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

2

The solar system consists of Sun, 8 planets dwarf planets (3) Planetary satellites ( >100) comets and asteroids Asteroid 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

3

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 4

Solar system …..

• 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 ???)

5

What is planetary transit?

• A transit of Venus occurs when Venus passes directly between the sun and earth.

• TOV is rare and has a strange pattern of occurrence

• Pattern occur in pairs. i.e. 8 years separation followed by a gap of 105.5 or 121.5 years and so on

• The pattern repeats after 121.5—8—105.5—8 yrs……

• Since the invention of telescope (yr 1608) only seven TOVs have occurred.

• The last TOV of this century would occur on June 6, 2012

TOV-2004 image

6

1631 1639

1761 1769

1874 1882

2004 2012

8 yrs

8 yrs

8 yrs

8 yrs

2117 2125

8 yrs

• 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

7

Why TOV is so rare??

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

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!!!!

8

9 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!!!

10

Historical Significance of TOV

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

(1473-1543) (1571-1630)

11

• 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

• 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.

1 A.U. ????

Distance measurements with Parallax Method

12 http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html#c1

Parallax is an apparent displacement in the position of an object viewed from different locations

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)

13

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

14

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!! 15

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.

16

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.

17

World-wide visibility of TOV-2012

18

TOV-2012 from India

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.

19

Bangalore

Delhi

Chennai

Mumbai

Leh

http://transitofvenus.nl/wp/

How to observe ?

20

Projection method

Binocular /Telescope covered with Mylar filters

Sun-screens

• 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.

21

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……

22

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.