The Moon and Mercury

Copyright © Karl Dahlke, 2022

Just as each element is a familiar friend to a chemist, so each planet and moon in our solar system is familiar to an astronomer. One can almost imagine standing on the surface: the local gravity, the atmosphere, the temperature, the day night cycle, various geological features, volcanoes nearby, and other moons or rings in the sky. I already touched on Venus, with its choking carbon dioxide atmosphere, 90 times as dense as earth's, and its broiling temperatures, hot enough to melt lead. I also mentioned Titan, the largest moon of Saturn, with methane in its cryogenic hazy orange-tinged atmosphere, methane that rains down into lakes and rivers. I'm not going to describe every planet and every moon in detail, that would be rather tiresome, and we have other fish to fry - but I would like to spend a little time on the moon, our nearest neighbor, and then on Mercury. They are both airless worlds in proximity to the sun, with slow rotations, so they have some similarities. They both have the same frigid night time temperature, as each rocky surface, with no atmosphere to protect it, radiates its heat out to empty space. Even their days are similar, baked under the relentless sun, although Mercury gets much hotter, as you might imagine.

The moon presents the same face, the same hemisphere, to us at all times. This can bee seen with the unaided eye. Certain dark and light regions create images, sometimes called the "man in the moon", and those regions never rotate out of sight. This is clearer with your telescope. Look at the moon every night for a month, weather permitting, and note the same features, the same mountains, the same craters, the same seas.

The moon still has phases. The entire disk is visible during a full moon. The half moon waning shows the left side of the disk, and the half moon waxing shows the right side of the disk, but it's always the same landscape with different sections illuminated.

Go into a dark room with a friend, Kathy, and set up your light source as usual to represent the sun. Tell Kathy to stand still while you walk around her. She is the earth and you are the moon. As you walk around her, turn very slowly so that you always face her. In a realistic simulation she would be spinning, like the earth, but she might get rather dizzy so you don't have to do that part. Whenever she looks at you she sees your face, not the back of your head. You are making this possible by turning in synchrony as you walk around her. She might say you aren't spinning, because she always sees your face, but you are spinning, you rotate exactly once for every trip around. Walk around her once and spin around once, so that you always face her. Walk around twice and you have rotated twice, etc.

Now switch places and let her walk around you, while always facing you. You are the earth and Kathy is the moon. If you have the light source set up properly, shining like the sun from a distance, Kathy will go through phases. Sometimes you see her whole face, sometimes half the face, sometimes just a sliver, but it is always her face, never the back of her head.

Watch the moon in the night sky for a couple weeks, through your telescope if you like, and think of Kathy, and you'll see that the model is correct. The moon always faces the earth, and the phases of the moon are sunshine and darkness sweeping across the lunar landscape. The moon travels around the earth every 29.5 days, and it spins every 29.5 days to remain in sync - thus a day on the moon is 29.5 days on earth. If you lived on the moon, you would experience 2 weeks of sunshine and 2 weeks of darkness, and that is a lunar day.

The far side of the moon, that we cannot see, is sometimes called the dark side of the moon, but that is a misnomer. It receives sunshine and darkness, just like the near side, as the moon spins on its axis. When the moon is full, the far side is indeed dark. During a half moon, half of the near side is lit and half of the far side is lit. When the moon is new, or barely a sliver, the far side is bathed in sunshine. So Pink Floyd was correct - “There is no dark side of the moon, really.”

It seems a strange coincidence that the moon should turn just enough to keep one face towards us at all times, year after year, millenia after millennia. This is called a locked orbit, and it's not unusual at all. If a moon is close to its host planet, gravitational interactions slow its rotation until it is locked, with one face toward the planet at all times. Isaac Newton explained why, in his treatis on gravity and orbits. Our moon is not that close to the earth, but it was long ago. It was close enough to set one face toward earth, and once this happens, the orbit remains locked even as the moon pulls away. And why does the moon pull away from the earth? Isaac Newton explained that as well. With current technology we can measure this effect; the moon pulls away from the earth by 4 centimeters a year.

