How Yomi calculates your chart

The Yomi Ephemeris

An ephemeris is a table of where the planets actually are at any moment. Astronomers have kept them for centuries; every serious observatory runs on one. Yomi's is called the Yomi Ephemeris, and it starts with the best source there is: NASA JPL Horizons, the planetary position system used for actual spacecraft navigation.

Yomi runs on that data. Daily positions for Mercury, Venus, and Mars, weekly for Jupiter through Pluto, covering 1900 to 2050. When Yomi says Mars is at 14 degrees Scorpio, that number is JPL's number, computed from the date, time, and place. The Sun and Moon are computed from classical astronomical theory and verified against JPL Horizons to a few hundredths of a degree.

The accuracy, measured

Every position the engine produces is validated against 19,932 JPL Horizons reference points spanning 1900 to 2050, plus independent probes at the hardest moments: retrograde stations, sign ingresses, the places where approximations break. These are the worst-case errors.

For scale: 0.024 of a degree is less than 1/12,000th of the zodiac circle. The Moon covers its worst-case error in about seven minutes of motion.

And you don't have to take any of this on faith. JPL Horizons is public. Pull any date at ssd.jpl.nasa.gov/horizons, compare any planet. The numbers match.

Your chart, not your Sun sign

Your birth chart is a snapshot of the sky at the moment and place you were born. Yomi places every planet in a sign and in a house. Houses divide the sky into twelve areas of life: work, relationships, home, money, and so on. Yomi uses the Placidus system, the most widely used house system in Western astrology.

To place the houses and the Moon correctly, Yomi needs your birth time. If you don't know it, Yomi uses noon as a default and tells you which parts of the reading depend on the time.

Transits, timed to the minute

Transits are where the planets are now compared with where they were at your birth. When a moving planet reaches a meaningful angle to one of your birth placements, that angle is an aspect. Yomi detects aspects deterministically and times them to the minute.

Conjunction

Two planets at the same point. Their themes combine.

Square

A 90-degree angle. Tension that asks for action.

Trine

A 120-degree angle. Ease and flow between two themes.

Opposition

A 180-degree angle. Two forces pulling against each other.

Applying / separating

Whether an aspect is still building or already fading.

Orb

How close the angle is to exact. Tighter orbs hit harder.

From the math to your reading

The engine hands over the facts and nothing more: which planet is transiting, the natal placement it touches, the aspect, the direction, the house, the exact timing. That is the part that has to be true, and it comes straight from the validated positions above.

A writing layer turns those facts into plain language about your week, and a validator checks every statement against the real sky before it ships. Anything the chart doesn't support is rejected, so a reading can't reference a placement that isn't there. The math is the truth underneath; the reading is the translation.

How it stays honest

Two things. The math is deterministic: the same birth data and the same moment always produce the same chart and the same aspects, with nothing random in between. And the engine is versioned: the full validation suite runs on every change, and when the engine improves, your stored chart is recomputed automatically. Accuracy claims on this page describe the engine as it runs today, not as it ran when it was first built.