What is an ephemeris? How planet positions are found, step by step

Every birth chart starts with an ephemeris, yet most people have never seen one. It is the part that says where the planets were. This guide explains what an ephemeris is and how it finds a planet. Then we work out Robin’s Sun and Moon from scratch, one step at a time, so you can see exactly what sits behind your own chart.

What is an ephemeris in astrology?

An ephemeris is a table or a program that gives the position of the Sun, Moon and planets for any time you choose. The word comes from the Greek for “diary”. The plural is ephemerides. Printed ones list a position for each day. Software ones work out the position for the exact minute you ask about.

Astronomers use ephemerides to aim telescopes and plan space missions. Astrologers use the same numbers and then add meaning on top. The astronomy is not in dispute. A planet is at a particular spot in the sky or it isn’t. What that spot means for a person is a different question, and we cover it in our guide to reading a birth chart.

Here is what comes out. For each body you get an ecliptic longitude, which is an angle from 0° to 360° measured round the zodiac. Divide by 30 and you have the sign. The remainder is the degree inside it. That is all a placement is: an angle, cut into twelve slices.

How are planetary positions calculated?

Nobody looks up at the sky. A program works the positions out from the laws of motion, then adjusts them to match what an observer on Earth would see. The steps, in plain order:

1. Fix the moment. Time must be on a single clock. Astronomers use UTC, so a birth time in Chicago or Lagos has to be converted first.
2. Place the planet and the Earth. Each orbit is described by a long list of repeating terms, a bit like adding many waves together. Planet series of this kind are called VSOP87. The Moon needs a much longer list, because the Sun keeps tugging at it.
3. Point from Earth to the planet. Subtract one position from the other and you have a direction.
4. Correct what we would see. Light takes minutes to arrive from a planet, so we see where it was a little earlier. Earth’s own motion also bends the apparent direction slightly (this is called aberration).
5. Turn it into the zodiac’s frame. The tropical zodiac is tied to the equinox of the date, so the position is measured along the ecliptic as it stands at that moment.

Our accuracy page describes the same steps for Charthood: the apparent position of each planet on the ecliptic of date, corrected for light travel time and aberration, which is the frame astrologers use.

Worked example: Robin’s Sun and Moon

Robin is one of our fictional example people. Robin was born at 8:30am on 14 March 1990 in Chicago. Here is every step the engine takes, with the Sun also done by hand.

Step 1: Local time to UTC

On that date Chicago was 6 hours behind UTC, because summer time had not started. So 8:30am becomes 14:30 UTC on 14 March 1990. The engine reads that offset from the IANA time zone database for the birthplace. Nothing else in the chart can be right if this step is wrong.

Step 2: The Sun, by hand

The Sun is the easy one, because it can be done with a short formula from Jean Meeus’s Astronomical Algorithms, which many astronomers keep on the desk. It is the “low accuracy” version, good to about a hundredth of a degree. We ran it for the birth moment:

So by hand the Sun sits at 353.7525°. Eleven whole signs is 330°, which takes us to the end of Aquarius, and the rest is 23.75° into the twelfth sign. That puts the Sun at 23°45′ Pisces. Minutes are cut off rather than rounded, as printed ephemerides do.

Step 3: The Sun and Moon, by engine

The Moon is not a job for pencil and paper. Its motion needs dozens of terms, so we let the engine do it. Here are both bodies side by side with NASA’s figures for the same moment.

The hand sum is 19″ from the engine and 19″ from NASA. That is a fraction of a minute of arc, so the sign and the degree match, though the printed minute can be out by one. The engine is 0.3″ from NASA for the Sun and 0.7″ for the Moon. The Moon moves so fast that its 0.7″ is the same as about 1.4 seconds of clock time.

Step 4: Longitude to sign

Now cut the angle into slices. The Sun’s 353.7473° divided by 30 is 11.792, so it has passed 11 whole signs and is 23.75° into the next. That is 23°44′ Pisces. The Moon’s 209.0439° gives 6 whole signs and 29.04° more: 29°02′ Libra.

Robin’s Pisces Sun, by contrast, sits nearly 24° into its sign and is safe from edge trouble. The wheel on the site draws both placements from these same numbers.

Swiss Ephemeris, JPL Horizons and astronomy-engine

Three names come up whenever people argue about accuracy. They are not rivals. They do different jobs, and Charthood uses two of them in different ways.

That quote matters because a chart prints positions to the nearest arcminute. An error under one arcminute never changes a printed degree by more than a step. Our own checks are much tighter than that claim, as the table shows.

Why do two ephemerides give different numbers?

When two sites show different charts for the same person, the planets are rarely the cause. Good ephemerides agree to a few arcseconds. The usual causes are further up the chain:

• A different clock. One site handled the time zone or summer time rule for your birth year and the other didn’t. An hour’s error moves the Moon by half a degree and the rising sign by about 15°.
• A different house system. Placidus and Whole sign put planets in different houses. Our accuracy review against NASA shows the same chart in four systems.
• A different node. Some sites show the mean lunar node, others the true one. They can differ by more than a degree.
• Rounding. One tool rounds 29°02′ to 29°03′ while another cuts it. That is a display choice, not an astronomy gap.

Common mistakes

• Reading a printed table as if it were your birth minute. Many printed ephemerides give one position a day, at a fixed time such as midnight UTC. For a fast body like the Moon you must adjust for your birth time.
• Mixing up local time and UTC. Robin’s 8:30am is not 8:30am UTC. Always say which clock you mean.
• Using the wrong zodiac. The tropical zodiac, which Western astrology uses, is tied to the equinox. The sidereal zodiac is tied to the stars and sits more than 20 degrees away. Check which one a tool uses.
• Expecting the sky to settle the meaning. An ephemeris tells you where Mars was. It cannot tell you what Mars in a sign says about a person. That part is interpretation, and a matter for reflection rather than proof.

What to do with this

You now know what happens in the first second after you press “make my chart”: the clock is converted, an ephemeris is read, and an angle becomes a sign. Charthood does that for you, and prints the UTC moment it used so you can check it. If you want to see how far the numbers can be pushed before they break, read the edge-case stress test linked above. If you just want your own placements, try the birth chart calculator and then read it in seven steps.