What local space is
At the moment you were born, every planet stood somewhere on the circle of the horizon around you: high overhead, low in the east, or below your feet on the far side of the Earth. Its direction, measured in degrees clockwise from north, is its azimuth. Local space turns each azimuth into a line on the map: a great circle that leaves your birthplace in the planet's direction and runs all the way around the world. The astrologer Michael Erlewine developed the idea in the early 1970s, and his first article on it appeared in 1977; Steve Cozzi's Planets in Locality became its standard book.
Local space or astrocartography?
Both put your chart on the map, in different ways. Astrocartography draws where on Earth each planet was rising, setting or overhead at your birth, so its lines cross the whole globe wherever you were born. Local space lines all start from one place and fan out like the spokes of a wheel, so they describe directions from that place: a trip, a move, a view from a window. Many astrologers use both: astrocartography to choose a city, local space to read the directions around it. You can see both at once: on the astrocartography map, open the Lines tab and switch on local space lines, and each planet's direction is drawn from your birthplace all the way round the Earth, over its astrocartography lines.
Reading the lines
Start with the planet: travel or a move in a planet's direction is read as bringing its themes, Venus for love and pleasure, Jupiter for growth and luck, Saturn for work and hard lessons. Then the places: the calculator lists the big cities within 60 km of each line, in the planet's direction and the other way, since a line runs both ways around the Earth. Some astrologers also read the lines inside a home, as a compass for rooms: Venus's direction for rest, Mercury's for a desk.
Relocated local space
The directions depend on where you stand. Seen from another city at the same moment, the planets sit at different azimuths, so astrologers draw a new compass for each home. Add where you live now to the calculator and it draws your relocated local space: the sky of your birth, seen from your door.
How it is worked out
The planets come from the site's own ephemeris, the same one behind every chart here. For each one, the calculator turns its right ascension and declination at your birth into an azimuth and an altitude for the place, a result that matches the independent Astronomy Engine library's horizon calculation to well under a millionth of a degree. It then follows each great circle and lists the big cities (100,000 people or more, and capitals) that lie within 60 km of it and more than 150 km from the start. The directions are geocentric, as local space charts usually are, so the Moon's can differ by up to a degree from the Moon you would have seen. The sky turns a degree every four minutes, so an exact birth time matters. Your details stay in your browser.