How it works
How astrocartography works
At the moment you were born, every planet was rising somewhere on Earth, overhead somewhere else, and setting somewhere else again. Astrocartography draws those places as lines on a world map. Live near a line and that planet's themes get louder in your life.
1 · The big picture
Who you are, when, and where
Iris reads you in four layers. Each one works on a different timescale, and every screen in the app follows the same order.
2 · Where a line comes from
Four points in the sky
Stand anywhere on Earth and the sky has four special points. A planet sitting on one of them is at its most powerful for that spot. Every line on the map is a planet sitting on one of these four points.
AC · Ascendant
Identity
Rising. First impressions, vitality, and how you show up somewhere new.
DC · Descendant
Relationships
Setting. Partners, the people you attract, and what others see in you.
MC · Midheaven
Career
Overhead. Reputation, status, ambition and your public life.
IC · Imum Coeli
Home
Underfoot. Private life, roots, family and emotional foundations.
3 · From the sky to the map
One planet, four lines
Mark every place on Earth where Venus was exactly overhead at your birth and you get a straight north–south line: the Venus MC line. Directly opposite, on the other side of the globe, is the IC line. The rising and setting lines curve, because how early a planet rises depends on how far north or south you are.
Lisbon
- Venus MC5 mi · 99%
Example birth chart, not yours. Tap a city to see which lines run within 500 miles of it and how strong each one is there. Solid lines are AC and MC; dashed lines are DC and IC.
4 · Distance
A line is a band, 500 miles each side
You don't have to stand on a line to feel it. Its influence is full strength right on the line and fades evenly to nothing at 500 miles (about 805 km) on either side.
5 · Reading a line
The angle says where, the planet says what
Ten planets plus Chiron, each with four lines, makes 44 lines. Read a line by joining its two halves: the area of life from the angle, and the energy from the planet.
Sun MC
Career recognition and public success.
Moon IC
A deep sense of belonging and ancestral connection.
Saturn MC
Heavy responsibility, authority and long-term building.
Mars AC
High physical drive and fast living, or high friction.
6 · Asking about your life
Seven focus areas, each fed by its own lines
When you ask Iris for the best places for love or career, it only looks at the lines that matter for that question. The first four areas also count every line on their angle, whatever the planet.
7 · Why your map is yours
The same line feels different for different people
Two people can both have a Venus DC line through Paris and have very different experiences there. Iris judges each planet by how it sits in your birth chart: whether you were born by day or night (sect), how comfortable the planet is in its sign (dignity), and which houses it rules.
Person A
Born at night, when Venus is the chart's helpful planet. Venus in Taurus, its own sign.
Venus DC in Paris: easy, supportive
Person B
Born in the daytime, so Venus is out of sect. Venus in Virgo, its fall.
Venus DC in Paris: still about love, with more effort
Uranus, Neptune, Pluto and Chiron lines show their meanings but aren't scored helpful or difficult yet, because the Hellenistic rules don't cover them.
8 · When
Transits switch places on and off
Your lines stay where they are, but planets moving through the sky today pass over the points that made them. While they do, a place gets busier. Iris shows these windows with exact dates.
9 · The math
How each line is calculated
Every line on your map comes from spherical astronomy, not a lookup table. These are the same equations the Iris engine runs on your birth moment.
- 1
Find where each planet really is
From the birth date, time and place, Swiss Ephemeris gives each body's position on the sky as right ascension α (east–west, like longitude on the sky) and declination δ (north–south, like latitude on the sky). The lines use these, not the zodiac degree. It also gives Greenwich sidereal time θ₀: how far the sky had turned over Greenwich at that moment.
birth moment → α, δ for each body, and θ₀ - 2
How far a planet is from overhead
At longitude λ the sky overhead is turned by the local sidereal time. The planet's hour angle H says how far it has moved past overhead: 0° means overhead, 180° means underfoot.
LST = θ₀ + λ · H = LST − α - 3
MC and IC lines: straight meridians
The planet is overhead where H = 0, which happens along one line of longitude. Latitude doesn't enter the equation, so the MC line runs straight from pole to pole. The IC line is the same meridian on the far side of the globe.
λMC = α − θ₀ · λIC = λMC + 180° - 4
AC and DC lines: on the horizon
The planet is on the horizon when its altitude is zero. Solving the altitude equation gives the hour angle H₀ at which it rises (−H₀) and sets (+H₀). H₀ depends on latitude φ, which is why these lines curve.
sin(alt) = sin φ sin δ + cos φ cos δ cos H = 0
⇒ cos H₀ = −tan φ · tan δ
λAC = λMC − H₀ · λDC = λMC + H₀ - 5
Where the curves stop
Far enough north or south, a planet never rises or never sets that day, so there is no horizon crossing. That happens when |tan φ · tan δ| > 1, which for a planet at declination δ is beyond latitude 90° − |δ|. At that edge the AC and DC curves join: on the side where the planet never sets they meet the IC line, where it just grazes the horizon underfoot, and on the side where it never rises they meet the MC line.
no AC or DC line where |tan φ · tan δ| > 1 - 6
How strong a line is at a place
Iris measures the great-circle distance d from the place to the nearest point on the line. For a meridian line that has a closed form. Strength then fades evenly over the fixed 500-mile orb.
dmeridian = R · asin(|sin Δλ| · cos φ), R = 3,959 mi
strength = max(0, 1 − d / 500 mi)
Worked example: the Venus AC line at Lisbon's latitude
- δ
- +18°
- φ
- 38.7°N
- λ_MC
- 9.0°W
- cos H₀
- −tan 38.7° × tan 18° = -0.260
- H₀
- 105.1°
- λ_AC
- 9.0°W − 105.1° = 114.1°W
- λ_DC
- 9.0°W + 105.1° = 96.1°E
So in this example chart, Venus was rising over the western United States at Lisbon's latitude while it stood overhead in Lisbon. Repeat this every half degree of latitude and you have the whole curve.