts measure them from a fixed point among the stars. Because the equinox drifts slowly against the stars, the two zodiacs are now about 24 degrees apart, and the gap is called the ayanamsa. The consequence is that most people's Sun, Moon and Ascendant are one sign earlier in a Vedic chart than in a Western one.

Neither is wrong. They are measuring from different zero points, as explained in sidereal and Western astrology. What is wrong is a chart that does not say which it used, or a reading that mixes the two. AsterNym's Western chart is tropical and its Vedic chart is sidereal with the Lahiri ayanamsa, and both pages say so.

Words you will meet on the wheel

Reading one without drowning

A full chart has ten or more bodies, twelve houses, twelve signs and dozens of angular relationships, which is more than anyone can hold at once. The traditional order of approach exists for a reason.

Start with the Big Three (Sun, Moon and rising) which is most of what a first reading is anyway. Then note which houses are occupied and ignore the empty ones for now. Then look for the tightest aspects, because closeness is astrologers' own measure of importance. Then stop.

Almost everything a beginner needs is in that sequence, and almost everything that makes charts feel impenetrable is the material after it.