Choosing an astrocartography calculator

The astronomy is rarely the problem

Every serious calculator, online or on the desktop, works from NASA/JPL's planetary ephemerides, usually through the Swiss Ephemeris. Astrodienst's AstroClick Travel on astro.com, one of the oldest online, draws the Sun, Moon, planets and lunar nodes and explains a line or a crossing when you click it. Between tools like these the planetary positions agree to well under an arcsecond, and an arcsecond moves a line about thirty metres. One minute of birth time moves it about 28 km. So when two maps disagree, look at the birth data and the settings before the astronomy.

Time zones and old clocks

The commonest error on any map is an hour of daylight saving applied, or not applied, to an old birth. Rules before 1970 are patchy, wartime clocks ran an hour or two ahead in much of Europe and North America, and small towns didn't always follow their capital. A good calculator shows the offset from universal time it used and lets you change it. If it doesn't show it, you can't check it.

Zenith Map: local time goes through the IANA time-zone database, wartime and pre-1970 rules included, the offset it used is shown with every chart, and you can override it.

An uncertain birth time

Few people know their birth time to the minute, and a time rounded to the quarter hour can move a line about 200 km either way. A calculator that draws one crisp line from a guessed time looks more certain than the record behind it. Look for a way to say how sure you are, and for the map to show the doubt.

Zenith Map: you say whether the time is exact, within 15 minutes, within an hour or unknown, and the globe draws bands where a line could be. An unknown time is cast for noon and marked unreliable. What a doubtful time does to a map.

How the lines are drawn

There are two ways to draw a line. In mundo, the standard set by Jim Lewis, puts it where the planet was physically on the meridian or the horizon. The zodiacal method uses the planet's degree of the zodiac instead. For the Sun the two agree; for the Moon, Pluto and Chiron, which sit well off the ecliptic, they can differ by degrees. Neither is wrong, but a calculator should say which it uses, and two maps drawn by different methods won't match.

Zenith Map: in mundo by default, with the zodiacal method as an option, and the Moon's parallax available in expert mode. The methodology lists every choice.

Crossings and parans

Where two lines cross, the latitude through the crossing is a paran, and astrocartography reads the pair along all of it. A map that only draws lines leaves those latitudes out, and they are some of the most useful information on it: a paran's latitude barely depends on the birth time, so it survives a doubtful time better than any line.

Zenith Map: crossings show wherever two drawn lines meet; with Pro, parans are drawn as bands of latitude and count in the city scores. What each pair means.

Distance and ranking

A line on a map is a band, not a wall, and a tool that ranks cities has to decide how fast a line fades with distance. Ask whether it says. Look for the orb, or the curve, written down, and for a ranking that shows its working rather than a bare score.

Zenith Map: a line counts in full on it, about 60% at 250 km and not at all past 1,000 km, and every city score lists the lines and crossings behind it.

The readings

Most calculators attach a paragraph to each line. Read a few. Good ones say what the line tends to do, what it costs and what else to check, such as the planet's condition in your natal chart; weak ones promise love on every Venus line and doom on every Saturn one.

Zenith Map: one written reading per planet and angle in a working astrocartographer's voice, with its version printed under it, plus a page for every line.

Your birth details

A birth date, time and place identify a person well, and people enter their partner's, their children's and their parents'. Check whether a tool needs an account, whether it keeps the charts on its servers, and how you delete them.

Zenith Map: the chart is calculated in your browser, and nothing about your birth leaves it unless you save a chart to an account. The privacy policy says what is kept and for how long.

What's free

Some tools draw every planet for free and charge for readings; others give a few planets and sell the rest. Neither is a trick, but know which you're looking at before you judge a map by what it leaves out.

Zenith Map: the map is free with the Sun, Moon, Venus and Jupiter lines, readings for any place, the overall heatmap and your top three cities. Pro adds all 21 bodies, the lenses, parans, Local Space for every body and a partner's lines.

A test you can run on any calculator

Enter the same birth in two tools and compare the Sun's MC line, which every tool draws the same way. Good calculators put it within a small fraction of a degree of each other. A gap of about 15° means one of them applied an hour of time zone or daylight saving the other didn't, and 7.5° means half an hour. Check the offset each one used before trusting either.

Then compare the Moon. If its MC line differs while the Sun's agrees, the tools draw by different methods; if only its rising and setting lines differ, by up to a degree, one of them corrects for the Moon's parallax.

Questions