Moon Reveals New Way to Find Oceans, Land on Other Earths

Article written: 7 Apr , 2009
Updated: 24 Dec , 2015


An Australian doctoral researcher using a backyard telescope has made a potentially big discovery: Earth’s oceans and continents shine differently on the dark side of the moon.

Now, Sally Langford, a doctoral candidate in physics at the University of Melbourne, is suggesting the “earthshine” of planets around other stars could provide long-distance windows into their surface features.


Langford's setup for moon observing. Credit: Stuart Wyithe, second author, also a physicist at the University of Melbourne.

Langford and her colleagues, from Melbourne as well as Princeton University, have shown for the first time that the difference in reflection of light from the Earth’s land masses and oceans can be seen on the dark side of the moon, a phenomenon known as earthshine. Their paper appears in this week’s edition of the international journal Astrobiology.

This is the first study in the world to use the reflection of the Earth to measure the effect of continents and oceans on the apparent brightness of a planet. Other studies have used a color spectrum and infrared sensors to identify vegetation, or for climate monitoring.

The researchers peered at the dark side of the crescent moon using a 20 cm (8 inch) telescope, on the bigger side of what most amateur astronomers use in their yards.

For three years, Langford took images of the Moon to measure the earth’s brightness as it rotated. Observations of the Moon were made from Mount Macedon in Victoria, for around three days each month when the Moon was rising or setting. The study was conducted so that in the evening, when the Moon was a waxing crescent, the reflected earthshine originated from Indian Ocean and Africa’s east coast. In the morning, when the Moon was a waning crescent, it originated only from the Pacific Ocean.

“When we observe earthshine from the Moon in the early evening we see the bright reflection from the Indian Ocean, then as the Earth rotates the continent of Africa blocks this reflection, and the Moon becomes darker,” Langford said.

Langford said the variation revealed the difference between the intense mirror-like reflections of the ocean compared to the dimmer land.

“In the future, astronomers hope to find planets like the Earth around other stars,” Langford said. “However these planets will be too small to allow an image to be made of their surface. We can use earthshine, together with our knowledge of the Earth’s surface, to help interpret the physical makeup of new planets.” 

LEAD IMAGE CAPTION: Earthshine on a crescent moon. Credit: Edward W. Szczepanski, Houston Astronomical Society (click on the photo to visit Szczepanski’s page)

Source: University of Melbourne. The paper is available here.

25 Responses

  1. Yael Dragwyla says

    Rob — The dark side of the Moon is the side that is turned away from the Sun. When the Moon is full, the dark side is the far side of the Moon; when the Moon is new, the dark side is the near side of the Moon. This is so because the Moon rotates relative to the Sun, though not to the Earth. The far side of the Moon is dark only half the time. There seems to be a confusion of terminology on your part here.

  2. Greg says

    Looks like she has earned her Ph.d. If she keeps this up a promising career lies ahead. Well done. This also happens to be a timely discfovery which should spur interest in getting bigger and better observatories up and running. Who will be the first to discover an ocean world with land masses? We should have an answer within our lifetimes.

  3. middenrat says

    Singular research, I am most impressed. That’s around 100 trips up and down the mountain for the images she needed.
    I’m a little bit confused how this insight can be applied. Are we then to study the moons of exoplanets for comparative exposures? That seems impossible, then what is so innovative about studying the subject’s spectrum directly (apart from the novelty of primarily being able to capture an image)?

  4. Greg says

    My understaqnding is that you could look at for dimming of the object itself as it rotates and interpret that as surface features. Say for instance you have an H20 signal in your spectral lines and on continuous or successive readings you detect dimming, you could interpret that as a change in surface features. If theexoplanet is warm enough for liquid water than the surface topology change detected is likely to be a continent. Of course you will need a really sensitive instrument to measure such dimming within an acceptable range of error or confidence interval. This research should place contraints on this and such an instrument may be built within our lifetimes.

  5. solrey says

    “Earth’s oceans and continents shine differently on the dark side of the moon.”
    I wondered about that statement, considering that the ‘dark side’ of the moon is the side that always faces away from Earth, the farside, and is just radio signal ‘dark’ (anxious times for early apollo missions). It’s a great album, but there is no ‘dark side of the moon’. It even says ‘dark side of crescent moon’ at one point. There are simply the “night” and “day” sides as seen from Earth. It wasn’t phrased ‘dark side’ in the paper so it must be this authors phrasing. Could that be confusing to folks, maybe?

