by Knut Jørgen Røed Ødegaard
The transit of Venus June 8, 2004 will be exploited by researchers to do tests
that otherwise are impossible. In addition to studies that can help us find
life on exoplanets, the Sun itself will be examined.
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This is how Venus will be on the solar disk on June 8, 2004. In just over
6 hours Venus will cross the solar disk.
Illustration: Institute of Theoretical Astrophysics, University of Oslo
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Several characteristics of the Sun are surprisingly hard to measure. But these
characteristics are important for our understanding of the Sun, its cycles
and how it may affect climate on the Earth and maybe also for forecasting
storms from the Sun that can be dangerous for instruments and humans in
space.
When Venus move onto the solar disk, we will see the completely dark and
cold surface (actually the clouds) against the Sun. The darkest things we
normally can see against to solar surface are sunspots. But they are in reality
far from being dark, we are fooled by the contrast between the spots and the
surrounding surface of the Sun. While most of the solar surface is 5800 degrees
C hot, the spots can be as "cold" as 4000 degrees. The temperature contrast
causes the spots to radiate much less than the rest of the surface.
For our eyes the spots seem to be black. If we had been able to cover all of
the solar surface, except the spots, we would have been dazzled by its light!
Venus will give the astronomers a unique opportunity to calibrate their
instruments and therefore more easily and accurately estimate the
brightness of various areas of the solar surface.
LIMB DARKENING
Different layers in the Solar atmosphere have different temperatures.
Deep in the atmosphere it is hot, where most visible light originates it is
around 6000 degrees C and further up the temperature drops to about 4500
degrees. Even further out the temperature rises quite rapidly, but the gas
density is so low there that very little light is emitted.
When looking towards the brim of the Sun, we look through thicker layers of
gas and we are not able to see as far into the atmosphere as we can in the
middle of the solar disk. The gas we see is therefore cooler and less bright.
The brim of the Sun is therefore less bright than the central part of the
disk - this phenomenon is called limb darkening.
How dark the brim is depends on the not too well known temerature distribution
in the upper layers of the atmosphere. A group of scientists i Nice in France
will measure the limb darkening in these layers during the transit of Venus
in visible light of different colors. This will give important information
about the distribution of temperature and density i the upper atmosphere.
DIAMETER OF THE SUN
The French-Belgian-Swiss space project PICARD will in a few years time
measure the diameter of the SUn with great precision in order to find out
how it varies during the solar cycle. Knowledge of the limb darkening in the
upper atmosphere is crucial for this project.
More about the PICARD project
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When looking at this picture of the Sun, we notice that the limb is darker
than the central areas. The phenomenon is called limb darkening. At the limb
we cannot see the deep and hot layers we see
at the central parts. We don't know the amount of limb darkening at the
outermost brim. The sunspots are apparently dark, but still much brighter
than the dark side of Venus will be during the transit!
Photo: Big Bear Observatory
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NB! Information about more research projects with connection to the transit
will be given here later on.
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