Essen, horizontal view |
Internet ProjectObserving, Photographing and Evaluating theTransit of Venus, June 8th, 2004 |
Essen, equatorial view |
By evaluating pictures taken during the transit of Venus (or Mercury, respectively) you get all results relative to the sun's angular radius ρ_{S} . For instance, by comparison of simultaneously taken pictures of different observers you get the relative parallax effect f. To be able to derive its absolute value Δβ you must know the angular radius of the Sun:
Of course, you know ρ_{S} roughly (about 15 arcminutes) and you can find its exact value - even for the day of interest. But:
There are, at least, two simple possibilities for this measurement:
Sunspots on the ground | Light rays causing the spots on the ground |
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The spots are pinhole camara pictures of the sun! Therefore, the angular radius ρ_{S} of the Sun can be determined by measuring the linear radius r of a spot and its distance d to the hole of its origin:
Mounting of a binocular for observing and measuring the sun's movement | The both projection of the Sun (during an eclipse) | Projection with ... | ... Solarscope |
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Having the optical system fixed you will soon remark that the Sun's picture tends to wander over the sheet.
Start | "Third contact": t_{1} | "Fourth contact": t_{2} |
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Therefore, the angular radius of the Sun must be:
The situation of the above picture, but now projected to the plane of
meridian.