Astronomy and the Bible;

112 ASTRONOMYANDTHEBIBLE

thecircuitoftheheavens,theirmotionwouldbede-
tectedinhalfayear.
"Withmachinerymore delicateeventhanthis,and
better adaptedto thepurpose,and of akind some-
what different, Bessel once more renewed the re-
search after the unattainable parallax of the fixed
stars. His great instrument, called the heliometer,
was mounted as early as 1829, but a multitude of
causesandsomeunsuccessfuleffortsdelayedhisprin-
cipal operations up to August, 1837. Three great
principlesguidedhiminhisselectionof 61 intheSwan
as the star on which to perform his observations.
First, itwasaffectedbyaverygreatpropermotion,
... whichindicatedittobeamongthenearestofall
the stars. Second, its duplex character adapted it
especially totheinstrumenthe wasaboutto employ.
Third, the region occupiedby 61 Cygni contains a
numberofminute stellarpoints, closeto the double
star,andpresentingadmirablefixedpoints,towhich
therelativemotionofthetwocomponentsofthestar
tobemeasuredmightbereferred.
"Withthese advantages,anda magnificentinstru-
ment,Besselcommencedhisobservations. Hemeas-
uredthedistancefromthecenterofthelinejoining
thetwostars,totwoofthesmallstellarpoints,which
servedhimas points of reference;andthis kindof
observationwasrepeatednightafternight, whenever
the starswere visible, from themiddle of August,
1837,uptotheendofSeptember, 1838. Theentire
series ofobservations wasthen taken and corrected
foreverypossibleknownerror, andincase anyap-