cumstances.  The  sky  must  be  very  clear ;  there  must  be  no
moon  visible ;  the  observer  must  be  away  from  the  lights  of  a
city ;  the  point  where  the  phenomenon  appears  must  not  be  in
or  near  the  Milky  Way.  For  the  latter  reason  the  Gegenschein
is  not  visible  in  June  or  July,  nor  in  December  or  January.
Even  under  the  most  favorable  circumstances,  the  observer
must  have  some  practice  in  seeing  a  faint  light  in  order  to  dis-
tinguish it.  Its  cause  is  still  involved  in  mystery.
CHAPTER  VII


THE   SUN


1.  Particulars  about  the  Sun.  —  The  sun  is  a  globe  whose
diameter  is  more  than  a  hundred  times  the  diameter  of  the
earth.  Hence  the  distance  round  it  is  more  than  a  hundred
times  that  round  the  earth.  Because  the  volumes  of  globes
vary  as  the  cubes  of
their  diameters,  it  fol-
lows that  the  volume
of  the  sun  is  more
than  a  million  times
that  of  the  earth.
More  exactly,  it  is
1,297,000  times  that
of  the  earth.  You
understand,  without
further  explanation,
that  the  sun  looks
small  because  of  its
great  distance  of  93,-
000,000  of  miles,  a
distance  which  the
swiftest  train  would
not  run  in  a  hundred  FlG-  56.  — Showing  how  an  image  of  the  sun

may  be  thrown  on  a  screen  with  a  spyglass,
years.
The  Sun's  Density,  Mass,  and  Gravity.  —  The  density  or  spe-
cific gravity  of  the  matter  composing  the  sun  is  less  than  that
of  the  matter  composing  the  earth.  The  mass  of  the  sun  is

103


104


ASTRONOMY


about  333,000  times  the  mass  of  the  earth,  instead  of  a  million
times  and  more,  as  it  would  be  if  it  had  the  same  density.
