5 ms·

> 2. Prime Meridian mark (that goes through the Royal Observatory in Greenwich) is off by 102.5 m No it ain't, it's right on the meridian by definition. Some o

> 2. Prime Meridian mark (that goes through the Royal Observatory in Greenwich) is off by 102.5 m No it ain't, it's right on the meridian by definition. Some other co-ordinate system's 0° meridian is adjacent & almost coincides, but for a 102.5m offset.

georgecmu 10y ago
it's right on the meridian by definition What definition do you refer to? The real explanation for the discrepancy is that due to local gravity effects, the local vertical at the observatory does not intersect the Earth's rotation axis. https://en.wikipedia.org/wiki/Prime_meridian_(Greenwich) https://en.wikipedia.org/wiki/Prime_meridian_(Greenwich) When the Airy transit circle was built, a mercury basin was used to align the telescope to the perpendicular. Thus the circle was aligned with the local vertical or plumb line, which is deflected slightly from the normal, or line perpendicular, to the reference ellipsoid used to define geodetic latitude and longitude in the International Terrestrial Reference Frame (which is nearly the same as the WGS-84 system used by GPS). While the local vertical defined at the Airy transit circle still points to the modern celestial meridian (the intersection of the prime meridian plane with the celestial sphere), it does not pass through the Earth's rotation axis. As a result of this, the ITRF zero meridian, defined by a plane passing through the Earth's rotation axis, is 102 metres to the east of the prime meridian. A 2015 analysis by Malys et al. shows the offset between the Airy transit circle and the ITRF/WGS 84 meridians can be explained by this deflection of the vertical alone; other possible sources of the offset that have been proposed in the past are smaller than the current uncertainty in the deflection of the vertical near the observatory.
- falsedan 10y ago
  > What definition do you refer to? The one that goes through the marker. If there's any ambiguity due to another meridian having the same name, I trust you can distinguish them with your common good sense.
  - LeifCarrotson 10y ago
    When a tree falls in the forest, does it make a sound? What a silly question: It's just semantics! Of course, it does release acoustic energy, when its motion causes vibrations in the air. If there is no one near enough for this energy to move their eardrum and be perceived by the brain, no one will hear the tree fall. Likewise, are these landmarks in the right location? George is saying that the 4 Corners monument and the Greenwich meridian were supposed to be at certain points, and they were installed at different points. You're retroactively redefining those points to be "where the monuments are". It's perfectly reasonable to talk about the difference between the intended position and the actual position, and when the intended position makes more sense - for example, going X degrees east or west of the meridian should put you at X degrees. If the meridian does not go through the axis of rotation of the earth, so that measurements at other points do not put you at X degrees, it's sensible to say that the meridian is off.
- gpvos 10y ago
  So with WGS-84 and the ITRF, we changed to a different coordinate system with a meridian that goes 102.5m east of the Greenwich observatory. This was done to keep continuity in UT1 time.[0] [0] http://link.springer.com/article/10.1007%2Fs00190-015-0844-y http://link.springer.com/article/10.1007%2Fs00190-015-0844-y

georgecmu 10y ago

it's right on the meridian by definition What definition do you refer to? The real explanation for the discrepancy is that due to local gravity effects, the local vertical at the observatory does not intersect the Earth's rotation axis. https://en.wikipedia.org/wiki/Prime_meridian_(Greenwich) https://en.wikipedia.org/wiki/Prime_meridian_(Greenwich) When the Airy transit circle was built, a mercury basin was used to align the telescope to the perpendicular. Thus the circle was aligned with the local vertical or plumb line, which is deflected slightly from the normal, or line perpendicular, to the reference ellipsoid used to define geodetic latitude and longitude in the International Terrestrial Reference Frame (which is nearly the same as the WGS-84 system used by GPS). While the local vertical defined at the Airy transit circle still points to the modern celestial meridian (the intersection of the prime meridian plane with the celestial sphere), it does not pass through the Earth's rotation axis. As a result of this, the ITRF zero meridian, defined by a plane passing through the Earth's rotation axis, is 102 metres to the east of the prime meridian. A 2015 analysis by Malys et al. shows the offset between the Airy transit circle and the ITRF/WGS 84 meridians can be explained by this deflection of the vertical alone; other possible sources of the offset that have been proposed in the past are smaller than the current uncertainty in the deflection of the vertical near the observatory.

falsedan 10y ago
> What definition do you refer to? The one that goes through the marker. If there's any ambiguity due to another meridian having the same name, I trust you can distinguish them with your common good sense.
- LeifCarrotson 10y ago
  When a tree falls in the forest, does it make a sound? What a silly question: It's just semantics! Of course, it does release acoustic energy, when its motion causes vibrations in the air. If there is no one near enough for this energy to move their eardrum and be perceived by the brain, no one will hear the tree fall. Likewise, are these landmarks in the right location? George is saying that the 4 Corners monument and the Greenwich meridian were supposed to be at certain points, and they were installed at different points. You're retroactively redefining those points to be "where the monuments are". It's perfectly reasonable to talk about the difference between the intended position and the actual position, and when the intended position makes more sense - for example, going X degrees east or west of the meridian should put you at X degrees. If the meridian does not go through the axis of rotation of the earth, so that measurements at other points do not put you at X degrees, it's sensible to say that the meridian is off.
gpvos 10y ago
So with WGS-84 and the ITRF, we changed to a different coordinate system with a meridian that goes 102.5m east of the Greenwich observatory. This was done to keep continuity in UT1 time.[0] [0] http://link.springer.com/article/10.1007%2Fs00190-015-0844-y http://link.springer.com/article/10.1007%2Fs00190-015-0844-y