Potted history of timekeeping from the BBC
This is from a series of things that made the modern economy, it says on the web page: Tick tock: The importance of knowing the right time: http://www.bbc.co.uk/news/business-39129620 In one short page it manages to mention everything but DST :) and it includes a picture of Harrison No 1. (If you're ever near Greenwich I recommend visiting the Royal Observatory, taking the obligatory foot-in-both-hemispheres selfie and then going to look at the clocks.)
From the BBC web page though, I particularly noticed this:
Some financiers recently calculated it was worth spending $300m (£247m) drilling through mountains between Chicago and New York to lay fibre-optic cables in a slightly straighter line. That sped up communication between the two cities' exchanges by three milliseconds.
jch
"John" == John Haxby <john.haxby@oracle.com> writes:
John> (If you're ever near Greenwich I recommend visiting the Royal John> Observatory, taking the obligatory foot-in-both-hemispheres selfie John> and then going to look at the clocks.) And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later. I found the same thing when I went to Quito. The HUUUGE equator marker is wrong, and they acknowledge that. They have a museum about 200 meters north of that where you have to pay to see their *new* equator line (and some local history stuff). But again, the joke is on the tourists... the actual equator line is just outside the front door of the museum (slightly further north), before you even pay. :) -- Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095 <merlyn@stonehenge.com> <URL:http://www.stonehenge.com/merlyn/> Perl/Unix consulting, Technical writing, Comedy, etc. etc. Still trying to think of something clever for the fourth line of this .sig
On Tue, 4 Apr 2017, Randal L. Schwartz wrote:
"John" == John Haxby <john.haxby@oracle.com> writes:
John> (If you're ever near Greenwich I recommend visiting the Royal John> Observatory, taking the obligatory foot-in-both-hemispheres selfie John> and then going to look at the clocks.)
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
I always thought that the meridian line as marked is 0° in *OSGB36*, not WGS84 which is what GPS uses: http://theconversation.com/heres-why-the-greenwich-prime-meridian-is-actuall... http://metro.co.uk/2015/08/15/oops-the-greenwich-meridian-line-is-in-the-wro... cheers, Derick
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
And...if Google Maps is correct, the Arizona/New Mexico/Colorado/Utah Four Corners Monument is not at the four corners. [Insert here the mandatory reference to convoluted DST in the region.] @dashdashado https://www.google.com/maps/place/Four+Corners+Monument/@36.9989687,-109.045... On Tue, Apr 4, 2017 at 11:07 AM, Derick Rethans <tz@derickrethans.nl> wrote:
On Tue, 4 Apr 2017, Randal L. Schwartz wrote:
> "John" == John Haxby <john.haxby@oracle.com> writes:
John> (If you're ever near Greenwich I recommend visiting the Royal John> Observatory, taking the obligatory foot-in-both-hemispheres selfie John> and then going to look at the clocks.)
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
I always thought that the meridian line as marked is 0° in *OSGB36*, not WGS84 which is what GPS uses:
http://theconversation.com/heres-why-the-greenwich-prime- meridian-is-actually-in-the-wrong-place-46302 http://metro.co.uk/2015/08/15/oops-the-greenwich-meridian- line-is-in-the-wrong-place-5344251/
cheers, Derick
On Mon, 10 Apr 2017, Arthur David Olson wrote:
And...if Google Maps is correct, the Arizona/New Mexico/Colorado/Utah Four Corners Monument is not at the four corners.
Yes, indeed. Using the Google maps measurement tool shows it to be about 12 meters (39 feet) SSE of the actual four-corners
[Insert here the mandatory reference to convoluted DST in the region.]
@dashdashado
https://www.google.com/maps/place/Four+Corners+Monument/@36.9989687,-109.045...
