WCP2984

[1]¹

71 Highbury Hill

London N.

24.6.10

Sir A. R. Wallace.

Dear Sir

I have been reading your interesting book 'Man's place in the universe',² and I thought you would kindly forgive me for troubling you with a difficulty I have.

I am a retired Colonel R. E, and have no pretensions to scientific knowledge, but my difficulty is this: In your book (as in [2] other books I have read in popular astronomy) the parallax of a star is spoken of as the difference in position between the observed place of the star at a given time and its observed position 6 mos later when the earth is at the opposite side of its orbit, and it is often said that this is the longest base line obtainable.

In this however no allowance is apparently made of the motion through space of the sun with its attendants planets, though apparently [3] in 6 mos. the earth will have travelled inward with the sun, as great a space as the diameter of its orbit, and therefore the actual base line would be nearly 1 1/2 times the diameter of the orbit of the earth.

Also by taking observations at intervals of some years would not a base line be obtained of a length, ten, twenty, fifty, or a hundred or more times longer than the diameter of the earth's orbit?

Of course this base line being in a certain direction could be of little use for the measurement of the distances of stars in or nearly in that direction, but as there are stars [4] in every direction could there not be many in suitable positions of which the parallax could be easily ascertained by so long a base line though undiscoverable from the much shorter base line of the earth's orbits' diameter?

If you could kindly answer this question of an unscientific man I should be greatly obliged Please excuse my troubling you.

Yours truly | S L Jacob.³ [signature]

15 miles per second⁴

60 60	86,400 73 x 5	180,000,000 diam 480,000,000 orbit
3600 12	259200 604800	Sun’s motion a year.
43200	secs. a day 6307200 5	82 moon suns 3
86,400	31,536,000 [illeg.] a year	160 480 mill. miles a year