Tuesday, July 10, 2007

Shrinking Earth

No, this is not about a resuscitation of old ideas about the history of planet Earth, but these days I could learn that the Earth Is Smaller Than Assumed, according to geodesist from the University of Bonn who have discovered that the blue planet is really smaller than originally thought. Well - not really, I would say: these guys are talking about 5 millimetre, or 0.2 inch.

Anyway, this accurate result is really impressive! It results from the combined analysis of radio signals from distant quasars, observed by a worldwide net of more than 70 radio telescopes. Characteristic features in the radio signals from quasars are received at slightly different times at different places on Earth, and the combination of these measurements using the technique of Very Long Baseline Interferometry allows a very precise determination of the relative distance of the radio telescopes: These relative distances can be deduced up to 2 millimetre on 1000 km, or up to 2 parts per billion (ppb). From the network of radio telescopes distributed all around the globe, it is possible to calculate its dimension very precisely. This analysis, accomplished with improved precision over previous similar work by the Bonn geodesist, yields a diameter of the Earth 5 millimetre smaller than supposed so far. According to a report in the New Scientist about this result, the total diameter of the Earth at the equator is around 12,756.274 kilometres (7,926.3812 miles).

Axel Nothnagel of the University of Bonn, who heads the team that provided new and more accurate data about the diameter of the Earth. (Credits: University of Bonn Press Release, July 5, 2007, Frank Luerweg)

A propos shrinking Earth: Earth was shrinking by a huge step, in a metaphorical way, 45 years ago today, as I heard this morning on the radio: On July 10, 1962, TELSTAR was launched from Cape Canaveral, the first communications satellite which allowed live TV broadcast between Europe and North America, bridging by the speed of light a distance that is steadily growing by 18 millimetre per year...

The TELSTAR communications satellite, launched 45 years ago today (Source: Wikipedia on Telstar)

PS: The paper by the Axel Nothnagel team is: The contribution of Very Long Baseline Interferometry to ITRF2005, by Markus Vennebusch, Sarah Böckmann and Axel Nothnagel, Journal of Geodesy 81 (2007) 553-564, DOI: 10.1007/s00190-006-0117-x. If someone can tell me where I can find the 5 millimetre in that paper, I am very grateful ;-)


  1. So North America is drifting away from Europe...

  2. this satellite looks really odd. It reminds me of the living brain in the Captain Future series


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  4. Consistent signficant figures in the equatorial radius hint at competence - in New Scientist!

  5. Just browsing the internet, very interesting blog

  6. Hi Stefan,
    Measuring parts in a billion always makes me feel awed. Wouldn't thermal variations along the places where two radio telescopes are anchored cause more than a part in a billion variation in distance between them? What about the distance to the center of the earth - no parts per billion periodic variation there? I was told that gyroscopes in the basement of a building can measure sunrise by the small change in tilt as one side of the building warms up compared to the other.

    Yet somehow we define a meaningful number. I think we have to say the earth's diamater is such-and-such when averaged on such-and-such time scale, otherwise the parts per billion precision carries less meaning.


  7. There was a cool song inspired by Telstar. http://en.wikipedia.org/wiki/Telstar_(song)

    My dad used to play an 8-track tape (yeah, I'm that old!) with that song on it. I always liked it because of its spacey sound.

  8. Hi Rae Ann:

    Indeed :-) I too thought 'Telstar, wasn't that a song?'. It's kind of funny these songs which refer to technological progress in communication/video/audio - Video killed the radio star, radio gaga, - then there's a song of that women talking to her husband in outer space (can't recall a title), there must be plenty of songs based on phone calls, also there are some German songs as well e.g. Hör ma (Sabine, Sabine, Sabine) (as far as I know the only song featuring a women with my name). Are there songs about the internet? The only thing that come into my mind is 'virtual insanity' by Jamiroquai ;-)



  9. Dear Bee,

    So North America is drifting away from Europe...

    Somehow... but there are even faster drifts. Here is a cool map.

    Thank you for the link to the Captain Future website - didn't know that such a thing exists. That was a really great SciFi series :-)

    The Telstat satellite looks quite funny indeed, especially with this antenna, like a curly hair. It could come straight from a children's book.

    Best, stefan

  10. Dear Arun,

    I agree with you - although I doubt that local temperature may have such a big influence. But I guess that tidal motion will be much larger, for sure on the level of several centimetre on 1000 km.... Gravitational wave experimentalist working on the big interferometers should know, and also accelerator physicist... look here: The fundamental [tidal motion] has a period of approximately 44 ks (or a frequency of 22.6 µHz) with a peak-to-peak amplitude of up to 0.57 m relative to the centre of the Earth.

    As I understand it, these numbers are averages, both over the statistical fluctuations, and over systematic effects such as temperature or tidal variations.

    I mean, if you have an statistical error of 2 mm for 1000 km, you can talk sensibly about the 5 mm "shift" for the 10000 km or so diamter only as an average.

    Best, stefan

  11. Dear RaeAnn,

    There was a cool song inspired by Telstar.

    Yes, in that radio notice about the Telstar launch 45 years ago, they also played that song - and I remembered it also from way time back ;-)

    Dear Bee,

    then there's a song of that women talking to her husband in outer space (can't recall a title)

    then this website seems to be what you are looking: www.midomi.com :-)

    It was mentioned by chance in stern.de today.

    Best, stefan

  12. Dear Stefan, now I feel terribly embarrassed that I didn't think of the tides.

    That is why silence nourishes wisdom. At least the appearance of wisdom.


  13. 2 milimetres per 1000 km
    or 2 parts per billion
    is that like the gap between the crest and trough on a passing gravitational wave?



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