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When viewed silhouetted against the Sun, Mercury’s disk (seen at left on November 9, 2006) is clearly smaller than that of Venus (seen at right on June 5, 2012). Other dark markings are sunspots.

Credit
Montage Marc Jobin/Espace pour la vie; Mercure : Brocken Inaglory/Wikimedia Commons; Vénus : Marc Jobin
Copyright
GFDL
Source
Espace pour la vie