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CERN | Geneva | Switzerland

View Blog | Read Bio

Is the moon full? Just ask the LHC operators

Last weekend, I had the pleasure to be shift leader for ATLAS. It was a real pleasure for many reasons: being right in the middle of the action, surrounded by an international team of enthusiastic and dedicated people, and taking part in great teamwork. The shift crew (about ten people plus dozens of experts on call) must keep the detector running smoothly, tackling every problem, big or small, as fast as possible.

Data was coming in at a high rate and all sub-detectors were humming nicely. Not a glitch in hours so we were getting slightly sleepy nearing the end of the shift around 22:00. So when a colleague from the trigger system (the system that decides which events are worth keeping) called to inquire about recurrent splashes of data, I was rather puzzled.

I quickly went around, asking a few shifters to check their system. Nobody had a clue. Then I took a closer look at this plot that I had not scrutinized before since everything was so seamless.

The two lower curves in beige and green show the instantaneous luminosity measured by the two largest detectors operating on the Large Hadron Collider (LHC), CMS and ATLAS. This is a measure of how many collisions are happening per second in each experiment from the two beams of protons circulating in opposite direction in the LHC tunnel. If you look closely at these curves, they both have small dips at regular intervals. Since both ATLAS and CMS were registering these dips, it had to be coming from a common source, the LHC.

So I called the LHC control room to find out what was happening. “Oh, those dips?”, casually answered the operator on shift. “That’s because the moon is nearly full and I periodically have to adjust the proton beam orbits.”

This effect has been known since the LEP days, the Large Electron Positron collider, the LHC predecessor. The LHC reuses the same circular tunnel as LEP. Twenty some years ago, it then came as a surprise that, given the 27 km circumference of the accelerator, the gravitational force exerted by the moon on one side is not the same as the one felt at the opposite side, creating a small distortion of the tunnel. Since the moon’s effect is very small, only large bodies like oceans feel its effect in the form of tides. But the LHC is such a sensitive apparatus, it can detect the minute deformations created by the small differences in the gravitational force across its diameter. The effect is of course largest when the moon is full or during the new moon, when the sun and the moon combine their tidal forces for being all aligned with the earth. But the same happens twice a day like the tides and the operators must correct for it.

What came as a surprise to me was to witness the dynamic aspect of it. As the moon was rising in the sky, the force it exerted changed ever so slightly, but even these infinitesimal changes were big enough to require a periodic correction of the orbit of the proton beams in the accelerator to adapt to a deformed tunnel. And each time the operator corrected the orbit, he also had to reoptimize the beams position to maintain the best collision rate by doing a small scan of the two beams to realign them, which caused the dips in the luminosity plot.

Other surprising disturbances were also observed in the LEP days like one that appeared every day at fixed times. It took months and a train company strike to figure it out. These perturbations were created by the passage of the fast train linking Geneva to Paris, the TGV, since it releases a lot of electrical energy into the ground. The LHC is also sensitive to the hydrostatic pressure created by the water level in nearby Lake Geneva that also deforms the tunnel shape.

Life is full of surprises when dealing with such a sensitive piece of equipment.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

Addition to the initial post

Since there have been so many questions on this, let me try to be clearer.

Why is this effect bigger during the full moon?

As Paolo explained in his comment, it’s because when the moon is full (just like when the moon is new as many pointed out) the tidal effect due to the sun reinforces that due to the moon (they are all aligned). So it is not because the moon is closer to the earth, this distance being rather fixed. It is just that the sun and moon combine forces on full and new moon.

Sorry for not mentioning the new moon in the initial blog, but it works just like with the tides: full moons and new moons bring the biggest tides, this is what my comment meant. But I agree: it was misleading.

Here is the graph used by the LHC operators to compensate the accelerator displacement. Each up and down represents a day, with a high and a low tides. The external modulation comes from adding in the position of the moon with respect to the earth and sun during the month. Since the moon takes 28 days to go around the earth, twice a month it is aligned with the sun. This occurs at full moon and at new moon. This is also when the tides are the strongest.

Is this effect there only on full moon?

No. The LHC operators have to correct for this effect every day, following the tides. Just as Christopher Grams, Phil Plait and others suggested. But on full moon, the effect is bigger. When I observed it myself during my shift on Saturday June 2nd, it was made even more visible because the operator took the opportunity to optimize the beam position every time he applied a correction to the orbit since it was a big correction.

What is moving?

