July 19, 2018

Pluto at Opposition 2018: Transit Dreaming

Earth and the Moon in transit as seen from Pluto.

Credit: Starry Night.

What sorts of celestial scenes would you witness, if you could magically sit on some far flung space rock? An interesting upcoming alignment was recently brought to our attention by our friends over at Earth & Sky and astronomer Anne Verbiscer at the Department of Astronomy at the University of Virginia on the NASA New Horizons blog, prompting us to take a closer look at a unique event that will go unwitnessed by human (or robotic) eyes: a transit of the Earth and the Moon on July 12th, 2018 (as reckoned in Universal Time)… as seen from Pluto.

This alignment occurs because the 2018 opposition of Pluto sees it very near one of its two ecliptic crossing nodes. Orbiting the Sun once every 248 years in a highly inclined orbit tilted 17 degrees with respect to the Earth’s path, these crossings occur during alternate spans of 87 versus 161 years. In fact, it was during the last node crossing back in 1930 when Clyde Tombaugh spotted Pluto, drifting through the star fields of the astronomical constellation of Gemini the Twins. It’s sobering to think that in just over eight decades since its discovery, slow-moving Pluto has only moved seven constellations (we’re count the pesky non-zodiacal constellation Ophiuchus) eastward to Sagittarius the Archer in 2018.

The orbital nodes of Pluto. Graphic credit: Anne Verbiscer.

Incidentally, the fact that Pluto was near a node and the ecliptic plane–right where you’d expect a planet to hide—very probably upped Mr. Tombaugh’s chance’s of spotting it. Pluto was also moving towards perihelion 29.7 astronomical units (AU) from the Sun on September 5th, 1989, meaning it reached its maximum brightness of magnitude +13.7 during oppositions right around that year. In 2018, it’ll only reach magnitude +14.2 at opposition, as it heads toward aphelion 49.3 AU from the Sun in February, 2114.

What would you see on Pluto on July 12th of this year, staring back at the Sun? We’ll assume you’re equipped with a life support system to brave the brisk Plutonian realm of high noon, and a solar filtered telescope tuned for the dim, -19 magnitude Sun, brighter than a Full Moon but about 1/30,000th the brightness of high noon on Earth. Old Sol would only appear an arc minute across, barely showing a discernible disk to the naked eye. See that tiny 0.5” dot? That’s the Earth, taking about 10 hours to span the disk of the Sun. The Moon is tinier still, at a diminutive 0.2” arcseconds across as it accompanies the Earth on its trek. (better pack a really powerful telescope).

Pluto’s large moon Charon would be the top draw in the Plutonian sky, at only 11,800 miles distant and appearing an amazing four degrees across (that’s eight times larger than a Full Moon here on Earth!) as it went through its cycle of phases once every 6.4 days. Giant Charon can eclipse the tiny Sun on as seen from Pluto as well, which will next occur starting in 2107 AD. From Earth, we’ll see a series of mutual occultations of the pair around the same time, as Pluto and Charon alternate passing one in front of the other.

An amazing view: Pluto backlit by the Sun as seen from New Horizons in 2015 shortly after flyby. Credit: NASA/New Horizons.

What about NASA’s New Horizons spacecraft, just 4.5 degrees from Pluto as seen from the Earth? Unfortunately, that angle is juuust far enough off that Earth will miss transiting the Sun from its point of view. And even if it were, New Horizons Long Range Reconnaissance Imager (LORRI) imager—were it equipped to stare at the Sun—is equivalent to an 8” Schmidt-Cassegrain amateur telescope, a decent size, to be sure, but it would still have a tough time resolving a 0.5 arcsecond disk crossing the face of the Sun.

New Horizons will visit Kuiper Belt Object Ultima Thule coming right up on New Year’s Day, 2019.

After 2018, Pluto spends the next 161 years in the southern celestial hemisphere.  Mark your calendars: Stick around until–you guessed it January 12th, 2178 and again on January 13th, 2179 AD, and any would-be Plutonian colonists can at least witness a transit of Earth and the Moon across the Sun… for real.


  1. [...] Earth and the moon transit the sun. The video below, credited to David Dickinson of the blog AstroGuyz, shows what an Earth transit would look like on [...]

  2. [...] Earth and the moon transit the sun. The video below, credited to David Dickinson of the blog AstroGuyz, shows what an Earth transit would look like on [...]

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