The transit of the 8th of June 2004
| Times for England (British Summer Time)
First contact (1): 07 h 20 min
Second contact (2): 07 h 40 min
Maximum: 10 h 23 min
Third contact (3): 13 h 04 min
Fourth contact (4): 13 h 24 min
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In order to observe the transit from its beginning, you will need a clear Eastern horizon. At 07 h 20 BST, the Sun will be only 11° above the horizon. At the end of the transit, at 13h04 BST, it will be more than 60° above the horizon.
The apparent diameter of the Sun will be 0.525° or 31.5'.
The apparent diameter of Venus will be 0.016° or 1' i.e. 3% of the Sun's diameter.
How to observe the transit?
Observing the Sun can be very dangerous. LOOKING DIRECTLY AT THE SUN CAN CAUSE PERMANENT BLINDNESS. Use extreme caution.
First method: naked eye observation
The eclipse of the 11th August 1999 was much talked about in the press, the Sun can ONLY be observed with the naked eye with appropriate filters that are in an excellent condition. At the time of the eclipse it was easy to find special "solar eclipse viewers". For the transit of Venus, the same type of filters should be used
Second method: by projection
To observe the transit you will need either a pair of binoculars or a small
telescope in order to project the image onto a piece of white card. A second
piece of card should be attached to the instrument (as shown in the photo on
the right) to provide shade and ensure a clear image.
This is the safest method, providing that no one looks through the instrument. Someone should stay with the telescope at all times to ensure that no one is tempted to look through it!
Using this method it is easy to see the black disc of Venus against the white disc of the Sun. |
Observing the Sun by projection (image SAB)
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A special instrument, the "solarscope", has been developed for observing the Sun by direct projection onto a screen. Information can be found at the
solarscope site or at specialised shops, it costs about sixty pounds.
Third method: telescope with a filter
| Some telescopes are sold with a solar filter than can be attached to the eyepiece. This type of filter SHOULD BE AVOIDED. The heat from the Sun can cause the filter to crack. A full aperture solar filter should be used. This can either be bought or homemade from Mylar film. Those who have a H-alpha filter for observing the Sun's chromosphere can observe Venus just before and just after the transit, as it crosses the solar chromosphere.
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Observing with a full aperture filter (image SAB)
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(How to photograph the transit?
With a 24x36 camera, the diameter of the Sun on the negative will be equal to the focal length in mm, divided by 109. This will give a diameter of 0.5 mm with a 50 mm objective or 3 mm with a 300 mm telephoto. The apparent diameter of Venus will be 1/30 of the diameter of the Sun. For those wishing to capture this historic event with a 24x36, a telephoto is a must. The best alternative would be a telescope. But a webcam attached to telephoto is an excellent alternative.
First method: camera (normal or digital) fitted with a telephoto lens
Make a Mylar filter ensuring that ABSOLUTELY NO LIGHT passes through the filter and enters the telephoto lens.
Photograph the sun, adjusting the exposure time manually. You will need to take a number of photos, with different exposure times (unless you have a high performance light meter that is able to determine the exposure time for you).
Second method: hand held camera with a telescope
| The simplest solution is to use a telescope fitted with a filter but the result is not guaranteed: the photograph will be taken via the eyepiece, with a hand held camera. The camera can be put onto automatic. This could be a good solution with a digital camera as the image can be seen instantaneously. A support can be made in order to mount the camera in front of the eyepiece.
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Photo on the left: Transit of Mercury taken with a hand held digital camera in front of the eyepiece of a telescope. A sunspot can be seen at the bottom left of the image. Mercury is the small black dot towards the top right of the image (image A.Chataux/SAB) |
| Another possibility is to photograph the image of the Sun projected onto a screen by a telescope (see above: how to observe, second method). |
Photograph of a projected image of the (image SAB)
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Third method: camera attached to a telescope
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The telescope will serve as a telephoto. The camera without its objective should be attached to the telescope (without an eyepiece). A T adapter will be needed to make the connection, this can be bought at most photographic shops. With a telescope with a focal length of 1,5 to 2 meters the Sun will almost fill the negative of a 24x36 camera. Therefore the automatic exposure time can be used.
The latest digital cameras, with interchangeable lenses, usually have a smaller captor, the focal length equivalent to a 24x36 can be obtained by multiplying the focal length by a correction factor. This needs to be taken into account in order to be sure that the whole of the Sun fits onto the image. |
The Sun photographed with a reflex camera at the eyepiece of telescope with a focal length of 1.5m the 25 April 2000 (photo PC/SAB) |
Fourth method: webcam + telephoto lens
Webcams are being more and more used for astronomy purposes. You will need a webcam that can be taken apart and attached either to a telephoto lens or to a telescope. Information can be found on the astrocam web site
A webcam attached to a 200 mm telephoto lens (protected by a filter) will allow you to capture the whole of the Sun. A 300 mm or above, will only give half of the Sun. Make an AVI film with a software such as QCFocus, this can be downloaded from the web for free.Other software such as
AVI2BMP (this can also be downloaded) will allow you to convert the film into individual BMP images. Chose the best image(s) and then treat them with an image treatment software.
