Venus Ingress

Transit of Venus: 8 June 2004

Venus Egress

What is a Transit

Venus follows an orbit around that Sun that lies inside that of the Earth's. As a result it has the potential to come between us and the Sun, an alignment known as an "Inferior Conjunction". On average this happens every 584 days, but because the orbits of the two planets are slightly inclined with respect to each other, Venus normally passes above or below the Sun at inferior conjunction and is lost in the Sun's glare. Only when Venus and Earth happen to arrive at the intersect point at the same time does Venus actually pass in front of the Sun. This has to happen within a day or two of 8th June or 8th December.

The last transit by Venus was in December 1882, but like London buses, you have a long wait then two come at once. They follow a predictable pattern of intervals of 121.5, 8, 105.5 and 8 years, then the cycle repeats itself. The next transit will be in 2012, and will be best seen from the Pacific region. This year's one is better for Europe. After 2012 there will be 105.5 year wait until the next one.

Why Transits are Interesting

In the past, expeditions were organised to far flung corners of the Earth to calculate the distance to Venus by measuring the parallax apparent from observing the event from two points as far apart as possible. Edmund Halley (after whom the comet is named) suggested this method but the next transit on which it could be tried was not until after his death. The most famous of these was in 1769, when Captain Cook was sent to explore the southern Pacific, and observed the transit from the island of Tahiti.

From a measurement of the offset, the distance to the Sun could be calculated using geometry, and as soon as the actual distance was known, real numbers could be put to the relative distances to other planets in the solar system.

Once the diameter of the Earth's orbit was known, a similar method of parallax could be used to determine the distances to nearby stars, although the offsets are much smaller as the distances are vastly greater. This in turn lead to an estimation of the size of our galaxy, from which the distances to other galaxies and hence the size of the universe could be estimated.

Nowadays there are other ways of determining these distances so the event is interesting more for its rarity and historical significance than any real scientific purpose. Attention has turned more to scanning for planets in other star systems by looking for the slight drop in light as they pass in front of their suns.

A good account and explanation of transits can be found at Fred Espenak's NASA / Goddard Space Flight Center Transits page.

The Event


Venus graced the evening sky for several months prior to the transit, outshining all other stars and planets. All the while it was approaching us, but while its size grew larger more of the dark side was turned towards us.

This picture shows Venus increasing in size but decreasing in phase over the space of 2 months - click it or here to see them at full size.

Venus approaches the Sun

The April and May pictures were taken from the UK but we saw the transit itself from the Languedoc region of the south of France, near Mirepoix. Ryanair conveniently fly from London to the nearby fortified city of Carcassonne.

Carcassonne: if it looks like a fairy tale castle then this is nearer to the truth than you might imagine! Although Medieval in origin, an extensive restoration in the 19th century owes as much to romantic imagination as historical accuracy.

My 8cm Helios refractor, plus mount and tripod, made the journey with robust components in hold baggage and the delicate stuff in hand baggage - everything was well packed and survived the journey intact!

Observing the transit. The towel over my head is the easiest way of keeping sunlight off the screen of my portable computer, which I needed to run the webcam. The orange bag of stones acts as a counterwight - a little improvisation saves carrying or paying for 2.5 kilos of dead weight on the flight!

The day dawned fine, but with a layer of high cirrus cloud, which persisted on and off for much of the morning. Although it did not significantly diminish the brightness of the Sun it did reduce the contrast of surface features as seen in Hydrogen Alpha light - disappointing, as I had hoped to catch the transit with some solar activity at the same time. However, Venus itself was sufficiently large and prominent not to be affected.

Sunrise on transit day
Sunrise on transit day.

First and Second Contact

Kick off was First Contact, when the silhouette of Venus just starts to creep on the disk of the Sun. This was due at 05:20UT, but in the Central European Time timezone, with daylight saving time, this translated into a much more reasonable 7:20am. Times and other details taken from Fred Espanek (NASA / Goddard Space Flight Center)

I unfortunately missed the moment of First Contact (05:20UT) - my fault entirely as I was tracking the wrong quadrant of the Sun! Therefore, this time-lapse movie takes up the story 8 minutes after First Contact, with the silhouette of Venus already on the face of the Sun.

Venus covers its own diameter in about 20 minutes - this may seem slow but it translates into an overtaking speed relative to the Earth of 38,400 kilometers per hour (24,000 mph).

The colour of the Sun is not the redness caused by looking at it when close to the horizon, but through a Hydrogen Alpha filter. Tthis reduces the blinding light of the Sun by a factor of about 100,000 by cutting out all wavelengths apart from the red Hydrogen emission.

Ingress of Venus

This frame catches the moment of Second Contact (05:40UT), when the silhouette of Venus just breaks away from the edge of the Sun and is seen completely against the bright photosphere. There was much talk of the so-called "Black Drop" effect, when a piece of Venus appears to remain attached to the blackness beyond the edge of the Sun. However, there is no sign of it here - maybe it is a phonemon of visual observing only, which disappears when multiple webcam images are stacked and sharpened. Second Contact

This picture, taken one minute before Second Contact, looks like it shows the Black Drop but this is an artefact of the severe processing I have given it. By greatly increasing the contrast and brightness it is just possible to see a short curve of light, marked with the arrow.

This is sunlight refracting through the atmosphere of Venus - light is bent round the planet and gives a very faint outline to the dark side.

Sunlight refracting around Venus

Mid Transit

Once Venus moved away from the edge of the Sun there were few details to show it against - not just masked by the thin cloud but also not helped by solar activity being very low. Only two tiny sunspots were visible, quite unlike the Mercury transit two years earlier. To show Venus in the context of the whole Sun I reduced the magnification by using the prime focus image.

Venus in transit

The Sun is too big to be recorded in one go by the Philips ToUCam Pro webcam, so this picture is a composite of 2 images joined together.

The arrows indicate the entry (ingress) and exit (egress) points.

Click the image or here for a timelapse animation of the entire duration of the 6 hour transit, during which Venus crosses the southern part of the Sun. It runs at 600 times natural, but with some frames taken at irregular intervals, the display duration of each frame is timed in proportion to the interval until the next one. The first two frames, and the last, are composites using the magnified images taken at ingress and egress, with Venus shrunk to be the same scale as the remaining frames.

Venus was just visible to the naked eye, with a suitable filter of course, whilst in transit. This picture gives a good impression of what the view was like through an eclipse viewer. I took it, without the telescope, just after mid transit when Venus was furthest from the edge of the Sun (08:23UT).


The position of Venus is rotated to the right compared to the the other pictures. All the telescopic views are taken with an equatorial telescope drive so celestial north points straight up. This picture is the natural view, without polar alignment, so, with the Sun in the East, celestial North points towards the left.

Transit in White Light

Third and Fourth Contact

With Venus approaching the edge of the Sun it was time to reinstate the higher magnification. With the Sun now high in the sky the telescope was awkardly positioned the additional size of the Barlow lens meant the webcam was right up against the motor drive. In sorting this out and refocussing I lost valuable time and unfortunately missed the moment of Third Contact (11:05UT), when Venus makes contact with the far edge of the Sun.

This timelapse shows the final 15 minutes, up to just after Fourth Contact (11:24UT), when Venus finally left the face of the Sun. Only 8 years until the next time!

Egress of Venus

Other Observations

The transit was visible from anywhere where the Sun was up during the 6 hours it lasted. Europe, Africa and Asia were well placed to catch all of it, but Australia and the Americas caught only the beginning and end respectively. For a selection other pictures and reports try these sites...

Venus in 2004 Venus Gallery

This page last modified