DK Eyewitness - Astronomy

What is a meridian?
Mauna kea
Increasing use of artificial light and air
pollution from the world’s populous
cities have driven astronomers to the
most uninhabited regions of Earth
to build their observatories. The best
places are mountain tops or deserts
in temperate climates where the air is
dry, stable and without clouds. The
Mauna Kea volcano on the island of
Hawaii has the thinner air of high
altitudes and the temperate climate
of the Pacific. There are optical,
infrared, and radio telescopes here.
Meridian lines are imaginary coordinates running from pole to pole that
are used to measure distances east and west on Earth’s surface and in
the heavens. Meridian lines are also known as lines of longitude. The word
meridian comes from the Latin word meridies, meaning “the midday,” because
the Sun crosses a local meridian at noon. Certain meridians became important
because astronomers used them in observatories when they set up their
telescopes for positional astronomy.
This means that all their measurements
of the sky and Earth were made relative
to their local meridian. Until the end
of the 19th century, there were a
number of national meridians in
observatories in Paris, Cadiz, and Naples.
Computer-driven telescope
Telescopes have become so big that astronomers
are dwarfed by them. This 20-in (51-cm) solar
coronagraph in the Crimean Astrophysical
Observatory in the Ukraine is driven by
computer-monitored engines. A coronagraph
is a type of solar telescope that measures the
outermost layers of the Sun’s atmosphere (p.38).
Prime
meridian
The greenwich meridian
In 1884 there was an international conference in
Washington, DC to establish a single Zero
Meridian, or Prime Meridian, for the world.
The meridian running through the Airy Transit
Circle—a telescope mounted so that it rotated in a
north–south plane—at the Royal Greenwich
Observatory outside London was chosen. This
choice was largely a matter of convenience.
Most of the shipping charts and all of the
American railroad system used Greenwich as
their longitude zero at the time. South of
Greenwich, the Prime Meridian crosses through
France and Africa, and then runs across the
Atlantic Ocean all the way
to the South Pole.
Crossing the meridian
In 1850 the seventh Astronomer Royal of Great
Britain, Sir George Biddle Airy (1801–1892), decided
he wanted a new telescope. In building it, he
moved the previous Prime Meridian for England
19 ft (5.75 m) to the east. The Greenwich Meridian is
marked by a green laser beam projected into
the sky and by an illuminated line that bisects
Airy’s Transit Circle at the Royal Observatory.
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