STAR DISTANCE.

Auckland Star, Volume LXIX, Issue 221, 19 September 1938, Page 4

STAR DISTANCE.

FIXED CENTURY AGO. GERMAN ASTRONOMER'S DISCOVERY. ! HBW FIELD TO SCIENTISTS. (By CARLOS .T. VIDELA.) One hundred years ago, man was enabled to measure the distance between the em-th „ n d the .tar.. It was in IS.JS that the -rent liermnii nstrnmor, Friedrich Wilhrlm Hesscl. director ~f the Koenigshorg (Ihsn-v.tn, y, ascertained that the parallax ,if star No. lil „f the conMell.uion I v-inirt was n'.:i|, and by no doing opened mi immense Held t<. modern a*» ronomy, *'"'•'' "'»< day. ~.;,• „„.,..,. f„|lowIlly MesseTs fiii.it|.m... |~m . i,,.,.,, (I |, K , lM niei-mire t It«- iti^tiiiic-i-h separating our little [.land fmm thousands of heavenly bodies. Tlhih. tlii' almanacs tell us now | that Siiiu-, Hi,. j_, n .,it Mar of t lie winter sky. is nine light-yeai iwhv from u„ ; thHt. Vega, tin- bright -t.n in tin ufttellttiun |.yi;i. is twenty -even iight-yeHi-s distant ; or that it "would take n man, astride a beam of lijjht streaking through space nt the speed of lXli..'i2-| miles per second, ;ii)H years to reach Polaris, the North Stir." Seeing Events ol 1638. Or, putting it the other way round, if one of us wore now sitting .somewhere in Polaris, looking at the earth through a telescope powerful enough, he would actually see, at this moment, what took plare three centuries ajn, in the year ]«3S—the image, carried by light, would just now be arriving there. Our hypothetical <>bi*er\er would be able to *ee. to-day, Louis XIII. sitting on the throne of France, or Charles I. reigning in England, unaware that his head would one day bo chopped off. or the Dutch pfttrootw in New Amsterdam still talking mt Peter Mimiit. not knowing that Mayor T.a Cuardm would. :i(M) years Inter, occupy the place tli.-n held by Willern Kieft.

Bessel's mathematical genius put an end to a centuries-old dispute. Copernicus and Galileo started it. affirming that the earth revolve* around the sun and is not stationary. If such is the case, their adversaries said, and if the earth does turn around the sun in one year, then the stars must show some apparent yearly shifting, due to the movement of our planet. Inasmuch as such change of position had never been noticed, they added, the conclusion was evident that the earth does, not move, but is, instead, the centre of the celestial world.

Copernicus and Galileo met this objection by declaring that the distance between the earth and the stare is so immense that the apparent yearly shifting of the stars, caused by the movement of our planet around the sun, is extremely smalf, so small that only the most delicate measurements, taken with instruments of an accuracy then unknown, could reveal it. Galileo asserted that the day would come when this annual apparent shifting of the stars would be measured accurately, and that that would put an end to all doubt* about the earth revolving * round the sun. He died in 1642. nearly "30 years before the brilliant German mathematician r certained the exact parallax of the star fll Cygni, finally vindicating Galileo's judgment. This star-shifting can be graphically imagined. Visualise a glaes cone, on the base of which a diameter has been drawn, dividing the base into two half circles. Let us assume that the circle which is the rim of the base of the cone represents the earth's orbit (which, of course, is not circular but elliptical), and that one end of the diameter represents the point of the orbit where the earth is on January 1. The opposite end would be, naturally, the point of the orbit where the earth would be half a year later —July 1. Now let us place' the cone, base down, on a table. The vertex of the cone is supposed to be a , ' star, shining above the earth, which revolves around the sun. represented by • dot on the centre of the base. Angle Represents Parallax. We have then a triangtO— one of Itst sides is the line joining the- star to January 1, the other side is the line from the star to July 1, and the base is the diameter we have drawn. The yearly apparent shift of the star (technically called -parallax*) is the angle formed by the prolongation of the two sides of the triangle/ The further away a siar Is from the earth, the lotager the sides of our cone and our triangle, and the smaller the -parallax. 6o far, in fact, aro the stars, -that the nearest one to us, Alpha in -the constellation '•' Centaurus, only twenty-five trillion miles away, has a parallax of only .75 of a second. To visualise what this means, let us suppose that wo could see a one-yard stick standing straight at a distance of 120 miles, or roughly that between San Diego and -Los Angeles—that would be en are of about one second. The measurement of that minute angle was the keystone of the whole problem of measuring how far we are from the stars. Once this angle became known, the rest wu a very simple problem in high school mathematics; V the base of a right triangle (distance from sun to earth, or from centre dot to July 1 on our glass cone) is equal to the hypothsnuse (line from star to July 1), multiplied by the sine of the angle opposite the base (half the parallax). We know what the distance from the sun to the earth is (from 01 to 04 million miles); we know what the parallax is (it has been measured for us); and we know what the sine of the parallax is (we can find it in any one of at thousand tables). The rest is child's play—an equation with only one unknown. That hypothenuse is the distance to the star. Camera Displaces Heliometer. Up to Bessel's time, the difficulties that beset the measurement of the parallax were often considered insurmountable. Often the observation error was larger than the whole angle the astronomers wanted to measure. It is not unusual to find, in texts on astronomy published before Besscl, that the stars don't show any visible yearly shifting, and that consequently their distance from the earth should he considered infinite. Bessel's painstaking work centred around the small star called in the catalogues "61 Cygni," of the sixth magnitude, having the fasten apparent motion then known. His computation, .35 of a second, still stands in comparison with more modern observations. The German scientist used a heliometer for this task. Nowadays, the use of photographic plates has largely disk placed that instrument in the observe-iorietv-JN.AJJ.A.)