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I understand that all manned landings on the Moon were on the near side, so technically they could be visible from Earth.

Was it possible to observe landings and/or extravehicular activities (EVAs) with Earth-based telescopes? If not, why?

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    $\begingroup$ This is what the Apollo 11 landing site looks like from an orbiter decades later. Even the Hubble space telescope orbiting Earth has difficulties observing the Moon, because the parallax, the relative lateral movement, is large compared to the exposure time. $\endgroup$ – LocalFluff Jun 10 '15 at 12:22
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    $\begingroup$ I saw them on TV! 8-)} $\endgroup$ – Keith Thompson Jun 16 '15 at 20:45
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Short answer

No it hasn't been possible to see what was happening on the Moon, from Earth.

The angular size of an object like the Lunar Module is much much smaller than the best angular resolution available in 1969.

  • To imagine what happens when the resolution of the telescope is not sufficient, let's observe a strip of black and white lines with decreasing width:

    enter image description here

    When the resolution limit has been reached it is not possible anymore to separate the black lines from the white ones.

Seeing details of the Moon lander from Earth surface is like seeing details of a fly at a distance of 400 km, the height the ISS orbits around the Earth.


Angular size and telescope resolution

When looking at distant objects, the actual linear size itself is not sufficient to determine our capability to see them. What counts is their apparent size which takes into account the actual size and the distance. This apparent size is expressed as an angle: the angular size.

enter image description here

  • Seen from Earth (380,000 km), a 10 m-object like the LM has an angular size of about 1.5 x 10-6 degree, that is 0.0054 arc-second or 5.4 mas.

The angular resolution, which is the smallest angular size that an instrument can see, is proportional to the diameter of its mirror / lens.

  • The largest telescope at the time was the 5m-Hale telescope of Palomar observatory, with a theoretical resolution of 25 mas.

    The Hale telescope resolution was insufficient, by a factor of 5 to see the LM as a single spot. Only a spot! That's not exactly seeing the EVA details, and this wasn't yet possible...

    In addition, a telescope on Earth has its theoretical resolution limited by the effects of the air layer.

Effect of different angular resolutions on observing the cosmic microwave background (CMB):

enter image description here
CMB maps obtained by Planck, WMAP and COBE (source).

There are optical telescopes now under construction that will be able to get the required resolution to see the LMs left behind in the past, including the Giant Magellan Telescope or the European Extremely Large Telescope. They won't see the details, but they'll detect the overall volume.


Practical determination of the resolution

  1. Angular resolution vs diameter of the mirror.
    The larger the mirror, or the lens, the better the angular resolution. Similarly for a given optic diameter, the shorter the wave used for the sensor, the better the resolution. The angular resolution is a matter of diffraction, the best (theoretical) resolution that can be obtained from an instrument if given by the Rayleigh limit:

    θ = 2.52 x 105 x λ/D

    where:

    • θ is in arc-seconds
    • λ is the wavelength considered
    • D is the diameter of the mirror / lens
    • λ and D are in the same unit
  2. The minimum object size which can be resolved is:

    s = tan (θ / 3600) x d

    where:

    • s is the minimum object size
    • θ is the angular resolution in arc-seconds
    • d is the distance to the object
    • s and d are in the same unit
  3. Application for 5m-Hale telescope at 500 nm (green):

    Angular resolution
    θ = 2.52 x 105 x λ/D
    θ = 2.52 x 105 x 500-9 / 5
    θ = 25.2 mas

    Linear resolution at Moon distance
    s = tan (θ / 3600) x d
    s = tan (0.0252 / 3600) x 380,000,000
    s = 46.4 m

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    $\begingroup$ This really puts it into perspective nicely. People often think of the Moon as the closest body to us, but even at that, the distance is mind boggling; Jupiter can fit between us twice! Nearly three times. $\endgroup$ – Rein S Jun 11 '15 at 13:56
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    $\begingroup$ @ReinS: Because the presentation of the solar system is like this, while the proportions are actually these ones. I like this page :-) $\endgroup$ – mins Jun 11 '15 at 17:54
  • $\begingroup$ I accepted this one for the nice diagram. $\endgroup$ – Cedric H. Jun 11 '15 at 20:55
  • $\begingroup$ Can you add the links to the presentation vs. proportion stuff in your answer please? Comments will be deleted in the long run. $\endgroup$ – hiergiltdiestfu Jun 13 '15 at 11:57
  • $\begingroup$ @hiergiltdiestfu: The solar system map is remotely linked to the question, I don't feel like introducing it in the answer. Comments are not deleted as far as I know. $\endgroup$ – mins Jun 13 '15 at 12:16
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As several nations' governments and plenty of amateurs pointed radio frequency antennas at the sites and received signals, one might conclude that they were observed. Observation does not have to imply visible light observation. See this http://www.arrl.org/eavesdropping-on-apollo-11 My recollection from the time is that plenty of amateurs tuned in to the transmissions coming from the moon.

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    $\begingroup$ That is an interesting point. Welcome to Space Exploration. $\endgroup$ – kim holder Jun 10 '15 at 21:50
  • $\begingroup$ I was only thinking about visible light but that's an interesting point! $\endgroup$ – Cedric H. Jun 11 '15 at 8:06
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No they were not. Telescopes, even today cannot resolve that small a detail from the distance. LCROSS, orbiting the moon was able to barely resolve the lunar modules left behind.

More good details in this similar question and answer on the Astronomy site: Visibility of the Apollo-11 Module

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  • $\begingroup$ That's what I wanted to know. $\endgroup$ – Cedric H. Jun 10 '15 at 12:18

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