The Transit of Venus 6th June, 2012
Joe Cali & Geoff Sims

This report is composed of four sections
1.    Main report page for Joe & Geoff 
2.    Gallery of our friends images
3.    Animation page (  big file  3.7MB)  & link to Geoff's video file
4.    Explanation of why the path Venus appeared to be a curve rather than a straight line as shown on TV
  (this page)

Explaining the strange curved paths followed by Venus during the 2012 transit.  

I've had numerous questions about the strange curved trajectory the Venus took across the Sun's disk.

From Cairns Australia the planet took a particularly odd trajectory appearing to enter the Sun's disk and returning
along almost the same path. It was in fact even closer to a straight line than in my illustration below.

This is a question I received from a Cairns science teacher and is typical of the questions I've received :

Q:    I was initially expecting the “black dot” to travel from the left to the right side
of the Sun as observed from earth, as most of the pictures on the internet show.
This made sense as Venus orbits more quickly and we are all going around the sun
counter clockwise. What we saw was a little different, and more like this….
although for us perhaps a little less movement to the left than in the picture?

Let's split the problem into two parts and then deal with each in isolation.

The left to right effect
Don’t think left to right but rather that Venus was moving from east to west. Pictures on the internet and on TV are usually generated using a geocentric perspective.  That is the view from the centre of the Earth, north up. This coincidentally is the same as the view from the northern hemisphere when looking at the Sun in the south.

Note the reversal of east - west compared to a map.  In cartography, we are looking down,
in astronomy, we are looking up! If you have trouble with this, lay on your back, head to the north feet south looking up. East is to your left, west to your right. Now roll over and face down as in a map view.  The directions are reversed.
 If you are standing anywhere on the southern hemisphere looking north, you effectively are standing upside down relative to the geocentric or northern hemisphere frame of reference. 

Around midday when the sun crosses the meridian,  the north pole of the Sun points down to the north celestial pole some 17o below due north on the horizon when viewed at Cairns latitude.

Now the apparent direction of motion is right to left relative to the Sun’s orientation at midday.  

What about the curved trajectory
The north pole of the Sun points towards the north celestial pole all through the day. Therefore the Sun
was rotating all throughout the transit. The Earth turns 360o in 24 hrs or 15o per hr. Over the 6.5 hours
of the transit, the sky and Sun rotated about 97 degrees. Coincidentally, the transit followed a chord which
subtended 100o. For simplicity of illustration, I have used a right angle in the representation below. 
Just by pure coincidence, and only from a few parts of Australia, the rotation of the Earth and the angular
velocity of the transit relative to the center of the solar disk.  In the diagram below, you can see that Venus
always appears to be at about "6 o'clock" with respect to the horizon regardless of the progress of the transit.

Consequently the planet doesn't appear to go anywhere when viewed in altitude and azimuth relative to the terrestrial horizon.  It appeared to
travel inward then outward along the same path.  Northern Australia is in a special location and is the only region in the world that sees this path that travels
and out along the same path.  

The photograph below was taken with an equatorial mounting.  These mountings have one axis aligned parallel to
the Earth's axis of rotation. They are driven with a motor to rotate at 15o per hour to effectively cancel out the Earth's rotation in one movement.
You can see that when the Earth's rotation is removed the planet has transited along a straight chord.  

This flash slide show shows some of the weird paths that the transit took as seen from different parts of the Earth.

Go to my Transit of Venus observers report  

Joe Cali
June 2012