Ephemerides and Natal Chart Structure: Assembling the Astronomical Pieces

This is the astronomy course’s eleventh lesson. The first ten took a machine apart piece by piece: ecliptic, Moon phases, ASC–DSC, Midheaven and IC, geographic coordinates, and time zones. Now we assemble those pieces and see how birth details produce a complete natal chart. You will then have the foundation for natal astrology.

An ephemeris: a reference for planetary positions

One final piece: how does software know Jupiter’s degree in Sagittarius at your birth? It uses an ephemeris.

An ephemeris is a table of astronomical data listing the Sun’s, Moon’s, and planets’ ecliptic coordinates at different times. Earlier astrologers consulted thick printed tables. Today high-precision computerized data, such as the widely used Swiss Ephemeris or this platform’s algorithmic engine, calculate an object’s position for a requested time.

The central point is that planetary ecliptic positions depend almost entirely on time. At the same instant, the Sun occupies nearly the same ecliptic degree whether viewed from Taipei or London. With an accurate time, an ephemeris establishes every planet’s sign and degree.

The four layers of a natal chart

With all the parts in place, a natal chart combines four layers:

This table almost reproduces the first ten lessons’ contents. Each lesson explains a layer: the ecliptic and degrees give layer one; horizon and meridian give layers two and three; aspects are further calculations from layer one.

From input to finished chart

Walk through the process once to understand what chart software does:

1. Collect inputs: birth date, birth time, and birthplace. All three matter; the last lesson explained why location is required.
2. Convert to universal time: use the birthplace’s time zone, including historical and daylight saving rules, to convert local time to UTC.
3. Obtain planetary positions: use UTC with an ephemeris to find ecliptic coordinates for the Sun through Pluto, plus points such as the lunar nodes. This establishes layer one.
4. Calculate the four angles: combine time and geographic coordinates to find the ecliptic’s intersections with horizon and meridian: Ascendant, Descendant, MC, and IC. This establishes layer two.
5. Divide the houses: use the angles and a house system, such as Placidus or whole sign, to divide the chart into twelve houses. astro.how uses Placidus; the house systems lesson explains the differences. This establishes layer three.
6. Mark aspects: calculate pairwise planetary angles and mark conjunctions, oppositions, trines, squares, sextiles, and other relevant relationships. This establishes layer four.

After six steps, a natal chart appears: twelve signs around the outside, twelve houses inside, planets distributed through them, and aspect lines between planets. This is the diagram you will later learn to read.

Tools calculate numbers; you interpret meaning

Everything so far is astronomical calculation. Ephemerides, geographic coordinates, UTC, and angles are verifiable; tools worldwide should produce consistent numbers. Astrological interpretation is another layer built on top of this precise foundation. If the foundation is wrong, even an elegant reading lacks support. Learning this astronomy carefully is more useful than rushing to memorize sign traits.

Remember the underlying principle: a chart is a calculated snapshot of the sky describing patterns and tendencies rather than fixing your destiny. Accuracy helps you see clearly rather than become bound by predictions.

You can now assemble birth details into a complete natal chart. Two advanced choices of coordinate system remain. Next we discuss precession: why the equinox slowly shifts backward, and how tropical and sidereal zodiacs differ.