If you remember the conservation of energy, you might object! This seems like a free lunch. The moon is gaining potential energy, climbing to a higher orbit - but where does that energy come from? The answer is the earth; the earth's rotation slows down to push the moon away. Our day was 10 hours long when the moon was newly formed and close at hand. Since then our day has stretched to 24 hours, while pushing the moon a quarter million miles away. The energy of the earth-moon system is conserved. NASA has measured the earth's rotation, and our day is increasing by 1.7 milliseconds every century, in order to push the moon outward 4 centimeters a year.

Visiting the Moon

Stand on the moon at night, (which no astronaut has ever done), and look up. There is no air to obscure your view, no atmosphere, no clouds. You see stars, more stars than you will ever see on earth. With no air currents to jostle the starlight, the stars do not twinkle, they hold their position as if affixed to the fabric of space. Do the stars seem cold, or is the cold penetrating your space suit? Since there is no atmosphere to act like a blanket, the heat radiates away, and over the course of a lunar night, two weeks on earth, the temperature drops to -280 °F, -173 °C. Your suit has a heater in it, so you're ok, but you can feel the extreme cold through your feet as they touch the ground. In a flash, and without warning, the sun peeks over the horizon. Without air, there is no red orange sunrise to herald the break of day. The sun is suddenly there, and your shadow is absurdly long as it stretches across the moonscape. The heat of the sun is welcome, as it quickly warms one side of your metal suit. The ground takes much longer to heat up however, since it is currently colder than any place on earth, and the sun is striking it at an oblique angle. The land is grayish-white, with nothing but rocks and dust. There is no life, and almost no color, and yet there is beauty. “Magnificent desolation.” declared Buzz Aldren, the second man to set foot on the moon.

Over the next week, the sun rises overhead and bakes the ground until it reaches 260 °F, 127 °C. It's easier to generate heat then to pump it away, e.g. a furnace is simpler than an air conditioner, so you best be gone, or protected, before the moon becomes as hot as a griddle. So how did our 12 astronauts survive? They always landed at lunar dawn, when the ground was cold and the sun was rising. It takes 3 or 4 days for the moon to become dangerously hot, and if they wanted to stay longer, their suits were reflective and their boots well insulated. Still, they had to finish their experiments and head back into space before their lunar module became uncomfortably warm. If we ever develope a permanent moon base, we need to protect the occupants from the broiling heat of the lunar day and the deep freeze of the lunar night. Active heating and cooling is one option, but thermal inertia is another. Construct a large reservoir of water near the moon base. The water is contained in one or more sealed tanks, so it doesn't evaporate into space. During the lunar night, the water freezes, and all that latent heat is available to keep the moon base warm. During the lunar day, the ice melts, pulling heat away from the moon base. I'm skating past a lot of engineering challenges here, but active heating and air conditioning has its challenges as well, especially when there is no air to carry heat away from the coils. Large fins, hotter than the moon's surface, must radiate heat away, and that isn't very efficient.


The terminator is the line between dark and light, the line between day and night. It's hard to tell if you're standing on the terminator on earth, because our atmosphere scatters light all about. The blue of twilight and the red orange sunset obscure the line between day and night? Also, the terminator sweeps across the earth at 1,000 miles per hour at the equator, and almost as fast in the temperate zones. Even if the line was well defined, it would race under your feet faster than a jet plane.

The terminator is sharp and clear on an airless world such as the moon. The ground is lit on one side of the line, and dark on the other. (This ignores hills and valleys.) On the moon, the terminator moves at 9.5 mph, 15 kph, the speed of a bicycle. As the sun is setting, you can see the terminator heading toward your feet. It passes beneath you just as the sun dips below the horizon. The ground around your feet goes dark, but the ground to the west is still lit. The terminator sweeps the light away like a giant eraser traveling to the west. After an hour the terminator is over the horizon, and all is dark.

