WCP2803

[1]¹

68, Redcliffe Square,

London. S.W.

March 15, 1901

Dear D^r Wallace,

Perhaps I may be able to convey in a few words some idea of the principle by which Copernicus and Kepler were able to estimate the proportionate distances of the planets.

It depended — to put it very roughly — upon the magnitude of the loops in their apparent paths or more generally, upon the difference in the effects of [2] the earth’s real motion on the seeming motion of each along the zodiac.

These would naturally be on a scale inversely proportioned to the distance of the body displaced by perspective.

Fundamentally, and neglecting complications of sundry kinds, the method is that of "annual parallax", still in vogue for determining the distances of the fixed stars.

There are excellent Chapters on Copernicus and Kepler in Mr Arthur Berry’s "Short History of Astronomy" [3] published in 1898 (see especially pages 115, 118), and Grant’s standard "History of Physical Astronomy" cannot fail to give the requisite explanations: but I have no copy by me for reference.

It is most gratifying to me to hear that you have read my books with approval. Words of commendation from such a laurel[?] are not easily forgotten.

Believe me to be | faithfully yours | Agnes M. Clerke [signature]