callaharj wrote:alter-ego wrote:...
"By this means, the sun's parallax may be discovered, to within its five hundredth part, which will doubtless seem surprising to some: but yet, if an accurate observation be had in both the places above-mentioned, it has already been shown that the duration of these eclipses of Venus differ from each other by 17 entire minutes, on the supposition that the sun's parallax is 12½ seconds. And if this difference be found to be greater or less by observation, the sun's parallax will be greater or less nearly in the same ratio. And since 17 minutes of time answer to 12½ seconds of the sun's parallax; for each second of the parallax there will arise a difference of upwards of 80 seconds of time; therefore, if this difference be obtained true within 2 seconds of time, the quantity of the sun's parallax will be got to within the 40th part of one second; and consequently his distance will be determined to within its 500th part; at least if the parallax be not found less than what I have supposed it; for 40 x 12½ is 500."
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Excerpt from
A New Method of Determining the Parallax of the Sun, or His Distance from the Earth
Dr. Edmund Halley, 1716
I've stared at this paragraph. I've read it. I've researched it. For hours. I still have no idea what the hell he's talking about. Would someone dare to elucidate this paragraph in layman's terms? =\
I'm probably going to regret even trying to explain, but since I have to explain things to myself by expressing them in layman's terms (since I understand no other terms), I'll at least try. If nothing else, if I have completely misunderstood this whole thing, those who do understand it will notice and will be better able to correct me.
So, parallax. It has to do with how things seem to "move" or "shift" in relation to their relative distance or position as seen from vantage points here on Earth.
Hmmm. I guess that didn't make it much clearer. All right, let's start by saying you are in your car, driving past a wide open landscape. Right next to the road there are some trees. In the mid-distance, there is, for some reason, a large boulder. Far on the horizon is a mountain range. Okay. The trees next to the road will whoosh by really fast as you drive along the road. The boulder in the mid-distance will seem to move much more slowly, as you overtake it. The mountain range will barely seem to move at all, although if you drive for an hour or even several hours, you will notice a change.
The objects that you pass will seem to slip behind you fast or slowly depending on how far away they are. Now imagine that you drive for half a mile, and then you stop. You make a careful note of where that boulder in the mid-distance seems to be in relation to the far-away mountain range. Then you drive the same way back again for half a mile and stop. Now the boulder seems to have moved in relation to the mountain range. You can use triangulation to figure out how far away the boulder is. You know the baseline: that's half a mile, the distance you drove with your car. Then you can measure how the angle to the boulder has changed as you moved along your baseline.
In this illustration, the boulder is the nearby star, and the mountain range is the stars in the distance.
For the Venus transit, my impression is that you could possibly measure the distance to the Sun by seeing how fast Venus, whose orbital velocity was known, passes over the face of the Sun as seen from one northerly and one southerly position on the Earth. The Sun is not seen to be in the same position from the northern hemisphere of the Earth as it is from the southern hemisphere, and the farther north and south you get, the greater the change in the Sun's apparent position will be. Also, my impression is that, perhaps, one hemisphere will see a slightly longer Venus transit that the other hemisphere. If you know the distance in the north-south position between the two measuring points, then you get a "baseline" on the Earth from which you are measuring the transit. (I think.)
I believe that the size of the orbit of Venus was known quite well in the days of Edmund Halley. So you had the size of Venus's orbit (which, as it happens, is almost circular, more so than the orbit of the Earth), and you also had the orbital velocity of Venus and two measuring points on the Earth. Venus will seem to transit faster from one of these measuring points than from the other, and since the crucial distance between these two measuring points is also known, you can start doing the math.
Which I won't; I'm a math illiterate.
Ann
A Picturesque Venus Transit
