Happy Discovery Day, Neptune!

^{Voyager image of Neptune taken on March 9, 1993.
Image Credit: NASA/JPL}

Happy Discovery Day, Neptune! The last 150 years wouldn't have been the same without you.

After the discovery of Uranus in the 18th century, astronomers noted that its position in the night sky was slowly migrating from its predicted orbit. Gravitational tugs from the giant planets Jupiter and Saturn were not great enough to explain the differences between the calculations and the observations. Studies of Uranus' orbit done independently by French mathematician Urbain Jean Joseph Le Verrier and English astronomer John Couch Adams predicted the existence of a planet beyond Uranus. On September 23, 1846, at the Berlin Observatory, Johann Gottfried Galle viewed Neptune within one degree of Le Verrier's prediction. Neptune was the first planet discovered through scientific calculations.

The elusive Neptune was observed by others before Galle. In England, James Challis had observed Neptune twice in August of 1846, but did not compare his observations with a star chart. The star chart would have shown that Neptune was changing position relative to the other stars, and could only be a planet. John Herschel saw it in July of 1830. Even Galileo Galilei observed Neptune on December 28, 1612, while tracking Jupiter's moons. The solar system's eighth planet was almost discovered before the seventh!

More Cool Stuff

The search for Neptune by Le Verrier, Galle, and others is the subject of the article "Neptune's Discovery 150 Years Later" in the September 1996 issue of Astronomy. Astronomy magazine can be found online at:

http://www.kalmbach.com/astro/astronomy.html

The story of Neptune's discovery can also be found at the Web site of the Royal Observatory in Cambridge:

http://www.ast.cam.ac.uk:80/pubinfo/leaflets/solar_system/section3.13.html

The above image was found at the JPL "Browse Image Archive." Once there choose the "Planetary Missions and Instruments" link, then scroll down to the Voyager mission:

http://www.jpl.nasa.gov/archive/images.html

Learn more about Neptune from the Windows to the Universe project. Pick your interest level, then click on the "Solar System" button:

http://www.windows.umich.edu

The Observatorium has information on how orbits work, as well as Java simulators that will allow you to experiment with satellite orbits:

http://observe.ivv.nasa.gov/observe/exhibit/reference/module/orbits/orbits.html

http://observe.ivv.nasa.gov/observe/exhibit/reference/module/orbits/orbit_sim.html

Check out other observations in the Observation of the Week Archive.