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Imagine... a new unmanned lunar lander at Sinus Medii, north of the remains of something which was once a 43 kilometer diameter crater, thanks to the I.A.U. officially known as Oppolzer. Now, this lander has only one purpose: to aim a beam of reflected sunlight toward Earth, via a disc shaped flat mirror on top of the lander. Telescopic observers on Earth could see a starlike dot of light just after local sunrise (the sunrise terminator at Sinus Medii). At the lander itself, the sun's disc is already visible just above the eastern horizon. The lunar surface around the lander is still very dark (perhaps here and there a sunlit rock or boulder). Now here's a question for enthusiasts of mathematics and optics: what should be the diameter of this disc shaped mirror to observe such a starlike dot of reflected sunlight? One meter? Ten meters? Less than one meter? Just fifty centimeters? More than ten meters? Note: a rather small mirror called a Heliotrope is capable of reflecting a beam of sunlight over ten or more kilometers. The diameter of such a small mirror is only five centimeters. An observer at ten or more kilometers distance could see a rather bright point of light: the beam of reflected sunlight from the Heliotrope. Also note that such an experiment is always performed in broad daylight, when everything around the observer is illuminated by bright sunlight. The sunlit surface of the moon, when seen at First Quarter, is way too dark compared with the sunlit daylight circumstances on Earth. Which means: the starlike point of reflected sunlight, coming from the mirror on the moon, is much more distinct. The telescopic observers watch the First Quarter Moon when their sunset (at their location on Earth) happened more than two hours ago: they are in nocturnal circumstances and their eyes are adapted to darkness. To make sure that the telescopic observers on Earth are watching an artificial object from Earth, the mirror on the lunar lander should move a little bit, to "shift" the beam of sunlight across the face of Earth, in repetitive motion. Projected toward Earth, the angular diameter of the beam of sunlight is the same as the angular diameter of the sun's disc, seen from Earth and/or the moon. Now, what would the telescopic observers see? A pulsating point of light slightly to the east of the moon's morning terminator. Question: can they see such a point of light during Full Moon too? Yes, but it would look not at all that distinct as during First Quarter Moon. If I was the "big boss" of NASA, I would send an unmanned lunar lander (with movable disc shaped mirror) straight to Sinus Medii on the Moon, a.s.a.p., because the number of people who can't or won't believe the fact that something from planet Earth is capable of performing a landing on the moon's surface is growing day by day! Telescopes aimed at the moon could change their way of thinking. Note: if NASA has a plan to launch such an unmanned lunar lander with movable disc shaped mirror on top of it... I already have a name for it: Project PHAROS. Again: is all of this something brand new or is such a lunar mirror already an old proposal? I know of Carl Sagan who wanted to drop a nuclear bomb on the moon, to see what would happen up there (believe it or not, he really wanted to do this!). I keep it much more "gentle" by sending a lander with sunlight reflecting mirror toward the moon's Sinus Medii... Needless to say that I want to talk about this topic on the daily workfloor. Alas, my colleagues are still waiting for 'THAT' moment when I start talking about Earthly subjects, such as... soccer, or... the trade-union, or... vacation. |
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> Telescopes aimed at the moon could change their way of thinking You sound about as optimistic as the people who think that 400,000 conspirators all remained silent for the past 55 years and that a fiercely rival country which could track the spacecraft with radar never tried to claim otherwise. Personally, I'd take the billions spent on that mission and send another telescope into orbit instead of wasting my time on those with a fervent desire to remain stupid |
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| Moreover, It's gonna be taxpayer's money used for this. Do not worry about proving things like this. But If I were the "big boss", instead of sending out another telescope with those billions, I would leave NASA and use the money for things that are more beneficial for humans. |
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Moreover, It's gonna be taxpayer's money used for this. Do not worry about proving things like this. But If I were the "big boss", instead of sending out another telescope with those billions, I would leave NASA and use the money for things that are more beneficial for humans. https://spinoff.nasa.gov |
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| But, sending a telescope or a rocket into space is not beneficial, explain how that is beneficial. It costs billions, remember that. |
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But, sending a telescope or a rocket into space is not beneficial, explain how that is beneficial. It costs billions, remember that. If you don’t support it, that’s fine. I’m a big proponent of supporting STEM, exploration, and pushing humanity’s knowledge on all fronts. I believe that far outweighs how the NASA budget (~0.5%) could be used otherwise. |
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| There is a line between useful knowledge, and useless knowledge. |
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| lol, guy on Astrophotography website doesn't recognize that orbital telescopes advance human knowledge. Hubble led to literally new ways of finding breast cancer |
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Brent Newton: Really? If that's true, that would be very good. Can you explain? |
