Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Week 4.  Galileo.

excerpts from
A Perfit Description of the Coelestiall Orbes
according to the most aunciente doctrine of the
Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations
by Thomas Digges (1546?-1595)

To The Reader

Having of late (gentle reader) corrected and reformed sundry faults that by negligence in printing have crept into my fathers General Prognostication, among other things I found a description or Model of the world, and situation of Spheres Celestial and Elementary, according to the doctrine of Ptolemy, whereunto all Universities (led thereto chiefly by the authority of Aristotle) sithens have consented.

But in this our age, one rare wit [Nicholas Copernicus (1473-1543)] (seeing the continual errors that from time to time more & more have bin discovered, besides the infinite absurdities in their Theorickes, which they have bin forced to admit that would not confess any mobility in the ball of the earth) hath by long study, painful practice, and rare invention delivered a new Theorick or model of the world, showing that the Earth resteth not in the Center of the whole world, but only in the Center of this our mortal world or Globe of Elements, which environed and enclosed in the Moon's Orb, and together with the whole Globe of mortality, is carried yearly round about the Sun, which like a king in the midst of all reigneth and giveth laws of motion to the rest, spherically dispersing his glorious beams of light through all this sacred Celestial Temple.

And the earth itself to be one of the Planets, having his peculiar & strays courses turning every 24 hours round upon his own Center, whereby the Sun and great Globe of fixed stars seem to sway about and turn, albeit in deed they remain fixed.

So many ways is the sense of mortal men abused, but reason and deep discourse of wit having opened these things to Copernicus, & the same being with demonstrations Mathematical, most apparently by him to the world delivered.

I thought it convenient, together with the old Theorick [the Ptolemaic system], also to publish this, to the end such noble English minds (as delight to reach above the baser sort of men) might not be altogether defrauded of so noble a part of Philosophy.  And to the end it might manifestly appear that Copernicus meant not as some have fondly accused him, to deliver these grounds of the Earth's mobility only as Mathematical principles, feigned & not as Philosophical truly averred....

Thus much for my own part in this case I will only say:  there is no doubt but of a true ground truer effects may be produced than of principles that are false, and of true principles falsehood or absurdity cannot be inferred.

If, therefore, the Earth be situate immoveable in the Center of the world, why find we not Theorickes upon that ground to produce effects as true and certain as these of Copernicus?  Why cast we not away those [equant circles] and motions irregular...[?]  Why shall we so much dote in the appearance of our senses, which many ways may be abused, and not suffer our selves to be directed by the rule of Reason, which the great GOD hath given us as a Lamp to lighten the darkness of our understanding, and the perfect guide to lead us to the golden branch of Verity amid the Forest of errors...[?]

The Globe of Elements [the terrestrial realm] enclosed on the Orb of the Moon I call the Globe of Mortality because it is the peculiar Empire of death....

In the midst of this Globe of Mortality hangeth this dark star [Earth] or ball of earth and water, balanced and sustained in the midst of the thin air only with that propriety which the wonderful workman hath given at the Creation to the Center of this Globe, with his magnetical force vehemently to draw and hale unto itself all such other Elemental things as retain the like nature.  This ball, every 24 hours by natural, uniform and wonderful sly & smooth motion rolleth round, making with his Period our natural day, whereby it seems to us that the huge infinite immoveable Globe should sway and turn about.

The Moon's Orb, that environeth and containeth this dark star and the other mortal, changeable, corruptible Elements & Elementate things is also turned round every 29 days 31 minutes 50 seconds, 8 thirds, 9 fourths, and 20 fifths, and this period may most aptly be called the Month.  The rest of the Planets' motions appear by the Picture, and shall more largely be hereafter spoken of.

Herein, good Reader, I have waded farther than the vulgar sort Demonstrative & Practice, & God sparing life, I mean though not as a Judge to decide, yet at the mathematical bar in this case to plead, in such sort as it shall manifestly appear to the World whether it be possible upon the Earth's stability to deliver any true or probable Theorick & then refer the pronouncing of sentence to the grave Senate of indifferent discreet Mathematical Readers.

