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  • 7/31/2019 Transit of Venus 2012_2


    Indian Institute of Astrophysics


  • 7/31/2019 Transit of Venus 2012_2



    Solar System

    Planetary motion

    Why transits happen?

    Conjunctions & orbits

    TOV -why it is so rare? Historical significance of TOV

    Sun-Earth distance measurements

    AUThe yardstick to measure the Universe

    When, where and how to view the TOV?

    Safety issues and precautions for observing TOV


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    The solar system consists ofSun, 8 planetsdwarf planets (3)Planetary satellites ( >100)comets and asteroidsAsteroid belt containing small

    rocky objects between the orbit of Mars and Jupiter

    All these objects are gravitationally bound to the Sun and

    revolve around it.

    Planets are very small compared to the space between them.

    The size of the astronomical objects can vary over a large range

    terrestrial or

    rocky planets Jovian or

    gas planets


    Solar system

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    Planetary motion Planets move around the

    sun in a

    near-circular path calledorbit

    Each planet has its orbit

    All planetary orbits are not

    exactly in one planeOrbits of different planets

    do not intersect 4

    Solar system ..

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    Transit is the passage of a planet across the solar disc/face asseen from Earth.

    Transit can be observe only for inferiorplanets (i.e. Mercuryand Venus ) of the solar system.

    Inferiorplanets are those which orbit the Sun inside theEarth's orbit.

    When Mercury or Venus passes between us and the sun, wecall that event an inferior conjunction.

    If they are on the far side of the sun (but seen in the same areaof the sky), we call that a superior conjunction.

    All conjunctions are not transit (why ???)


    What is planetary transit?

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    TOV-2004 image


    1631 1639

    1761 1769

    1874 1882

    2004 2012

    8 yrs

    8 yrs

    8 yrs

    8 yrs

    2117 21258 yrs

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    Why TOV is so rare??

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    For transit to occur Sun-Venus-Earth should be in straightline

    If not, then Venus would be seen either going above orbelow the sun

    That explains why we dont see transit every 1.6 year.

    But then why dont we see transits every 8 years?

    The reason is that Venus takes 224.7 earth days and notexactly 225. So every fifth alignment (see explanation innext 2-slides) is little short!!!!



    3.4 deg




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    105.5 or 121.5 years

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    Historical Significance of TOV

    Sun-earth system as a base-line to measure distances to other stars using parallax



    Correct model (16th Century) of solar system

    - Nicholas Copernicus

    Laws of planetary motion and solar system modelwithout scale

    - Johannes Kepler(17th Century)

    Keplers 3rd law (T1/T2)2= (R1/R2)3 gives relative

    distances of planets from the Sun.

    Absolute distances were not fixed Venus-Sun distance = 0.7 AU

    Mercury-Sun distance = 0.4 AU

    Earth-Sun distance = 1.0 AU

    Mars-Sun distance = 1.6 AU

    Jupiter-Sun distance = 5.2 AU

    Saturn-Sun distance = 10 AU

    Knowing the solar parallax (i. e., the angle subtendedat the Sun by the Earth's mean radius.)and the meanEarth radius allows one to calculate the AU.

    1 A.U. ????

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    Distance measurements with Parallax Method


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    The transits of Venus of 1631 and 1639

    Arkan Simaan, PHYSICS EDUCATION 39 pg 247 (2004)


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    The transits of Venus of 1761 and 1769

    Multiple observations were planned around the world.

    French and English scientists took the main lead.

    Main challenges: wars, long journeys, natural hazards, regulating clocks,precise longitude/latitude info.

    Le Gentil de La Galaisire set out to Pondicherry to observe 1761 TOV buthe did not succeed as the Pondicherry port has fallen to British by that

    time. He waited for 8-years but again failed due to overcast inPondicherry in 1761. By the time he returned to France after 11 years hisrelatives had sold his properties thinking he is dead!

    In 1761, more than 120 measurements were made, from over 60 sites.Solar parallax varied between 8.28 and 10.60 seconds of arc.

    With more than 150 observations from 77 sites in 1769. Solar parallaxwas between 8.50 and 8.88 seconds of arc.

    Uncertainly due to black drop effect, longitude etc


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    The transits of Venus of 1874 and 1882

    Friedrich Bessel had measured (in 1838) the annual parallax of thestar 61 Cygni.

    The distance from Earth to Sun thus became the baseline formeasuring the universe.

    Newly developed photographic methods were used first time for

    TOV In addition, Johann Galle had succeeded (in 1875) in measuring

    the solar parallax with considerable precision (8.873), using theasteroid Flora.

    French, British and American astronomers organized expeditions toall parts of the Earths surface from which the transit would beobservable.

    William Harkness analyzed the results of several observations andfound 8.794 seconds of arc for the solar parallax.

    TOV-2004 was more of an educational value!!15

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    How to measure 1 A.U. ?


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    Transit of Mercury

    Mercury is the innermost planet of the solar system

    It takes about 88 days to go around the sun. It require additional 28 days for to catch up

    with Earth.

    In other words the time from one inferior conjunction to the next inferior conjunction is

    about 116 days. So why we don't see a transit of Mercury every 116 days?

    Mercurys orbit is tilted by about 7 deg w.r.tEarths orbit (ecliptic plane)

    From our point of view, a great majority of the time Mercury passes above or below the

    sun when it is at inferior conjunction.

    Mercury's orbit crosses the ecliptic only twice a year: in early May and in early

    November. So a transit can only occur if Mercury is at inferior conjunction at those times.

    On average 13 transits per century, separated by intervals ranging from 3.5 to 13 years.


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    World-wide visibility of TOV-2012


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    Scientific objectives of TOV-2012

    To understand the relationship between apparent diameter of the Venusduring the transit with its known diameter. The comparison will help toestimate the size of exoplanets detected around other stars.

    To improve and develope reliable exoplanet detection techniques bymeasuring the dip in Sun's brightness. The robust models will then helpeliminate the false detection signals that may be arising due to theappearance of star-spots or intrinsic variability of the host star.

    To deduce the composition of exoplanet atmosphere by comparing it tothe spectroscopic studies of the Venus' atmosphere during the transit.

    To improve our understanding of the climatic conditions of the Venus'atmosphere.

    Estimating the scattering noise of the instruments and telescope optics.


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    TOV-2012 from India

    Wednesday 06 June 2012 TOV will already be inprogress when the sun-risesso viewers from India will

    miss the 1st

    and 2nd










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    How to observe ?


    Projection method

    Binocular /Telescope

    covered with Mylar filters


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    Do not look directly into the Sun.

    Do not use sun-glasses

    Do not see the sun through an unfilteredoptical devices e.g., cameras, telescopes &

    binoculars etc.

    Do not use discarded X-ray sheets, glossysurface, welding glasses etc.


    Retina does not have pain receptors so it is easyto damage it or sustain a permanent eye injury

    by looking directly into the Sun

    For eye safety

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    Disclaimer: The information provided in these slides is collected from many web sites,

    journals and magazine articles. Readers are requested to double check the accuracy of

    various accounts from other independent sources.

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