108 ASTRONOMYANDTHEBIBLEmove your finger,butopenyour lefteye, andclose
your right. Thefingerwillseem tochangeits posi-
tiontotheright,andcoveranotherspotonthewall,
asatB. Inthelanguageofastronomy,thisapparent
change in the position of your finger is called its
parallax.
The apparent changein theposition ofthe moon
iscalledthelunarparallax,thatofthestarsiscalled
thestellarparallax.
Inthemeasurementofthetrianglebeforementioned,
thevalueofthebaselineisknown,beingmarkedby
the position of thetwoobservers. Bythe methods
oftrigonometry,giventhebaselineandtheopposite
angles, it is a very simple matter to ascertain the
lengthofeitherside,whichinthiscasewouldbethe
distancefromtheobservertothemoon.
"Parallax, then,ingeneral,is theapparentchange
intheplaceofanobject,occasionedbytherealchange
intheplaceofthespectator."
Withthe sun, the moon, and the planets, a base
lineequaltothediameteroftheearth,orabouteight
thousand miles, has sufficed to give a sensible and
measurableparallax;butwhenweattemptbythisbase
linetodetectamovementonthepartofthestars,we
failutterly. Ifweviewastarwiththeproperinstru-
ment, and measure its position in the heavens, and
moveeightthousandmiles fromthatspotandmake
another measurement, we find our two angles the
same. With a base line only eight thousandmiles
in length, we can detect no apparent movement on
thepartofthestar;ithasnoparallax. Ifthestar