I was reading today that, in research published in The Astrophysical Journal, molecular oxygen has been discovered in the Markarian 231 galaxy, 561 million light-years from Earth. A light-year, which measures distance in space, equals about 6 trillion miles.

And yet we can't photograph the Apollo landing equipment 243,000 miles away.

Why is it that we can better observe this far-away oxygen than stuff nearby on the surface of the Moon?

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    $\begingroup$ But the Moon landing stuff has been photographed. $\endgroup$
    – ProfRob
    Commented Feb 22, 2020 at 16:42
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    $\begingroup$ This appears to be an invitation to discuss, rather than a question to answer. Please take the tour to see how this differs from a discussion forum. $\endgroup$
    – James K
    Commented Feb 22, 2020 at 19:27
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    $\begingroup$ One has nothing to do with the other. The molecular oxygen was discovered by spectroscopy. $\endgroup$ Commented Feb 22, 2020 at 21:48
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    $\begingroup$ We have absolutely nothing to image anything in that galaxy, not even a star system. The molecular oxygen was discovered by spectroscopy (we have seen photons what is given out exclusively by molcular oxygen). $\endgroup$
    – peterh
    Commented Feb 23, 2020 at 0:10
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    $\begingroup$ as for "Good luck getting Excel to calculate that!": i.sstatic.net/cc7My.png $\endgroup$
    – uhoh
    Commented Feb 23, 2020 at 3:27

1 Answer 1


The Moon landing sites have been photographed, but from orbit around the Moon. For example: https://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html It is legitimate to ask why not from the Earth?

A lunar module has a diameter of about 4m. At a distance of 400,000 km, this subtends an angle of 0.002 arcseconds.

The absolute best angular resolution you can obtain from the surface of the Earth, using massive telescopes and adaptive optics imaging is about 0.1 arcsecond.

i.e. You cannot resolve anything on the Moon's surface, using a telescope on Earth, that is much smaller than about 200 m in diameter.

The observations you refer to (see Wang et al. 2020) are of a galaxy, taken at microwave wavelengths (so hardly comparable). A galaxy is far away, but very big. The observations were able to resolve emission from the central 20 kpc (about 60 thousand light years) diameter region of this galaxy. The galaxy itself is at a distance of about 180 Mpc (about 600 million light years), so the angular resolution involved, using a bit of basic trigonometry, was about 22 arcseconds, which is set by the instrumentation they were using (i.e. the signature they found could be more concentrated than that). Thus this central region in Markarian 231 subtends an angle of 22 arcseconds at the Earth, which is roughly 10,000 times the apparent size of a lunar module on the surface of the Moon.


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