Tycho Brahe

The teaching

Taught from his own books, above all De nova stella and his book on the comet of 1577, with his oration on the mathematical arts.

Measure first

Tycho teaches that the heavens are known only by measuring them, exactly and again and again. His instruments are large, solid and fixed, finely divided, fitted with sights and set true, so that each can check the others; his observations are taken on every clear night, year after year, and not only at the moments the books single out. He corrects even for the air, which lifts a star near the horizon above its true place, and makes tables of how much. Only from such a record can astronomy be renewed, since the old tables, Ptolemy's and Copernicus's alike, had drifted from the sky.

The test of distance

A body near the Earth seems to shift against the fixed stars when it is seen from different places, or from one place as the Earth turns, and the nearer it is the more it shifts; the Moon shifts plainly. A body that shows no such shift lies far beyond the Moon. By this test, made with his own instruments night after night, he placed the new star of 1572 among the fixed stars and the comet of 1577 among the planets, and from the two placings drew two teachings.

The first is that the heavens change. The schools held that above the Moon nothing comes to be or passes away; a new star that blazes and fades among the fixed stars shows that it does. He read the star as a sign as well, as astrologers did: its colour went from a white like that of Venus and Jupiter to the red of Mars and at last to the dull hue of Saturn, and he took the colours for the natures of what it signified.

No solid orbs

The second teaching comes from the comet. It moved through the region of the planets, crossing the paths where the schools set the solid, crystalline shells that were thought to carry the planets round. No comet could pass through such shells, so there are none: the heavens are open and fluid, and the planets move through them freely. With the orbs gone, the old picture of the heavens as nested hard shells had to be given up, whatever system took its place.

His system of the world

In his own system the Earth stands still at the centre, the Moon and the Sun go round it, and the five other planets go round the Sun. He gives his reasons. The Earth is a heavy, sluggish body, unfit for the swift motions Copernicus gives it, and Scripture speaks of it as at rest. And if the Earth went round the Sun, the fixed stars should show a yearly shift; his instruments, the best of the age, found none, so the stars would have to lie at a vast distance, with an enormous empty space beyond Saturn and each star of an enormous size, which he judged against reason.

His system keeps the economy of Copernicus's arrangement while keeping the Earth still, and every measurement of his day could be fitted to it.

Looking up, looking down

Over his work stood two mottoes: by looking up I see downward, for astronomy, and by looking down I see upward, for chemistry, which he called a terrestrial astronomy. The seven metals answer to the seven planets, and the work of the furnace below to the motions of the heavens above, so the two arts are one study seen from two sides. In the laboratory beneath his observatory he made chemical medicines, among them remedies against the plague.

Those medicines are history, not counsel. The chemical remedies of his age were made with metals and salts that are poisons, and none of them is to be made or taken.

The stars incline

Astrology, for Tycho, is the use to which astronomy is put, and it is only as good as the measurements beneath it. In his oration on the mathematical arts he defends it: God set the lights in the heaven “for signs, and for seasons, and for days, and years”; the Moon moves the tides and the Sun the seasons; and so the stars act on bodies and incline the mind. But they do not compel it, and a wise man can resist what they incline him to. He cast nativities and yearly forecasts as his office asked, and laid the failures of astrologers to the bad tables they worked from.