I admit: I really like the idea of this meme. It’s neat to compare our planetary neighbors, particularly from a perspective we don’t get to see in real life. I give the author kudos.
Although this meme isn’t particularly Stupid or Bad, there are some minor issues to discuss – pedantic issues, really. In most cases I wouldn’t even bother bringing them up, but I really love astronomy and I want to talk about the planets for a bit. If you only want to read the posts where I aim my Snark-O-Matic 5000 at the most offensive memes ever dreamt of, come back in a couple of days. I’ve got some memes in the hopper that are sure to steam your socks.
Let’s start by talking about how we see the Moon. Obviously we cannot see the entire surface of the Moon – not even during a full moon – because one lunar hemisphere is always turned away from us. But it’s worse than that: we cannot even see the entire Earthward hemisphere. Because the Moon is spheroidal instead of a flat disk, our line of sight lies tangent to its edge at a point that is somewhat ahead of the cross-sectional disk. Perhaps the following not-to-scale illustration will make this more clear.
The yellow portion of the Moon is the portion that is visible to the observer; the pink portion is not visible because it is blocked from the observer’s line of sight. Notice that less than half of the Moon’s surface is visible at any given moment. If this diagram were to scale you would see that it’s not much less than half, really we can see more than 49.5% of the lunar surface depending on the position of the Moon in the sky and in its orbit.
(In reality, we see about 59% of the Moon’s surface over time due to libration, which allows an observer on Earth to see the Moon from different perspectives at different times.)
If the Moon were larger, however, we’d be able to see even less of its surface. If the Moon were the size of Jupiter (or if you were to replace the Moon with Jupiter, as per this meme), you’d only be able to see about 41% of its surface. It wouldn’t look the same as it does through a telescope, because you’re not really up close when you look through a ‘scope. The cloud bands, which are pretty much parallel in real life, would appear to bow outward from the center-point of Jupiter’s face. I think the meme got that part right, although I’m not sure if the amount of bowing is proper.
The size of Jupiter’s disk is also problematic. At its average distance, the Moon has an angular diameter of 0.55279 degrees. In other words, if you were to draw a circle all the way around the sky and divide it into 360 equal parts, the Moon would be a little bit wider than half a part.
Jupiter’s angular diameter, if it were centered only 384,400 km from Earth, would be a staggering 21 degrees to the people directly beneath it (slightly smaller if you were seeing it close to the horizon). To give you some perspective, hold both of your fists at arm’s length, side by side. That’s about how wide Jupiter would appear in Earth’s sky if it were one lunar distance away. Never mind the ferocious tides, constant earthquakes, and violent windstorms resulting from the relative proximity of such a huge gravitational influence; the sight would be spectacular!
Perhaps even more spectacular than this meme depicts. The angular diameter of Jupiter would be about 38 times greater than that of the present-day Moon. The Jupiter disk depicted in this meme is actually too small! In reality, Jupiter’s disk would be about as large as the orange circle in the modified image below.
Saturn’s equatorial radius is 60,264 kilometers (meaning its angular diameter would be about 18 degrees; smaller than Jupiter but still considerably larger than what is depicted here). But its rings…Saturn’s rings would truly be a sight to behold. Saturn’s ring system has an outer radius of about 181,000 kilometers, so it would stretch nearly halfway from Saturn to Earth. Assuming that Earth was orbiting around Saturn (for that would be the way of it) in the plane of its rings, the ring system would span an incredible 43 degrees across the sky, wider than four fists held side by side at arm’s length. If Saturn appeared “sideways” in Earth’s sky, with one edge of the ring system touching the horizon, the opposite edge would be halfway up the sky.
So the sizes may not be exactly accurate, but let’s leave that behind and talk about brightness.
Planets reflect a lot of light; that’s why we can see them. The brightness of a planet depends on four things: its distance from the Sun (which determines how much light it receives), its albedo (which determines how much light it reflects), its phase angle relative to the observer (which determines what portion of the sunlit face we can see from Earth), and its distance from Earth (which determines how much reflected light reaches Earth). Calculating the brightness of a planet is hard enough when the planet is where it’s supposed to be, let alone when you place the planet in an imaginary position 384,400 kilometers from Earth. The numbers I’m about to spew out should be considered estimates at best.
Let’s start with Mercury. If Mercury were centered on a point 384,400 kilometers from Earth, it would be farther from the Sun, so it wouldn’t receive so much light. But…it would be much, much closer to Earth, so that would make it seem much brighter. Also, Mercury has a higher albedo than the Moon, which is another way of saying that it’s more reflective of visible light. Put those factors together and you find that a full Mercury, as seen from Earth, would be between 1.5 and 2 times as bright as the full moon. I see that the creator of this meme has added a bit of a glow around Mercury to simulate the scattering of Mercuryshine in Earth’s atmosphere, but I’m not sure it’s enough. The reflected light of Mercury at the Moon’s distance would be more than sufficient to read by, and the sky would be significantly brightened by it.
The memer did a better job with the Venus picture, obviously understanding that Venus is a highly reflective planet and would brighten Earth’s “night” sky dramatically. In fact, Venus would be almost 45 times brighter than the full Moon. When fully illuminated by sunlight, it would cast an eerie glow across Earth’s night sky…sort of a twilight glow somewhere between night and day.
If Jupiter were present at the Moon’s current position, and if it were at opposition (on the opposite side of Earth from the Sun), its reflected light would end all concept of “night”. It would be thousands of times brighter than the full Moon and more than one percent as bright as the Sun. Now that reflected light would be “spread out” across the illuminated face of Jupiter, which would fill an area about 1500 times the area of the Sun or Moon in Earth’s sky. In other words, the light density wouldn’t be painful to your eyes, but it would certainly put the kibosh on your plans to go stargazing. You’d probably want to invest in a good set of blackout shades if you plan to get any sleep.
Now I’ll repeat this: I think this meme is an interesting idea. I certainly don’t expect the author to engage in the hours of Photoshopping required to make the images look more realistic. But since the image is out there, and since knowledge is power, I thought it would be worth talking about.