Your guide to the transit of Venus
Don't miss the very last transit of Venus in our lifetime. Before you rush outside, brush up on when to see it, what to see, and how to see it safely.
The transit of Venus is one of the rarest cosmological events you'll ever see. In early June, Venus will pass between the Earth and the Sun for the last time for more than 105 years. The next transit will not happen until 2117.
The last transit occurred in 2004 — the first time that the event could be seen using modern space telescopes. Scientists hope the 2012 transit will give them more clues about the atmosphere of Venus and planets in other solar systems.
Where to see it
Australia, New Zealand, East Asia and the western Pacific are the best locations for viewing all of the 2012 transit, which starts soon after sunrise and finishes in the afternoon.
Most other parts of the world will see some of the transit, with the exception of West Africa and most of South America.
The transit will begin at 22:09 UTC (5 June 2012) and finish at 04:49 UTC (6 June 2012). Despite adjusting for time zone differences, your viewing time may vary by a few minutes depending upon your latitude and longitude.
What to look out for
There are five stages to a transit; external ingress (when Venus first touches the Sun), internal ingress (moment when complete disc of Venus is first in front of the Sun), greatest transit, internal egress (last moment complete disc is in front of Sun), and external egress (end of transit). These are sometimes referred to as the first and second contact points, midpoint, and third and fourth contact points.
The timing of the egress and ingress were critical to astronomers during the 18th and 19th centuries, as they provided a way to measure the distance of the Earth from the Sun. This measurement, known as the astronomical unit, allowed them to work out the scale of our solar system and the distances of each of the planets circling the sun.
While observing the transit you may notice two strange optical phenomena — the 'black drop' and aureole effects — which were both first reported during the transit of 1761.
The 'black drop' occurs just after internal ingress and before internal egress, when the black disc of Venus is almost touching the edge of the Sun. At this time the silhouette of Venus appears to stretch towards the edge of the Sun creating a teardrop shape.
Scientists spent the next two centuries investigating what caused the effect. Observations of the transit of Mercury in 1999 by ground-based and satellite telescope suggested it was caused by disturbances in the Earth's atmosphere creating diffraction patterns and the quality of the observing equipment.
This was confirmed during the transit of Venus in 2004, when some observers reported seeing the effect and others didn't. Most observers who didn't see the effect reported they had excellent visibility, with little to no atmospheric disturbance.
The aureole effect occurs just before internal ingress and after internal egress.
A thin arc of light traces the outer edge of the planet. This is caused by light from the Sun being refracted (bent) as it passes through the atmosphere of Venus. Immediately before internal ingress (or after internal egress), a bright spot of light appears where the silhouette of Venus touches the edge of the Sun. The aureole effect appears brightest near the poles of Venus. This may be because there is less cloud blocking light above the poles.
To see both effects you will require a medium to high power telescope with a filter that allows you to clearly see the Sun's photosphere.
Protecting your eyes
The Sun is a bright source of electromagnetic radiation, which includes visible light, infrared, ultraviolet and x-rays.
Our eyes normal reaction when we glance at the Sun is to look away. This reflex reaction protects the retina, a layer of cells lining the back of the eye.
These cells collect light and convert them to electrical signals, which are sent to the brain. Intense exposure to electromagnetic radiation can destroy the cells, and cause permanent eye damage, so it's important to protect your eyesight while viewing any astronomical phenomenon involving the Sun.
You can view the Sun directly by using specially made filters. Do not use sunglasses, smoked glass, exposed film or CD/DVDs as filters. While these objects may reduce the amount of visible sunlight your eye receives, they don't significantly reduce the amount of UV light or x-rays
Specially manufactured solar viewing glasses reduce all forms of light that are known to cause damage to the eyes. But it's important to note that while these glasses typically meet European and US safety standards, there are no Australian/New Zealand standards for solar filters. Also be aware that they are made of a thin polymer plastic that can be scratched or damaged. Do not use the filter if you suspect it is damaged.
An alternative is number 14 (or greater) welding glasses. If the glasses have been damaged, or you are unsure of their rating, do not use them.
If you own a telescope, there are a range of solar filters available such as Baader film and Hydrogen-alpha. The latter type allows you to see the structure of Sun, such as prominences and flares, in fine detail. You should only use filters that attach to the front of the telescope, which filter light before it enters the tube. Do not use filters that attach to the eyepiece.
There are a number of ways you can indirectly view the Sun. The simplest method is to make a pinhole camera. Using a sharp pin, piece a small hole in a piece of stiff cardboard. Stand with your back to the Sun and hold the card up, allowing light to pass through the hole and project onto a flat surface that is in the shade. Never look through the pinhole at the Sun. The greater the distance between the pinhole and screen the greater, but fainter, the image will be.
This method is extremely good at showing large sunspots or the partial stages of a solar eclipse. But as Venus is 32 times smaller than the Sun when viewed from Earth, it may be very hard to see.
To see a larger, brighter image, you can use a pair of binoculars or a viewfinder to project the image of the Sun.
Use the shadow cast by the binoculars to help you point them directly at the Sun. Never look at the Sun through the binoculars.
You can use a tripod and attachment to fix the binoculars in place, but be aware that the heat from the Sun passing through it may cause damage to the glue and lenses so give them a break every so often.
Published 23 May 2012