than 24 hours in the summer and be below the horizon for 24 hours or more in the
winter.
Lines of longitude are also known as meridians. As is the case with lines of lat-
itude, any number of lines of longitude can be drawn on a map or globe. Yet, there
are substantial differences between meridians and parallels. Meridians cross paral-
lels at right angles and as such, are north-south lines. Importantly, meridians
converge to the North and South poles. That is, the Earth distance associated with
one degree of longitude approximates 111 km at the equator and becomes progres-
sively less by latitude. At 45°the distance lessens to 79 km and at 90°the distance
is 0 because of the convergence of meridians.
Time is reckoned from the daily appearance of the sun overhead of the local
meridian. A day, apart from a few minutes’ variation caused by the elliptical Earth
orbit around the sun, is 24 hours long. This is because the sun appears to pass west-
ward over 15°of longitude per hour. It is not a coincidence that 24 hours multiplied
times 15°per hour yields a circle of 360°. Indeed, the configuration of clocks with
hands for hours, minutes, and seconds is an analog to Earth/sun relations.
Where is 0°longitude? Unlike latitude, there is no physical argument for any
meridian to garner this label. Originally, most cities reckoned their time using their
local meridians. As human society became more time aware and time dependent
the need for global timekeeping became vital. By the 19th century, matters of
increasingly rapid communication and navigation were confused by the plethora
of central meridians. A passenger traveling the transcontinental railroads of North
America was subject to a new time at every major stop! In 1884, an international
convention adopted a universal time scheme with time zones. The prime meridian
of the world was set as the longitude of the Royal Observatory at Greenwich,
United Kingdom. This was a result of the United Kingdom’s long-standing astro-
nomical observations at this location and the fact that the International Dateline,
at which point calendar days change any time it is crossed, was relegated to the
population-sparse Pacific Ocean.
Longitude is numbered as the angular difference east and west of the prime
meridian. Eastward, longitude is numbered from 0°to 180°, and this is the Eastern
Hemisphere, which contains Asia. Westward, the numbering is also from 0°to
180 °, and this is the Western Hemisphere, which contains the Americas. The
International Dateline nominally follows 180°longitude but deviates around
islands and countries so as to not be a nuisance to population centers.
Accurate measurement of latitude and longitude are essential to location and
navigation. Historically, latitude was measured by use of a sextant, a device that
determines the angle of the sun above the horizon. Using this device when the sun
is overhead at the local meridian allows a ready determination of latitude from
Latitude and Longitude 201