Imagine you are standing on the lunar surface, for instance, near Tranquility Base where Neil Armstrong landed.
You look up into the sky, and there it is…the earth…standing clear and clean in the lunar sky.
It’s in the lunar sky at the exact same location it was when Neil Armstrong landed on the moon. You see because the moon rotates around its axis at the same rate that it revolves around the sun, the earth will always appear at the same point in the lunar sky. It will, apparently, never move. It’s the same phenomenon that causes the same side of the moon to always face the earth.
On the lunar surface, if you look up to see the earth in the sky, you look in the same location day after day after day, and the earth will still be there. Over the course of a lunar month (27 days or so), the earth will change from a “full earth”, to a “waning crescent earth”, to a “new earth”, to a “waxing crescent earth”, to a “full earth again”. It will go through all the same cycles that the moon does from the earth. But it won’t move in the sky. All day long, all month long, it will stay in essentially the same location.
You see, the earth will move in the sky, but it won’t move across the entire sky. It moves in a few degrees of arc and bobs around inside those few degrees. In an area a bit bigger than your out-reached fist, the moon will move around over the course of a lunar month. It changes location a small amount, almost continuously.
Why is that? It’s because of lunar wobble.
You see, as our moon — or any planet or moon — revolves around its axis, its axis also moves around in a circular manner. Think of a spinning top. If you look at a top spinning on the ground, the top spins around the main axis, but the main axis also rotates a bit in a semi-circular pattern. The spinning top wobbles.
The same is true with all spinning bodies, including planets and moons. The earth does it, except the earth’s wobble is very slow. The earth takes 27,000 years to make one rotation in its wobble. It’s so slow, we don’t even notice it. It’s the reason why the north star changes over the course of many thousands of years, but other than that, we don’t even notice it.
Our moon, however, is different. It wobbles at a much faster rate. It wobbles in part because of the tidal lock gravitational pull the earth has on the moon. The result is the lunar wobble is about every 27 days, and the wobble amounts to a few degrees a month.
This is fast enough that it can be noticed. You can see the impact of this. From the earth, it means the portion of the moon facing the earth changes a little bit over the course of a month. Sometimes you can see a bit more of the eastern edge, sometimes a bit more of the western edge.
From the lunar surface, it means the earth, which is basically stationary in the sky, will move around a few degrees over the course of the moon.
And if you happen to be at a place on the lunar surface where the earth appears near the lunar horizon, it would mean that the earth would appear to rise and set regularly, every several days or so. You would see a rising and setting earth. That would be an amazing sight to see.
Impact on Belitopia
In the imaginary world of Belitopia, this impacts the communications satellites with BLA lunar base. Remember, the BLA base is on the far side of the moon, opposite that of earth. You cannot see the earth, ever, from the BLA base. This means you cannot communicate directly with the earth either. To avoid this problem, communications satellites are put into orbit at the L4 and L5 Lagrangian points and are used to communicate with the earth. For more information about what the Lagrangian points are, listen to season 1, episode 7 of Belitopia, “Apollo Lunar Bases p2”.
From the perspective of the BLA base, these two Lagrangian points are low in the sky, near the eastern and western horizon respectively. They appear in the sky at a stable location, just like the Earth does. But they will also wobble, just like the Earth does, due to the lunar wobble.
In the case of the L4/L5 Lagrangian points from the perspective of the BLA lunar base, it means the Lagrangian points will bobble up and down above and below the horizon…one point in the eastern sky and one point in the western sky. When the L4 point is above the horizon, the L5 point will be below the horizon. When the L5 point is above the horizon, the L4 point will be below the horizon. Over the course of a lunar month, these roles keep switching. At any given point in time, one of the two points is always above the horizon, and the communications satellite at that location is used to communicate with earth. The base keeps switching between the two, based on the time of the month and where they are in the cycle.
All because the moon is wobbling…