Anniversary Post: First Image of Far Side of Moon

First Picture of Other Side of MoonOn this day in 1959, the Soviet Luna 3 spacecraft transmitted the first ever pictures of the far side of the Moon. I thought we might take this opportunity to discuss why it is that the same side of the Moon is always facing us. Although I should tell you that this is not exactly true. I think we are able to see about 55% of the Moon’s surface, because it jiggles. But for all intents and purposes, we see the same moon each night. This is because it is tidally locked.

The Moon once rotated rapidly. But over time, the Earth’s gravitational field slowed it. The force from the Earth produces a bulge in the part of the moon that is directly facing the Earth — and also directly opposite (just like the Moon created tides on Earth). This has the effect of squishing down the sides, so that the moon looks like a football with the pointy end facing Earth. Of course, the deformation isn’t anywhere near that great. But that’s the basic idea.

While the Moon was spinning fast, the bulge was always slightly after the direct line. As a result, the gravitational field had a net torque on the Moon, slowing its rotation. The effect was very small. But it’s amazing what you can accomplish in a billion years. I used to tell my students to image the Moon (or any other tidally locked object like Mercury) as if it were a frying plan. The handle would always be facing just a little off center from the Earth, and would thus be constantly pulled slightly in the opposite direction of the Moon’s rotation.

Luna 3 was the first mission specifically meant to photograph the other side of the Moon. Luna 1, sent in January of that year, was meant to crash on the Moon. It missed. (Don’t laugh: we missed the Moon the first time we tried.) And it became the first human object to go into orbit around the Sun. Luna 2, sent in September, actually hit the Moon. Later, in February 1966, Luna 9 would be the first spacecraft to make a soft landing on the Moon (or any other place).

The radio signal on Luna 3 was so weak, that the spacecraft had to get almost all the way back to Earth in order to transmit its 18 images. The one above is the first transmitted back. I think we humans have become far too cavalier about this kind of stuff. What we now do in space is mind boggling. It’s always nice to go back five or six decades and see what we were doing and just how hard it was. Oh, and no one knows for sure what happened to Luna 3. But it probably burned up in the Earth’s atmosphere.

8 thoughts on “Anniversary Post: First Image of Far Side of Moon

  1. The tidal ‘lock’ of Mercury is bizarre, though. And the large eccentricity of its orbit just makes things stranger still:

    Mercury is gravitationally locked and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

    At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury’s angular orbital velocity equals its angular rotational velocity so that the Sun’s apparent motion ceases; closer to perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun’s normal apparent motion resumes.

    • Yes! I used to cover that in my planetary astronomy class. I’ll have to wait for a good excuse to write about that.

  2. I like the comparison to how our cell phones have X times the power of the computers that sent people to the moon. We have gone a great deal of distance in a very short period of time and it seems like we keep accelerating.

    • It is amazing, isn’t it! At the same time, we bomb hospitals and wedding parties. I think the trend is positive overall, however. Not that it matters to incinerated wedding guests, of course.

      • No, it doesn’t. Then again, it used to be we would firebomb entire cities during war and now we precision target. It still sucks but not as many people suffer.

        Although occasionally has some articles pointing out despite what the news thinks, things are not so bad depending on where you are. One of my neighbors was just in a car accident and fifteen years ago, he would have died. Instead he walked away bitching about the insurance companies.

        • Absolutely. I do think in an objective sense, we are progressing — and fast. The idea of Columbus’ genocide is unthinkable today. But I continue to bitch today so that tomorrow there is even less to bitch about. (Also: bitching is a natural human need that was strangely left off of Maslow’s hierarchy of needs.)

            • Well, insurance companies are certainly better today than they were 400 years ago. I think the idea of reinsurance is good — although clearly we should move to the point where the government does that. But cultural progress does not necessarily imply progress of all its parts. Look at the Republican Party!

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