Measuring the Stars.

Kaipara and Waitemata Echo, 11 December 1914, Page 3

Measuring the Stars.

(By EDGAR LUCIEN LAKKIN.)

"Will you give a list of all the stars whose distances have been determined (about forty-five), together with their distances and. t times for the transmission of light therefrom?" lam asked. The list is no longer forty-five or fifty stars whose distances are thought to be known with any degree of accuracy. Modern celestial ~r photography has brought the list down to seventeen whose parallaxes are greater than 0-5 of 1 second of arc. The word parallax must first be explained. A circle contains 1,----4i29(5,000 seconds, 21,600 minutes or 300 degrees. L * (;o out to a flat plain free from fee obstructions such as trees and ■ . houses, and draw an accurate circle L ; whose circumference is in length |te^|^ Jv.296,000 inches, almost 20.V miles. marks 1 inch apart, then each ;■/ ■''^C division would be 1 second of arc. :>F Then stand in the centre of this '. <• circle and the divisions could not • be seen without a powerful tele- ■. ■; . scope. : Go into space deeps to the nearest star, take along the great telescope from up here, turn around and look' back toward the sun ' and earth. 0 Of course the earth would be invis- . ible. Then the distance from the !^~-. earth to the sun, 02,832,000 miles, would be just equal to three-quar-ters of an inch on the circle. The distance from the earth to the sun as seen from any star expressed in seconds of arc is called parallax of the given star. From the centre, distant 3£ miles, try to measure with, a surveyor's instruments f, i, 1-5, 1-10, 1-100, 1-200, of a division on the distant circle, with accuracy then the task would be less than the herculean work of astronomers since the invention of the telescope in A..D. 16 K. No two surveyors could agree in theiif measures of these minute angles, but to measure the distances from the earth to even the nearer Stars, and nearer star clusters and nebulas, always require this degree of accuracy. The longest possible base line is '■ the sun's distance, 91,882.000 miles; but from remote faint stars such as tliose in the Milky Way this great distance shrinks to almost zero or nothing—so small that no instruments can be made that will detect it. Therefore the distances of these stars cannot be measured. MEASURING THE STARS. • Astronomers during the 300 years measured the positions of many stars with extreme accuracy, and again in six months. Meanwhile the earth has moved to the oppok^^^^fcSLte side of the sun, a straight distance of 185,761,000 miles. They were fiVX:(\ with astonishment to find that in many cases this distance vanishvd or appeared to be exactly zero, naught. Finally, after 2J centuries of in'v creased accuracy in.instruments, and of arduous labour, they found that a star, Alpha Oentauri, had a uarallax, a displacement in space -"Hie to the annual motion of the <?arth of f second, or £ inch on «>ur circle on the plain. Then by simple rules of trigonometry it was at once found that this nearest neighbour which our sun has is 275,020 times more distant than the sun, or in round numbers 25 trillion miles, now used as a yardstick to measure stars vastly more distant. Thus (forty or fifty stellar distances were measured under extreme difficulty. Then came photography. The entire celestial vault was photographed in 25,878 plates on the same scale. Millions upon millions of minute riots, all images of stars—sunswere developed on the negatives. These excessively minute dots can be measured over and over in high ■y^. power microscopes.