Phases & Seasons

Here's Looking at You!

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The Moon is a familiar object in our night sky, and the Apollo astronauts' photos have shown us what the Earth looks like from the Moon. What would Charon look like from Pluto, or Pluto from Charon? Charon is about half of Pluto's size and located about 17 "Pluto radii" away. This means that the disk of Charon is about 3.5 degrees across when seen from Pluto, and Pluto is twice that size (7 degrees) when seen from Charon.

This diagram shows one way to measure angles with your fingers. If you hold your arm out toward the sky, the width of a finger is about 1 degree. This means that from Pluto the disk of Charon is about 3 or 4 fingers across. Our Moon (and Sun, coincidentally) is about one-half of a degree across - the width of a skinny pinky finger. From Charon, the disk of Pluto is wider than an outstretched fist. Wouldn't that be an amazing sight in the sky!


Cycle of Phases

We are used to the Moon rising and setting every day, mainly due to Earth's daily spin. We also know how the Moon's appearance changes over a month. The 29 1/2-day lunar cycle of phases stems from the changing illumination by the Sun as the Moon orbits Earth.

moon phases

Because the Moon's spin period is the same as its orbital period around Earth, the same face of the Moon always faces the Earth. We never get to see the "backside" of the Moon from our planet.

Now try to think about what Charon would look like from Pluto - and Pluto from Charon. Just like our Moon, Charon's spin period is the same as its orbital period - 6.4 Earth days in Charon's case - and from Pluto you only see the one side of the moon. But Pluto also spins at the same 6.4-day rate, meaning that from Charon a person would only see one side of Pluto. This leads to an interesting situation - animated here - where a (red) person on the "backside" of Pluto never sees Charon and a (red) person on the "backside" of Charon never sees Pluto.

view Pluto

Moreover, from any particular location on Pluto, Charon appears to stay in the same place in the night sky for the whole 6.4 days - and Pluto appears fixed in the sky when viewed from Charon.

While Pluto/Charon does not appear to move when viewed from Charon/Pluto, the illumination by the Sun does change. So, both Pluto and Charon go through a 6.4-day cycle of phases. In this diagram the top panel shows the cycle of phases seen by the (blue) person on Pluto looking up at Charon at times 1-8 over the 6.4-day cycle.

In this diagram the top panel shows the cycle of phases seen by the (blue) person on Charon looking up at Pluto at times 1-8 over the 6.4-day cycle.

view Charon

The tilt of Pluto's spin (and Charon's orbit) leads to more subtle changes in the phases over Pluto's full year. New Horizons science team member Marc Buie of Southwest Research Institute shows more of these effects on phases of Charon on his website.

Because objects in the outer solar system never come between Earth and the Sun, we never see their "backsides" and so we'd never see a new or crescent Pluto from Earth. Wherever we are in our orbit around the Sun we'd see the sunlit side of a Pluto and it always looks nearly full. What a shame!

HST images

We can detect if Pluto's surface is smooth or rough by looking at it during different phases, just as you can tell the difference between a ball bearing and a cotton ball by how they reflect light at different angles. By flying behind Pluto - as scientists plan - the New Horizons spacecraft will take pictures of Pluto in phases we could never see from Earth.


'Tis the Seasons

Seasons on Earth are mainly caused by the 23-degree tilt of its spin axis. This axis points in a fixed direction toward Polaris (also known as the North Star or Pole Star). The diagram shows an oblique view of Earth's roughly circular orbit.

Earth tilt

Because Earth's axis tilts with respect to its orbital plane, the angle of the sunlight we receive in different places over the globe changes during the year. From March until September, when the North Pole points toward the Sun, the days are longer, the Sun is higher in the Northern Hemisphere, and we have northern summer and southern winter. Because Earth's orbit is very close to circular, the slight change in distance between the Earth and the Sun does not cause any appreciable change in sunlight. In fact, we are slightly closer to the Sun in December than we are in July.

seasons

This diagram refers to seasons experienced in the Northern Hemisphere. The Southern Hemisphere has equinox (equal night and day) at the same time, but experiences winter while it's summer in the north and vice versa.


Seasons on Pluto

Both tilt and distance affect seasons on Pluto. The planet's spin axis is nearly "sideways," tilted over 120 degrees, so the tilt alone leads to extreme seasons on Pluto. And because Pluto's eccentric orbit swings it between 29.7 and 49.3 AU from the Sun, the amount of sunlight varies by a factor of 280% over a Pluto year (rather than just a 5% variation on Earth).

Since planets move faster when closer to the Sun, Pluto's northern fall lasts much longer than the quickly passing northern spring.