A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Sun's surface. Although small sungrazers can completely evaporate during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong evaporation and tidal forces they experience often lead to their fragmentation. Up until the 1880s, it was thought that all bright comets near the Sun were the repeated return of a single sungrazing comet. Then, German astronomer Heinrich Kreutz and American astronomer Daniel Kirkwood determined that, instead of the return of the same comet, each appearance was a different comet, but each were related to a group of comets that had separated from each other at an earlier passage near the Sun (at perihelion). Very little was known about the population of sungrazing comets until 1979 when coronagraphic observations allowed the detection of sungrazers. As of October 21, 2017, there are 1495 known comets that come within ~12 solar radii (~0.055 AU). This accounts for nearly one third of all comets. Most of these objects vaporize during their close approach...

An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside the hull of the craft, an inboard motor is an engine enclosed within the hull of the boat, usually connected to a propulsion screw by a driveshaft. In international shipping the marine diesel engines are the largest most powerful engines ever produced. History The first marine craft to utilize inboard motors were steam engines going back to 1805 and the Clermont and the Charlotte Dundas. Harbour tugs, and small steam launches had inboard steam engines. In the 1880s the naphtha engine made its appearance and a few boat engines appeared. Such engines had low power and high fuel consumption. The gasoline (petrol) engine pioneer Gottlieb Daimler and Maybach built a four-cycle boat engine and tested it in 1887 on the Neckar River. Sintz in America built several commercially available engines from 1893. Sizes Inboard motors may be of several types, suitable for the size of craft they are fitted to. Boats can use one cylinder to v12 engines...

Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word stereoscopy derives from Greek στερεός (stereos) 'firm, solid', and σκοπέω (skopeō) 'to look, to see'. Any stereoscopic image is called a stereogram. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope. Most stereoscopic methods present a pair of two-dimensional images to the viewer. The left image is presented to the left eye and the right image is presented to the right eye. When viewed, the human brain perceives the images as a single 3D view, giving the viewer the perception of 3D depth. However, the 3D effect lacks proper focal depth, which gives rise to the Vergence-Accommodation Conflict. Stereoscopy is distinguished from other types of 3D displays that display an image in three full dimensions, allowing the observer to increase information about the 3-dimensional objects being displayed by head and eye movements. ...

Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6). The average density of the continental crust is about, 2.83 g/cm3 (0.102 lb/cu in), less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3 (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3 (0.10...