|Zitzewitz, Paul & Neff, Robert. Physics. New York: Glencoe/McGraw-Hill, 1995: 159.||"Pluto's average radius is 1.15 × 106 M."||2300 km|
|Microsoft Encarta 97 Encyclopedia. Microsoft, 1996.||"Pluto is about 2320 km in diameter"||2320 km|
|Yenn, Bill. Solar System. New York: Crescent, 1991: 8.||"The diameter of Pluto is 1375 mi (2200 km)"||2200 km|
|Smoluchowski, Roman. The Solar System. New York: Scientific American Books, 1983: 120.||"It's size is very difficult to establish, at present is 4000 km."||4000 km|
|Encyclopedia of Knowledge. vol. 15. USA: Grolier, 1992: 73.||"Pluto may have a diameter of about 2284 km (1416 mi)"||2284 km|
|Hamilton, Calvin J. Pluto and Charon. Views of the Solar System. Los Alamos National Laboratory, 1997.||Equatorial radius (km) 1,160||2320 km|
Discovered by Clyde W. Tombaugh on February 18, 1930 Pluto is the ninth planet from the Sun, it is the smallest, most remote planet known in the Solar System. Given how far away it is, for many years very little was known about the planet. Until in 1978, when astronomers discovered a relatively large moon orbiting Pluto and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another. This enabled astronomers to determine Pluto's size fairly accurately. However, it wasn't until 1994, when with the use of the Hubble Space Telescope, astronomers were able to determine it's size even more precisely.
All of the results gotten, were pretty close except for the result received from a book published in 1983. This could be easily explained, by the fact that as stated above, Pluto's size was not accurately determined until 1985. Therefore, this source should not be trusted.
The measurements for the diameter of Pluto given in the remaining four sources were very close. In fact, if all of them would be rounded to the nearest hundreds place, three out of four of them would agree that the diameter of Pluto is 2300 km. The difference in the results could be explained by Pluto's methane atmosphere, which may lie many kilometers deep, making the diameter figure uncertain.
Although, if wanted a more precise answer, it would be better to go with the result from the Encarta 97 Encyclopedia, which is that Pluto is 2320 km across. This is because it is the most recent source and therefore, the most accurate. The reason is due to the fact, that every year new and better techniques are discovered for determining the sizes of planets. So, the newer the source, the more precise it is.
Anna Chernovetskaya -- 1997