On Earth, the longitude of the Royal Observatory in Greenwich,
England is defined as the 'prime meridian,' or the zero point of longitude.
Locations on Earth are measured in degrees east or west from this
position. The prime meridian was defined by international agreement in
1884 as the position of the large 'transit circle', a telescope in the
Observatory's Meridian Building. The transit circle was built by Sir
George Biddell Airy, the 7th Astronomer Royal, in 1850. (While visual
observations with transits were the basis of navigation until the space
age, it is interesting to note that the current definition of the prime
meridian is in reference to orbiting satellites and Very Long Baseline
Interferometry (VLBI) measurements of distant radio sources such as
quasars. This 'International Reference Meridian' is now about 100 meters
east of the Airy Transit at Greenwich.)
For Mars, the prime meridian was first defined by the German
astronomers W. Beer and J. H. Mädler in 1830-32. They used a small
circular feature, which they designated 'a,' as a reference point to
determine the rotation period of the planet. The Italian astronomer
G. V. Schiaparelli, in his 1877 map of Mars, used this feature as the
zero point of longitude. It was subsequently named Sinus Meridiani
('Middle Bay') by Camille Flammarion.
When Mariner 9 mapped the planet at about 1 kilometer (0.62 mile)
resolution in 1972, an extensive 'control net' of locations was computed
by Merton Davies of the RAND Corporation. Davies designated a
0.5-kilometer-wide crater (0.3 miles wide), subsequently named 'Airy-0'
(within the large crater Airy in Sinus Meridiani) as the longitude zero
point. (Airy, of course, was named to commemorate the builder of the
Greenwich transit.) This crater was imaged once by Mariner 9 (the 3rd
picture taken on its 533rd orbit, 533B03) and once by the Viking 1
orbiter in 1978 (the 46th image on that spacecraft's 746th orbit,
746A46), and these two images were the basis of the martian
longitude system for the rest of the 20th Century.
The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) has
attempted to take a picture of Airy-0 on every close overflight since the
beginning of the MGS mapping mission. It is a measure of the difficulty
of hitting such a small target that nine attempts were required, since
the spacecraft did not pass directly over Airy-0 until almost the end of
the MGS primary mission, on orbit 8280 (January 13, 2001).
Photo Credit: NASA/JPL/Malin Space Science Systems
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