What is a transit of Venus?

Steven van Roode By Steven van Roode

In its course around the Sun, the planet Venus overtakes the Earth every 1.6 years. It then passes between Earth and the Sun. But not on every occasion the planet is seen passing exactly in front of the Sun. Because the orbit of Venus is slightly tilted, the planet usually passes above or below the solar disk when it overtakes Earth. Very rarely we are granted to see Venus projected unto the Sun as a small, black silhouette. This so-called transit of Venus will happen on June 5 and 6, 2012 for the very last time this century.

The rarest of astronomical phenomena

You may be familiar with some well known, but rare phenomena astronomers can predict in our solar system: the comet of Halley returns every 75 to 76 years and will come into view again in 2061. The planet Mercury is seen roughly every 3 to 13 years in front of the solar disk. A total lunar eclipse occurs less than twice a year. But a transit of Venus is much rarer: they occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The last pair was in 1874/1882, the current pair is 2004/2012 and the next pair is scheduled for 2117/2125. So, this year’s transit of Venus is your last chance to see this phenomenon – it will be your grandchildren who will watch he next one!

Learn more about the periodicity of the transits of Venus

What a transit of Venus looks like

Venus is about the same size as the Earth, but even though the planet is nearest to Earth at the time of a transit, it still looks very small at that distance. During the transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. It takes about 6 hours for the planet to move from the left side of the Sun (ingress) to the right side (egress).

When the transit is about to commence, the planet comes into contact with the disc of the Sun. The planet then enters slowly upon the disc of the Sun until it touches the solar limb on the inside. From this point Venus advances across the Sun’s disc and when the planet’s black silhouette reaches the other side it touches the limb of the Sun on the inside first. Then the planet begins to pass off the Sun’s disc and finally comes into contact with the solar limb for the last time and the transit is at an end.

Learn more about the general circumstances of the 2012 transit of Venus

Where and when to be to observe the transit of Venus

The transit of Venus lasts for about 6 hours and occurs on Tuesday June 5 and Wednesday June 6. When you’re living in North America, you will see the first part of the transit before sunset on June 5. European observers will see the last part of the transit after sunrise on June 6. When you’re in the Pacific, East Asia and above the Arctic Circle, you can see the transit in its entirety. The following map shows the world visibility of the transit:

Get the dates and times for your location

How to observe the transit of Venus

Basically, you can observe the transit in two ways: either by direct viewing (unaided eye or through a pair of binoculars or a telescope), or by indirect viewing (projecting the Sun’s image). Either way, you should be cautious not to look at the Sun without proper protection: the Sun’s light is so intense, it blinds you instantly if you do.

Direct viewing can already be done with special eclipse shades. Venus will be just large enough to discern its silhouette on the Sun. If you are planning to use binoculars or a telescope to watch the event, you should apply a special solar filter to the front end. These devices will provide a magnified image of the Sun, so you can follow the transit up close.
When you want allow large crowds to watch the transit, projecting the Sun’s image is more convenient. You can use binoculars or a telescope to do this. Building your own Sun Funnel makes it impossible for someone to look accidently through the eyepiece.

Six ways to observe the transit safely

What to look for

Every astronomical phenomenon has its own signature features: solar eclipses have Bailey’s Beads, lunar eclipses have the reddish hue of the moon. What about the transit of Venus? Does it have any special phenomena worth looking for? There are several, and we will mention the two most important here. Both happen at the start and the end of the transit.
When the disk of Venus is only partially on the disk of the Sun, a thin arc of light can be seen around the part that is off the Sun. This effect is called the aureole. It’s sunlight scattered by the dense atmosphere of Venus. First observed in 1761, it proved the existence of such an atmosphere.

When Venus touches the solar limb on the inside, a greyish hue lasting a couple of seconds can be seen between the limbs of the Sun and Venus. This effect is known as the black drop. It arises from the blurring by the earth’s atmosphere and the diffraction of light in the telescope. In history, this effect formed a great hindrance for astronomers to determine the times of the transit’s start and end with great accuracy. Its cause was a mystery for a long time, until a full explanation was found from the observations of the 2004 transit of Venus.

Learn more about the aureole and the black drop

International projects you can join

Watching the transit of Venus together with others adds to this special experience. Working together with people all across the world will make it a lasting memory, not only for yourself, but for future generations as well. There are several international observing projects you can join.

Measure the Sun’s distance – This project aims at recreating a historical experiment. After a call by Edmond Halley (yes, the same guy of the comet), astronomers observed the times of the beginning and end of the transit from places far apart in order to find the distance to the Sun by means of trigonometry. In the eighteenth and nineteenth century this experiment induced many hazardous and epic voyages, of which James Cook’s 1769 voyage to Tahiti is the best known example. By now the distance to the Sun has been measured accurately using radar, and the experiment has lost it’s scientific significance. But for amateurs it’s still a great challenge to recreate these measurements with today’s technology: a special free phone app allows you to join the project.

Experimental archaeology – Do you own an antique telescope? Than you are invited to join an experimental archaeology project. Reading the historical accounts of astronomers of yesteryear, questions about their observations of the transit of Venus arise. For example, it’s not quite clear whether some eighteenth century astronomers actually saw the aureole or if they observed an illusion caused by the poor quality of their optical equipment. Carefully watching the 2012 transit of Venus with antique telescopes may help to resolve these issues.