Fictional Lunar colony - information requested

[oliverh72]

Hi all,

I'm working on writing a science fiction novel set on the moon. A key element of this is the city that has been built there. It is right near the north pole, near to where a large (fictional) cache of ice has been found.

What I need to know is this: the city exists within a giant dome (approx 10 miles across or about 80 sq miles total) which is set on an enormous maglev track. Ahead of the city on the track is a solar panel array. Both the city and the panels move around the track, which is circular, and orbits the pole of the moon.

The idea is that the solar panels stay at a constant speed, staying continuously in the sunlight. The city on the other hand, connects with the panels to "recharge the batteries" (so to speak) for 12 hours, then disconnects to slow down enough to move across the line of light into the darkness, thereby creating "night". Basically, the city would move back and forth across this line every 12 hours, simulating an earth-like day.

Here's the question. How fast would both of these things be moving? From what I've been able to gather, even if you were at the equator, the moon only spins at about 10 miles/hour. Assumedly, that would also be the rate of the light/dark line that passes over the planet, since the spin and orbit are the same - about 29.5 days. So assuming you were near the pole, that speed would be reduced to almost nothing... or a couple miles per hour, max?

Am I right?

Secondly, if the dome were (alternatively) in a fixed position near the pole and assuming it was not hidden beneath the edge of a crater - ie: it had a reasonably unobstructed view of the earth and the sun - the length of that city's "night" would be 14.75 earth days or about 350 hours, correct? Also, how much light would the earth be contributing - ie: how dark would it be?

Thanks for your help!

Oliver

[AndyG63]

The Moon's synodic period is 29.5306 days. Given the diameter of the Moon, the "point of midday" moves at about 15.41 km/s around the equator. That speed, as you note, reduces with latitude, and it does it like this:

V = 15.41*cos(latitude)

So it drops to walking speed north (or south) of about 75 degrees, and becomes zero at the pole - there, you'd rotate, not translate! Incidentally, why use maglev? Why not just have a giant crawler-transporter wandering around a track of crushed regolith?

I'm not sure what you mean regarding the dynamics of your 24-hour-day city's movement, so I can't comment on that.

Meanwhile, for the fixed city at the pole, the sun and Earth would always be low in the sky - and occasionally dip below the horizon due to the Moon's axial tilt (1.54 deg) and orbital inclination (5.145 deg relative to the ecliptic).

When the Earth was "up", you'd see it go through phases, and this would naturally change the amount of light cast by it. But for a full Earth, since it's about three times as reflective as the Moon, and around 13 times the apparent size, it could be 40 times brighter than a full Moon: a full four magnitudes, making it shine around -16.7. Easily bright enough to read small text by, and you'd have some colour vision too.

Andy

[oliverh72]

Thanks for that information!

As for the dynamics of the city's movement - it would be moving so that it could stay near to the light/dark line of the sun... if that makes any sense. Since that line would be very stark, it would be easy to see and follow.

The idea is that the city would be attempting to mimic a standard 24 hour earth day cycle, as opposed to the approximately 708 hours of the moon's noon to noon cycle. So the city would follow the line and leapfrog back and forth across it every 12 hours, simulating a 24 hour day...

Am I way off base here? Would that work?

[Aussie]

Not a clue about the calculations you want there, but there is water ice at the north pole of the moon. Not fictional I get what you meant though, the ice you're talking about is fictional.

[skybadger]

Hi

I ust read an Alistair Reynolds with cities on crawlers to stay beneath a satellite. I also just read Ben Bova regarding establishing the first colony on the moon. Involved getting solar panels to the pole to power the extraction of water from icy regolith and the use of nanobots to extract He3.

I think that the speed according to the cos law will be very slow since you are so near to the pole and also the boundary will be wider tha you expect, due to diffraction ( since no atmosphere for refraction) and the finite width of the sun leading to an umbral zone.

Mike

[The Warthog]

I am not suer what the axial tilt of the moon to the sun is, but if it is more than a degree or two, your city at the north pole would be in darkness for half the year, just as the north pole of the earth is. I just looked it up; the axial tilt of the moon is 1.5º, which leaves some high peaks in sunlight all year 'round, but at ground level there would be darkness for half the year. Some trigonometry should tell you whether that would be a great problem. The upper reaches of your dome may be high enough to catch sunlight, and the upper parts of your rotating solar cell array may be able to reflect light into the city when the floor of the ciry is in shadow.

I'd suggest a heavy soft plastic substance for the dome, as it could resist rupture by micrometeorites and be easily resealable should a larger object hit it. It could be supported by the air inside, eiiminating the need of a great structure.

Let me know when you get it built; I'll gladly move in. This is better than a dream castle in the air~