Pluto has moderately eccentric orbit e = 0,2488 during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun.
Pluto has orbital period of 247,94 years.
The New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first ever, and to date only, spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons.
The Pluto–Charon barycenter came to perihelion on September 5, 1989.
These makes us conclude that when the New Horizons on July 14, 2015 made its measurements of Puto's mean temperature Pluto was only 26 years away from the perihelion, so Pluto was still pretty much closer to the sun than its semi-major axis.
On July 14, 2015 the dwarf planet was approximately at 32.9 AU from the Sun.
And this partly explains the difference between the calculated and the measured Pluto's mean surface temperatures.
Charon (Pluto's satellite): Charon in true color, imaged by New Horizons in 2015
The faster a planet rotates (n2>n1) the higher is the planet’s average (mean) temperature T↑mean:
Tmin↑→ T↑mean ← T↓max
06.11 | 11:57
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