Thursday, March 17, 2050

Seasons and Latitudes

When the New Horizons probe made its Pluto flyby in 2015, the imaging team at NASA found itself, for one last time, with new worlds to map. They followed the set of longitudes and latitudes for Pluto and Charon established by the International Astronomical Union in the late 20th century. Pluto's north pole (hidden on the dwarf planet's nightside, and therefore unmapped) was at positive 90 degrees, while its south pole was at negative 90 degrees. A line drawn between the two of them, running through the spot on Pluto's equator directly beneath Charon (known as the sub-Charon point), was Pluto's prime meridian, the zero degree longitude. Three hundred sixty degrees were marked out, running east from the prime meridian, the direction of Pluto's rotation. Given Pluto's circumference of 3751 kilometers, that means that each degree of latitude, and each equatorial degree of longitutde, is about ten and a half kilometers long.

The International Astronomical Union had also established that Pluto's north pole was on the same side of the ecliptic as Earth's north pole, despite the fact that the Plutonian pole rotating counterclockwise was the one below the ecliptic. To get a sense of how that works, take a look at a standard globe of the Earth. It's probably resting on a stand, and turns freely on its axis, which is tilted 23.5 degrees from true to represent the tilt of the real Earth's axis compared to its orbital plane. Go ahead and start spinning the globe slowly on its axis, with the north pole turning counterclockwise, just like the real Earth does. Now pick up that globe and tilt the axis even further, until it's a full 122 degrees from true. You'll notice that the north pole is now angled toward the floor. According to the IAU, this means that the north pole has now become the south pole, and vice versa. Antarctica is now at the Earth's north pole, while North America and Eurasia are now in the southern hemisphere. The new south pole still rotates counterclockwise, just like it did when it was the north pole, but since east and west switched places at the same time north and south did, the Earth's rotation is now considered retrograde, moving from east to west. This is the situation on Pluto: tilted 122 degrees from its orbital plane, the south pole is the one that rotates counterclockwise.

Pluto's orbit is such that it passes through one of its equinoxes just two years before it reaches perihelion, its closest approach to the Sun. This is what happened during its most recent equinox in 1987: Pluto's northern hemisphere (which, you'll recall, rotates clockwise, unlike Earth's northern hemisphere) passed from summer to fall, and its southern hemisphere passed from winter to spring. This meant that Pluto's north pole, which had been in direct sunlight for 124 years, slipped into night, and the south pole, which had been in darkness for 124 years, slipped into daylight.

Because of Pluto's unusually eliptical orbit, this equinox goes by a lot faster than the other equinox, which occurs during aphelion, and as a result winter and spring are shorter in the southern hemisphere than summer and fall. Winter and spring in Pluto's southern hemisphere "only" last 52 years each, while summer and fall last 72 years each. Spring was about half over in Pluto's southern hemisphere when New Horizons made its flyby, and the summer solstice was coming up fast when the Hades brought me to Pluto 23 years later.

Summer on Pluto isn't like summer on Earth, even accounting for the fact that Pluto is so much colder. Because Pluto's axis of rotation is tilted 122 degrees from its orbital plane, Pluto is basically rolling on its side as it orbits the sun, with its south pole pointed 32 degrees "below" its orbital plane, and its north pole pointed the same amount above it. When spring comes to Pluto, the pole peeks over the edge of the world, and travels across the dayside until the fall equinox, when it slips over the far edge into the nightside. During the summer solstice, the Sun is only 32 degrees away from the azimuth, or "directly overhead" point, at the pole, which is the closest it ever gets there. This was the reason that the mission planners at the International Space Agency decided to establish our main base at Pluto's south pole, and this was the reason I rode the Cerberus down to Point South after detaching from the Hades. When the Cerberus touched down at Point South on August 22, 2038, I had come to the place where I would be spending the majority of my remaining years.

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