Aspicio Ephemeris

Who is actually keeping track of the planets

NASA does, through its Jet Propulsion Laboratory. These are the figures used to fly spacecraft across the solar system and put them in orbit around other worlds, and JPL publishes the whole thing for anyone to use. It also runs a free public service that will tell you where any body in the solar system was at any moment you care to name.

Aspicio computes every position on this site from that file directly. Because JPL’s service answers the same questions independently, we can mark our own homework against it. That is what this page is.

What everyone else uses

Almost every astrology site and app you have used, astro.com and astro-seek among them, computes with one program: Swiss Ephemeris. It is careful, long-standing work by people who know exactly what they are doing, and for ordinary use it is more than good enough.

It is also a step removed from the source. Its files have to stay small enough to ship, so it stores a compressed version of NASA’s data rather than the data itself, and compression costs precision. That is the whole reason for the gap below. Not better astronomy on our side, just a shorter route: we read NASA’s file, and they read a copy that has been squeezed to fit.

Every check, marked

Ten bodies, every twenty-five years from 500 BC to 2600. For each one we asked NASA where the body was, then asked ourselves, then asked Swiss Ephemeris. Every square below is one of those questions, and the paler it is, the closer that answer sits to NASA’s.

Each figure is how far that answer sat from NASA’s, in millionths of an arcsecond. A chart prints positions to the nearest arcminute, and one arcminute is sixty million of these, so the whole table fits inside a rounding error of the finest thing a chart shows. Even at our worst we are 13 times nearer than Swiss Ephemeris is at its worst.

So does any of this change your chart?

No, and that is the point. Nothing on this page will ever move a placement you read. What it buys you is that the foundation under your chart is one you can check rather than one you have to take on trust, and that when Aspicio and another site disagree, you can find out which of them is wrong.

Going back further

For an ancient chart the planets are the easy part. The hard part is the Earth itself. It spins very slightly unevenly, so the further back you go, the less certain anyone is about what time it actually was when something happened. That can only be pieced together from the eclipses ancient observers wrote down.

We use the most recent reconstruction of it, published in 2021. Swiss Ephemeris still uses the one before, from 2016, and around the year 1000 the two disagree by about three minutes. Even on the newer one, a chart for Caesar’s era has a Moon good to roughly 2.5 arcminutes, and no ephemeris ever written can do better than that. We would rather tell you so than print a figure we cannot stand behind.