Tag Archives: Transit of Venus


Transit of Venus 6th June 2012 by Gerard Keyzer

Categories: Education, Members, Tags: ,


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


Transit of Venus, Background and Explanation by Gerard Keyzer

Categories: Education, Members, Tags: , , ,

The Transit of Venus

What is it that makes an event in nature special? Is it the rarity of its occurrence, the immensity and sheer wonder of the spectacle, or possibly the effect it has on mankind? There are arguments for all these criteria and there is also personal significance. There will be a Transit of Venus on June 6th this year and while it is
not a visual spectacle that excites the imagination of the general populace the way a Total Solar Eclipse does, it is an event that holds great significance for us all.
Firstly, it is a truly rare event. Transits of Venus occur in pairs eight years apart but then do not recur for 105.5 years and on the next cycle 121.5 years. Total Solar Eclipses occur at the rate of three every two years. Few people alive to see this transit will see another. Transits of Venus were unknown until Johannes Kepler, the great planetary mathematician, predicted an occurrence on December 6th, 1631. Venus appears as a small round black dot when crossing the face of the Sun, invisible to the naked eye. As Galileo had turned the newly invented telescope on the heavens for the first time in 1609 it had never been observed before. Due to a small error in parallax mathematics Kepler did not predict the second of the transit pair occurring eight years later but it was independently predicted and observed by English amateur Jeremiah Horrocks on 4 December 1639. As he was not well known and did not publicise his calculations it is believed he and his friend were the only two people on the planet to see this one.

Picture of the Transit of Venus
Right – The Author’s photo of the Jun 8, 2004 Transit
In the eighteenth century, advances in telescope design and instrumentation continued at breakneck speed. Planetary positions and orbits were plotted more accurately and predictions could be mathematically refined. Around this time we see the great nations of Europe exploring the world by sea for trade riches and power. The Spanish, French, English, Dutch and Portuguese had been sending explorers to all corners of the globe for a hundred years. Enormous riches and influence, not to mention new territories to claim, were the spoils for the adventurous, however many of these explorers and crew perished because they became lost. Unable to accurately ascertain their longitude they could miss their target by many miles or hundreds of miles, running out of water and food or becoming becalmed in unknown waters. It was easy to find your latitude, simply measure the angle of the Sun above your horizon at local noon but to find your longitude was more difficult. The story of the search for the best method is brilliantly told by Dava Sobel in her book Longitude. If a navigator could know accurately the time at a given point on the globe, say Greenwich or Paris, he could compare it to noon at the meridian of his location.

Every hour difference was equal to 15 degrees of longitude. Many solutions were proposed but ultimately those who had accurate timepieces, unaffected by temperature, barometric pressure or the motion of a ship, would be the most astute navigators. The chronometer and its use for accurately plotting one’s position was obviously a momentous advance for mankind but what has it got to do with the Transit of Venus? As you will see both our own history and the development of astronomy are connected by this event.
After observing a transit of Mercury in 1677, the brilliant English astronomer, Sir Edmund Halley, proposed that the next transit of Venus could be used to determine the distance of Venus from the Sun, and by simple trigonometry the distance from Earth to the Sun.

Why was this important to astronomy?

Astronomers had noticed that some stars, when measured at different times of the year, appeared to move slightly against the  background stars. They had known of this effect for quite some time as the superior planets (outside Earth’s orbit) would appear to move in reverse against the stellar background for a short period when Earth went past them in its own orbit. This effect was known as parallax. See the diagram below.

By checking a stars position at six monthly intervals an astronomer would be measuring the baseline of a triangle the diameter of Earth’s orbit or twice the length of the Earth – Sun distance. Knowing that distance accurately meant the parallax distance to some stars and perhaps measurement of the scale of the visible universe would be within their grasp.

Above: The transit appears differently from separate locations on Earth – (courtesy of Exploratorium TV)
Halley proposed to use a smaller measure of parallax to find this Venus-Sun/ Earth-Sun distance. He posited that two observers timing the Transit from distant locations on Earth could create a long enough baseline using parallax measures to create a  heoretical angle from Earth through Venus to the Sun. Johannes Kepler, who formulated the three Laws of Planetary Motion, had also calculated, with his third Law, the ratio of a planets distance from the Sun compared to the time taken for its orbit. Kepler had proven that the ratio of Venus’ distance to the Sun compared to Earth was 0.72. By multiplying the apparent Venus /Sun angle by 0.72 we arrive at the Earth/Sun angle. Let’s not worry about the actual equation here but remember we have now worked out the angle and the length of one side of our triangle (the distance between our observers on Earth) and we can use our High School maths (remember Sine, Cos and Tan?) to calculate the distance accurately. This method is the same as used by all surveyors to determine distance with a theodolite.

Diagram of the Timing of a Transit – (courtesy of Quasar Publishing)

Of course, unless the observers in different parts of the world knew their positions accurately they could not determine the distance between each other. Most of the advanced seafaring powers and their respective scientific societies equipped and commissioned Transit expeditions to all corners of the globe. This is where our recent history and the event of the Transit

Venus black drop effect

intersect. For the Transit of 1769 the English Navy purchased a coal carrying ship called the Earl of Pembroke , and joined with the Royal Society in commissioning a Research survey to Tahiti and the South Pacific. As it was nominally a Naval vessel it could only be commanded by a ranking Naval officer so James Cook, chosen for his navigational skills and his surveying prowess,
was duly promoted to Lieutenant and commissioned to observe the Transit of Venus from Tahiti on June 3, 1769. As the Commander of the ship he was entitled to be called Captain. From Tahiti Cook was to continue to New Zealand to observe a Transit of Mercury on 9 November and map the North and South islands while there. Here he was to open his sealed orders that instructed him to continue west where he discovered and mapped the east coast of Australia. On this first tour of duty Cook navigated with the use of accurate Moon and star charts supplied by the Royal Observatory at Greenwich, using a sextant to calculate angles and lunar distance. On his second voyage of discovery in 1771 he had the benefit of a replica of Harrison’s famous
chronometer to calculate his longitude, but no Transit to time on this occasion. This chronometer cost £400 or approximately £59,000 in today’s currency.
When all the timing, positional, and angular measures were calculated the results from the earlier Transit of 1761 and those of 1769 gave a figure for the Earth/Sun distance that only varied from our modern measure by about 3%. Inaccuracies crept in because of poor seeing, poor timing, poor calculation of locations, and an anomalous effect of the Transit known as the Black Drop that made it difficult to see the exact second the planet entered or exited the disc of the Sun. Around a century later in 1874 and 1882, the research was conducted primarily with the new technique of photography but was again slightly marred by the discovery that Venus had an atmosphere which generated further inaccuracies in timings.

So, is the event rare? It certainly is! Is it an immense and wondrous spectacle? Perhaps only when you consider the implications. Does it have a profound effect on mankind? The event itself, perhaps not, but the observation generated an enormous tide of activity in the affairs of men. It pressured advances in technology, navigation, timekeeping, measurement and the science of Astronomy. Is it personally significant? To us, who will not see another, it may be. If you have an inkling of the history it must affect you. Personally, it is one of many aspects of the Universe above our Earth that astonish and compel me.
Clear Skies.