Lunar madness

I had  hoped to calculate from the JPL ephemeris how the regular 100,000 and 400,000 cycle  in the eccentricity of the earth-moon orbit around the sun  would also effect the moon-earth orbital eccentricity. Such variations could dramatically change the tidal forces acting on earth and play a leading role in triggering interglacials.

The seminal work on solar system dynamics has been done over decades by the French group at Observatoire de Paris. The latest paper La2010: A new orbital solution for the long term motion of the Earth gives the most accurate orbital parameters for the earth over the last 20 million years. However it also shows just how difficult it is to disentangle the moon’s orbit around the earth from their barycentre orbit around the sun, making it almost impossible to predict any long term changes in tides. Their quoted  error in calculating the moon’s eccentricity the last 1 Mya is ±0.03 compared to ±0.00000005 for the earth eccentricity around the sun!


So it is indeed quite possible that major changes in the moon’s orbital parameters will occur as the eccentricity of the Earth-Moon barycentre increases every 100,000 years. However it is almost impossible to calculate how large is the effect.

LA2010 also published some very accurate eccentricity values for the earth over the last 20 million years and into the future. I plotted these out and could not help but notice that the earth’s next 400 y cycle looks almost exactly the same as that which occurred roughly 3 million years ago. That was during an when  glaciations followed the 41,000 year obliquity cycle. Exactly the same eccentricity cycle is observed nearly 5 million years ago. There is therefore a larger 2.8 million year super-cycle in the earth’s eccentricity .

LA2010 calclations of the earth's eccentricity over a 4 million year period spanning the present day.

LA2010 calcluations of the earth’s eccentricity over a 4 million year period spanning the present day.

The graph below shows  the L04 stack sediment data from 3 million years  ago (colder is larger dO18). The larger the obliquity the larger are the extremes between the seasons. For large eccentricity the precession term then introduces a significant north-south asymmetry which depends on which particular northern/southern hemispheric season aligns with perihelion.

Combined fits compared to data 900-1500 ya

Combined fits compared to data 3 million to 1 million years ago

I am proposing that it was only when the obliquity cycle became insufficient to trigger an interglacial 800,000 years ago that the moon’s resonant tidal effect was needed to complete the job. This was the cause of the switch in phase to a 100,000 year glacial cycle.

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6 Responses to Lunar madness

  1. A C Osborn says:

    Can they really calculate 20 Million Years in to the Past?
    How can they possibly know what Celestial occurrencies there could have been during that time, ie impacts on the moon making minor changes in it’s position/orbit?

  2. Clive Best says:

    They claim remarkable precision for the earth’s eccentricity back as far as 50 million years. They even include general relativity corrections to solar system planets. Of course this assumes there have been no collisions with truly massive bodies over that period. The perturbations to the moon’s orbit however seem to be very uncertain. In addition the moon is more likely to have been impacted by large enough objects to change its orbit slightly.

  3. Roger Andrews says:


    Re the “switch in phase to a 100,000 year glacial cycle”, take a look at this plot of the LR04 d18O record (I’ve inverted the Y scale so that glacials appear as troughs and interglacials as peaks and eyeballed elapsed times to the nearest 5,000 years):

    The troughs show the ~100,000 year glacial cycle beginning maybe 700,000 years ago. The peaks, however, show a dominant ~80,000 year interglacial cycle that falls apart only after the last interglacial 120,000 years ago. The average cycle length is 82,000 years, i.e. 41,000 times two.

    It seems that glacials and interglacials may march to the beat of a different drummer.

  4. E.M.Smith says:

    There are also stadials and interstadials to consider. Someone decides that it wasn’t warm enough long enough to be an interglacial and the stadial category gets applied… yet the period and direction were correct… and a 41k warm spike is removed from the record of interglacials…

    We only exit the glacial mode when everything lines up enough to melt the north pole in summer. You are correct that the shift to 100k was because the 41k event was not quit enough all alone all the time. Some of the 41k points still show up in the stadial events.

  5. Euan Mearns says:

    Clive, I count 10 eccentricity cycles with widely varying amplitude per million years. But there is that wee hook to the left of your blue dashed line at 0. Could that be the Younger Dryas? And on this scheme earth’s orbit is still becoming more circular and the next glaciation is thousands of years away. Could this prolonged period of near circular orbit explain the anomalous long Holocene interglacial? (and global warming?)

  6. Clive Best says:

    That wee hook is indeed the end of the last ice age !

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