    So about all this tells us regarding gleaning the ‘physical makeup’ of rocky exoplanets is that water is more reflective than land. No kidding, who’da thunk? Figuring that out is going to help someone get a doctorate? I know it takes a lot of hard work and dedication to earn a post-grad degree, but this is pretty much like a no-brainer, imo. So if a rocky exoplanet has at least some, cyclic photometric variability, that means that there is likely a combination of liquid and land. What if the surface is mostly sand and the water is murky or otherwise have low albedo? Would an “ice age” climate result in undetectable photometric variance due to a mostly homogenous, high albedo? I guess if we’re only looking for a ‘twin’ in our current configuration this would work but it seems like an awfully narrow scope.

  6. Rob says


    the ‘dark side’ of the moon is always dark. that’s why it’s got the name that it does. you’re confused with this and the ‘far side’ of that moon; that’s the side that we don’t ever see.

    I guarantee doing some photometry isn’t going to get her a PhD. That’s going to come from all the other research she’ll have done over the course of her studies.

    and if it was a no brainer, why didnt you put out a paper years ago?

  7. mike says

    following your line of thinking Rob the Earth also has a dark side

  8. John says

    I can fully see how this effect provides ephemeral changes in the amount of light reflected from the Earth to the moon and back. I don’t see how it can do much more than that. Surely other factors come into play. I think of short term changes in cloud cover and sudden snowfalls over large areas. On a seasonal basis I would expect there is a change in vegetation as leaves fall or new growth occurs.

    If there were a sufficiently technologically advanced civilization on a hypothetical planet around one of the stars that are members of the Alpha Centauri system, imagine what they would see looking at our solar system. Venus, Earth and Mars would all show phases that would overwhelm this effect completely. That aside, Venus would be fairly consistent in the amount of sunlight reflected, regardless of the rotation of the clouds or “continents” it has. Mars would not show much more and Earth would MAYBE show minor variations on a daily basis but they would be obscured by changing weather patterns that would make it difficult to determine our day length. The amount of sunlight visible from reflected sunlight due to changing weather and daily rotations would be minuscule compared to the changes in the phase of Earth, as seen from there.

    Am I missing something? I very often do.

  9. Daniel N says

    I believe ‘dark side’ as used in this article is referring to the shadow earth cast on the moon, as opposed to the lit side (crescent) that is receiving sunlight directly. The ‘far side’ is always far but not always dark. =)

  10. UKDave says

    Don’t know why this is so confusing…..

    Dark side of the moon = the side of the moon (and there always is one) thats not illuminated by the sun. Sometimes we get to see a bit of it – sometimes all of it, and sometimes none at all.

    It’s not referring to the far side, or the portion of the moon obscured by the by earhs shadow during a lunar eclipse.

  11. Todd Coolen says

    If we could measure such a minute variation in the reflection of an exoplanet, would we not be able to also look at the exoplanet with enough detail to determine the composition? I am not an astronomer. At first this idea seems very inventive and executeable, but the data could only be used in concert with other data to compile a reasonable existence of liquid… and that liquid could be a sea of methane.

    I still say “Well Done!” though. It may get some more prying minds to dig deeper into the research and come up with more unique ideas of identification.


  12. omnivorr says

    I blame all this confusion on Pink Flloyd

  13. Feenixx says

    I cannot follow this at all.
    OK, for “dark side”, read “night side”, which is the side acing Earth during a new moon, and the side facing away from Earth during a full Moon, and some of both on other days … I figure that’s what it means.

    But how this method can apply to study exoplanets – I have no idea, and I can’t follow:
    “However these planets will be too small to allow an image to be made of their surface. We can use earthshine, together with our knowledge of the Earth’s surface, to help interpret the physical makeup of new planets.”

    To me, this reads something like “the planets are too small to study, so we are going to study how the light bounces off their moons, applying what we learned about how the earthshine bounces off our own moon.”

    I’m scratching my head…

  14. Jon Hanford says

    ‘UKDave Says:
    April 8th, 2009 at 1:38 am

    Don’t know why this is so confusing….. It’s not referring to the far side, or the portion of the moon obscured by the by earhs shadow during a lunar eclipse.’ The far side of the moon is never obscured by the earth’s shadow (during a lunar eclipse) because the far side of the moon NEVER faces the earth. I don’t know why this is so confusing too……. this is high school science.

