Hot answers tagged

54

LOCKS The answer is one that is used extensively today in canals and rivers: Locks. Locks are basically double water-tight gates. If you want to go from high water to low, you sail in, the gates are closed behind you, and water is led or pumped out until it is level with the lower water within/outside the harbor. Vice versa for going from low water to high. ...


36

Some years ago I went to Scarborough (Yorkshire) and visited the local harbor during low tide: all the ships were sitting on wet sand, as shown in the painting below (for some reason I can only find stock photos) If you are able to predict the tides, it's something you can live with. If instead you want to ensure water access at every time, you can resort ...


30

No tidal locking The moon still hasn't developed a tidal lock with the planet. So, the hemisphere of this moon facing the planet is always different. Because of its geological history, this moon's surface has areas with very different albedo, think something like Ceres bright spots, but more widespread. This way, its brightness has some spikes only in ...


26

Just take a look how communities with much higher than average tides handle the issue IRL. e.g. these boats are in the Bay of Fundy, which due to quirks of geography has the worlds highest tides at up to 53ft: As you can see, people just accept that their boats will be grounded for a significant chunk of time. Large ships can be flat bottomed and designed ...


23

Under the right conditions, shadows can be cast under the light reflected by Venus. And this is what a body covered by water and plants look like from space. Consider that it will be way closer than Venus is to Earth, and stay assured that it will be enough of a light source at night. You can also see it by yourself: go out and look at the new Moon, if you ...


21

The moon's albedo is around 0.1, meaning it reflects about 10% of the sunlight that hits it, for a peak illuminance of a few tenths of a lux (sources on albedo and lux). Even if the moon were a perfect mirror, it would only reflect 10x as much light and would only cast a few lux, making its illuminance during a full moon only a few hundredths of a percent of ...


21

This could be achieved by placing the moon in a slightly inclined orbit outside an inner semi-transparent ring system. (side view) For most of the year, people living on the planet will view the moon through the rings (which need to have some thickness, a "smoke ring"). They will see a dimmed version of the regular moon phases. But two times every ...


20

Zeus Wins As a reader of this story, my first question is: "Did the author read The Moon is a Harsh Mistress, by Heinlien?" You said "yes", so I don't get why Zeus is unable to throw rocks at Earth, or any other target. It has: Robots Energy All the resources of Luna The mere threat of throwing rocks should be enough for humanity to ...


15

Let's break it down by type of system, as planet-moon or planet-planet systems will have different constraints from star-planet systems. Planet-moon/planet-planet system Looking at planet-moon or binary planet systems, the answer to your question will always involve standing on a decent sized moon and looking up at the host planet. A moon by definition has ...


13

Could it have a Titan size moon? Well, maybe. Titan's only twice as big as our own moon, after all... not like ten times bigger or anything. Our moon does seem to be a bit unusual, as it wasn't formed at the same time as the Earth, but in a subsequent collision with another planet. It wouldn't be beyond the realms of possibility for a larger moon to have ...


13

Planetary albedo is the percentage of light/radiation reflected back into space by a stellar object. Using accumulated satellite data going back to the 70's, the Earth's average albedo is around 0.30, or about 30% of the light/radiation Earth receives is reflected back into space. If the Earth were an ice covered world, the estimated albedo would instead be ...


13

Given the extreme tide experienced globally, smaller boats as we know them would likely fall out of favor, and the daily driver would be overtaken by amphibious vehicles, which can negate the need for traditional docks entirely. Several answers here include floating docks, which I think will just have to be the reality of larger cargo and cruise vessels, ...


12

So, you didn't specify exactly what "half the size" means, so I'll go with the most conservative reading of the term, which would be "half the radius". This is smaller than "half the mass" or "half the apparent area" and the like. It won't make much difference though. Our Moon has a radius of 1737km, which will be the ...


12

I work with some ports that have high tidal ranges, it's a big issue, but it's a problem for which many solutions have been developed. If the vessels are small, they can simply rest on the seabed when the tide goes out. If the vessels are large then you can schedule the ship movements so they won't ground. If the port would completely dry out during low tide ...