The accelerator is moving, not the proton beams. The accelerator is moving with the earth crust. We all know the moon creates tides. This happens because the moon pulls on the ocean as it circles around the earth. The earth crust feels the same pull but since water is much easier to move than the earth crust, almost nobody ever notices the small earth deformations. But the LHC operators do because the accelerator is both very large and very precise. They see the protons moving outside of the accelerator orbit, as the accelerator shifts.

To recap: The LHC accelerator was pulled away from the proton beams that circulate inside it by the tidal forces. The operator corrected the orbit to bring back the protons in the center of the beam pipe. Each time he did that, since it was a big correction, he also swept the beams with respect to each other to find the place of maximum collisions. This sweeping of the beams caused small dips in the luminosity plot (the measure of the collisions rate), making it very visible.I hope this helps.

Thanks to all of you who offered clarifications or asked the right questions!



  • Paul

    I’m confused as to why this should indicate a full moon rather than the lunar perigee.

  • Thank you for this exciting article! But I am confused a little bit. When moon is full, it does not mean it is closer to the Earth, except the ‘Supermoon’ case. Maybe this is an axtra amount of light that causes the anomaly?

  • George T. Fleming

    You explained how gravitational forces due to the Moon affect the LHC and mentioned how the effect becomes strong as the Moon passes over head, which it does once per day plus 49 min. But, you did not explain what this has to do with the phases of the Moon, and why the effect is stronger on the day of the full Moon than other days. Presumably, the answer involves not only tidal forces due to the Moon but also the Sun. Then, I would expect the effect you mentioned to be just as strong during the New Moon as the Full Moon.

  • Chris R

    Why would the effect be greater during a full moon? Is the moon always closer to the earth when it’s full?

  • Paolo

    Sorry, I’m not a physicist and I don’t really understand why the effect is stronger when the moon is full: because it’s also closer? Can you explain a bit? Thanks!

  • Photon

    “The effect is of course largest when the moon is full.”

    I’m sorry but I don’t quite see this as obvious, do you mind explaining how the moon being full increases the difference in gravitational pull across the LHC? I would have guessed that, just like the oceans, the greatest ‘spring’ tides can also occur at the new moon.

  • Paolo

    In the meanwhile I read a bit of wikipedia and I think I understand: it’s because when the moon is full (or when the moon is new, for that matter) the tidal effect due to the sun reinforces that due to the moon (they are aligned)

  • David

    This effect would surely not be dependent on whether the moon is full, but on the relative position of the moon with regards to the LHC? The gravitational infuence of the moon does not change depending on whether it is full or not, as this is only an illumination of the surface; the moon is ‘obviously’ still there when it is new. Presumably the observed effect results from the Earth’s rotation or the fact that the moon was recently at perigee?

  • The LHC collides proton-proton. The LHC could collide antiproton-antiproton. You could know whether matter and antimatter are real world gravitationally indistinguishable in a single day of running. This is a huge payback for a small investment offered 13 times each year. Twice the differential signal obtains from proton-antiproton.

    Double up! A nearly total solar eclipse covers Geneva at 1030 hrs (CET local time) Friday 20 March 2015.

  • KJ

    I’m confused about why this effect would be greater when the moon is full. Because regardless of what phase the moon is in, its mass is the same. Its distance from the Earth also does not depend on whether or not it is full. The phase of the moon affects only how much of the moon we can see, not mass or distance from Earth.

  • James Jurack

    Is this really an effect of phase or rather of orbital distance? Why should the percentage of the visible surface of the moon being illuminated change its gravitational pull on your tunnel? Wouldn’t it exert more force at perigee rather than when full?

    Unless this has something to do with the position of the moon in the sky — the fact that it passes overhead at a certain time of day, which /does/ change with its phase. If that were the case, though, I’d expect the dips to happen every day, just at different times depending on the lunar phase.

  • Jason Walton

    “The effect is of course largest when the moon is full.”

    Erm… Why is that? A full moon exerts the same gravitational strength as any other moon. A full moon has the distinction that it is on the far side of the earth from the sun and thus exerting a force opposite the sun (which is why it causes spring tides, unlike when the moon is halfway waning or waxing, where it’s pulling at right angles to the sun and causing neap tides), but if that were the determining factor I’d expect this effect to be equally strong at new moon, when the moon is pulling “with the sun”?

