Is a free astrology chart accurate? We checked ours against NASA

Put the same birth details into two free sites and sometimes you get two different rising signs. So which free astrology chart do you trust? And is a free one ever as good as a paid one?

We ran the test on our own free chart, because it’s the one we can open up completely. We checked every planet against NASA’s JPL Horizons system, then went looking for where charts actually go wrong. The planets turned out to be the easy part. The clock is where it falls apart.

Is a free astrology chart accurate?

Usually, yes, for the planets. Any calculator using a proper ephemeris will put the planets in the right place to well under a minute of arc. Price has nothing to do with it. What varies is how carefully a site handles your birth time: the time zone, summer time rules for your birth year, and the house system it draws with.

The test: one moment, checked against NASA

JPL Horizons is NASA’s own ephemeris service, used to plan spacecraft. We asked it for the apparent position of each planet along the zodiac at nine moments between 1931 and 2039. One of them is 14:30 UTC on 14 March 1990, the moment Robin was born in Chicago. Here’s our free chart beside NASA for that moment.

The biggest gap here is Venus at 6.0″. Across all nine moments and all ten bodies, the worst is 20.3″. A chart prints positions to the nearest minute of arc, which is 60″, so you’d never see any of these differences on the wheel. They run as automated tests every time our code changes, and the full method is on our accuracy page.

Most big astrology sites use the Swiss Ephemeris from Astrodienst, which is also excellent. Either way, if a site uses a real ephemeris, the planets are not where your chart goes wrong.

Where charts really go wrong: the clock

A chart needs your birth moment in universal time. To get there, a calculator has to know what the clocks were doing in your birthplace on your birthday. That’s harder than it sounds. Here are three real cases, cast with our engine.

Trap 1: the winters the UK didn’t go back

From 1968 to 1971 Britain kept its clocks an hour ahead all year, a trial called British Standard Time. A baby born at 9am on a January morning in 1970 was born at 8am GMT. Assume plain GMT and the chart is an hour out: Aquarius rising instead of Capricorn.

Trap 2: wartime double summer time

In the summers of the Second World War, UK clocks ran two hours ahead of GMT. Someone born at 9am on D-Day, 6 June 1944, has Cancer rising. A calculator that only knows about ordinary summer time gives them Leo. Same person, same minute, different chart.

Trap 3: morning and evening swapped

The simplest one. Robin was born at 8:30am. Typed as 8:30pm, the chart gives Libra rising and a Scorpio Moon instead of Taurus rising and a Libra Moon.

We handle the first two by reading the clock history for your birthplace from the IANA time zone database, the same one your phone uses. Lena’s 4:50am birth in Berlin in January 1963 becomes 03:50 UTC, because Berlin was an hour ahead of Greenwich that winter. But the database is honest about its limits:

Big cities are well documented. Small towns before 1970, less so. If you were born then, check the UTC time your chart says it used against what you know about local practice. We explain more about why birth time matters in our rising sign birth time study.

The honest disagreement: house systems

Here’s a difference that isn’t an error at all. There are several ways to divide a chart into twelve houses, and they put some planets in different houses. If one site uses Placidus and another Whole sign, they will disagree, and both are right on their own terms.

Compared with Placidus, Whole sign moves 2 of Robin's points, Equal moves 6, Porphyry moves 0. Equal houses shift the most here. They draw the 10th house exactly 90° from the ascendant, and in Robin’s chart that lands more than 20° away from the real Midheaven, so the top half of the chart slides over. Our guide to the twelve houses shows the cusps for every system on one chart.

A five-point check for any free chart

Use this on any free astrology chart, ours included. It takes two minutes.

1. Does it show the time zone offset it used? If it doesn’t, you can’t check the conversion. Ours prints the offset beside your birth time, just under the chart’s title.
2. Does it pick the time zone from the birthplace? You shouldn’t have to tick a summer-time box. That’s how the 1970 and 1944 traps happen.
3. Does it say which ephemeris it uses? Swiss Ephemeris, astronomy-engine or NASA data are all good signs.
4. Does it name the house system, and let you change it? Placidus by default is normal. Being able to switch is better.
5. Does it warn you when the time is unknown? A good chart drops the rising sign and houses rather than guessing, and flags a Moon that changed sign that day.

Our verdict

A free astrology chart can be every bit as accurate as a paid one. The astronomy is a solved problem, and the numbers above show it. What separates a good chart from a bad one is the boring part: getting your birth time into universal time correctly, and being open about the house system. Check those two things and you can trust the wheel.

If you’ve now got a chart you trust and want to know what it says, our step-by-step guide to reading a birth chart walks through one from start to finish.