11

No. The moon is simply too small to be useful as a mirror. As you're no doubt aware, moonlight is already reflected light from the sun. Lunar dust is fairly dark, with an albedo of about 11-12%. In other words, about 8/9ths of the light that falls on the moon doesn't reach Earth. With a perfect mirror, you could deliver all of that. The problem is, the moon ...


10

Not At All, For Two Reasons First, if a moon's ring system extended far enough to overlap the location of its parent planet, the rings wouldn't form in the first place, since no ring can form where a planetary body sweeps them out (part of why Saturn's moons don't fall within its ring system). So the rings wouldn't rain down on the planet, because if they ...


10

First thing to remember is that when you're close to a spherical object, you can only see a portion of its surface (the horizon). Therefore the anglular diamater of some body with radius $r$ viewed from a distance from the surface of $h$ will be $2\sin^{-1}\frac{r}{r + h}$. (this also means that the maximum angular diameter of a celestial object (such as the ...


10

If your moon had an orbit significantly inclined to the plane of the ecliptic (i.e. the planet's orbital plane), it would only be full close to the ecliptic a couple times a year (depending on its orbital period relative to the planet's, perhaps you'd have occasional years with one or two more or fewer such full moons -- one pair would be split around the ...


10

You're doing it wrong First, bear in mind that optics used to be a normal branch of physics where you had to learn all the impossible things you can't do. Now, well, this is by no means the strangest thing I've read recently: Anti-Solar Cells: A Photovoltaic Cell That Works At Night Next, well, if you want to bounce light off the Moon to a point on Earth, ...


9

First of all, for lunar regolith we can neglect reflection and just consider scattering. This paper simulated the scattering properties of the lunar surface, and came to values around 22% of reduced reflectance for incidence angles between 32 and 57 degrees. I haven't been able to find a comparable chart for talcum, beyond generic statement like "...


8

You could see Tokyo on the moon with a cheap telescope. So using a cheap telescope I found on amazon, one could get 40 times magnification. The full moon is 1/2 a degree in the sky. Or 30 arc minutes. The human eye can see about 28 arc seconds of resolution. So your basically getting 62 "pixels" of moon for the naked eye in horizontal and vertical ...


8

The first thing on the moon that could be visible from earth is already there. A mirror. Will anyone notice, 100 feet away, something else Armstrong left behind? Ringed by footprints, sitting in the moondust, lies a 2-foot wide panel studded with 100 mirrors pointing at Earth: the "lunar laser ranging retroreflector array." Apollo 11 astronauts ...


8

The Moon albedo is 0.11, Saturn albedo is 0.342. So Saturn reflects about 3 times more light the Moon per unit surface. Since it appears in the sky 7 times larger than the Moon, overall it will look about 21 times brighter than the Moon, if it orbits the star at the same distance Earth does. If instead it orbits at the distance of Saturn, we drop into the ...


8

Everything is brighter. https://commons.wikimedia.org/wiki/File:Ellip-orbit.gif Your planet and its moons are in an elliptical orbit around its star. When it is close the star is brighter. Because of that, reflected light from the moons is also brighter. The more elliptical the orbit is, the shorter the time near the star is.


7

I think that a planet that can hold an Earth like atmosphere can keep two spherical moons with no big issues. Let's break down your question in sub-problems. First: what is the smallest a body can be while becoming spherical? For a body to become a sphere, it has to have sufficient self-gravity to pull it into a spherical shape. This depends on what the ...


7

The obvious answer is "there used to be an atmosphere, which has since been lost". Tectonic activity, while suitable for explaining large-scale structures like the Valles Marineris, cannot account for small-scale branching structures like your river valleys. There is, however, another way in which we can use Martian features as inspiration: ...


7

You're describing a 1:2:6:12:12:36:52 "Laplace resonance" There are lovely systems with orbital resonances in real life. The calendar you need must be based on the least common multiple of these numbers. So we factorize each one: -;2;2x3;2x2x3;2x2x3;2x2x3x3;2x2x13. Now we string together enough factors that each one has enough pieces to work ...


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