This is a gentle reminder of a truly rare event that is on our horizon. This month’s Astro Flier will be our last
publication prior to this celestial wonder so you need to be prepared. For night- time observers, on other pages in the magazine I have reprinted – with kind permission of Geoff Smith, Observations Officer of the ASNSW, an observing list he published for the South Pacific Star Party. The list is entirely relevant to our sky for the next month or so, bearing in mind that the time advances by two hours per month (completing a full circuit in a year) and the area of sky will move 15 degrees further west than at the same time last month.

Jupiter is moving completely behind the Sun from the Earth and as such is no longer a viable target. Mars of course is easy to find near Regulus in Leo and for all its’ tiny apparent diameter is shining like a bright ruby at mag. 0.2. Saturn has also passed opposition and is in our northern sky throughout the night. It is imperative that we take advantage of the steady seeing and dustless skies to view the celestial beauty of this gas giant, we had to wait six months as we will now have to await the return of Jupiter. Venus is moving in front of the Sun next month!

The Transit of Venus on 6th June is perfectly timed for Eastern Australia and we get to see the whole event as it lasts 6½ hours. This means that even should cloud obscure our view the chances are we will see at least some part of the event. We have had a brief outline of the historical significance at an earlier meeting but it is personally significant to all of us as we will never see it again (I would have to live until I was 159 years old! Jack will be 187!) Anyone who doesn’t catch a glimpse this time will not see it ever, nor will 95% of people born this year.

Scientifically speaking this Transit commemorates one of the first real global and multilateral
scientific endeavours, one that was not primarily based on the desire for profit, but more the
advance of knowledge.

In 1619 Johannes Kepler, mathematician and astronomer, had used the meticulous observing
notes of Tycho Brahe to formulate his Three Laws of Planetary Motion. The Third law, which
concerns us here, tells us that the cube of a planet’s mean distance from the Sun is directly proportional
to the square of that planet’s orbital period. The orbital periods of the planets were easily calculated,
even by naked eye observers lacking truly accurate measuring tools. By using the Third
Law and the length of the Venus year they knew that Venus’ distance from the Sun was 72.3% of
the Earths’ distance.

If astronomers had a way of measuring one such distance accurately, the distances to all the planets could readily be calculated. Once the distances were known, the planets apparent size (angular) could be measured and the actual size could be calculated. Once the actual size of a planet was known, the orbits of that planets’ moons could give us the true mass of the planet. Astronomers were taking tentative steps on the second stage of measuring the local universe. But most importantly, if the true Earth – Sun distance could be calculated, we could begin to measure, using parallax, the distance to the nearest stars!

Parallax is the apparent shift in an objects position when viewed from different angles. In its’ simplest form our paired eyes use parallax to gauge the distance of objects we view. In astronomical terms it was seen as a means to measure the distance to the nearest stars. If an object was observed at six month intervals to move against the background of distant stars the angle could be calculated and the distance extrapolated. The baseline of the triangle would be the diameter of the orbit of Earth or twice the Earth-Sun distance.

Sir Edmund Halley, in 1677, proposed that it would be possible to calculate the Venus-Sun distance
by using the same principal if the Transit was viewed from two widely separated locations on Earth. Well
planned, well funded, well staffed, and well provisioned expeditions set off at the behest of the major European
powers to observe the Transits of 1761 and 1769. The British expedition to Tahiti, led by Lieutenant
James Cook, led to the mapping of New Zealand and the east coast of Australia. The expeditions met with
varying degrees of success, encountering issues such as cloud, local hostility, harsh conditions of travel, poor
navigation (positioning of observer) and timing issues. As well as inaccurate timepieces that were used to
calculate longitude, they also experienced “ the Black Drop”, blurring of edges due to the atmosphere and
poor optics, that affected the precise recording of the ingress and egress time that was essential to the parallax measure. Nevertheless, when all the data was collated and calculations were made, the results gave a figure within 3% of the distance known today!

The Transit we will Observe

The Transit begins when Venus appears to move on to the surface of the Sun and this is due at 8:16 am in
Wollongong, one second later than in Sydney. The most significant timing is ingress, interior or second contact, occurring at 8:34 am. I suggest being well set-up by 7:30 am with a clear view of the east. A comprehensive article appears on pp 8,9, 10 of Astronomy 2012 For most of us the safest way to view is by projecting the solar image . This view can be shared with many bystanders and the image can be easily
photographed by individuals. A music stand to hold your target screen would be useful.

SAFETY! Do not look at the Sun through any optical device without full aperture solar filter at the
objective end of the scope!
These are available from any astronomical supply shop and fit completely over the dew shield. SECURE IT
IN PLACE. If you project the Sun image remember that unprotected sunlight will heat the inside of any scope tube in minutes, will melt any adhesives between lenses, ruin eyepieces and burn anything in its way. If your eye gets burned, you will be blinded.

For those of you imaging, make sure the project is well planned. While the Transit lasts a long time, critical stages don’t last more than a few seconds. I suggest trying to photograph first and second contact, and/or third and fourth contact with a few shots in between. More experienced astrophotographers could take a shot every 10 or 15 minutes and create a sequence later on. One of the best things you can do is photograph
the occasion, i.e. the equipment, other viewers and friends, and the location. These will be your lasting memories. There will be thousands of photographs taken that will be better than yours so ABOVE ALL, DON’T WORRY, enjoy the occasion and the spectacle. You will be playing a small part in the history of astronomical observing.
I will be observing first and second contact from my observatory in Kangaroo Valley and then going to the KV Showground at 10 am to show interested locals. After that it’s off to a local High School so feel free to contact me if you want to join in.

Clear Skies