RAFT
EMA Bonn:
Mercury Crossing
the Sun Disk
May 7th, 2003
On Wednesday, May 7th, 2003, a rare sky event happened: On its orbit around the sun, Mercury, the innermost planet, crossed the sun disk, as seen from the earth. Astronomers call this kind of event a transit. It started at 7:13 h and ended by 12:32 h.
We took the opportunity to observe this rare event at the EMA Bonn during classes. On the eve, we installed and adjusted a telescope on the roof terrace above the arts room. Special filters were mounted onto the objectives of the main telescope and the search telescope in order to be able to observe the sun without risk of eye damage. In addition, we tested the digital camera that was to take pictures of the individual phases of the Mercury transit.
The weather was good to us. In the course of the night, the clouds dispersed, and before the beginning of classes, the still low sun was promisingly clear. It continued to be so during all the morning. Only occasionally, slight hazes dimmed the view. Students of all ages (and the teachers, of course) could take a look through the telescope and experience the mercury transit (images 1,2,3).
During the transit, Mercury appears in front of the bright sun as a very small black disk. Its apparent diameter is approximately only 1/150 of the diameter of the sun disk. Thus, you have to use a telescope in order to be able to observe the transit.
We used a reflector telescope Meade ETX-105EC. Its mount was parallactical, i.e. its axis was adjusted in line with the axis of the Earth (images 3,4). By this mount, the north-south orientation of the sun disk, as observed through the telescope, is constant while following the sun's movement with the telescope. Thus, it is easy to follow Mercury on its orbit. The intensity of the sunlight entering the telescope was strongly reduced by a type A-VIS glass sun filter with a transparency of 0,001%. The search telescope was protected against the sunlight by a so-called Baader foil.
Between the phases when groups of students could directly observe the sun and transiting Mercury, images were taken. For this, the camera was mounted onto the eyepiece (image 5). For the shots we used a Canon Digital Ixus camera in combination with a 20mm Super Ploessel ocular. The ocular and the camera were firmly connected to each other by a self-made mount. All images were taken using an automatic release in order to avoid blurring.
During the morning, image series were repeatedly uploaded, in the context of our RAFT project, virtually live onto a web site that was provided to us by the Fraunhofer Ges.
In our reflector telescope, the light is reflected, for sake of comfort, by a plane mirror to the eyepiece which is directed upwards. One obtains an upright but side-inverted image of the transit. This may be easily verified by looking at the position of the sun-spots. When processing these images in the computer, the side inversion was undone in order to obtain the real orientation of the sun and the Mercury orbit.
A typical example is image 6 which was taken at 9:29 h. It shows a section of the sun with the comparatively tiny Mercury disk in the upper right and two sun-spots, of which one is apparently larger and the other smaller than the Mercury disk. These two sun-spots are welcome reference points. They stay more or less in place during the observation period of four hours, as a full rotation of the sun (and of the sun-spots which are fixed on its surface) around its axis takes some 27 days
The camera being frequently mounted and removed between the direct observation phases, a certain twisting of image planes could not be prevented. However, this effect was corrected during image processing by rotating the images as necessary. The reference line was taken to be the line connecting the two sun-spots, and the applied image rotation provided for a constant angle between the lower image border and the reference line (image 7). Thus, the Mercury orbit is easily displayed. Image 8 gives sections of shots taken at five different times that were patched into one single picture. Each section shows the Mercury disk. One sees the rectilinearity of Mercury's orbit. The varying distances between the Mercury disk images relate to different time spans between the takes. As expected, the Mercury disk crossed the sun disk at constant speed.
Image 9 shows Mercury's disk shortly before leaving the sun disk. This image demonstrates the excellent conditions for observation. The shadow of Mercury's disk is clearly identified as circular. It is impressive to note how tiny Mercury appears against the sun. Taking into account Mercury's position between Sun and Earth during transit, at a reduced distance from the Earth, its actual size is even smaller by one third.
By 12:32 h, the sky event was over. It was an eventful morning, after all. The next transit of Mercury that may be observed in Germany is scheduled for May 9th, 2016. Without the pictures taken by Mrs. Fritzsche, of the observatory and the students by the telescope, the memory of the event would be only half as pleasant. Further, Mrs. Fritzsche uploaded the images of the Mercury transit that were taken by the author in the context of our RAFT project, onto the web site of the Fraunhofer Ges.. Many thanks!