another planet. It was discovered by Christiaan Huygens, who included it in a drawing of Mars in 1659. He used repeated observations of the feature to estimate the length of day on Mars. The feature was originally known as the Hourglass Sea but has been given different names by different cartographers. In 1850s, Angelo Secchi called the feature Atlantic Canale (later he called it Scorpion and Cook Sea or Cook Canal). In Richard Proctor's 1867 map it is called then Kaiser Sea (after Frederik Kaiser of the Leiden Observatory). Camille Flammarion called it the Mer du Sablier (French for "Hourglass Sea") when he revised Proctor's nomenclature in 1876. The name "Syrtis Major" was chosen by Giovanni Schiaparelli when he created a map based on observations made during Mars' close approach to Earth in 1877. Seasonal variations Syrtis Major was the object of much observation due to its seasonal and long-term variations. This led to theories that it was a shallow sea and later that its variability was due to seasonal vegetation. However, in the 1960s and 1970s, the Mariner and Viking planetary probes led scientists to conclude that the variations were caused by wind blowing dust and sand across the area. It has many windblown deposits that include light-colored halos or plumose streaks that form downwind of craters. These streaks are accumulations of dust resulting from disruption of the wind by the elevated rims of the craters ('wind shadows'). Nili Patera Caldera Nili Patera is a 50 km diameter caldera at the center of the Syrtis Major Volcanic Complex. It and Meroe Patera located to the south are the primary named calderas within a nested caldera complex developed by multiple eruption and collapse events. In the Northeast quadrant of Nili Patera is a 630 m tall volcanic cone named Nili Tholus, on and around this cone is a light-tone lava flow of chemically evolved lava with multiple occurrences of relict silica sinter deposits created by a formerly active hot spring system. Moving sand dunes and ripples Nili Patera was the subject of a 2010 study into moving sand dunes and wind ripples. The study showed that dunes are active and that sand ripples are actively migrating on the surface of Mars. A following study also showed that the sand dunes move at about the same flux (volume per time) as dunes in Antarctica. This was unexpected because of the thin