How bright is the moon, really?

Posted: October 17, 2017 by oldbrew in moon, research, solar system dynamics
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Researchers aim to find out. It’s an interesting question as ‘our Moon’s average visual albedo is 0.12’, similar to soil or asphalt, and yet songwriters can describe ‘the light of the silvery Moon’.

The “inconstant moon,” as Shakespeare called it in Romeo and Juliet, is more reliable than his pair of star-crossed lovers might have thought, says Phys.org.

Now researchers at the National Institute of Standards and Technology (NIST) plan to make the moon even more reliable with a new project to measure its brightness.


Scientists put the moon to work daily as a calibration source for space-based cameras that use the brightness and colors of sunlight reflecting off our planet to track weather patterns, trends in crop health, the locations of harmful algal blooms in oceans and much more. The information sent from Earth-facing imagers allows researchers to predict famines and floods and can help communities plan emergency response and disaster relief.

To make sure that one satellite camera’s “green” isn’t another’s “yellow,” each camera is calibrated—in space—against a common source. The moon makes a convenient target because, unlike Earth, it has no atmosphere and its surface changes very little.

The trouble is that, for all the songs written about the light of the silvery moon, it’s still not understood exactly how bright the moon’s reflected light is, at all times and from all angles. Today’s best measurements allow researchers to calculate the moon’s brightness with uncertainties of a few percent—not quite good enough for the most sensitive measurement needs, says NIST’s Stephen Maxwell.

To make up for these shortcomings, scientists have developed complicated workarounds. For example, they must periodically check the accuracy of their satellite images by making the same measurements multiple ways—from space, from the air and from the ground—simultaneously.

Or, if they want to compare images taken at different times by different satellites, they have to ensure that there is some overlap during their time in space so that the imagers have the chance to measure the same part of the planet at roughly the same time.

But what happens if a research team can’t get a new camera into space before an old one is retired? “You get what’s called a data gap, and you lose the ability to stitch together measurements from different satellites to determine long-term trends,” Maxwell says.

Really knowing how bright the moon is—with uncertainties of much less than 1 percent—would reduce the need for these logistically challenging solutions and ultimately save money. So NIST is setting out to take new measurements of the moon’s brightness. Researchers hope they will be the best measurements to date.

“Brightness” here means, specifically, the amount of sunlight reflecting off the surface of the moon. Its apparent magnitude is about 400,000 times smaller than the Sun’s, but the moon’s exact brightness depends on its angle with respect to the Sun and Earth. And those angles follow a complex pattern that repeats roughly every 20 years.

Continued here.

Comments
  1. oldbrew says:

    Wikipedia talks of ‘the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions.’ But it still has a very low albedo.

    The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. This is partly due to the brightness enhancement of the opposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full moon.

    Additionally, colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions.

    http://en.wikipedia.org/wiki/Moon#Appearance_from_Earth

    Is this convincing?

  2. USteiner says:

    As a sailor I surely admire the beautiful picture of a sailboat in the middle of the night, throwing a little bow wake.

    Wait a minute! A sailboat with not a single sail set throwing a bow wake? They won’t by any chance be running a …. Oh my God, nooo that can’t be!

  3. USteiner says:

    The solar “constant” is known as 1376 W/m^2. How much is the lunar “constant”?

    It must be changing daily (due to the moon phases) plus seasonally (due to the distance to the sun). How much?

    [reply] see ‘albedo’ link at start of post

  4. ren says:

    Is the Agung volcano “waiting” for another geomagnetic storm?
    Geomagnetic storm levels are likely on 24-25 Oct and 07-11 Nov and
    G2 (Moderate) levels are likely on 25 Oct and 10 Nov due to
    recurrent CH HSS effects.

  5. ren says:

    Forecast of geomagnetic activity and phases of the moon.

  6. ren says:

    The fluctuations in geomagnetic activity are also visible in the El Niño index.

    When activity increases the jet stream over the North Pacific Ocean becomes more latitudinal.

  7. USteiner says:

    [reply] see ‘albedo’ link at start of post

    Please help me to translate this into a Lunar Constant given in W/m^2

  8. BA2204 says:

    Ren, thanks for for the link. What connection, if any, does solar activity have with the Madden-Julian Oscillation of the Stratosphere? Brett

  9. oldbrew says:

    USteiner says: October 17, 2017 at 3:14 pm
    Please help me to translate this into a Lunar Constant given in W/m^2

    Same as Earth in principle. Distance variation from the Sun isn’t exactly the same as Earth, but not significant really.

    NASA fact sheet: Solar irradiance (W/m2) Moon 1361.0 Earth 1361.0 Ratio 1.000
    http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html

  10. USteiner says:

    :-/ sigh; language can be so difficult.

    What I want to know is the energy flux of the moonlight towards the earth. Measured the same way as the solar constant, so light from the moon perpendicular to the axis moon-earth, measured outside the earth atmosphere.

    The moon “glows” with solarconstant * albedo, i.e. 1367*0.12 = 164 W/m². And then I take this as coming from a surface of a ball with moon diameter, and taking the distance to earth into account I can determine the lunar constant.

    Is that really true? The moon has some strange reflexions as it seems to be the same brightness over the whole area that we can see. It would be dimmer at the edges if it were like e.g. a billiard ball.

    I suppose there are better ideas around than what I have layed out, and perhaps even measurements?