Instead of standing at the terminator, imagine the sun is directly overhead. It arcs across the sky at 9.5 mph, so if you wanted to keep it overhead, you could do so by riding west in one of those moon rovers. You could travel for hours, always keeping the sun directly overhead, until the rover ran out of charge, then you'd have to walk back to base.

Keeping Cool on Venus and Mercury

If you want to send an astronaut to Venus, the heat is a huge problem. On an outer planet you can always generate heat, by radioactivity if need be, but you can't create cool, so what to do? Let's say you have a source of energy, maybe a nuclear reactor, so that's not a problem. Plenty of power to cool the astronaut's home base. Thing is, an air conditioner works by pumping heat out of your house and into coils in the back yard, which lose heat to the outside air. That means the coils have to be hotter than the air, otherwise the heat can't flow "downhill". The air conditioner on Venus has to heat the external tubing to more than 900 °F, 500 °C, to work. With only modest changes in pressure, as per the compressor, we need a coolant that is liquid / gas in a temperature range from 0 to 500 °C. I don't know any substance like that, do you? I can't imagine how to build an air conditioner that would work on Venus, even in theory. I guess nothing will be landing on Venus any time soon.

In the 1960's and 70's, Russia successfully landed Venera probes on Venus, which sent back pictures of the surface. With no air conditioning, the probes lasted about an hour, more or less. Snap a few pictures, send them back to earth, then succumb to the heat. That's a lot of money, time, and effort for not much data. In contrast, NASA's Opportunity rover has been tooling around Mars for 13 years. One hour versus 13 years. Well we had to see the surface of Venus at least once, didn't we, to make sure the Three Stooges weren't there. So thank you, Russia, for doing that. Yes, there was a movie, Have Rocket Will Travel, wherein the Three Stooges went to Venus, but that's another story.

Now move to Mercury, which has a day night cycle. Daytime temperatures climb to 800 °F, 427 °C, but at night the temperature falls to -280 °F, -173 °C. Night on Mercury is the same as night on the moon, but daytime is quite another matter. Mercury is 3 times closer to the sun, and receives 9 times the solar energy. 9 times the heat and 9 times the light, with the sun looking 9 times as big and bright overhead. Remember that our atmosphere cuts and disperses almost half of the sun's energy, so you have to multiply the broiling sun of the Sahara desert by 15 to imagine the sun on Mercury. Active cooling is almost impossible under these conditions. construct a large, subterranean tank of water next to the house. At night, pipes carry pressurized ammonia (or some other liquid) through this reservoir, and out to tubes that are connected to radiator fins. Heat vents into space, whereupon the supercooled ammonia returns to the water, which then freezes into ice. During the day, the ammonia circulates between the tank of ice and your house. The ice melts, and keeps the house cool. You're taking advantage of the day / night cycle, averaging out the temperatures through the latent heat of the water ice. You need a big tank however, because a day on Mercury is 176 earth days long. That's a lot of heat to capture and then radiate back into space. This trick doesn't work on Venus, because its thick atmosphere keeps the entire planet the same temperature, day and night, all year round.

NASA's Messenger probe used a similar cooling strategy, on a much smaller scale, as it orbited Mercury for four years. Despite a sunshade, it absorbed heat from the sun while it was on the day side of the planet, then it radiated that heat away when it was on the night side. An elliptical orbit gave it more time on the night side to cool off. Sometimes the ellipse can be used to advantage.

The Sun Backs Up

Let's say you have a nice little house on Mercury, equipped with the aforementioned air conditioning. As you look out the window at the blazing sun, perhaps you recall the words of Isaiah 38:7-8.

“This is the Lord's sign to you that the Lord will do what he has promised - I will make the shadow cast by the sun go back the ten steps it has gone down on the stairway of Ahaz.” So the sunlight went back the ten steps it had gone down.

