January 11, 1955
[sent January 18]
Dear Professor Einstein:
Am I right or wrong in the following: A comet grazing the sun can experience an el.-magn. effect without violating Keplers 3rd law, because:
1. A static potential difference between the sun and a body on an orbit would also produce an inverse square relation which can be hidden in the gravitational effect.
2. The magnetic component of the effect would produce acceleration. And actually an unaccounted for acceleration is observed in comets passing close to the sun; this effect was studied on Comet Encke. (J. Zenneck, Gravitations in Encyclop. d. Mathem. Wiss. vol. V, part I, p. 44).
3. Even assuming a comet as a neutral body partly consisting of ionized gases, and a solar protuberance as a collection of ions of one sign on a neutral sun, we would have in a grazing comet a conductor passing through an electrical field.
By the way, Kepler himself regarded the motion of the planets and comets on ellipses as originating wholly in the sun, and for a time thought of magnetic action (electricity was not yet known; but Gilberts book on magnetism already appeared in 1600). Kepler wrote:
[Sol] trahendo et repellendo retinet, retinendo circumducit (Opera omnia, VI, 345).
Actually Keplers idea of a magnetic field reaching from a primary to a satellite can be checked as follows:
If the lunar daily librations in latitude follow the rotation of the polar magnetic field of the earth around the geographical pole, then the magnetic field of the earth reaches sensitively to the moon. Among lunar daily librations are some unaccounted for. According to H.T. Stetson of M.I.T., a magnetic needle slightly follows the sun.
As to Lexells comet: It was removed by Jupiter from a parabolic orbit to an ellipse of 5½ (five and a half) year period, and at the next passage it was sent away on a hyperbolic orbit. This I mentioned; you have thought it impossible, even after reading this in Newcombs astronomy.
You have asked me: what do the specialists say about the shape of the sun. I quote Donald Menzel of Harvard Solar Observatory (Our Sun, 1950, p. 39): but the measures are as likely as not to indicate a polar diameter greater than the equatorial, which we are indeed loath to believe.
With all good wishes,