(20) http://commons.wikimedia.org/wiki/Image:Gravitation.png. Public Domain.
(21) NASA.http://nix.larc.nasa.gov/info;jsessionid=c21rl6idgm4cc?id=
PIA11800&orgid=10.jpg. Public Domain.
(22) A Solar Flare.. Public Domain.
(23) A Spring Tide.. Public Domain.
(24) A Neap Tide.. Public Domain.
(25) Earth’s Magnetic Field. GNU-FDL.
(26) A Solar Eclipse.. GNU-FDL.
(27) http://en.wikipedia.org/wiki/Image:Phases_of_the_Moon.png. GNU-FDL.
(28) Photo of a Total Solar Eclipse.. GNU-FDL.