  • Terry

    You seem to be on two different subjects here…

    Moon phase refers to the amount of shadow cast on the moon by the earth blocking the sun, the moon rising refers to its physical location in orbit around the earth. As I understand it one of these creates a gravitational pull on the earth (location), the other does not (shadow).. so how is a ‘full moon’ responsible for this hydrostatic pressure?

  • Israel Galan

    Surely what affects the LHC is the position of the Moon and its distance, not the phase which is only the part if the Moon that falls out of the shadow that the Earth projects.

  • Dave

    The fullness of the moon will not change it’s grvitational attraction on the LHC.
    The relative position and the distance of the moon, maybe…

  • Chris R

    That’s silly, why would the effect be larger when the moon is full? Does the amount of light reflected affect the mass of the moon in some way?

  • Christopher Grams

    Hi Pauline,

    I was wondering if it is not necessary to adjust the proton beam orbits every time the moon passes by, which would be every single night, because a ‘full moon’ is merely when the earth’s shadow is not on the moon?



  • Steph

    I don’t get it. Full moon doesn’t mean that the moon is closer… I agree with a ~28 days cycle, but the sentence “The effect is of course largest when the moon is full.” doesn’t make any sens to me.

  • Martijn

    Wow, nice job! I feel stupid for forgetting some ; in code while the LHC is being annoyed by the moon hahahahahaa

  • “The effect is of course largest when the moon is full.”
    I would expect the effect to be similarly large at new moon also.

  • Daniel Brown

    Is it accurate that the full moon creates this effect? That it’s greatest effect is when the moon is full? I would imagine it had little to do with a full moon at all but it’s position in the sky and distance from the Earth.. It’s effect should be discernible while the moon is in certain particular positions, during the day or night and when the moon is in any phase…

  • Anders

    Just a question, maybe a stupid one. Moon is “full” when the light of the sun lights it all the way. But when it’s not full, that means the visible side does not bathe in the sunlight. So, my question is: what has this to do with the gravity? Doesn’t the moon orbit there anyway, causing the tides and so on..? So why does it matter if it’s full or not. Thanks!

  • Wow, I’d never expected the LHC to be affected by this. Eye opener, great artice!

  • Jörg


    From the article: “The effect is of course largest when the moon is full.”

    I think this is dead wrong as the moon’s gravity effect on the LHC is not dependent on its phase.

  • Daniel Brown

    Forget it, the sun is in opposition. Should have mentioned that though Pauline haha

  • Why is the effect largest when the moon is full? That doesn’t mean that it’s closer, it’s just a matter of its relation to the sun.

  • Troy Carson

    “The effect is of course largest when the moon is full.” Maybe I am missing something, but I don’t understand this statement. The moon does not change in size but only in how much is illuminated by the sun. Therefore it shouldn’t matter if how much is illuminated but only the position of the moon.

  • Marshall Eubanks

    I don’t understand why the operators are required to correct for this manually,. These tidal deformations are certainly calculable, and a search reveals that this was first mentioned in 1992 ( http://www.nytimes.com/1992/11/27/us/moon-is-blamed-for-blips-in-a-particle-accelerator.html ) which should be enough time to include it in the software.

    By the way, I calculated once that the LHC might be able to see the polar motion as well as the lunisolar tides – do they see any small ~ 200 day terms ?

  • Jerry

    I know that the LHC is affected by the position of moon and the tides in Lake Geneva, but it would be in a period of 24 hours, not as frequent within a single run as observed in image for this article. The periodic dips in the total instantaneous luminosity of the ATLAS and CMS experiments are due luminosity optimizations ~15 minutes after the start of the stable beams and every ~2hrs after that.

    If it’s a matter of the moon, the optimizations should appear when the moon is over Geneva-area and not when the moon is on the other side of the planet. And these optimizations occur for every single run as request by the shift leader of either experiment or as instructed and agreed upon in the LHC run coordination meetings.

  • Steve Grant

    Would the effects not also be equally noticeable at a new moon too? I thought they were similar in terms of gravitational effects? Actually my laymans intuition would suggest that the gravitational effects would be STRONGER during a new moon, given that the Moon and Sun are ‘pulling’ the same direction; and conversely in opposite directions during a Full Moon.

    Like I said, just a layman here.

  • Usolis

    That just doesn’t seem quite right. The Moon being full means that it would be on the opposite side of the Earth to the Sun, and therefore cancel somewhat with the Sun’s gravity. A new moon would however produce the opposite effect of strengthening the gravitational pull towards the Sun.

