If there were two earth-like planets in a binary system and one of them had landmasses, and the other one had oceans covering a significant amount of its surface, could you see the continents of the first planet reflected onto the oceans of the second one?

If so, could this also work with oceanic moons?

  • 1
    $\begingroup$ You're basically talking about a version of Earthshine, which is visible (barely) and easily visible by telescope. space.com/6556-earth-reflection-varies-moon.html or planetshine en.wikipedia.org/wiki/Planetshine The Moon is a very bad reflector. It's the color of dark asphalt. Oceans should be much better but clear enough to see continents . . . tough call. Maybe someone else can answer. On seeing the planet's reflection off the other planet's ocean - sure. That seems likely. $\endgroup$
    – userLTK
    Oct 15, 2017 at 1:18
  • 2
    $\begingroup$ The planets would have to be very close. A good answer would have to explain how realistic ocean surfaces scatter more light into a fairly narrow cone centered on the specular direction while land scatters light more evenly. $\endgroup$
    – uhoh
    Oct 15, 2017 at 4:44
  • $\begingroup$ videosanimation.com/post/121373738567/animation-night-day still looking for the original source. Probably images from some geostationary weather satellite. Reflection of the Sun indicates some degree of directionality from water, but the viewing planet would have to be really big and close to see much. $\endgroup$
    – uhoh
    Feb 8, 2019 at 7:50

1 Answer 1


The surface of the ocean isn't smooth, so there would not an image formed.

Look at how the Earth's oceans reflect the sun.

enter image description here

(credit: NASA/ESA)

Obviously the sun is reflected by the ocean, but the rough surface of the ocean scatters the light enough for no image to be formed. It would be the same if there were a planet being reflected: No detail would be visible.

  • $\begingroup$ Well... and really drifting off into Worldbuilding.SE, if the putative planets had no atmosphere, the ocean surfaces would be a lot smoother. Granted, we'd have to make them oceans of some fluid with a very low partial pressure, etc. $\endgroup$ Oct 16, 2017 at 14:56
  • $\begingroup$ You mean a real planet Mercury? $\endgroup$
    – James K
    Oct 16, 2017 at 16:09
  • $\begingroup$ Well, that would certainly do it :-) $\endgroup$ Oct 16, 2017 at 18:35
  • 1
    $\begingroup$ The question doesn't really ask about a crystal-clear image. The majority of the light is going to be reflected within a fairly narrow angular deviation from specular. If the two planets are close and the planet you were standing on had large continents or any albedo modulation (e.g. green vs brown) you'd could recognize some pattern. The diffuseness of the reflection is also weather dependent - a period with low wind speed will result in a calmer, flatter sea and narrower diffuseness. $\endgroup$
    – uhoh
    Oct 16, 2017 at 20:40
  • $\begingroup$ That's Italy (up is roughly south) in this photo flickr.com/photos/nasa2explore/29185512413/in/photostream (also here i.stack.imgur.com/hYrps.jpg) next to the reflection of the Moon. FWHM might be 50 km at a distance of 400 km or even less. I'll keep looking for a better one, just because it's fun. $\endgroup$
    – uhoh
    Jan 24, 2018 at 9:02

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .