[AstroNet] Fwd: Measuring the Astronomical unit by parallaxmeasurement on Mars

Tue Oct 6 12:16:18 SAST 2009

Dear All,

This was done a little over a year ago in a collaboration between the Garden Route Centre of the ASSA and the University of Milan, the results were published in the European Journal of Physics
Eur. J. Phys. 30 (2009) 35–46

Kind regards

Case

********************************************************
Case Rijsdijk                 Tel:   +27 44 877 11 80
"Entilini"                         Fax   +27 0866 844 195
1156 Langvlei Ave.        Cell:  +27 83 444 24 94
Kingfisher Close             e-mail: particles at mweb.co.za
Wilderness 6560
South Africa                    "Enough is a feast"
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  ----- Original Message ----- 
  From: Christian Hettlage 
  To: astronet discussion group 
  Sent: Tuesday, October 06, 2009 11:20 AM
  Subject: [AstroNet] Fwd: Measuring the Astronomical unit by parallaxmeasurement on Mars

  Dear All,

  Please find below an opportunity for measuring the distance to Mars. Please contact John directly in case you are interested. Thanks!

  Kind regards,

  Christian

    Von: "John Clark" <>

    Datum: 3. Oktober 2009 14:53:32 GMT+02:00

    An: <assa at saao.ac.za>john.clark at johnclarkastronomy.co.uk

    Betreff: Measuring the Astronomical unit by parallax measurement on Mars

    Antwort an: <john.clark at johnclarkastronomy.co.uk>

    Dear Sir,

    I am a UK-based amateur astronomer with an interest in solar system measurements.  I have spent some time over the last few months estimating the length of the astronomical unit by timing eclipses of Jupiter’s moon Io, essentially reversing an observation by Roemer who discovered the finite speed of light by timing such eclipses.  I measured the speed of light in a university laboratory, and back-calculated the AU from my own observations of Io over a couple of years.

    This method worked, but it is difficult to get an accurate value.  My value was 20% low compared to the accepted value.

    My impression is that the method of Cassini and Richer (1672) is likely to be more accurate, but it requires simultaneously measuring and comparing the position of Mars from two faraway locations on Earth.  This measurement would have to be done to within at least one second of arc.  This is not as hard as it sounds, but it obviously isn’t trivial.

    I wondered if there is anyone in South Africa who is interested/crazy enough to collaborate with me to try this measurement during the forthcoming opposition of Mars?  It would then be at its closest to Earth, so that the parallax would maximise.

    Based on a rough estimate, it is about 10,000 km from the UK to SA over the Earth’s surface, so the chord length (straight line distance) is about 9,000 km.  Mars will be at about 100,000,000 km away at its closest approach, so we are looking for a parallax of 9,000 parts in 100,000,000 or about 18.6 arc seconds. A measurement good to 1 arc second would therefore have an error of 1 in 18.6 or 5.4%.  Obviously a measurement good to 0.5 arc sec would have half that error.  With good amateur equipment that should be feasible.  

    I can help with the maths needed to post-process the photos.

    Yours sincerely,

    John.

    Dr John D. Clark

    323 Wootton Road

    King's Lynn

    Norfolk 

    England

    PE30 3AX

    +44 1553 679378

    john.clark at johnclarkastronomy.co.uk      http://www.johnclarkastronomy.co.uk      

    Measure Solar System Objects and their Movements for Yourself! by John D. Clark

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