oz time

Thanks. I'm planning a road trip next May to go to the Dayton Hamfest and the Air Force Museum. If we're ambitious and the weather is good, on the way back we might swing by the Steven Udvar-Hazy museum in Chantilly VA, where they have an intact SR-71. (Of course I might have a back seat full of oscilloscopes.) ;)

Cheers

Phil Hobbs

It's not so much trying to kill you, as capable of killing you if you put yourself in the wrong place at the wrong time.

Crocodiles and sharks aren't any more lethal than grizzly bears.

The Air Force Museum in Dayton OH is really good. Their SR71 is sitting under the wing of the only remaining XB-70. The Valkyrie is simply *huge*. The B36 is amazing too (as is its original tire). Well worth the time if you're anywhere close.

:)

Clifford Heath.

Nope. The courtyard line is 102 meters away from the real prime meridian line. See Fig 2 at: I'm told that visitors to the observatory often bring their GPS receivers or smartphone and try to find the real prime meridian. It's probably easier to look for the garbage bin marker: Here's the actual bin:

I know people like that. ;)

Cheers

Phil Hobbs

Things get very nasty, if you want to do geodetic measurement with millimeter accuracy, especially over a longer period o time. The EurAsian plate floats around about 25 mm/year. The plate is not completely rigid, so parts of it (such as Scandinavia or British Isles) may move millimeters/year relative to the Eurasian plate.

If the 0 meridian would be defined to always be the physical line in Greenwich town, the line would have moved 8 m during its existence due to EurAsia drift. The coordinates on other continents would have moved by 8 meters in addition to the local continental drift.

When the GPS coordinates are locked to the geoid, all continental drifts are relative to it and you only need to consider the plate movement of your own continent.

How does one define the "geoid"? What is used as a reference?

Rick C.

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By observing orbiting satellites to determine the center of gravity of the Earth and also any of the bulges.

You can't use the C of G as a reference for points on the surface of the geoid. Are you suggesting using bulges?

Seems to me it only makes sense to use an arbitrary point on some continent and refer everywhere else to it. There was opposition to Greenwich first time round, believe it or not...

Mike.

Center of gravity is just that, the center. What is to say the "bulges" don't also move?

Rick C.

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That's the only thing I can think of. I know there is a wobble to the orientation of the Earth's axis. Does that also mean the center of rotation also wobbles or does the entire Earth actually shift as the axis wobbles?

It seems like a real chicken and egg problem. What do you use as a reference to indicate anything else is moving or not?

Rick C.

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For paper maps, maybe. For satnav the natural thing to use is the geoid.

Cheers

Phil Hobbs

They're continually being measured. (Google 'satellite geodesy'.)

The bulges can be used to define a coordinate system, like the flats on a hex nut.

Cheers

Phil Hobbs

I suppose one arbitrary reference is as good as another.

Rick C.

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As long as they don't change. Are they not changing? If they were moving in some manner, how would we know?

Rick C.

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Well, the rotation of the Earth is also defined in terms of the geoid, IIRC. Thus we'd only measure the differential motion of the bulges, which is all we'd really care about anyway.

Cheers

Phil Hobbs

That's not merely a circular argument but a spherical one! ;-)

Mike.

Continental drift is happening, and we can now measure relative positions accurately enough to see it happening. The North American continental plate is moving away from the mid-Atlantic ridge at 25 mm per year.

The Indo-Australian plate is rotating - the Australian end is moving northwards at 56 mm per year, while the Indian end is only doing 37 mm per year.