Starship Asterisk*

A Picturesque Venus Transit

Explanation: The rare transit of Venus across the face of the Sun in 2004 was one of the better-photographed events in sky history. Both scientific and artistic images flooded in from the areas that could see the transit: Europe and much of Asia, Africa, and North America. Scientifically, solar photographers confirmed that the black drop effect is really better related to the viewing clarity of the camera or telescope than the atmosphere of Venus. Artistically, images might be divided into several categories. One type captures the transit in front of a highly detailed Sun. Another category captures a double coincidence such as both Venus and an airplane simultaneously silhouetted, or Venus and the International Space Station in low Earth orbit. A third image type involves a fortuitous arrangement of interesting looking clouds, as shown by example in the above image taken from North Carolina, USA. The next transit of Venus across the Sun will be in 2012 June.

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How the heck did the CIA get into an APOD? Must have been the spooky music.

...
"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."
...

Excerpt from A New Method of Determining the Parallax of the Sun, or His Distance from the Earth
Dr. Edmund Halley, 1716

Beyond wrote:How the heck did the CIA get into an APOD? Must have been the spooky music.

You noticed that also?
I see there is going to be another transit in June!

Odd that no sunspots are visible, this was at the middle of the last solar cycle.

The "double coincidence" hyperlink is of particular astronomical interest, as the odds of that happening are astronomical.

I think it is marvelous that nowadays there are so many people around the world seriously looking into the so many astronomical events compared to only a handfull in previous times!!!

Redbone wrote:Odd that no sunspots are visible, this was at the middle of the last solar cycle.

By mid-2004, Cycle 23 was well on its way towards Solar Minimum, with sunspot activity reduced to about one-third of what it had been at the previous maximum, in 2001. Keep in mind that today's APOD, while aesthetically pleasing, is an image of the Sun made through clouds, and shows seriously reduced contrast and resolution of solar features. Transit images made telescopically, under clear skies, do show some small sunspots and filaments on the Sun, although there were no major sunspot groups visible that day.

what is the "viewing clarity" of a camera? i've heard of aperture, focal length, focal ratio, magnification, transmission, resolution and even weird stuff like strehl ratio and zernicke zones ... but viewing clarity? i really have to guess. is that like the weather? the camera doesn't have viewing clarity because it's cloudy? the lenses are maybe dirty? there's still vaseline on the lens from the joan rivers portrait shoot? i'm stumped here.

drollere wrote:what is the "viewing clarity" of a camera? i've heard of aperture, focal length, focal ratio, magnification, transmission, resolution and even weird stuff like strehl ratio and zernicke zones ... but viewing clarity? i really have to guess. is that like the weather? the camera doesn't have viewing clarity because it's cloudy? the lenses are maybe dirty? there's still vaseline on the lens from the joan rivers portrait shoot? i'm stumped here.

In a technical sense, it doesn't mean anything. As used, I think it just means that the better the optics, the better the data. No surprise there.

For some reason I can't quite wrap my mind around the idea that you measure the distance to the Sun with the help of the Venus passage. I don't doubt, however, that it can be done, and that it can be explained, too, to someone who is less dense when it comes to mathematics than I am.

And I so admire those smart people who figure out ways to probe the depths of space that no one else had thought of, or managed to carry out, before them.

Ann

Ann wrote:For some reason I can't quite wrap my mind around the idea that you measure the distance to the Sun with the help of the Venus passage. I don't doubt, however, that it can be done, and that it can be explained, too, to someone who is less dense when it comes to mathematics than I am.

This is a way of getting the change in observed direction (parallax) of Venus as seen between two widely separated places n Earth, so that trigonometry gives the distance. At the time this was thought of (1700s), there was no really accurate way of measuring small angles well with telescopic accessories. However, the Sun as background provides a good reference. If a transit is observed from locations on Earth well separated north-south (more accurately, perpendicular to the relative motion of Venus and Earth), the angular shift can be derived from the difference in duration of the transit as observed at the two locations. In practice, this accuracy could not be reached because of the "black-drop" effect, due to blurring in the Earth's atmosphere.

A transit of Venus was one of the reasons Captain Cook was in Tahiti. The importance was that not only did that give the distance of Venus - since extant observations gave the relative sizes of planetary orbits with high precision, one distance established in absolute units gave the scale of the whole plan of the Solar System , including the size of the Earth's orbit. Later uses of the method into the 20th century included asteroid parallaxes from across the Earth, especially Eros at a very close approach. Those results were finally superseded by radar ranging to the nearby planets.

Orin wrote:You noticed that also?

Orin, i notice lots of things that i don't remember later

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."
...

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? =\

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."
...

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

I live in Argentina, the transit will not be visible from my location. The nearest place to see the transit is a 6 hours flight.

It already started all the transit hype and I feel terrible. Argh!

Coming in 2012: Our Last Transit of Venus
Universe Today | Nancy Atkinson | 2011 Oct 17

On June 5, 2012 in the US and June 6, 2012 in Europe, a transit of Venus across the face of the Sun will be visible from many parts of the world. This is a very unusual and exciting event and another transit of Venus won’t be visible from Earth for another 120 years. And so, this has been called the last transit of our time, as likely no one alive today will still be around in 120 years. A transit can tell us more about Venus, the Sun and give us more information about finding extrasolar planets. A group of filmmakers from the Netherlands are working on a documentary of this event, and they are looking for professional and amateur astronomers and historians to participate.

“The film will highlight three groups of people: scientists who will observe the Transit to study Venus and exoplanets, amateurs and students who will redo the experiment of determining the size of the Solar System and professional and/or amateur historians with the intention to observe the Transit with 18th and 19th century instruments,” say the filmmakers from Lightcurve Films. “Feel free to contact us about the project if you are interested to collaborate.”

callaharj 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."
...

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? =\

To summarize, Halley concluded that the sun's distance could be determined to 1 part in 500 assuming: 1) A transit timing difference accuracy (between the two observers) ≈ 2 sec, 2) The total transit difference time ≈ 17minutes, and 3) The sun's parallax ≈ 12.5 arc seconds.

His assumptions were based on an historical collection of sun/earth distances by other methods to estimate the sun's parallax = 12.5 arcseconds, his own experience at timing a Mercury transit contact point = 1 second, the angular size of Venus, along with the solar latitude of the transit are known, and that orbits and Sun are circular in shape.
- Applying Kepler's 3rd law, the (projected) distance between the two observers and the measured angular separtion of the two transit paths (relative to the sun) => the Earth / Sun distance is calculated
- The reason why the transit times are different for two properly positioned observers (Northern and Southern Hemisphere positions) is the transit occurs along different chords of a circle which have different lengths. As Halley pointed out, the difference in transit timing for these two paths is significant and easily measurable in this manner, leading to a relatively accurate parallax determination of 1 part in 500 for a 12.5 arcsecond parallax.

************************************
Kudos Ann,
You're to be commended on your explanation, and attempting to make it intuitive. The only correction I'll make is that Venus' orbit was not known either (as NGC3314 alluded to, and the video explicitly pointed out). Kepler's laws coupled with a creative observation method was the breakthrough for this key distance determination.

Kudos to both of you! That definitely helped in understanding it...thanks for taking the time to help =).