Download Transit of Venus

Indian Institute of Astrophysics
Bangalore-560034
Content…
 Solar System
 Planetary motion
 Why transits happen?
 Conjunctions & orbits
 TOV -why it is so rare?
 Historical significance of TOV
• Sun-Earth distance measurements
• AU –The yardstick to measure the Universe
 When, where and how to view the TOV?
 Safety issues and precautions for observing TOV
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Solar system
The solar system consists of
Sun, 8 planets
terrestrial or
dwarf planets (3)
rocky planets
Planetary satellites ( >100)
comets and asteroids
Asteroid belt containing small
rocky objects between the orbit of Mars and Jupiter
Jovian or
gas planets
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…
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Solar system …..
• Planetary motion
 Planets move around the
sun in a
near-circular path called
orbit
— Each planet has its orbit
— All planetary orbits are not
exactly in one plane
— Orbits of different planets
do not intersect
http://www.sercc.com/education_files/aer_spring_01.html
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What is planetary transit?
• Transit is the passage of a planet across the solar disc/face as
seen from Earth.
• Transit can be observe only for inferior planets (i.e. Mercury
and Venus ) of the solar system.
• Inferior planets are those which orbit the Sun inside the
Earth's orbit.
• When Mercury or Venus passes between us and the sun, we
call that event an inferior conjunction.
• If they are on the far side of the sun (but seen in the same area
of the sky), we call that a superior conjunction.
• All conjunctions are not transit (why ???)
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• A transit of Venus occurs when Venus passes directly
between the sun and earth.
• TOV is rare and has a strange pattern of occurrence
TOV-2004 image
• Pattern occur in pairs. i.e. 8 years separation followed by a
gap of 105.5 or 121.5 years and so on
1631
8 yrs
1639
• The pattern repeats after 121.5—8—105.5—8 yrs……
1761
8 yrs
1769
• Since the invention of telescope (yr 1608) only seven
TOVs have occurred.
1874
• The last TOV of this century would occur on June 6, 2012
8 yrs
1882
2004
8 yrs
2012
2117
8 yrs
2125
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Why TOV is so rare??
• Venus passes between Earth and Sun 5 times in 8
years so why don’t we see transit every 1.6 year??
• Reasons
a.
Mismatch in orbital periods (Earth: 365.25 days,
Venus=224.65 days)
b. Inclined orbital planes

Venus’ orbital plane is tilted by about 3.4 deg w.r.t. ecliptic plane
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

For transit to occur Sun-Venus-Earth should be in straight
line
If not, then Venus would be seen either going above or
below the sun
June
Earth
Sun
3.4 deg
Venus
December

That explains why we don’t see transit every 1.6 year.

But then why don’t we see transits every 8 years?

The reason is that Venus takes 224.7 earth days and not
exactly 225. So every fifth alignment (see explanation in
next 2-slides) is little short!!!!
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http://lasp.colorado.edu/home/wp-content/uploads/2012/05/NEW_Paper_Plate_Transit_of_Venus.pdf
Since Venus takes 224.7 earth days and not exactly 225. So every fifth alignment is little out of
step from red line!!!! In other words the “5-point star of passes” rotates clockwise every year a
little bit.
In 2004 the 5th pass alignment was just ahead of red-line and Venus passed across one edge of
the sun.
On 5-6 June 2012, the 5th pass would be little behind the red-line and Venus will be seen
crossing the other edge of the Sun.
In next 8-year (i.e. 2020) the Earth-Venus will be too far from red-line and altogether miss the
disk and hence no transit.
It will take another 105.5 (121.5) years for the E-V to come close to red-line. That will happen
in 2117 and then 2125 AD. That is why it is a life time opportunity!!!
105.5 or 121.5 years
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Historical Significance of TOV
• Correct model (16th Century) of solar system
- Nicholas Copernicus
• Laws of planetary motion and solar system model
without scale
- Johannes Kepler (17th Century)
• Kepler’s 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
(1473-1543)
(1571-1630)
1 A.U. ????
• Knowing the solar parallax (i. e., the angle subtended
at the Sun by the Earth's mean radius.)and the mean
Earth radius allows one to calculate the AU.
Sun-earth system as a base-line to measure distances to other stars using parallax
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Distance measurements with Parallax Method
Parallax is an apparent displacement in the position of an
object viewed from different locations
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html#c1
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The transits of Venus of 1631 and 1639
•
In 1629 Kepler predicted the Venus transit in 1631
Dec but did not live to see his prediction come true as
he died in yr 1630..
•
Pierre Gassendi made observations from Paris but
missed the transit ( since it was night time in Europe)
•
Jeremiah Horrocks & William Crabtree realized that
2nd transit would happen in 1639…They made three
observation..
•
Edmond Halley, observed a transit of Mercury from
Saint Helena (1677). He came up with a revolutionary
procedure, replacing measurements of angles by
measurements of times.
•
Joseph-Nicolas Delisle refined Halley’s method. It was
only necessary for two astronomers to record a single
contact. This meant that useful measurements could be
made from many more points
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 Galaisière set out to Pondicherry to observe 1761 TOV but
he 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 in
Pondicherry in 1761. By the time he returned to France after 11 years his
relatives 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 parallax
was 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 the
star 61 Cygni.
• The distance from Earth to Sun thus became the baseline for
measuring 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 the
asteroid Flora.
• French, British and American astronomers organized expeditions to
all parts of the Earth’s surface from which the transit would be
observable.
• William Harkness analyzed the results of several observations and
found 8.794 seconds of arc for the solar parallax.
TOV-2004 was more of an educational value!!
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How to measure 1 A.U. ?
Method proposed by Edmond
Halley in yr 1621
Observe the TOV from two
different location A & B on Earth
Determine distance AB
Measure angular separation 
Use trigonometric relations to
determine E-V distance
Use known relative distance
Between S-E and S-V to finally
compute 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?
Mercury’s orbit is tilted by about 7 deg w.r.t Earth’s 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 Venus
during the transit with its known diameter. The comparison will help to
estimate the size of exoplanets detected around other stars.
• To improve and develope reliable exoplanet detection techniques by
measuring the dip in Sun's brightness. The robust models will then help
eliminate the false detection signals that may be arising due to the
appearance of star-spots or intrinsic variability of the host star.
• To deduce the composition of exoplanet atmosphere by comparing it to
the 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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Delhi
Leh
Mumbai
Chennai
 Wednesday 06 June 2012
 TOV will already be in
progress when the sun-rises
so viewers from India will
miss the 1st and 2nd contact.
Bangalore
TOV-2012 from India
http://transitofvenus.nl/wp/
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How to observe ?
Projection method
Binocular /Telescope
covered with Mylar filters
Sun-screens
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Retina does not have pain receptors so it is easy
to damage it or sustain a permanent eye injury
by looking directly into the Sun
For eye safety……
• Do not look directly into the Sun.
• Do not use sun-glasses
• Do not see the sun through an unfiltered
optical devices e.g., cameras, telescopes &
binoculars etc.
• Do not use discarded X-ray sheets, glossy
surface, welding glasses etc.
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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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