<P> His works include a detailed description of what he termed the' non-natural features' of the planet's surface, including especially a full account of the' canals,' single and double; the' oases,' as he termed the dark spots at their intersections; and the varying visibility of both, depending partly on the Martian seasons . He theorized that an advanced but desperate culture had built the canals to tap Mars' polar ice caps, the last source of water on an inexorably drying planet . </P> <P> While this idea excited the public, the astronomical community was skeptical . Many astronomers could not see these markings, and few believed that they were as extensive as Lowell claimed . As a result, Lowell and his observatory were largely ostracized . Although the consensus was that some actual features did exist which would account for these markings, in 1909 the sixty - inch Mount Wilson Observatory telescope in Southern California allowed closer observation of the structures Lowell had interpreted as canals, and revealed irregular geological features, probably the result of natural erosion . </P> <P> The existence of canal - like features was definitively disproved in the 1960s by NASA's Mariner missions . Mariner 4, 6, and 7, and the Mariner 9 orbiter (1972), did not capture images of canals but instead showed a cratered Martian surface . Today, the surface markings taken to be canals are regarded as an optical illusion . </P> <P> Although Lowell was better known for his observations of Mars, he also drew maps of the planet Venus . He began observing Venus in detail in mid-1896 soon after the 61 - centimetre (24 - inch) Alvan Clark & Sons refracting telescope was installed at his new Flagstaff, Arizona observatory . Lowell observed the planet high in the daytime sky with the telescope's lens stopped down to 3 inches in diameter to reduce the effect of the turbulent daytime atmosphere . Lowell observed spoke - like surface features including a central dark spot, contrary to what was suspected then (and known now): that Venus has no surface features visible from Earth, being covered in an atmosphere that is opaque . It has been noted in a 2003 Journal for the History of Astronomy paper and in an article published in Sky and Telescope in July 2003 that Lowell's stopping down of the telescope created such a small exit pupil at the eyepiece, it may have become a giant ophthalmoscope giving Lowell an image of the shadows of blood vessels cast on the retina of his eye . </P>

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