This seems unlikely in a literal sense, the amount of energy needed to stop and reverse Earth's rotation is almost unimaginable, but maybe the prophets were imagining life on Mercury. As mentioned earlier, a day on Mercury is 176 earth days long. In other words, Mercury spins very slowly on its axis. That's 88 days of intense sunlight as the sun creeps across the sky, and 88 days of darkness. But watch what happens around noon, assuming your house is located at just the right longitude. The sun has passed the zenith moving west, then it slows to a stop, backs up, now traveling west to east, and passes over your house again. This happens for 8 earth days, then it stops and resumes its normal east to west trajectory, passing over your house once again. From here the day proceeds normally, as the sun creeps to the west and down to the horizon, where it quietly sets.

The same 8 day retreat happens 88 days later, beneath your feet, but you don't notice it because you are now on the night side of the planet.

Now how can this be? How can the sun "back up"?

The orbit of Mercury is elliptical, which means it zips around the sun faster when it is close to the sun, and slower when it is farthest from the sun. At its closest, it moves around the sun faster than it spins on its axis. The apparent eastern motion from revolution exceeds the apparent western motion from rotation. From the perspective of your house, the sun seems to back up. It doesn't go forward (westward) again until you are well passed perihelion.

Relocate your house 90 degrees to the east, and you will see the sun set in the west, then rise in the west, just for a couple earth days, then set in the west again.

A rolling Outpost

The terminator on Mercury moves at a stately 2 miles per hour, or 3 kilometers per hour. You could walk westward at an easy pace and keep the terminator just in front of you, separating light from dark. As mentioned earlier, the terminator will stop, and back up for 8 days, when Mercury is closest to the sun. Then, after 8 days of retreat, the terminator travels westward again. This happens once every 88 days.

With this in mind, picture a road, or track, running all the way around mercury, forming the equator. An outpost with 6 scientists on board rolls along this track at 2 miles an hour heading west. It stays in position 100 miles post sunset, 100 miles east of the terminator. Under the hot sun, mercury reaches 500 °C, but during the long night it drops to -173 °C. We can't survive either extreme, but if we pick the right point in the day night cycle, maybe 100 miles past sunset, the ground has cooled to room temperature. Our outpost rolls along the track at 2 miles an hour, and the ground beneath is always a comfortable 70 °F. You can go exploring for a few hours, then return to the outpost using a little golf cart, which travels at 15 miles an hour. If the golf cart breaks, or runs out of juice, you better run, cause you need to catch up with the outpost.

It's not clear how to build the road in the first place, or how to maintain it. Our roads buckle under the summer winter cycles; imagine the day night cycles on mercury! It's probably not practical, but it's fun to think about.

If our outpost got stuck in a pothole, or had some other mechanical breakdown, Then it is carried, by Mercury's rotation, inexorably into the night, and gets colder and colder over the next few weeks. Well we can survive cold better than we can survive heat. Generate heat using an on-board nuclear reactor or some such while we repair the rolling outpost. Once repaired, it can roll at 5 miles an hour, so eventually it returns to its proper location. But if we can't repair it, we have a long cold night ahead of us, which we might survive, but we surely wouldn't survive the hellish day to follow. Keep that train rolling!

All this is rather silly. A manned outpost on Mercury would certainly exist in a crater at the north pole, permanently shielded from the sun's rays; nothing else is feasible. Some of these craters have substantial ice deposits, supporting human life, and, (after electrolysis), providing rocket fuel to get back home.


What does the element mercury, the liquid metal that measures temperature and atmospheric pressure, have to do with the planet mercury? Almost nothing, except both are associated with quickness.

Mercury is the fastest planet, relative to the sun, and relative to the stars. Greek astronomers watched it flit from one side of the sun to the other, and associated it with Hermes, the messenger god. Hermes is depicted as a graceful youth wearing a winged hat and winged sandals, indicating speed. The Roman equivalent of Hermes is Mercury, thus we named the planet Mercury after its Roman god.

The metal that we know as mercury was originally called quicksilver, since it is silvery in color, with the quickness of a liquid. The mysterious metal was also called mercury after the same messenger god. That is the only connection between the two. There are no deposits of mercury on Mercury.