|Arny, Thomas. Introductory Astronomy: Second Edition. Boston: Addison-Wesley, 1979: 180.||"Pluto's rotational period is 6.39 days, longer than that of any of the giant planets."||6.39 days|
|Comparative data for the planets. Compton's Online Encyclopedia. 1999.||"Sidereal period of rotation (Earth days:hrs:min), 6:09:17"||6.387 days|
|Weissman, Paul. The solar system and its place in the galaxy: Fig.1. Encyclopedia of the solar system.1999.||"Pluto, Rotational Period, 6.387 days"||6.387 days|
|Seidelman, K.P. Explanatory Supplement to the Astronomical Almanac.Mill Valley: University Science Books, 1978: 707.||"Sidereal orbit period (hrs), Pluto, 158.28"||6.595 days|
|Pluto Factsheet. NASA/Goddard Space Flight Center.||"Sidereal rotation period (hrs), Pluto, 153.2928"||6.3872 days|
At the present time, Pluto is the planet in our solar system furthest away from the sun. Our Earth's sidereal rotation period, otherwise known as an Earth day, is approximately 24 hours long. Just imagine a day about six and a half times longer than that!
Well, that is a Plutonian day. Instead of 24 hours, a day is around 153 hours. All the sources I have found give around the same amount of time for a Plutonian day, give or take 5 hours or so.
You might be wondering how the sidereal rotation of Pluto is found. Well, as with other planets, a surface anomaly that is recognizable (with the help of powerful telescopes, of course) is recorded at a certain point. Once the same surface anomaly is seen again at that same point, the time it took for it to show up again and therefore for the planet to make one full rotation is recorded as the sidereal rotational period of that planet.
Crissy Bilyk -- 1999