    What isn’t factored in however is CERN’s position on the surface of the Earth relative to these events (which of course changes throughout the day just as the tides do). At Midnight During a full Moon the effect would be considerably different than at noon. I would rather suspect that the variation is much larger on a daily basis than on a lunar cycle just as a neap tide is only about 20% lower than normal.

  • Forbes Hirsch

    What on earth does the phase of the moon (full or otherwise) have to do with the gravitational effect? I think you were being led on.

  • Mike Schriber

    This makes absolutely no sense. Why would the gravitational pull of a full moon be different than any other phase? Only the altitude of the moon over the Earth effects it’s pull (on the Earth) and that has no correlation with the phase of the moon.

    The tidal effect of the moon on a large object like the LHC occurs regardless of the phase or altitude of the moon and would have to be constantly adjusted for as the moon’s position changes relative to the LHC.

    A scientist should know better.


  • J

    Why does the phase of the moon change the strength of the effect?

  • Tibs


    “Full” means we’re seeing the fully lit side of the moon. A more well-lit moon does not mean that it exhibits more gravitational pull.

  • Steve

    A full moon isn’t any closer to the Earth than during the other phases. Sometimes a full moon coincides with the closest approach to the Earth and sometimes it doesn’t.

  • Bill Broadley

    Seems like it’s most likely to be the moon position relative to the LHC and not the amount of light bouncing off the moon. So isn’t it mostly that the moon is overhead, not that it’s full (or not)?

  • Cliff

    Perhaps a little explanation is in order as to why it is worse when there is a full moon.

    It would seem to me to be worst when the moon is closest. at new moon phase, and at noon in the day time.

  • Rob Baston

    Marvellous — so the LHC also detects that the Weyl tensor is not zero near Geneva when the moon is full! Seriously, how measurable is the effect? Is it enough to detect variations from Newtonian gravity or not?

  • DC Whitworth

    I know I’m being picky but this article implies that the gravitational effect of the moon on the earth varies with its phase which isn’t the case.

  • Good Day from Canada

    Why would only the “full” moon affect the LHC?

    Would it not be twice per day like the ocean tides?

    Very curious
    Fred Wagner
    Brockville, Ontario, Canada

  • william

    Why is the effect largest when the moon s full? Surely that’s just to do with how much of the moon’s lit surface is visible from our point of view?

  • Hi- Love this blog! But I must point something out: it’s not the Moon’s phase that’s the main effect, it’s the location in the sky. The tides across the LHC tunnel’s diameter will be largest when the Moon is in the plane of the tunnel; that is, when it’s rising. It’ll be minimized when it’s at the highest point it gets in the sky (if it were overhead, the tidal force across the circle would be zero or close to it).

    When the Moon is full, its tides are added to those from the Sun, which are about half as strong. That may be what the operator was talking about. But this happens when the Moon is new as well! At that time it’s aligned with the Sun, and their tides add constructively. I wonder if this effect is noticeably larger when the Moon is at perigee as well; tides are strongest then.

    If you find out more, please email me! I’d love to write about this for my blog. 🙂

  • David Hollebon

    A fine example of careful scientific observation of small effects. Just the kind of article which inspires the young to take an interest in science.

  • Pauline Gagnon

    Thanks to everybody who criticized or offered clearer explanations. I have inserted an addendum in the blog to clarify the most ambiguous points.

  • BannedFromBAUT

    According to the article, tidal forces distort the geometry of the LHC sufficiently to require operator intervention. Do these tidal forces also disturb the proton orbits?

  • BannedFromBAUT

    Apologies. Incomplete question.
    Something about this tidal distortion issue provoked a thought that if the distortions can be compensated-out by software, could the resulting bias signal be incorporated into a gravity wave detection scheme?

  • Paul

    I’d hardly describe a 7% variation as “rather fixed”.

  • The tidal forces do! They depend on the Sun-Earth-Moon angle, so I hope you see the point.

  • Tim G. Meloche

    I was interested in the discussion about gravitational influences on the CERN machine and so I wrote the following article to help in analyzing the data:

    CERN Proves Unified Principles of Nature Exist

    the European Organization for Nuclear Research Operations has been
    working toward solving the big unknowns of the Universe which include
    the search for the unified principles that underlie all of nature.

    the CERN experiment is up and running the sensitive machine requires
    operators to periodically adjust the alignment of the proton beams so
    that the two moving opposing beams make consistent collisions in order
    to collect uniform data sets. The multiple flows of protons are
    controlled inside a 27km vacuum tube by superconducting magnets
    strategically placed along its length. The adjustments required for
    consistent collisions are described by Indiana University physicist Pauline Gagnon in a Livescience article by Clara Moskowitz.

