Posted on 07/20/2025 1:07:31 PM PDT by MtnClimber
Explanation: About 1,300 images from the Lunar Reconnaissance Orbiter spacecraft's wide angle camera were used to compose this spectacular view of a familiar face - the lunar nearside. But why is there a lunar nearside? The Moon rotates on its axis and orbits the Earth at the same rate, about once every 28 days. Tidally locked in this configuration, the synchronous rotation always keeps one side, the nearside, facing Earth. As a result, featured in remarkable detail in the full resolution mosaic, the smooth, dark, lunar maria (actually lava-flooded impact basins), and rugged highlands, are well-known to earthbound skygazers. To find your favorite mare or large crater, just follow this link or slide your cursor over the picture. The LRO images used to construct the mosaic were recorded over a two week period in December 2010.
“Seems like a near impossibility that both the moon’s rotation and its orbit around Earth would be precisely the same.”
Given geometry and gravity it is a certain.
The earth’s rotation will never cause the moon to break free from earth’s gravity. If the sun retains the same mass and size and never swallows the earth and moon, as it almost certainly will, the earth and moon will become mutually tidally locked in about 50 billion years, with a period of 48 current days. That means that days will be about 48 x 24 hours long, and the moon will remain in the same place in the sky at any point on earth’s surface (more or less), but undergo phases once every 48 x 24 hours, which will become the length of the day than rather than the current 29.53 x 24 hours. At which time, one month will equal one day of 24 x 48 hours.
The wobble is called libration. The main source of libration is due to the fact that the moon’s orbit is elliptical and approximately Keplerian. If the moon’s orbit were circular and uniform, and the rotational rate uniform, then the moon would not appear to librate. The real moon’s orbit is elliptical, and its orbital rate is faster when it is nearer to the earth and slower when it is further away, but it’s rotational rate is more uniform, so it does not appear to keep the same aspect towards the earth at all times. There is also a smaller component of physical libration. The uneven composition of the moon means that it is tidally locked with the heavy side facing earth. As libration swings it away from this minimum energy configuration, like a pendulum, the effect of earth’s gravity is to pull it back, but this effect is small compared to effect of the elliptical orbit.
Why do the craters look like direct perpendicular hits?
Chandler wobble characterizes the unforced motion of uniform ellipsoid of rotation. Euler had calculated a period of 11 months, but the measured effect is 14. Simon Newcombe showed that Euler had treated the earth as a rigid body. If account is taken for Young’s Modulus, one can estimate Young’s modulus of the earth’s crust from the period of Chandler wobble.
Thanks for all of that! That wobble is wild. But even with that it doesn’t vary much from holding that one face towards Earth.
It’s still a bizarre phenomena even with the mathematical physics of it explained.
Thanks!
You are right about the phases of the moon. But the same side of the moon always faces Earth. At the new moon, the dark side of the moon faces Earth(the same side always facing Earth) and the far side is illuminated. We just can’t see it from Earth.
I think I get it now. But it still seems weird to me! :)
It is tidally locked. Think of the moon as a pendulum. The minimum energy configuration is with the bob or weight at the lowest point. The only two bodies in the solar system that are known to be mutually tidally locked are Pluto and its “moon” Charon. Charon and Pluto are more of a double planet than moon and major body, the center of mass of the Pluto-Charon system is well outside of Pluto.
The center of mass of the Earth-Moon is only about one third of the way below the surface. The Earth and moon actually orbit a point that lies about 4000 kilometers from the center of the earth on the line connecting the center of mass of the moon and earth. It is the center of mass of the Earth-Moon system that orbits the sun. Both the earth and moon orbit their center of gravity every 27.3 days with respect to fixed stars, 29.53 with respect to sun.
Now that I get that it’s also a bit strange to think that a solar day on the Moon is 28 Earth days long.
Thanks! That is quite complicated but very interesting.
Am I right to picture that that would make the true path of the Earth-Moon orbit around the Sun look scalloped if the path were visible?
True path with respect to what?
Let’s say the Earth-moon orbit the sun, although that is a slight simplification, it’s good enough for our purposes.
The center of mass of Earth-Moon orbits the sun in an elliptical orbit. That orbit looks very round to your eye. The Earth and moon orbit each other around their center of gravity which is deep under the surface of the earth. Their orbit with respect to one another is tilted about 5.2 degrees with respect to their orbit with respect to the sun. Twice a lunar month, the moon’s orbit crosses the plane of the earth’s orbit around the sun, once ascending (south to north), once descending (north to south). If it happens to cross near the time of a new moon, when the moon is between the earth and sun, the moon will cast a shadow on the earth and we will have a solar eclipse. If it passes when the earth is between the sun and moon, the moon will pass through the earth’s shadow and we will have a lunar eclipse.
Enjoy!
It is also slowly but steadily, and tragically,moving away from us.
Oops! Upon further review I see you already covered that.
You were too fast for me. 😊
#7 you are close to the answer. We are in a diorama in some alien kids classroom project.
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