Thursday - August 20, 2015

I Feel The Earth Move Under My Feet

You are now 334 feet further West than you thought you were. Congratulations.

Once called the Prime Meridian of the World, the invisible line running north to south that divides the world into Eastern and Western hemispheres passed through the Airy Transit Circle — a 19th-century telescopic instrument at the Royal Observatory at Greenwich, England.

However, this line of longitude now runs 334 feet (102 meters) east of where it did. What made it shift? A change in finding out which way is down — from using a basin of liquid mercury to relying on satellites around Earth, researchers have found.

“Zut alors!” cry the perfidious Fwench, always jealous that the Prime never ran through Paris (because their Empire sucked), “tres oui! Knowing how to get down properly is what being Fwench is all about!”

The article goes on to tell the hapless reader the difference between latitude and longitude, wobbly gravity, the importance of a decent pocket watch (but does not mention our hero Mr. Harrison, you blighters!), and the dangers of getting it wrong:

One example of such a disaster happened in 1707, when four British warships and more than 1,400 lives were lost because storms forced the fleet’s navigators off course, making them believe they were safely to the west of the island of Ushant instead of closing in on dangerous rocks near the Isles of Scilly*.

This is all supported by Official Science and a link to actual published papers containing lots of advanced maths, complex graphs, and impressively big words, so you know it’s correct:

Astrogeodetic and gravimetric DoV determination
Natural coordinates [Φ, Λ, H] are related to geodetic latitude, φ, longitude, λ, and ellipsoidal height h, by (Heiskanen and Moritz 1967):
ξ=Φ−φη=(Λ−λ)cosφN=h−H(a)(b)(c)⎫⎭⎬⎪⎪,
(2)
where, ξ is the meridional component of the DoV, η the prime vertical component of the DoV and N is the geoid undulation. These equations relate the geodetic quantities [φ,λ,h] to the physical quantities [Φ, Λ, H] associated with the gravity field. If the geodetic coordinates [φ,λ, h] of a point have been determined with respect to a given datum, and if the natural coordinates [Φ, Λ, H] have been determined also (e.g., from astronomical observations and from spirit leveling), then one can obtain the deflection components (ξastro, ηastro) and the geoid undulation (N) at that point from Eq. (2). Today, such a geometric determination can be realized by making astronomical observations of Φ and Λ at a point whose geodetic coordinates [φ,λ,h] have been determined from GNSS positioning, and whose orthometric height, H, has been determined from spirit leveling. Such a geometric determination yields astrogeodetic DoV components that are specific to the particular datum to which the geodetic coordinates, φ and λ, refer.
...

Fig. 2

The reference ellipsoid from a point on its minor axis above the north pole. O = spheroid axis passing through the geocenter, Eq = ground level equipotential surface, Sph = spheroidal parallels, OA ′= geodetic meridian plane at K, SA = astronomical meridian plane at K

This actually makes perfect sense, even if it wasn’t featured as the centerfold in PNAS magazine. The one thing we’re left wondering is just how often Huma gets to control Hillary’s other handheld device.

Posted by Drew458     on 08/20/2015 at 06:43 AM   
Filed Under: • Amazing Science and Discoveries •  
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