Birth Chart Calculator

What do you need for a birth chart?

A birth chart needs a date, a clock time and a place. The time to use is the local time on the birth record, on a 12-hour or 24-hour clock. The place takes two steps: the country, then a city or town from a list of more than 230,000 places. A small town missing from the list can go in as coordinates, latitude first.

Why do the birthplace and the exact time change the chart?

Place and time set the Ascendant, the degree of the zodiac rising in the east at birth, and each birthplace has its own horizon. The zodiac turns past it once a day, so the Ascendant moves about one degree every four minutes on average. A clock off by 15 minutes can move it about 4 degrees, enough to change the rising sign or push a planet into the next house.

The Ascendant's rate is uneven. At the latitude of New York, Aries rises in a little over an hour and Libra in about two and a half, so the same 15 minutes costs more in some signs than in others.

The place does not move the planets. Positions here are geocentric, measured from the center of Earth, so two people born at the same instant in Chicago and Cairo share their Sun, Moon and planets degree for degree. Their Ascendants and houses differ, and so do their Midheavens, where the zodiac crosses the north-south line overhead, because all of these are measured from the birthplace. One degree of longitude east or west shifts them as much as four minutes on the clock.

How are time zones and daylight saving handled?

The calculator reads the offset from UTC, the world's reference clock, in the time zone records of the place, daylight saving time included. A birth record gives local time, and the chart needs UTC. An hour of error moves the Ascendant about 15 degrees.

Before anything is calculated, the form prints the conversion to UTC. A birth in Sydney at 10:00 AM on March 21, 1955, gets the line "10:00 AM local, Australia/Sydney, UTC+10 = 00:00 UTC" above the button. A chart is only as good as that line. If you know a different UTC offset was in force that day, type it in by hand and it replaces the time zone.

Clock rules before 1970 are only partly recorded for many places, and before 1967 towns in the US and Canada often set their own daylight saving time. In both cases the form warns that the offset may be off and leaves room to type another. Before 1883, when most places kept their own time by the Sun, the offset comes from the longitude of the birthplace, unless the records hold a standard time, as for Britain. A time the clocks skipped or showed twice, such as 1:30 AM on the night they go back, stops the form until one reading is chosen.

What is a house system, and why do they differ?

Houses divide the sky around the birthplace into twelve sections, and a house system is the rule for drawing the lines. Changing the rule moves no planet through the zodiac. It changes which house the planet falls in. A Sydney birth at 10:00 AM on March 21, 1955, has the Moon at 18°58′ Aquarius, in the 9th house under Placidus and the 10th under Whole Sign.

The systems disagree because they divide different things. Whole Sign, Equal and Porphyry divide the zodiac itself. Whole Sign makes each sign one house, starting with the sign of the Ascendant, Equal counts 30 degrees at a time from its exact degree, and Porphyry cuts each quarter between the angles into three. Placidus, Koch and Alcabitius divide time, following how long a degree takes to rise and cross the sky. Regiomontanus and Campanus divide circles in space, the celestial equator and the prime vertical. Under Whole Sign and Equal, the Midheaven can land outside the 10th house.

Placidus is the default here, the usual choice in English-language astrology. Inside the polar circles, where some degrees of the zodiac do not rise at all, this calculator does not draw Placidus, Koch, Regiomontanus or Campanus houses. It draws Porphyry and says so under the wheel.

What changes if the birth time is unknown?

Without a birth time, the calculator leaves out the Ascendant, the Midheaven and the houses, because all three need the clock. It places the planets at noon local time on the birth date.

A noon chart puts the Moon in the wrong sign for about 1 birth in 9. The Moon moves about 13 degrees a day, so a noon position can be more than 6 degrees off. When the Moon stays in one sign all day, the result gives that sign and its range of degrees. When the sign changes, the result names both signs and the time of the change. Aspects to the Moon are left out either way.

The other planets move far more slowly, so their signs usually hold. When one changes sign on that date the result says so, with the time to the minute for the Sun and to about a quarter of an hour for Mercury, Venus and Mars. The slower planets get the date alone. A rough time such as "mid-morning" cannot pin down an Ascendant, but it can often settle a Moon sign: compare it with the time of the change.

What does the result show?

The result opens with the Sun, Moon and Rising signs, the wheel and a short reading of each sign. Then comes a table of 16 points: the Sun through Pluto, Chiron, both lunar nodes, Black Moon Lilith, the Ascendant and the Midheaven, each with its sign, degree, direction and house. Below that are the house cusps, every aspect with its orb, closest first, and the readings of the planets and Chiron. The lunar nodes are the two points where the Moon's orbit crosses the Sun's yearly path. Black Moon Lilith is the point of that orbit farthest from Earth. These are mean positions, which smooth out the orbit's wobble, and a switch shows the true nodes. The printout repeats the birth data and the conversion, so a chart on paper can be checked later.

The positions are astronomy. The short readings beside them come from astrological tradition, which this site describes as tendencies, and they are meant for self-reflection and entertainment.

Is the birth data sent to a server?

No. The calculation runs in this browser, and the date, time and city or coordinates are not sent to a server. Choosing a country downloads that country's place list, so the host can see which country was requested. A browser's network panel can confirm that no request carries what was typed. Inputs stay in the tab so a reload keeps them. They are normally cleared when the tab closes, and Start over clears them at once. AstroJane sets no cookie, and the address of the page stays the same.