    Livescience article’s summary is that the magnetic fields controlling
    the protons paths/orbits require adjustments to compensate for
    gravitational field fluctuations induced by the changing alignment of
    the moon and sun with respect to CERN’s location throughout the year.

    Moskowitz and Pauline Gagnon suggest that it is the “planet’s crust
    stretches toward the moon” causing the protons paths/orbits to require
    adjustments. It is true that the gravitational tug from the moon and sun
    does move the earth’s surface and this can be measured with an accurate
    satellite system.

    the CERN machine is located many meters under the surface and the
    planets crust movement would generally be uniform across the CERN
    machine. Moreover the proton beams are housed in a vacuum tube and are
    magnetically manipulated to not touch the tube in their 27 Km orbits.
    The magnetic control feature of the machine over the proton beams in the
    tubes is designed to keep the protons in the center of the tubes. The
    protons magnetic centering system of the machine is physically
    independent of any movement of the entire region were the CERN machine
    resides. Moreover the CERN machine would equally lift with the earth’s
    crust movement or a growing number of machine foundation fractures would be observed through time.

    Livescience article misses a critical feature of how gravity works at
    the atomic scale and how this affects the operation of the CERM machine.
    The protons inside the vacuum tubes are simply behaving similar to the
    behavior to the nucleus inside a typical hydrogen atom making up part of
    the ocean when there is a gravitational induced lift.

    movement of the protons in the tubes is subtle yet predictable and the
    CERN system has shown the ability to build a data set of the required
    adjustments co-related to the changing alignment of the moon and sun to
    CERN throughout the year. This data set is an unexpected gift for those
    trying to understand the force of gravity at the atomic scale.

    new data set in real time can now be compared to another set of data
    produced by a secondary gravity field experiment set-up in multiple
    locations along the 27 Km long system. This secondary experiment would
    create a data set for the fluctuating weight of a simple common mass
    with respect to the changing alignment of the moon and sun in the CERN
    gravitational field throughout the year.

    results of comparing data sets would lead to a very accurate
    mathematical description of atomic gravitational fluctuation induction
    from orbiting gravity signatures like our moon and sun.

    simple picture is an exciting unexpected result for scientist trying to
    solve for the mechanism of gravity at the atomic scale. The subtle
    adjustments required for smooth operation through the year demonstrate
    how this multi-billion Euro machine is connected to the natural world in
    its day to day operation.

    theory behind the principles of atomic gravitational fluctuation is
    simple yet powerful in the quest to mathematically describe the unified
    principles underlying physic and nature. The CERN program will continue
    to provide cutting edge research to the World.

    gravitational fluctuation occurs in all the atoms of the periodic
    table. The electron spheres of each allow the nuclei to subtly move
    within each sphere in response transiting gravity signature like the
    moon. The daily ocean tides are a natural example atomic gravitational
    fluctuation. The movement in our oceans and atmosphere are enhanced due
    to their relatively deep fluid state.

    cooled vacuum tubes and magnetic fields carrying the protons at CERN
    create a similar environment compared to a proton within it electron
    sphere in a stable hydrogen atom when it is co-related to a transiting
    gravity signature like the moon. The kinetic energy of the protons at
    CERN offers an opportunity to analysis the effects of an additional
    experimental parameter compared to the relatively stationary nuclei in
    the weight fluctuation experiment.

    combined data sets present an excellent opportunity to form a forward
    moving enthusiastic and dedicated sub-team from the approximately 10,000
    currently working with the CERN machine, who are intent on solving
    gravity mathematically at the atomic scale.

    About the Author

    Tim G. Meloche obtained a formal education in Aerospace Engineering
    from Ryerson University in Toronto, Canada (1983). Tim is a scholar in
    the study of new and past discoveries made through observations and
    experimental analysis by the many throughout history. A life time
    combining both educational paradigms has been utilized in the pursuit to
    resolve modern day problems in physics. Through time and continued
    education Tim worked towards formulating principles of energy-matter
    interactions that are in harmony with experimental and observational
    analysis. His efforts have led to a unified scientific method to better
    analysis and understand physics at both the atomic and astronomical
    scales. He continues the quest for expanded knowledge and to bring the
    “Unified Principles of Physics and Nature” into academics for all to
    understand and benefit from.