+------------------+--------------------------+----------------------------+ | Paul Goyette | PGP Key fingerprint: | E-mail addresses: | | (Retired) | FA29 0E3B 35AF E8AE 6651 | paul at whooppee dot com | | Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd dot org | +------------------+--------------------------+----------------------------+
On Tue, Apr 4, 2017 at 11:07 AM, Derick Rethans <tz@derickrethans.nl> wrote:
On Tue, 4 Apr 2017, Randal L. Schwartz wrote:
> "John" == John Haxby <john.haxby@oracle.com> writes:
John> (If you're ever near Greenwich I recommend visiting the Royal John> Observatory, taking the obligatory foot-in-both-hemispheres selfie John> and then going to look at the clocks.)
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
I always thought that the meridian line as marked is 0° in *OSGB36*, not WGS84 which is what GPS uses:
http://theconversation.com/heres-why-the-greenwich-prime- meridian-is-actually-in-the-wrong-place-46302 http://metro.co.uk/2015/08/15/oops-the-greenwich-meridian- line-is-in-the-wrong-place-5344251/
George Kaplan and his colleagues went into why the "Greenwich Meridian" moved pretty exhaustively. https://www.researchgate.net/publication/282478531_Why_the_Greenwich_meridia... Regards Marshall Eubanks
cheers, Derick
Well... I'm close to Dublin now, will give a try on GMT... I went to Belem do Para, Brasil, close enough to que ecuador line... It's sooooo daaaaaaamn hot! I hope I NEVER go back there. Maybe somewhere cold (is there any high mountain over the ecuador line?) but never back to Amazonia! On 10 April 2017 at 17:24, Marshall Eubanks <marshall.eubanks@gmail.com> wrote:
On Tue, Apr 4, 2017 at 11:07 AM, Derick Rethans <tz@derickrethans.nl> wrote:
On Tue, 4 Apr 2017, Randal L. Schwartz wrote:
>> "John" == John Haxby <john.haxby@oracle.com> writes:
John> (If you're ever near Greenwich I recommend visiting the Royal John> Observatory, taking the obligatory foot-in-both-hemispheres selfie John> and then going to look at the clocks.)
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
I always thought that the meridian line as marked is 0° in *OSGB36*, not WGS84 which is what GPS uses:
http://theconversation.com/heres-why-the-greenwich-prime-mer idian-is-actually-in-the-wrong-place-46302 http://metro.co.uk/2015/08/15/oops-the-greenwich-meridian-li ne-is-in-the-wrong-place-5344251/
George Kaplan and his colleagues went into why the "Greenwich Meridian" moved pretty exhaustively.
https://www.researchgate.net/publication/282478531_Why_the_ Greenwich_meridian_moved
Regards Marshall Eubanks
cheers, Derick
-- *Pablo Santiago Sánchez* ZCE ZEND006757 phackwer@gmail.com (61) 9843-0883 http://www.sansis.com.br *"Pluralitas non est ponenda sine necessitate"*
On Tue 2017-04-04T07:35:21 -0700, Randal L. Schwartz hath writ:
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be. At least, I presume my modern smartphone's GPS is fairly accurate, as well as having looked it up on Google Earth later.
The meridian is exactly as it has always been. The GPS is telling where the meridian is for a satellite looking down to the center of the earth. The Observatory is where the meridian is for someone looking up at the stars. It is the same meridian because the meridian is a direction, not a place, and throughout its history the International Time Bureau did a good job of keeping it constant Full analysis is in this paper from 2015 https://link.springer.com/article/10.1007/s00190-015-0844-y by Malys, Seago, Pavlis, Seidelmann, and Kaplan -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
Randal L. Schwartz wrote:
And you should also note that the location where they charge you to take that photograph is about 20 meters to the west of where it should be.
