Observing the Transit of Venus – What we learned…


by Pat Browne

  The Transit 'to' Venus event was graced with perfect weather for viewing the last Transit of Venus for this century. Because we gathered together by car-pooling or cycling, the transit to the event was also a fun ride! … see

We were treated to a view that was not processed or guided electronically. At our setup, there was no camera/computer recording the event, nor was there a motor drive to keep the telescope tracking at a particular rate. The  image was projected onto a white screen as  the light from the Sun and Venus came to focus, emerging out the eyepiece in a 17.5" Dobsonian reflector telescope. The staging of this display was provided by our hosts Rolf and Linda Meier.

However, we were given a demonstration of the intensity  of the sunlight, by the incineration of a leaf, and the lighting of a  firecracker cake candle just by holding these objects close to the eyepiece: A  clear signal to emphasize the need for caution when doing solar observing!

P1030164As we leisurely observed this event which started at 18:04 pm for our latitude, and ended when the sun began to set shrouded by clouds …  

we pondered the following …


 1. Why do we need to re-adjust the position of the scope?

Because the earth is turning at 'sidereal rate': roughly 15 arc-secs/sec. Without the aid of a tracking platform, the position of the telescope must be 'nudged' to track th Sun. So , like the 'Fool on the Hill', we see the "Sun going down, while the eyes in our head, see the world spinning round".

2. Is the path of Venus across the Sun (the 'chord') a straight line or curved?

Venus is travelling on its orbit, along the ecliptic, on a curved path; however for this relatively short distance that we are viewing its path, the line is effectively a straight line.

Here is the overview of the trajectory – courtesy Stellarium


Here is a snapshot of a partial animation ot the chord trajectory along the sun – courtesy EarthCentredUniverse software.



3. What are the sunspots we see on the Sun?

These are  dark, cooler areas on the photosphere of the sun. They are known to have strong magnetic properties and a given sunspot has a magnetic polarity. The sunspots usually form in groups, to complement their magnetic polarity. The cooler temperatures can be 3000 degrees Kelvin cooler than the surface which is around 6300 degrees K.










4. What is the Black Drop effect?

We saw this teardrop shape just after second contact of Venus inside the limb of the Sun.  This has historical significance, as this effect made it difficult to know the exact time when the rim of venus is tangent and inside the limb of the Sun. This point was needed to accurately measure the trajectory of Venus across the disk of the Sun. 


 When these observations were taken at different latitudes, calculations of the parallax angle determined the distance to Venus, and proportionally, the distance to the Sun


4a. What is the significance of the distance to the sun, the Astronomical Unit  (A.U)?

This is the first rung in our 'distance ladder' which we use to determine distances in the neighborhood of our galaxy, for example, to the nearest stars. It is used as a baseline in trigonometric parallax to measure nearby stars . In the early 20th century, and more recently with the Hipparcos satellite mission, we  measure the nearby stars against the background of much more distant stars from two different times separated by 6 months.  Comparing the two images, the nearby star will shift position between the two images. This angle of shift is the parallax angle. The angle of parallax is the vertex of the triangle whose base is the Astronomical Unit or the Sun/Earth distance.



5. About how fast are we seeing Venus travel?

If we were able to see the entire transit, this would span roughly 6 hours . A *very* rough calculation we did was, knowing the diameter of the sun, and approximating the chord to the diameter we could say, 1,392,000 km/ 6 is roughly 200,000 km per hour.

This is a back of the envelope calculation . The actual value is actually 1/2 that… but it is impressive to watch. (According to Wikipedia: Average orbital speed 35.02 km/s * 3600 s/hr = 126,000 km /hr)

 Thanks to everyone for ordering such perfect weather. the clouds rolled as cyclists rode off into the sunset…IMG_2565



Finally, here's the John Phillip Sousa Transit of Venus march: 


You can hum it to the next generation who, like Helen Sawyer Hogg (a great Canadian 20th century  chronicler of the Transit of Venus), may probably miss out on this  fleeting experience!

 Clear Skies!