Re: [time-nuts] zero crossing of venus
On 6/5/12 5:20 PM, Tom Van Baak wrote: Attached are two snapshots of a NASA live feed -- an interesting reminder about the difficulty measuring timing signals with great precision. When you look closely, the leading edge of the sun is rather ill-defined, not unlike many 1PPS pulses. I suppose with enough photos, modeling, and image processing one could pinpoint when the transit (zero crossing) really occurs to great precision. Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? Thanks, /tvb Speaking of this.. does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? I assume it makes use of some astronomical time measure to determine when Venus enters and leaves from different viewing places. but that would require a clock that can time from night (when you get an astronomical measurement) to day reasonably accurately. Or, do you measure the position of the sun in the sky (something that's fairly easy to do) But maybe not.. maybe it's more about where it enters and leaves the solar disk (in an angular sense, i.e. what's the length of the chord) positionally, in which case the time is less important. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
On 6/6/2012 9:09 AM, Jim Lux wrote: does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? http://transitofvenus.nl/wp/getting-involved/measure-the-suns-distance/ ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
People have tried to time these transits for a LONG time. No one has had very good success because of the physics. First venus has an atmosphere the refracts sunlight. There is also an optical illusion when two circles are close we think we see them overlap. It was once important to measure this so they could determine the size of the solar system but no days we can bounce radars of the planets and directly measure distance and velocity Yes venus is a disk, not a point. That is why they define 1st and 2nd contact as when each edge of tens crosses the edge of the sun. There are four events to time. A good way to measure this today is with a video camera. The camera takes 30 frames per send and later you can figure out in which frame each contact was made. And then you look at the time code to figure out when On Tue, Jun 5, 2012 at 8:56 PM, Andrew Rodland and...@cleverdomain.orgwrote: Jim Lux jimlux@... writes: On 6/5/12 5:20 PM, Tom Van Baak wrote: Attached are two snapshots of a NASA live feed -- an interesting reminder about the difficulty measuring timing signals with great precision. When you look closely, the leading edge of the sun is rather ill-defined, not unlike many 1PPS pulses. I suppose with enough photos, modeling, and image processing one could pinpoint when the transit (zero crossing) really occurs to great precision. Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? Thanks, /tvb or do something like compare the centroid of venus to centroid + radius of sun (or segment thereof.. ) it's pretty easy to get 0.1 pixel centroid precision, from what the star tracker, tiny moon finder folks tell me. Drifting off topic, but the (apparent) radius of Venus is actually significant here, being about 1 minute of arc — about 1/30th the apparent radius of the Sun. In the case of the current transit of Venus, there are 18 minutes between the time when the leading edge of Venus touches the Sun's disc from the outside, and the time when the trailing edge of Venus touches the Sun's disc from the inside. So if you want to talk precision, don't treat Venus as a point. It's too nearby for that. Andrew ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
Jim, Look at: http://www.venus2012.de/venusprojects/photography/basicideas/basicideas.php http://www.didaktik.physik.uni-due.de/~backhaus/Venusproject/mercury2003.htm http://www.exploratorium.edu/venus/question4b.html John WA4WDL -- From: Jim Lux jim...@earthlink.net Sent: Wednesday, June 06, 2012 9:09 AM To: time-nuts@febo.com Subject: Re: [time-nuts] zero crossing of venus On 6/5/12 5:20 PM, Tom Van Baak wrote: Attached are two snapshots of a NASA live feed -- an interesting reminder about the difficulty measuring timing signals with great precision. When you look closely, the leading edge of the sun is rather ill-defined, not unlike many 1PPS pulses. I suppose with enough photos, modeling, and image processing one could pinpoint when the transit (zero crossing) really occurs to great precision. Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? Thanks, /tvb Speaking of this.. does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? I assume it makes use of some astronomical time measure to determine when Venus enters and leaves from different viewing places. but that would require a clock that can time from night (when you get an astronomical measurement) to day reasonably accurately. Or, do you measure the position of the sun in the sky (something that's fairly easy to do) But maybe not.. maybe it's more about where it enters and leaves the solar disk (in an angular sense, i.e. what's the length of the chord) positionally, in which case the time is less important. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
