Birth chart calculator: the sky you arrived under

Each detail you enter does different work. The date carries most of the planets: the Sun moves about a degree a day, and Jupiter and everything beyond it move a fraction of that. The time matters most for the Moon, which covers about 13° a day, and for the rising sign and the houses, which turn through the whole zodiac in a day. The place sets the local clock, and its latitude and longitude set the rising sign and the houses.

This chart uses the tropical zodiac, the usual one in Western astrology, where 0° Aries is the point the Sun reaches at the March equinox and each sign is a 30° slice counted from there. Precession has moved those slices nearly a full sign away from the constellations that share their names, so a sign here is a stretch of the zodiac and not a group of stars.

You get ten bodies and two angles. Asteroids, Chiron and the lunar nodes are left out.

Read it from the rim inward. The outer band is the zodiac, twelve signs marked with degree ticks. Just inside it sit the planets, each with a small tick at its exact degree. Further in come the house lines and their numerals, and the open centre holds the aspect lines.

The zodiac runs counter-clockwise, and the wheel is turned so the rising degree sits at the left, on the line marked ASC. That line is the horizon: DSC at the right is the western end, planets above it were in the visible sky, and planets below it were under the ground. The other axis, from MC to IC, marks the meridian.

Most chart reading comes down to a three-part phrase. The planet names a theme, the sign a manner, the house an area of life. Take Mars in Gemini in the 7th house. Mars is where heat and push come up. Gemini is curious, talkative, inclined to hold two views at once. The 7th is the people you pair with, face to face. Put together, that makes a question more than a statement: where might you notice your push coming out as quick talk and argument with the person across the table?

Two small marks are worth knowing. The degree says how far through a sign a planet has travelled, from 0° to just under 30°. Retrograde means the planet appeared to move backward against the stars as seen from Earth at that time, an effect of the viewing angle as Earth and the planet pass each other. The Sun and the Moon never do it.

Signs divide the zodiac. Houses divide the local sky at the moment of birth, which is why they need a time and a place. In Placidus, the system used here, the horizon and the meridian give the cusps of the 1st, 4th, 7th and 10th houses. The cusps between them come from dividing time: how long each degree takes to travel between the horizon and the meridian, cut into thirds. So Placidus houses come out in unequal sizes, more so the farther a birthplace lies from the equator. The system is named for Placidus de Titis, a seventeenth-century Italian monk.

Whole Sign, an older method found in Hellenistic astrology, makes the entire rising sign the 1st house, the next sign the 2nd, and so on round. Astronomy cannot settle which is right, since both are conventions about how to slice the sky. Because the cusps differ, a planet can change house between systems while its sign stays put.

An aspect is an angle between two points on the wheel, measured along the zodiac. Five are drawn here: the conjunction at 0°, sextile at 60°, square at 90°, trine at 120° and opposition at 180°. An angle counts if it lands within an allowance called the orb, which here is 8° for conjunctions and oppositions, 6° for squares and trines, and 4° for sextiles. Orbs are a convention, so another site can list a few more or a few fewer aspects from identical positions.

The geometry has its own logic. Signs a trine apart share an element. Signs a square apart share a modality, cardinal, fixed or mutable, but differ in element. Readers tend to take squares as friction and trines as ease, and the tighter the orb, the more weight most of them give it. On the wheel, line pattern and weight show the type, and the rising degree and the Midheaven count as points too.

The rising sign, or Ascendant, is the sign on the eastern horizon at birth. The whole zodiac passes in about 24 hours, so a sign takes two hours on average, and the average hides a lot. In London, Aries and Pisces cross in under an hour while Leo through Scorpio take nearly three.

If another site gives you a different rising sign, check these in order. The time comes first: the Ascendant moves about a degree every four minutes on average, so a rounded time near a boundary can tip it. Then the clock history: a chart needs the offset in force on that day in that place, daylight time included, and China, for one, ran daylight time from 1986 to 1991. This form shows the offset it used, so you can compare. Then the place: a degree of longitude is worth four minutes of sky.

Then the zodiac. Sidereal charts, the norm in Vedic astrology, sit about 24° behind tropical ones, which often means a different sign altogether. House systems get blamed for this more than they deserve. Placidus, Koch, Equal and Whole Sign share one Ascendant; the cusps differ, and with them the houses your planets fall in.

The farther from the equator, the more uneven the rising gets. In Reykjavík, at 64°N, Aries crosses the horizon in about 13 minutes and Leo takes about three and a half hours.

Placidus stops working at about 66.56°, north or south, the polar circles. It depends on dividing the time a degree spends above the horizon, and inside those circles some degrees neither rise nor set on a given day, which leaves no time to divide. For a birth there, in Tromsø at 69.6°N for example, this chart switches to Whole Sign houses and says so on screen. Charts drawn inside those circles are less stable in general: a small change in the time can move the angles a long way.

The calculation runs entirely in your browser. The page loads Astronomy Engine, an open-source astronomy library, converts your local time using the time-zone history of the place, and works out each position there.

Your date, time and chosen city stay in your browser and never go into a URL, unless you choose to save the chart to your account: a save stores the date, time and place there, so the chart opens again. Using the chart does cause three kinds of request, and none of them carries those details. The browser loads the country's public list of cities, the same file for everyone, so that request names the country; the search then runs on that list on your device. It loads the card images only when you open a placement, so those requests name a card. And if you go on to a reading with the default, "Answer with this card", the reading link carries the card of your sun sign.

We check the arithmetic against Swiss Ephemeris, run in its built-in Moshier mode. The test suite holds 45 charts from 1900 to 2026, at latitudes from 60°S to 66°N. Every body lands within 0.01° of the reference, about what the Moon covers in a minute, and the Ascendant, the Midheaven and all twelve Placidus cusps land within 0.001°. The fixture also includes charts inside the polar circles, checked against Whole Sign houses.

Each placement carries three cards, read from the table of correspondences of the Golden Dawn, the London order founded in 1888 whose attributions later tarot writers widely took up.

The first is the planet's card. The Sun is The Sun, the Moon is The High Priestess, Mercury is The Magician, Venus is The Empress, Mars is The Tower, Jupiter is the Wheel of Fortune and Saturn is The World.

The second is the sign's card: Aries The Emperor, Taurus The Hierophant, Gemini The Lovers, Cancer The Chariot, Leo Strength, Virgo The Hermit, Libra Justice, Scorpio Death, Sagittarius Temperance, Capricorn The Devil, Aquarius The Star, Pisces The Moon. That last one surprises people, since the Moon in the sky takes a different card.

The third is the decan's card. Each sign splits into three decans of 10°, and the Golden Dawn gave them the 36 numbered minors from two to ten. The suit follows the element: Wands for fire, Cups for water, Swords for air, Pentacles for earth. The number follows the modality: cardinal signs hold 2 to 4, fixed signs 5 to 7, mutable signs 8 to 10. So Venus at 14° Taurus reads The Empress, The Hierophant and the Six of Pentacles, the middle decan of a fixed earth sign.

Uranus, Neptune and Pluto work differently. The Golden Dawn's table gives them no card, and it assigns the three trumps now linked to them to elements instead: The Fool to air, The Hanged Man to water, Judgement to fire. Pairing Uranus with The Fool, Neptune with The Hanged Man and Pluto with Judgement is a modern attribution, and the panel labels it modern.

These are pictures to think with. Death for Scorpio or The Tower for Mars names a correspondence, a way into that sign's or planet's imagery, and says nothing about any event.