« on: Yesterday at 12:04:55 »
Quote from: syphrum
Recently a twice in a lifetime opportunity occurredIf we were orbiting a star 100LY away, which was on the plane of Venus' orbit, it would not be a "twice in a lifetime" opportunity, it would be an opportunity which repeats every 7 months.
What makes it so rare on Earth is that there is a narrow band where the plane of Earth's orbit crosses the plane of Venus' orbit, and it is rare that both Venus and Earth are in that narrow band on the same day.
if the transits had been across a star say 100 LY away how much would we have learnt ?If we had a telescope, it had better be above any CO2 or N2 in the atmosphere, otherwise we would miss 96% and 4% of Venus' atmospheric composition, respectively. A satellite observatory is strongly recommended!
The amount of light blocked by Venus is (0.95/109)2, or 0.0076% of the Sun's area. With a decent model of the age, structure and diameter of the Sun, this would give you an estimate of the diameter of Venus.
Of the light blocked by Venus, a tiny sliver of a percentage would have been modified by passing through the atmosphere of Venus. It would be very hard to detect this change in spectrum, even if your space telescope had a very large diameter.
It may be better to take measurements about 4, 5 and 6 months after the transit, where a considerable amount of the light falling on Venus would be reflected in our direction as the illumination of Venus changed from crescent to almost full. Venus reflects about 70% of the light falling on it, which gives a much better chance of working out its atmospheric composition from the subtle changes in spectrum (compared to the transit).
Unfortunately, these spectrum readings would be muddied by the reflection from Jupiter: Jupiter has 136 times the visible area of Venus, but only 1.8% of the light intensity, and half the albedo, producing a similar brightness to Venus.
But the most successful approach may be to use a coronagraph, which blocks out the light from the star, allowing imaging of multiple orbiting planets at once. Unfortunately, an effective space-based coronagraph would require two satellites in orbit, maintaining a precisely fixed orbital relationship relative to the distant star for a long exposure - a very difficult problem in orbital mechanics! A coronagraph has the advantage that it does not rely on the observer having a particular orbital alignment with the planetary system.
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