On 6/6/12 7:02 AM, Mike S wrote: On 6/6/2012 9:09 AM, Jim Lux wrote: does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? http://transitofvenus.nl/wp/getting-involved/measure-the-suns-distance/ Of course, back in the 1700s, they didn't have nice iPhone apps to give good time hacks.. I see that Halley's scheme relied on measuring the length of the chord of the transit, which could be done geometrically (at least from the drawing, it looks that way), without needing time involved. http://www.transitofvenus.nl/halleysmethod.pdf seems to say that a precision of around 1 second for the duration between contacts would do. (of course, here in southern California, we couldn't do it, because the sun set before 3rd contact) Delisle's technique seems to require synchronized clocks. How well synchronized? A good math treatment of the technique would be nice to find, then one could take known clock and measurement accuracy and figure this kind of thing out. I think this has more of the info http://www.venus2012.de/venusprojects/contacttimes/basicidea/basicideatimes.php but I'm still looking through it. I'm also interested in how did they get their absolute time hack for the Delisle technique. It's an astronomical measurement, so maybe the lunar distances or Jovian moons approaches would work, but then you have to have a stable enough clock to last from night until day. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
On Wed, Jun 6, 2012 at 6:09 AM, Jim Lux jim...@earthlink.net wrote: Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? I'm thinking few milliseconds is the best we can do, ever. Especially from the ground. By 1874 thy had decent clocks and even photography and good telescopes. The problem is that the exact time is not well defined visually. The infamous black drop effect prevents accurate timing. This is where Venus and the Sun appear to be moving closer at a slow rate and then as they get closer, venus jumps. does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? I think it is as simple as a similar triangles geometry problem. There was a recent article on this in Sky and Telescope. Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
In message CABbxVHs=nhLies1VRYO5Qhk2uF48AAseHZBTV69-CorER=z...@mail.gmail.com , Chris Albertson writes: does anyone have a reference to the math and process used to measure distance from earth to sun using transit of venus? I read about it many years ago in a book about Einstein (as lead-up to the mercury stuff). The measurements mentioned was the time it took Venus to pass the Suns limb, and the time it took the center of Venus to pass across the Sun. No math given. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
On 6/5/12 5:20 PM, Tom Van Baak wrote: Attached are two snapshots of a NASA live feed -- an interesting reminder about the difficulty measuring timing signals with great precision. When you look closely, the leading edge of the sun is rather ill-defined, not unlike many 1PPS pulses. I suppose with enough photos, modeling, and image processing one could pinpoint when the transit (zero crossing) really occurs to great precision. Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? Thanks, /tvb or do something like compare the centroid of venus to centroid + radius of sun (or segment thereof.. ) it's pretty easy to get 0.1 pixel centroid precision, from what the star tracker, tiny moon finder folks tell me. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
It's a Transit, not a 'zero crossing'. -John ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] zero crossing of venus
Jim Lux jimlux@... writes: On 6/5/12 5:20 PM, Tom Van Baak wrote: Attached are two snapshots of a NASA live feed -- an interesting reminder about the difficulty measuring timing signals with great precision. When you look closely, the leading edge of the sun is rather ill-defined, not unlike many 1PPS pulses. I suppose with enough photos, modeling, and image processing one could pinpoint when the transit (zero crossing) really occurs to great precision. Does anyone know more details how this is done? Is the state-of-the-art at the millisecond level? microsecond? nanosecond? Thanks, /tvb or do something like compare the centroid of venus to centroid + radius of sun (or segment thereof.. ) it's pretty easy to get 0.1 pixel centroid precision, from what the star tracker, tiny moon finder folks tell me. Drifting off topic, but the (apparent) radius of Venus is actually significant here, being about 1 minute of arc — about 1/30th the apparent radius of the Sun. In the case of the current transit of Venus, there are 18 minutes between the time when the leading edge of Venus touches the Sun's disc from the outside, and the time when the trailing edge of Venus touches the Sun's disc from the inside. So if you want to talk precision, don't treat Venus as a point. It's too nearby for that. Andrew ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
