http://www.ck12.org Chapter 1. Introduction to Physical Science
FIGURE 1.
In the early 1800s, astronomers noticed
a wobble in Uranus’ orbit around the
sun. They predicted that the wobble
was caused by the pull of gravity of an-
other, not-yet-discovered planet. Scien-
tists searched the skies for the “missing”
planet. When they discovered Neptune in
1846, they thought they had found their
missing planet. After the astronomers
took into account the effects of Neptune’s
gravity, they saw that Uranus still had
an unexplained wobble. They predicted
that there must be another planet be-
yond Neptune. That planet, now called
Pluto, was finally discovered in 1930. Of
special note, as of 2006, the Interna-
tional Astronomical Union (IAU) demoted
Pluto from its planet status as it does
not meet one of the criteria for plan-
etary standards. For more information
see: http://www.nature.com/news/2006/
060821/full/news060821-11.htmlFIGURE 1.
This map shows the supercontinent Pan-
gaea, which was first proposed by Al-
fred Wegener. Pangaea included all of
the separate continents we know today.
Scientists now know that the individual
continents drifted apart to their present
locations over millions of years.