It's more complicated than that. The line on the ground (and laser beam at night) is genuinely lined up with the historical transit circle instrument which defines the meridian which in 1884 was selected as the international prime meridian. There is a discrepancy between that and where a GPS unit will read zero longitude, of about 5.3 arcseconds (about 350 ms of rotational time, and about 100 m on the ground at Greenwich). This discrepancy mainly arises from differing definitions of longitude, interacting with non-uniform mass concentration in the Earth. One would normally expect that if one erected a plumb line, and extended the vertical of that line to the north and south, then the resulting meridian plane would pass through the centre (and entire rotational axis) of the Earth. In fact it's not quite so. In the case of Greenwich, there is a relative mass concentration to the west, which gravitationally attracts any plumb bob, making a plumb line lean slightly, by some 5.3 arcseconds. A meridian plane based on this local vertical misses the centre of the Earth, passing about 100 m to the west of it. When a transit circle is installed, it is aligned very precisely to match the local vertical. In the case of the Greenwich instrument, installed in 1851, this process involved a dish of mercury to provide a horizontal plane. The result of the mass concentration is that, where we think of the transit circle being directed precisely upwards, this does not mean that it's looking precisely away from the Earth's axis. The direction in which the Greenwich instrument looks is 5.3 arcseconds to the east of that. All of its observations, which defined GMT and our prime meridian, are based on this local vertical direction. It is the direction of that meridian plane, not the location of the observatory relative to the Earth's axis, that defined the prime meridian. GPS takes a different view of longitude. It doesn't measure local vertical, but instead works in a strictly geometrical manner, producing coordinates in 3D space. When those coordinates are geometrically reduced to latitude and longitude, one's implied local meridian is strictly the plane passing through one's location and the Earth's axis. One's longitude, then, is the angle between this geometrically-defined meridian and the prime meridian, the latter necessarily still being based on the observations made using the local gravitational vertical at Greenwich. Thus it is that the GPS-based (geometric) longitude at the location of the Greenwich instrument is 5.3 arcseconds west. So neither the line on the ground nor GPS is wrong; they're just the zeroes of different ways of reckoning longitude. There is also presumably some discrepancy between the astronomical longitude of the transit circle and the modern prime meridian due to inaccuracy in historical switches between ways of realising the prime meridian, but this is probably two orders of magnitude smaller than the difference between astronomical and geometric longitude. -zefram
On Apr 4, 2017, at 7:43 AM, John Haxby <john.haxby@oracle.com> wrote:
This is from a series of things that made the modern economy, it says on the web page:
Tick tock: The importance of knowing the right time: http://www.bbc.co.uk/news/business-39129620
In one short page it manages to mention everything but DST :) and it includes a picture of Harrison No 1. ...
Interesting they would show No. 1 rather than No. 4, which is the one that actually solved the longitude problem. paul
On 4 Apr 2017, at 16:05, Paul.Koning@dell.com wrote:
On Apr 4, 2017, at 7:43 AM, John Haxby <john.haxby@oracle.com <mailto:john.haxby@oracle.com>> wrote:
This is from a series of things that made the modern economy, it says on the web page:
Tick tock: The importance of knowing the right time: http://www.bbc.co.uk/news/business-39129620 <http://www.bbc.co.uk/news/business-39129620>
In one short page it manages to mention everything but DST :) and it includes a picture of Harrison No 1. ...
Interesting they would show No. 1 rather than No. 4, which is the one that actually solved the longitude problem.
Possibly because it looks as though it means business. It did solve the longitude problem, it just wasn’t small enough to be a practicable solution. It’s in a glass case about a metre on a side and, if memory serves, still running. No.4 is too valuable even to clean but doesn’t look anything special. No.6 sold for several million[1] :) jch [1] https://youtu.be/Er_gYljbmno?list=PLW4Z0lilD4aHrIi8FLNI2oTn_hPF6xWED
participants (10)
-
Arthur David Olson -
Derick Rethans -
John Haxby -
Marshall Eubanks -
merlyn＠stonehenge.com -
Pablo Sánchez -
Paul Goyette -
Paul.Koning＠dell.com -
Steve Allen -
Zefram