Pluto Demoted Day on August 24 commemorates the day in 2006 when Pluto's status was downgraded from a full sized planet to a dwarf planet by the International Astronomical Union (IAU).
Discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, Pluto was the Solar System's 9th planet for 76 years. Given the name of the ancient Greek ruler of the underworld, Pluto was named by an 11-year-old girl called Venetia Burney.
250 Year Orbit
Because Pluto is very far from us - on average it is about 6 billion kilometers (4 billion miles) from Earth, little is known about the dwarf planet. In size, Pluto has a diameter of about 2250 kilometers (1400 miles), which is about two-thirds the size of Earth's Moon.
Pluto's orbit is highly eccentric. In other words, its orbit around the Sun is not perfectly circular. This means that the distance between it and the Sun varies over time. It takes Pluto about 250 Earth years to orbit once around the Sun, and about 6.5 Earth days to make a full rotation around its axis.
Demotion to Dwarf Planet
The IAU defines a planet as a celestial body that "orbits around the Sun, has a nearly round shape and has cleared the neighborhood around its orbit." Pluto was reclassified as a dwarf planet because it does not meet the third criteria to be a full-sized planet. It is not the dominant object on its orbit around the Sun - other bodies can be found in the region around its neighborhood.
How to Celebrate?
Celebrate Pluto's previous status as a planet by spendings some time in the day reading about planets and their characteristics.
Have children? Take them on a visit to your local planetarium and introduce them to the joys of astronomy. Teach them everything you know about the Solar System and about planets. Maybe you could make a model of the Solar System with them in order to understand how it works?