The  superiority  of  the  heliometer  over  all
other  devices  (except  photography)  for  measur-
ing small  angles  has  been  specially  brought  into
prominence  by  Sir  David  Gill's  researches  on
the  distance  of  the  sun  —  i.e.,  the  scale  of  the
solar  system.  A  measurement  of  the  distance
of  any  planet  fixes  the  scale,  and,  as  Venus
approaches  the  earth  most  nearly  of  all  the
planets,  it  used  to  be  supposed  that  a  Transit
of  Venus  offered  the  best  opportunity  for  such
measurement,  especially  as  it  was  thought  that,


Observation  107

as  Venus  entered  on  the  solar  disc,  the  sweep
of  light  round  the  dark  disc  of  Venus  would
enable  a  very  precise  observation  to  be
made.  The  Transit  of  Venus  in  1874,  in  which
the  present  writer  assisted,  overthrew  this
delusion.
In  1877  Sir  David  Gill  used  Lord  Crawford's
heliometer  at  the  Island  of  Ascension  to  measure
the  parallax  of  Mars  in  opposition,  and  found
the  sun's  distance  93,080,000  miles.  He  con-
sidered that,  while  the  superiority  of  the  heli-
ometer had  been  proved,  the  results  would  be
still  better  with  the  points  of  light  shown  by
minor  planets  rather  than  with  the  disc  of
Mars.
In  1888-9,  at  the  Cape,  he  observed  the  minor
planets  Iris,  Victoria,  and  Sappho,  and  secured
the  co-operation  of  four  other  heliometers.  His
final  result  was  92,870,000  miles,  the  parallax
being  8",8o2  (Cape  Obs.,  Vol.  VI.).
So  delicate  were  these  measures  that  Gill
detected  a  minute  periodic  error  of  theory  of
twenty-seven  days,  owing  to  a  periodically
erroneous  position  of  the  centre  of  gravity  of
the  earth  and  moon  to  which  the  position  of  the
observer  was  referred.  This  led  him  to  correct
the  mass  of  the  moon,  and  to  fix  its  ratio  to  the
earth's  mass  =  0.012240.