Farewell, and respect my travail as thou shalt see them tend to the advancement of truth and discovering the Monstrous loathsome shape of error.

The Old Theory

Although in this most excellent and difficult part of Philosophy, in all times have been sundry opinions touching the situation and moving of the bodies Celestial, yet in certain principles all Philosophers of any account of all ages have agreed and consented.  First, that the Orb of the fixed stars is of all other the most high, the farthest distant, and comprehendeth all the other spheres of wandering stars.

And of these straying bodies called Planets the old philosophers thought it a good ground in reason that the nighest to the center should swiftliest move, because the circle was least and thereby the sooner overpassed, and the farther distant the more slowly.

Therefore, as the Moon being swiftest in course is found also by measure nighest, so have all agreed that the Orb of Saturn, being in moving the slowest of all the Planets, is also the highest; Jupiter the next, and then Mars.

But of Venus and Mercury there hath been great controversy, because they stray not every way from the Sun as the rest do.  And therefore some have placed them above the Sun as Plato in his Timaeus, others beneath, as Ptolemy and the greater part of them that followed him....

The New Order of the Spheres

The first and highest of all is the immoveable sphere of fixed stars, containing itself and all the Rest, and therefore fixed as the place universal of Rest, whereunto the motions and positions of all inferior spheres are to be compared....  Copernicus by the motions of the earth salveth all, and utterly cutteth off the ninth and tenth spheres, which, contrary to all sense, the maintainers of the earth's stability have been compelled to imagine.

  • The first of the moveable Orbes is that of Saturn, which ... is also in his course most slow, & once only in thirty yeares passeth his Periode.
  • The second is Jupiter, who in 12 years performeth his circuit.
  • Mars in 2 years runneth his circular race.
  • Then followeth the great Orb, wherein the globe of mortality, enclosed in the Moon's Orb as an Epicycle, and holding the earth as a Center by his own weight resting always permanent, in the midst of the air is carried round once in a years.
  • In the fifth is Venus makinge her revolution in 9 moneths.
  • In the sixth is Mercury who passeth his circuit in 80 days.
  • In the midst of all is the Sun.
For in so stately a temple as this, who would desire to set his lamp in any other better or more convenient place than this, from whence uniformly it might distribute light to all, for not unfitly it is of some called the lamp or light of the world, of others the mind, of others the Ruler of the world....  Thus doth the Sun, like a king sitting in his throne, govern his courts of inferior powers....

The New Causes of Time

In this form of frame may we behould such a wonderful Symmetry of motions and situations, as in no other can be proponed.  The times whereby we the Inhabitants of the earth are directed, are constituted by the revolutions of the earth.  The circulation of her Centre causeth the year, the conversion of her circumference maketh the natural day, and the revolution of the Moon produceth the month.

The New Causes of Apparent Planetary Phenomena

By the only view of this Theoricke the cause & reason is apparent why in Jupiter the progressions and Retrogradations are greater than in Saturn, and lesse than in Mars, why also in Venus they are more than in Mercury.  And why such changes from Direct to Retrograde, Stationary, &c. happeneth notwithstanding more rifely in Saturn than in Jupiter & yet more rarely in Mars, why in Venus not so commonly as in Mercury.  Also why Saturn, Jupiter and Mars are nigher the earth in their Acronicall than in their Cosmicall or Heliacall rising.

Especially Mars, who rising at the Sunset, showeth in his ruddy fiery collor equall in quantity with Jupiter, and contrarywise, setting little after the Sun, is scarcely to be discerned from a Star of the second light.  All which alterations apparently follow upon the Earth's motion.

And that none of these do happen in the fixed stars, it plainly argueth their huge distance and immeasurable Altitude, in respect whereof this great Orb, wherein the Earth is carried is but a point, and utterly without sensible proportion, being compared to that Heaven....

The Question of Earth's Stability

But because the world hath so long a time been carried with an opinion of the Earth's stability, as the contrary cannot but be now very impersuasible, I have thought [it] good out of Copernicus also to give a taste of the reasons philosophical alleged for the Earth's stability, and their solutions, that such as are not able with Geometrical eyes to behold the secret perfection of Copernicus' Theorick, may yet by these familiar natural reasons be induced to search farther, and not rashly to condemn for fantastical, so ancient doctrine revived, and by Copernicus so demonstratively approved.

