THE TRANSIT OF VENUS.

Evening Star, Issue 3711, 14 January 1875, Page 3

THE TRANSIT OF VENUS.

To the Editor, Sib, —Your correspondents “Inquirer” and “ Rough Astronomical Notes ” are equally at sea regarding Professor Peters’s statement that the astronomers want to measure the thickness of a hair at a distance of 8,000 feet. Can either of them inform your readers what would be gained by the expedition if they could not measure a less angle than 3-10ths of a second ? or what 3-lOths of a second has to do with the sub ject at all ? The sun’s horizontal parallax ia already known to the nearest tenth of a second, and the object of the expedition >s to determine it to the nearest hundreth of a second, if possible. To do this it is clearly necessary to be able to measure an angle which ia less than -of a second, for all observers know that the possible error in any set of observations is equal to the least angle which can be measured by the instrument in use. In other words, the astronomers want to add a third figure to the expression of the sun’s mean equatorial horizontal parallax, and to be sure that the figure added is the nearest to the true figure. If “ Inquirer” will compute the angle subtended by a hair at 8 OOl) feet, he will fii-d that it agrees tolerably well with this limit of error.—l am, &c., Peertbingle, Dunedin, January 14.