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Exploring the Moon
Timeline of Lunar exploration.
http://www.space.edu/moon/
Is Mars Habitable?
A. R. Wallace's 1907 critique of Lowell's Book 'Mars and Its Canals'
http://www.wku.edu/~smithch/wallace/S730.htm
Neptune and Pluto
Mathematical discovery of planets
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Neptune_and_Pluto.html
Neptune's Discovery
Article in Astronomy Magazine.
http://www.astronomy.com/Content/Dynamic/Articles/000/000/000/119ipise.asp
Notes on Sunspot Observations
John Winthrop's 1739 notes, from the collected papers of Fredrick Kilgour.
http://www.oclc.org/pdemo/kg/p012.htm
Orbits and Gravitation
The development of the mathematics of planetary motion.
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Orbits.html
The Struggles to Find the Ninth Planet
A paper by Clyde Tombaugh relating his discovery of Pluto.
http://perso.wanadoo.fr/olivier.granier/meca/vulga/pluton/pluton.htm