This was posted before, over at the old WordPress site. I have since updated the pictures with the latest images of Pluto and Charon. Also, comparison to the view of the moon from Earth is included in each picture and a lot of reference lines were deleted. January 25, 2019
Steve Campbell December 2015
The full moon, especially when it is near the horizon has always been a most impressive sight and there are few that compare…at least on Earth. The truth is that your index finger at arm’s length can cover the moon. You will need to close one eye for this trick, though. If you saw Tom Hanks in Apollo13, you know the drill. The full moon covers less than one half of one degree of arc. By the way, the idea that the moon is somehow magnified by being near the horizon is just an illusion. If you measured the angle it “subtends”, you will come up with the same number when it is overhead or low in the sky. Call it a Jedi mind trick, if you like. Figure A is a simulation of the Moon, as seen from the Earth. So that we can make a fair comparison, the field of view here is 10 degrees across. I will keep that constant as we move around.
The lines are just the plotted orbits of the moon and other planets in the background.
Figure A: A simulation of the Moon as seen from Earth. The field of view is 10 degrees wide. That scale will be continued in the following images – until it doesn’t work. I’ll include the moon in those images as a reference.
In the neighborhood
Impressive as our full moon view may be there are other, more breath-taking vistas to be had, with a small matter of transportation to be solved. There is one that has actually been experienced by Apollo astronauts on eight missions – each with a crew of three – that reached the moon. So, that is exactly 24 people, right? Wrong, twenty one. John Young and Eugene Cernan both went to the moon twice and landed once. Jim Lovell went twice and alas, did not land either time. But all these guys saw the Earth from the moon, even if only 12 saw it from the surface. That sight is even more striking. Figure B is a simulation of the Earth as seen from the moon.
Figure B: Simulation of the Earth as seen from the moon. Same scale
As awe-inspiring as this view may be, you can still hide this behind a single digit, but you will need to use your thumb. The Earth’s atmosphere is not depicted here. That would make it more interesting, especially during a lunar eclipse. At that time, the Earth would be illuminated only by artificial lights and the occasional burst of lightening. If it were a total lunar eclipse, the rim of Earth’s atmosphere would be aglow with the red hues of every sunset and every sunrise in the world. The lunar landscape would be bathed in a soft red glow. Not bad, eh?
The Outer Limits
Until recently it was assumed that the Earth-moon system was the closest to a double planet, but that ended in 1978 when Pluto’s moon Charon was discovered. Compared to the Earth/Moon system, Pluto and Charon are much closer to the same size and much closer together. So, you would think –and be correct – that Charon would appear much larger from Pluto than the Moon from Earth. Figure C is a simulation (same scale) of Charon as seen from Pluto.
Figure C: Charon as seen from Pluto. Note that this is an actual image of Charon, from the New Horizon’s flyby.
As you can see clearly, Charon is a much more imposing figure than the Earth from our moon. To continue our arm’s-length-finger imagery: I am quoting a New Horizons researcher whose name escapes me when I say “It would take three fat fingers to cover up Charon.”
However, I would point out a few drawbacks to this one. First, the lack of color (as compared to Earth, at least) should subtract a few points. Second, Charon will not be seen as “full” except for twice in a Pluto year, when the plane of the Pluto/Charon orbit is aligned with the Sun/Pluto orbit. A Pluto year is about 248 Earth years. Third, the Sun is very dim out there by Pluto and the whole scene will be very obscure. Fourth, Charon is tidally locked, always turning the same face toward Pluto and Pluto does likewise to Charon. This means that Charon always appears in the same spot in the sky, unless you are in the other hemisphere, then you never see it anyway. And, finally that eclipse scenario we talked about with the Moon and Earth only happens around those times of “full Charon” (every ~124 years) and it has no atmosphere to make it more interesting. The alert reader (that guy, again) will point out that Pluto must be much larger as seen from Charon than the other way around. Correct. Figure D is just that scenarion.
Figure D: A simulation of Pluto as seen from Charon.
Again, the surface features are guesswork and not updated by the recent New Horizon probe’s imagery.
You see that, in our 10 degree wide view, Pluto has filled it, at least vertically. Clearly, this is the most stunning view, yet. One full hand, at arm’s length might not even cover this up.
As I note in the caption, this is not the real image of Pluto, which, as it turns out is much more interesting. However, most of the objections of the view of Charon apply here. It is very dark. Pluto will always be in the same spot in the sky, if it is in the sky, at all. Pluto will only be “full” once in 224 years and the eclipses will be about that rare as well. There will be a “rim of light” during a full eclipse, but it will be pale blue.
Now is where the real estate people would put the “hook” that draws you in to the thing they are really selling. As it turns out, there are much more amazing views available, much closer to home, far more dynamic, interesting and much better illuminated than those “outer limits” properties that you have been seeing! Let’s talk about the Jupiter neighborhood.
There is this moon called Io (pronounce by purists with a short “I” and by everybody else, with a long “I”). It is the closest of the four main “Galilean” satellites of Jupiter. Io is a small moon of Jupiter that is still 37% larger than Earth’s moon.
It has a view of Jupiter that is un-paralleled by any other self-respecting Jovian moon (i.e., with a significant surface gravity). I have summoned up another simulation at our same scale of 10 degrees across., in Figure E.
Figure E: Simulation of Jupiter seen from Io. Clearly a change of scale is in order!
This is an order of magnitude greater than what we have looked at previously. We need to back off of this 10 degree wide view. Figure F is a view with 45 degrees as the width of field. To show how much we have “zoomed out” please see the inset at lower left that shows the moon as seen from Earth at the same scale.
Figure F: Jupiter (in a section of sky that is 25 degrees across) as seen from its closest large moon, Io. Note the comparable view of the Earth’s moon – from Earth.
This is clearly the most spectacular view we have yet imagined. The arm’s-length comparison: Tom Hanks would have to hold up a pizza pan that is 13* inches across to cover this sight. And here, we will have a view of Jupiter that goes from “full” as you see it here, through a half-phase to a crescent, to “new” and back in less than two days. Add to that the fact that Jupiter rotates in about nine hours and the clouds are in constant motion and changing. And also, the colors (in comparison to our own moon) are vivid and diverse. When Io is eclipsed, passing behind Jupiter, there will also be lightening and auroras that should surpass anything seen on Earth from orbit.
*Ironic, that! 😉
I think that when it comes to spectacular scenic outlooks in the Solar System, we have found the ideal spot – Io. Remember to move to the side facing Jupiter and the view is always available. But, before you decide you would like to live there, I should say that I failed to mention that it is a very active place, geologically speaking – far more so than Earth. Io is prone to sudden outbreaks of hot molten Sulfur volcanoes. It also orbits in a radiation belt around Jupiter. And to quote astrobio.net (7): “The radiation in Jupiter’s belts is a million times more intense than in Earth’s belts.”.
So, you see that there is this great view at Io, but there are a few details that complicate things.
- Solar System Simulations: http://space.jpl.nasa.gov/
- Apollo Astronauts: https://en.wikipedia.org/wiki/List_of_Apollo_astronauts#Apollo_astronauts_who_walked_on_the_Moon
- Charon: https://en.wikipedia.org/wiki/Charon_(moon)
- Planetary Statistics: http://www.windows2universe.org/our_solar_system/planets_table.htm
- Moon comparisons: https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size
- More on Jovian moons: https://en.wikipedia.org/wiki/Moons_of_Jupiter
- Jupiter Radiation Belts: http://www.astrobio.net/news-exclusive/hiding-from-jupiters-radiation/