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Image on the left:
An untreated image of the Sun taken with a webcam attached to a telephoto lens covered with a Mylar filter.
The arrow indicates Mercury. The grey marks are due to dust on the captor. These can be removed with the aid of a "flat" (image PC/SAB). |
| Fifth method: webcam fitted to a telescope
Images on the right: Two images of the transit of Mercury (7 may 2003) taken with a webcam fixed to a 1.5 m focal length telescope, fitted with a solar filter. A number of AVI films can be made and then the images to be kept can be selected and saved. It is best to use a motorised telescope but it is not essential.(images
J.Nillon/SAB). |
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Measurements for amateurs
The method used in the 19th century consisted of measuring the duration of the transit from different locations and then applying very complex calculations. L'IMCCE (Institut de Mécanique Céleste et de Calcul des Ephémérides) will be collecting the observations from amateurs all over the world to carry out the same calculations. You can participate by visiting the following site: http://www.imcce.fr/vt2004/fr/
The CLEA recommends an alternative method which although less exact, allows you carry out the calculations for yourself. It uses techniques not available during the last transit in 1882. A summary of this method can be found at the following link: procedure for measuring the distance of the Sun
Brief description of the method
The method consists of obtaining two photos (taken at the same time) of Venus in front of the Sun taken from two locations sufficiently far apart and then superimposing them. The two images need to be orientated the same way, with north at the top for example. When superimposing the images there will be slight difference in the position of Venus with respect to the Sun. This difference, in arc minutes, together with the distance between the two locations will allow the distance of the Sun to be calculated.
Taking the photos or images:
1. Use a camera or webcam that gives an image of at least half of the Sun (camera without lens fitted to a 1.5 to 2 m focal length telescope or a webcam fitted to a 300 mm telephoto lens.
2. Set up the telescope.
3. Set up the camera or webcam as precisely as possible, the long side of the photo needs to be oriented east - west with south at the bottom. For this use the apparent movement of a star or the Sun itself. This should be carried before the transit itself and reference points should be marked on the camera or on the webcam and on the telescope in order to find the correct orientation quickly.
Check the orientation of the camera regularly as a poorly set up system can lead to field rotation.
4. Take an image of Venus each half hour (twelve images from 5h30 UT to 11h UT).
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5. For each image, note the height and azimuth of the Sun. A simple graduated device will be proposed in order to obtain the necessary measurements.
For more information see procedure for measuring the distance of the Sun.
If sunspots are visible this will help to correctly orientate the photos.
Variation :
If using a webcam, better definition images will be obtained with a lager focal length, but it will be difficult to superimpose the images.
To include the edge of the Sun, we suggest taking images with a webcam attached to a telescope having a focal length of approximately 1 m, at the following times: 09h45, 10h15, 10h45 (BST).
Calculation of the distance of the sun for amateurs (simulation)
Step one
| Simulated observation of the transit from Dijon at 10 h UT (12 h BST) |
Simulated observation of the transit from Reunion Island at 10 h UT (12 h BST) |
Superimposition of the two images |
There is a difference of approximately 0.5' between the two images.
| Second step
Determine the distance between the two parallels (Dijon-Sun) et (Reunion-Sun). To avoid calculations, put two sticks showing the direction of the Sun (you will need to know the height and azimuth of the Sun at the time of the observation). Measuring with a ruler with give a simple approximation of the distance. With a not very precise set-up, I found 8500 km.Another way is to use the Swiss web site fourmilab to obtain an image of the Earth as seen from the Sun at the appropriate times. Simply measure (on the image) the distance between the two locations and compare this with the diameter of the Earth.
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Third step: calculation of the distance of the Sun
| AB = 8500 km. AV = 0,28 UA. CV =
0,72 UA
The Tales theorem can be used to calculate CD:
CD = 8500x0,72/0,28 
21 860 km.
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What does the difference of 0.5' between the two images correspond to? Imagine that there were sunspots at points C and D. The difference of 0.5' is the angular distance between the two sunspots as seen from the Earth (not as seen from Venus as may be thought).
At what distance do you need to be, to see two points 21 860 km apart make an angle of 0,5'?
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Working with angles in radians:
0,5' en radian : 0,5/60*pi/180
0,0001454.
d = 21 860/0,0001454 150
000 000 km The answer is correct, but what will it be on the 8th June? Wait and see...
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