ens has
sought to establish its geophysical position, wherever that might be,
within the confines of the biosphere of planet earth - which we have
in more recent times, come to refer to as "the pale blue dot" within our
cosmos - since the very beginning of our species TIME on Planet Earth.
In geography, latitude (φ) is a geographic coordinate that specifies
the north–south position of a point on the Earth's surface. Latitude is
an angle which ranges from 0° at the Equator to 90° (North or South)
at the poles. Lines of constant latitude, or parallels, run east–west as
circles parallel to the equator. Measurement of latitude requires an
understanding of the gravitational field of the Earth, either for setting
up theodolites or for determination of GPS satellite orbits. The notion of longitude was developed by the Greek Eratosthenes
(c.276 BC/195 BC) in Alexandria and Hipparchus (c.190 BC/120 BC)
in Rhodes and applied to a large number of cities by the geographer
Strabo (63 BC – c.24 AD). But it was Ptolemy (c. AD 90 – c. AD 168)
who first used a consistent meridian for a world map in his
Geographia. Longitude is an angular measurement, usually expressed
in degrees and denoted by the Greek letter lambda (λ). Meridians
(lines running from the North to South poles connect points with the
same longitude. By convention, one of these, the Prime Meridian,
which passes through the Royal Observatory, Greenwich, England,
was allocated the position of zero degrees longitude and subsequently
TIME at that point, has become known as Greenwich Mean Time (GMT)
or ZULU TIME in accordance with the Z of zero being pronounced as
ZULU during the phonetic AlphaBET process of Air Transport Traffic-
Communications. The longitude of other places is measured as the
angle east or west from the Prime Meridian, ranging from 0° at the
Prime Meridian to +180° eastward and −180° westward. Specifically,
it is the angle between a plane containing the Prime Meridian and
a plane containing the North Pole, South Pole and the location in
question. [This then forms a right-handed coordinate system with
the z axis (right hand thumb) pointing from the Earth's center toward
the North Pole and the x axis (right hand index finger) extending from
Earth's center through the equator at the Prime Meridian.] If the Earth
were perfectly spherical and homogeneous, then the longitude at a
point would be equal to the angle between a vertical north–south plane
through that point and the plane of the Greenwich meridian. Everywhere on Earth the vertical north–south plane would contain the
Earth's axis. But the Earth is not homogeneous, and has mountains -
which have gravity and so can shift the vertical plane away from the
Earth's axis. The vertical north–south plane still intersects the plane
of the Greenwich meridian at some angle; that angle is the
astronomical longitude, calculated from star observations. The
longitude shown on maps and GPS devices is the angle between the
Greenwich plane and a not-quite-vertical plane through the point;
the not-quite-vertical plane is perpendicular to the surface of the
spheroid chosen to approximate the Earth's sea-level surface,
rather than perpendicular to the sea-level surface itself.
GPS is operated and owned by the US and the EU has decided that
being completely reliant on another country's GPS system would leave
them vulnerable if they and the US ever fell out. That's why they're
currently launching the first few of what will, when completed by 2019,
be a 30-strong swarm of satellites. Collectively, they're known as
Galileo - Europe's answer to the GPS. But while Galileo has emerged
largely due to politics, it will offer tangible benefits for you and
your pocket, too. Simply put, it will mean stronger signals and
better coverage, leading to a much more accurate and more
reliable service.
http://www.techradar.com/news/car-tech/satnav/galileo-how-europe-s-...
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Added by Michael Grove at 10:14 on November 25, 2013