Shanghai astrocartography

What Shanghai’s position does to a reading

Astrocartography is geometry before it is anything else, so where a city sits changes how it behaves. These are facts about Shanghai rather than about you — they hold whoever is reading.

How close is close, here

Distances here are measured as real ground: one degree of orb is 111 km, the same width everywhere on earth. That matters at Shanghai, because a degree of longitude here is only about 95 km — 14% narrower. Older software that measures overhead lines in longitude will therefore report a wider figure than we do for the same city. Neither is wrong; ours describes a distance you could actually walk.

Every line can reach this latitude

A rising or setting line is a horizon curve, and it exists only where the body actually crosses the horizon. At 31.23°N that means a body within 58.8° of the celestial equator. All ten bodies stay inside that, so every one of the forty lines can pass through Shanghai’s latitude. Nothing here is ruled out by geometry — only by your own chart.

The same geometry, in daylight

The equation behind a rising line is the equation behind sunrise, so you can watch it work. At the summer solstice Shanghai gets about 14 hours 2 minutes of daylight, and at the winter solstice about 9 hours 58 minutes. The further a place sits from the equator, the steeper that swing — and the more slanted its horizon lines run.

The clock against the sun

Overhead and underfoot lines are meridians — they depend on longitude, not on what a clock happens to show. Shanghai sits almost exactly on its own zone’s meridian, so midday on the clock and midday by the sun agree to within a few minutes. Any planet standing overhead here has its opposite line running down the far meridian at 58.5°W, half a world away.