  11. ren says:

    BA2204 says:
    October 17, 2017 at 5:30 pm
    Ren, thanks for for the link. What connection, if any, does solar activity have with the Madden-Julian Oscillation of the Stratosphere? Brett
    I not dealt with such a dependency. There are many factors that affect the weather. Important are the ones that initiate change.

    http://ds.data.jma.go.jp/tcc/tcc/products/clisys/STRAT/

  12. ren says:

    Currently, we have a very strong jumping speed of the solar wind, which causes weather anomalies. There will be more of them.

  13. oldbrew says:

    USteiner – re: ‘What I want to know is the energy flux of the moonlight towards the earth. Measured the same way as the solar constant, so light from the moon perpendicular to the axis moon-earth, measured outside the earth atmosphere.’

    Isn’t that what the researchers are trying to find out, although only from Mauna Loa?

    Quote: ‘it’s still not understood exactly how bright the moon’s reflected light is, at all times and from all angles.’

  14. ren says:

    I’m surprised Americans do not notice the disturbing pattern of the polar vortex.
    https://earth.nullschool.net/#current/wind/isobaric/10hPa/orthographic=-64.26,83.38,452

  15. Gamecock says:

    f8, 1/60, ASA 64.

  16. USteiner says:

    @oldbrew:
    I’d be satisfied with much less than “exactly”!

  17. oldbrew says:

    Probably back to this then 😐
    http://www.asterism.org/tutorials/tut26-1.htm

    Quote: ‘A graphical illustration of phase angle versus lunar brightness follows’

  18. E.M.Smith says:

    Oddly, moon photogaphers all all know the moon matches an 18% gray card. You must over expose it a bit from a direct meeter reading to gat a bright look like the eye sees at night.

    f16 at 1/asa speed gives gray. So you need f8 to f11 at 1/asa to get it looking white and bright.

    Maybe NASA needscto talk to some art photographers…

    @USteiner:

    The same exposure works for any lunar phase, so the lit portion always looks the same briteness This means all you need to know is the percent lit facing earth and the apparent lunar size. Phase of the moon and libration.

    http://aa.usno.navy.mil/faq/docs/moon_phases.php
    https://www-spof.gsfc.nasa.gov/stargaze/Smoon4.htm

    Nice video of libration effect here:

    https://en.wikipedia.org/wiki/Libration

    While total energy reflected is constant and libration doesn’t change that, the apparent size will change the intercept of that energy by the Earth.

    If you need precision beyond photography use adjusted for obvious optical changes, you will need to do your own research. I.e. 0.00001% error will not be found on discussion articles. That’s going to take things like change of reflective surface area as libration moves mountains and valleys in or out of the lunar disc visible, and changes in mare vs mountains visible at any libration.

  19. USteiner says:

    @oldbrew: Probably back to this then …

    Only if you can provide a translation from Magnitude into W/m². As this is not in sight, I‘ll give it a try in a different way.

    My interest stems from an über-green claim for a solar collector, which can also collect moonlight, insinuating “with output like at daytime”. This is obvious nonsense, but surprisingly my attempts to find data on the Lunar Constant – the energy flux of the sunlight reflected by the moon towards the earth – so far have failed.

    The Solar Constant for the moon is, on average, the same as for earth, taken here as 1367W/m². With an albedo of 0.12 as used in this thread the flux at the moon’s surface away from the moon is then 164W/m².

    Looking at the moon at e.g. full moon, you find that the center is as bright as the edges. (If the moon were a Lambertian reflector, the intensity would dim toward the edges). I take this to justify the assumption that every part of moon’s surface is radiating into every direction of space with those 164W/m².

    (and as @E.M.Smith has just pointed out, photographic experience suggests that this is also valid for other moon phases for the lit portion).

    Hence, with:
    mr = moon-radius = 1738km
    emd = earth-moon-distance = 384400 km

    the radiation arriving at the outer atmosphere of the earth would be reduced by the ratio of the moon surface to the surface of a sphere with radius emd, or:
    moon surface / emd-surface = 4 pi mr^2 / 4 pi emd^2 = (mr / emd)^2 = 2.04 * 10^-5

    Multiplying with 164W/m² gives 3.4 mW/m² (that is milli-Watt!). With that the Lunar Constant is ~400000 times smaller than the Solar Constant.

    As an aside: (http://onlinelibrary.wiley.com/doi/10.1029/2011JE003987/full )
    The hottest and coldest temps measured on moon are +100°C (boiling water for earth) and 100K. The first amounts to a thermal radiation of ~1000W/m², the second for ~5W/m². Even on the cold end there is more energy coming to earth than from the reflected light. It still feels cold during the night :-).

    Well, did I make a mistake? Anyone with better numbers?

  20. oldbrew says:

    The Moon also receives ‘earthshine’.
    http://en.wikipedia.org/wiki/Planetshine#Earthshine

    ‘It should be pointed out that as we reach new Moon, earthshine becomes a factor. Someone on the Moon sees a “full Earth” when we see a new Moon. As seen from the Moon, our Earth would look about 100x brighter than our full Moon. This is because of the Earth’s larger size and higher albedo. Imagine being on the Moon and seeing a full earthrise at magnitude –17.7, with earthlight dimly illuminating and casting shadows on the lunar scenery.’

    http://www.asterism.org/tutorials/tut26-1.htm

  21. oldbrew says:

    The moon is a harsh mistress—gravitational impacts on NSLS-II
    October 19, 2017

    Night and day, as the moon orbits around earth and the earth around the sun, the gravitational forces of these celestial bodies pull on the earth. This pulling force is what causes the earth’s sea levels to rise and fall, a phenomenon we call the “tides.” But did you know that land, too, experiences a tide?

    Read more at: http://phys.org/news/2017-10-moon-harsh-mistressgravitational-impacts-nsls-ii.html

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