  15. Roger Levinson says

    So the dark part of the Moon shows variations in the reflecting light from our planet. Cannot see how that helps much. Let us say that Alpha Centaura has a planet in the habital zone, There is no reflection from the planet itself unless it also has a Moon for this reflection to shine upon. Since we cannot even see planets of the size of planet Earth at this distance then it follows we are not going to see any possible moons if they exist let alone the reflections. Seems a complete waste of effort to me. Better surely to conentrate on something a bit more useful? Roger.

  16. RickE says

    This is a specious theory – I’m sure she enjoyed wasting her government grant –
    and three years.

  17. solrey says

    Using the phrase ‘dark side of the moon’ is inappropriate in a scientific article, imo. Unless they’re talking about the radio ‘dark’ farside. Even then just as a ‘tongue in cheek’ phrase. Lunar nightside would be the correct terminology.
    Here’s a very simple method for collecting a broader, more detailed dataset:
    Use archival data from any number of LANDSAT or GOES satellites. This would provide much more detail about earthshine since it would involve many different angles, equatorial, hemispheric and polar, so deserts and polar icecaps could be included. Different liquid to landmass ratio’s could be quantified. A good part of the earthshine, non-optical EM band could be studied as well.

    @jon hanford
    “I don’t know why this is so confusing too……. this is high school science.”
    In high school, that sentence would receive a C for grammar, while using the phrase ‘dark side of the moon’ would receive an F in science. 🙁

  18. Olaf says

    It is not confusing at all, it is really observing and measuring the dark part of the moon if you snap a picture of it.

    If you take a snapshot of the moon from Jupiter, then you will see part of the moon lit and part of the moon dark on that picture. That dark part can contain information that gets reflected back from the Earth?

    Imagine a distant planet that revolves every 1 year around it’s axis, we would have to wait one year to see the other part of it. But if it has a fast revolving moon around it, then we can look at the dark part of that moon and get information from the planet when it revolves around it every 14 days. No need to wait one year, we have at least some scientific infrmation coming in right now.

    Her Phd Is proving that this can work and how to do it. With a lot of matematics to extract the data.

  19. Olaf says

    Todd Coolen –
    “If we could measure such a minute variation in the reflection of an exoplanet, would we not be able to also look at the exoplanet with enough detail to determine the composition?”

    I don’t think you will get a very detailed view of the reflection, but it could give some additional clues that confirm other scientific data or prepare us for what might be just around the corner.

  20. Olaf says

    They have this old techinque of reconstructing the image of a CRT monitor just by looking at the reflection bounce bacik from something in front of it even if you canno see the screen.

    This is because the electron beam is like a laser moving sideways and up and down, so if you record the brightness and then try to reconstruct the electron beam path and put the measured brighness at the correct location and you could see a rough image.

    It is a bit simlar here too, the rotation of the moon and the rotation of the planet will change pixel brightness of the dark part of the moon you are imagining and depending how big this dark part is , in pixels, you could map these pixels in some chart by compensating for the rotation of both moon/planets… Interesting is that you might get a glimps of changing weather pattern since these brightness pixels will move independingly.

    Interesting technique,…

  21. solrey says

    @olaf, roger, feenixx, etc.
    In the actual paper, they talk about monitoring the light reflected, DIRECTLY towards us, off an exoplanet, not off of any moons it might have. The idea is that as a planet rotates on it’s axis, if the reflectivity/luminosity will vary cyclicly, that would indicate a surface made of both liquid and rock, i.e. oceans and land. Monitoring these changes would be a factor of rotational velocity, not orbital periodicity.
    Studying absorption lines would provide more detailed information about atmospheric and/or surface composition.
    Using earthshine reflected off the moon was just a convenient tool they used to study these cyclic luminosity variations.

    I still think satellite data would have been more comprehensive, but I guess any excuse to hang out in an observatory will do, eh?

  22. Jon Hanford says

    @ solrey: I don’t write my posts for grading by high school grammer teachers, and where in my post did I refer to the ‘dark side of the moon’?

  23. Dork Leader Standing By says

    Maybe they are smarter than us and don’t leave bright fn lights on at night.

  24. Chaz says

    @Hanford: It’s Grammar :p

  25. kammeres says

    An easy way to verify/validate Ms. Langford’s data would be to compare her photo times and reflectivity values with a full disc visible satellite photo from the same date & time. Hopefully she thought to do so at the time. If not, archived shots should be obtainable from NOAA. Soils, vegetation, water surfaces and clouds all have different albedos which contribute to the earth’s net reflectivity. Clouds have the highest value but are transitory in nature; thus, if a pattern in the reflectivity values indeed corresponds to when the full disc shot is primarily open water or primarily land mass, Ms. Langford has achieved something of definite value.

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