What the difference between astrology and astronomy is

Astronomy studies what celestial bodies are and how they move; astrology starts from those same positions and proposes a meaning for them. The first measures, the second interprets, and both work over the same table of numbers.

The confusion between the two words is not ignorance on the part of whoever asks: it is historical. For almost two thousand years they were one profession, practised by the same people, written in the same books. The parting is recent, and not everybody had reason to notice it.

The difference that matters today is one of method. Astronomy answers to experiment: one of its claims can be shown wrong by a measurement, and that is how it advances. Astrology is not organised that way — it is a tradition of reading, and what it offers is vocabulary for somebody's experience. Confusing the two harms both.

The two were one profession for two thousand years

The same author wrote the astronomy manual and the astrology manual of the West. Claudius Ptolemy, in Alexandria, in the second century after Christ: the Almagest organised the movement of celestial bodies and was the astronomical reference for fourteen hundred years; the Tetrabiblos organised interpretation. Two books, one desk.

It was not an isolated case. Astrology was a university subject in Europe, physicians consulted the sky before a procedure, and whole courts kept astrologers on the payroll. Johannes Kepler, who discovered that planetary orbits are ellipses, earned part of his living calculating charts and published a treatise in 1601 proposing firmer foundations for astrology — he thought the practice of his time was badly done, and wanted to mend it instead of dropping it.

Between one and the other lies a bridge that almost never comes up: the astronomers and translators of the Islamic world, who between the eighth and twelfth centuries kept, corrected and expanded that material. The Arabic name of dozens of stars astronomy still uses is the trace of that passage.

What parted the two

The parting was no quarrel, it was a change of requirement. From the seventeenth century onwards astronomy adopted a criterion astrology had no way of meeting: a claim only gets in if it can be tested and knocked down. Once that became the price of entry, one of the two stayed inside and the other stayed out.

Two things helped the distance grow. One was the exit from universities, which took away from astrology a common training and a discussion among peers. The other was the market: with no chair, the practice went to the newspaper and to the fairground, and the format that survived there is the shallowest of all — the column of twelve paragraphs, which is less than a hundred years old and came to stand, to outside eyes, for the whole tradition.

The precession objection, and what it knocks down

This is the honest objection, and it deserves an honest answer. The tilt of the Earth traces a slow cone, and because of that the March equinox point slides over the background of stars by about one degree every 72 years. The first to measure it was Hipparchus, in the second century before Christ, comparing his own observations with older records.

The accumulated effect: the March equinox, which in Ptolemy's day fell near the start of the constellation Aries, today sits almost 24 degrees further on, inside Pisces. That is where the line about your sign being wrong comes from.

What it really knocks down is less than it seems, and the reason is a definition. The tropical sign was never a piece of constellation. Ptolemy pinned the zodiac to the equinox point, and not to the stars: degree zero of Aries is where the Sun stands when it crosses the celestial equator in March, and that happens on the same date every year. The ruler moves along with the season, because it was tied to the season.

The other choice exists, and it is legitimate. Vedic astrology uses the sidereal zodiac, pinned to the stars, and that is why your Sun sign there is usually the previous one. The almost 24 degrees of difference between the two rulers are no mistake of either: they are two different decisions about where to put the zero.

That thirteenth sign

The band of sky the Sun travels through crosses thirteen constellations, and that is news to nobody. The Sun stands in front of Ophiuchus, the serpent bearer, from about 30 November to 18 December. The Babylonians knew this and chose twelve anyway, because they wanted twelve equal slices to match a calendar of twelve months.

The headline comes back from time to time with NASA's name attached, and NASA answers the same thing every time: it studies astronomy, not astrology, and it changed no sign at all. What exists is an educational text of theirs explaining the difference between the band of constellations and the twelve slices.

And there is the detail that closes the argument. The constellations come in very different sizes: the Sun spends about a week in front of Scorpius and more than a month in front of Virgo. A zodiac made of constellations would have twelve signs of unequal length, one of them lasting four times another. The twelve slices of thirty degrees exist precisely to avoid that problem.

There is a practical consequence of this that almost nobody mentions, and it is worth knowing before arguing with anyone. The boundaries of the 88 constellations of the sky were drawn by Eugène Delporte and published by the International Astronomical Union in 1930, so that astronomers would stop disagreeing about where one ends and the next begins. They are working lines, drawn to catalogue stars — and not equal slices of the Sun's path.

In other words: the constellation map the headline uses to say your sign has changed is the same age as the newspaper horoscope column, and it was made for another purpose. Comparing the two is like comparing a map of time zones with a relief map, and concluding one of them is wrong because the lines do not match.

What astronomy lends astrology every day

No birth chart exists without astronomy. Every planetary position comes out of an ephemeris — the table that says where each body stood at each moment —, and an ephemeris is a product of celestial mechanics, not of tradition.

Here that has a name and a number: the positions on this site are calculated with VSOP87, the same model of planetary orbits astronomy uses, to a precision on the order of one arcminute. Chiron, which is not part of that model, comes from a separately tabulated ephemeris.

That is why the first thing to demand of any reading is that the arithmetic be right. A chart with the wrong degree is no different school of astrology: it is an arithmetic error wearing the look of an opinion.

What neither can say about the other

Astronomy has no way of declaring astrology false by measuring a planet. What it can say, and does, is that there is no known physical mechanism for the effect astrology proposes, and that controlled tests did not find the effect. Those are two real limits, and neither is solved by a better telescope.

And astrology has no way of correcting astronomy. No reading changes an orbit, an eclipse date or the distance of a planet. When an astrological text contradicts an astronomical figure, the text is the one that got it wrong.

There is one case where the border shows up plainly for anybody: the date of an eclipse. It is known centuries in advance and to the second, and no school of astrology argues about the number. What gets argued is what it means — and that is exactly the line dividing the two.

Respect for the border is what makes it possible to use both. Here, the calculation is astronomy and the reading is astrology — and the limit of each is said out loud, because that is what separates a serious reading from a promise.

Frequently asked questions about astrology and astronomy