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There is a line between useful knowledge, and useless knowledge. There *may* be a cost benefit difference between more useful and less useful knowledge, but I would argue that ALL knowledge is useful. |
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Frank Alvaro: Is it useful to know that I can break glass with my tooth? Or that I will freeze to death in space? Or to know that a star is very far away? What would I do with that knowledge? Is it useful for you to know that I like to wear pajamas? |
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| List of retroreflectors on the moon...Very old news... https://en.wikipedia.org/wiki/List_of_retroreflectors_on_the_Moon |
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Yes, I know about the retroreflectors from Project Apollo, and I knew it (that subject) would appear here in this topic, but most of us here on Earth don't have powerful laser beams! We with our telescopes need the sun's reflected light from a slightly movable mirror in the central part of the moon's Earthfaced side. This point of light could be observed in a very large part of a continent, by thousands of telescopic observers and occasional visitors of public observatories, while the reflected laserlight from one or more retroreflectors is only seen by those who aim a ray of light from their own location somewhere on Earth. I can see it because I have a powerful laser and telescope, you have only your telescope so you can't see it. The purpose of the slightly movable mirror on the moon (the pulsating starlike point of light) is to give a very large part of Earth's population the occasion to see something through all sorts of (not-too-small) telescopes, without intervention of an apparatus which could produce a powerful laser beam. By the way, see also the geostationary satellites which could be seen as points of light by many telescopic observers, especially when they (those geostationary satellites) reflect sunlight toward Earth (in other words: a series of mirrors). There is such a photograph (of this series of points of reflected sunlight) in one of the APODs (Astronomy Picture Of the Day). P.S.: What a strange discussion this is! I only want to know how large such a mirror on the moon should be, to make it possible to observe the point of reflected sunlight through common telescopes (I mean: not the largest telescopes such as Keck, etc... just those of many amateur astronomers, say: 8 inch Dobsonians, or the medium class of Schmidt-Cassegrain reflectors) (read: backyard telescopes in or outside home domes). While we're talking about mirrors... For those who want to read something about an unbelievably absurd proposal, search the article CHARLES CROS in Wikipedia (the Martian Communication Mirror). |
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| Suddenly there is this silence... |
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(crickets) I think sometimes all the Astrobinners subscribed to a certain topic get this simultaneous thought that a person would reply to them on something they typed, so they wait; as a result, nothing at all is typed on the thread anymore... (crickets again) |
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A very curious phenomenon it is. Now, about this hypothetical flat disc shaped mirror on the unmanned lunar lander (or lunar rover)... I am not a mathematician, but I guess there must be some sort of calculation, or perhaps a formula, to get to know the most suitable diameter of this sunlight reflective disc. Or perhaps some sort of equivalent of a magnitude scale, say: a mirror of 1 meter shows a starlike point of magnitude 10, a mirror of 2 meter shows a starlike point of magnitude 5, and so on... I know that a project such as this one (the slightly movable mirror in Sinus Medii) is pure utopia (money-to-the-moon), BUT... who knows... (imagine: two or three Artemis-astronauts holding a disc shaped mirror of 2 meter diameter to reflect the sun's light toward Earth...). |
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But I cannot imagine seeing light coming off of a small mirror on the moon from earth through a telescope. I remember my astronomy book saying the biggest telescope on earth can only resolve detail as big as an average garage, on the moon. Might be irrelevant somehow, but if I cannot see a spot the size of a city on the moon with a normal consumer telescope, how will I see a spot of light coming off of even a 30 foot mirror? You got to remember resolving power. Maybe I totally misunderstand you?  EDIT: I found out, according to AI, a 10 inch telescope can see a reflection from a mirror 16.4 inches in diameter, which doesn't make any logical sense to me, but okay... |
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The optical effect of a Heliotrope (a small pocket-mirror to reflect the sun's light toward the horizon) (read: a rather simple device from the survival kit of an air force pilot who crashed somewhere at sea but survived the crash and is adrift on a small inflatable lifeboat), is something which should not be underestimated! Such a bright point of reflected sunlight could still be seen over a hundred kilometers! Keep also in mind that the moon's surface, shortly after local sunrise, is still very dark, and because of this dark background, the point of reflected sunlight is distinct! During the bright Full Moon it is "absent". Another fact is (or was) the visibility of the sunlight reflecting CSM (Command-Service Module) of Apollo 8 in December 1968 when the Dutch astronomer Gerard P. Kuiper observed Apollo 8 while it was very near the moon! (he observed and photographed it through the 154 cm NASA telescope in the Catalinamountains of Arizona). Source: Chriet Titulaer, Operatie Maan, page 145. See also: Wikipedia - Heliotrope (instrument), invented by the German mathematician Carl Friedrich Gauss. And also: M.G.J.Minnaert, Light and Color in the Outdoors, pages 17-18-19, item 14: Gauss's heliotrope (with drawing, showing two persons on the beach, doing experiments with a small sunlit mirror, instead of surfing or swimming, or whatever...). |