What reasons moved Aristotle and others that
Followed him to think the earth to rest
immoveable as a Center to the
whole world.

The most effectual reasons that they produce to prove the Earth's stability in the middle or lowest part of the world, is that of Gravity and Levity.  For of all other the Element of the earth (say they) is most heavy, and all ponderous things are carried unto it, striving, as it were, to sway even down to the inmost part thereof.  For the earth being round, into the which all weighty things on every side fall....  So much therefore, the rather shall the Earth rest in the middle, and (receiving all things into itself that fall) by his own weight shall be most immoveable.

Again, they seek to prove it by reason of motion and his nature, for of one and the same simple body, the motion must also be simple, saith Aristotle.  Of simple motions there are two kinds:  Right and Circular.  [I]t is mete to [the terrestrial] elements to attribute the right or straight motion, and to the heavens only it is proper circularly about this mean or Center to be turned round.  Thus much Aristotle.

If therefore, saith Ptolemy of Alexandria, the earth should turn but only by the daily motion, things quite contrary to these should happen.  For his motion should be most swift and violent that, in 24 hours, should let passe the whole circuit of the earth, and those things which by sudden turning are stirred, [and] to disperse things united, unless they should by some firm fastening be kept together....  [T]he Earth, being dissolved in pieces, should have been scattered through the heavens,... and much more, beasts and all other weights that are loose could not remain unshaken.  And also, things falling should not light on the places perpendicular under them, neither should they fall directly thereto, the same being violently in the mean carried away.  Clouds also and other things hanging in the air, should always seem to us to be carried toward the West.

The Solutions of these Reasons
with their insufficiency.

...But he that will maintain the Earth's mobility may say that this motion is not violent, but natural.  And these things, which are naturally moved, have effects contrary to such as are violently carried.  For such motions ... which by nature are caused, remain still in their perfect estate and are concerned and kept in their most excellent constitution....

But why should [Ptolemy] not much more think and misdoubt the same of the [universe], whose motion must of necessity be so much more swift and vehement than this of the Earth, as the Heaven is greater than the Earth....  For the more the motion should violently bee carried higher, the greater should the swiftness be, by reason of the increasing of the circumference, which must of necessity in 24 hours be passed over, and in like manner by increase of the motion the Magnitude must also necessarily be augmented.  Thus should the swiftness increase the Magnitude, and the Magnitude the swiftness, infinitely.  But, according to that ground of nature, whatsoever is infinite can never be passed over.  The Heaven therefore, of necessity must stand and rest fixed....

For a ship carried in a smooth Sea with such tranquility doth pass away, that all things on the shores and the Seas to the sailors seem to move, and themselves only quietly to rest with all such things as are aboard with them, so surely may it be in the Earth, whose Motion being natural and not forcible, of all other is most uniform and unperceivable, whereby to us that sail therein, the whole world may seem to roll about.

But what shall we then say of Clouds and other things hanging or resting in the air or tending upward, but that not only the Earth and Sea making one globe, but also no small part of the air is likewise circularly carried, and in like sort all such things as are derived from them or have any manner of alliance with them.  Either for that the lower Region of the air, being mixed with Earthly and watery vapors, follow the same nature of the Earth; either that it be gained and gotten from the Earth by reason of Vicinity or Contiguity....

Hereto we may adjoin, that the condition of immobility is more noble and divine than that of change, alteration, or instability, & therefore more agreeable to Heaven than to this Earth, where all things are subject to continual mutability....

Thus, as it is apparent by these natural reasons that the mobility of the Earth is more probable and likely than the stability.  So, if it be Mathematically considered, and with Geometrical mensurations every part of every Theorick examined, the discreet student shall find that Copernicus, not without great reason did propone this ground of the Earth's Mobility.

Go to:
  • Conversation with the Sidereal Messenger (1610), by Johannes Kepler (1571-1630)
  • Dialogue Concerning the Two Chief World Systems(1632), by Galileo Galilei (1564-1642)
Weekly Readings
Lecture Notes