The most obvious features on the surface of the Moon are the circular walled areas called craters. These have been known since telescopic observations began. The Earth has very few easily recognizable craters of any great size, although in recent years careful observations have revealed many more which are somewhat obscured by water, vegetation, erosion […]
A glance at any atlas shows that the Atlantic coastlines of Africa and (South America have very similar shapes. The coastline does not truly represent the shape of a continent, as it depends on the level of the oceans. A much more significant feature is the edge of the continental shelf where the sea bottom […]
In 1889. G.V.Schiaparelli announced that his extensive observations of Mercury had shown that the planet rotated on its axis in the same time. 88 days, that it took to revolve around the Sun and that as a result it always kept the same face turned towards the Sun. This result was confirmed by several other […]
On the Earth, a pivoted magnet will swing round to point roughly north-south and, if it is free to tilt, will take up an inclined position with the northern end pointing downwards in the northern hemisphere and upwards in the southern. Such a magnet can be used to plot the direction of the Earth’s magnetic […]
The interiors of the planets arc totally inaccessible to direct observations. The deepest bole drilled into the Earth goes to a depth of about l0 km, just a small fraction of the 6378km distance to the centre. Information about the interior must therefore come from indirect methods and in practice the most important method by […]
The best known part of the Solar System is, naturally enough, that part of it nearest to the Earth. Mercury, Venus, the Earth and Mars are known as the TERRESTRIAL PLANETS, but only because of their position in the inner Solar System, not because they are just like the Earth. The Moon, by comparison with […]
The Earth and the Moon throw long conical shadows into space and when one of these bodies moves into the shadow cast by the other there is an ECLIPSE. The shadow has two parts: the total shadow, called the UMBRA, and the partial shadow, termed the PENUMBRA. The Sun is totally obscured to an observer […]
The Moon is in orbit around the Earth, and, like a planet orbiting the Sun, its-motion is along an ellipse with semi-major axis 384 390km and eccentricity 0.0549. Because of the ellipticity of the orbit the distance-between the Earth and the Moon varies between a minimum of 356400km and a maximum of 406700km. The point […]
As the Earth rotates on its axis, the stars appear to move round the sky in the opposite direction. Since the Earth is travelling around the Sun, the Sun as viewed from Earth appears to move against the background of stars. Because of this it is possible to measure (or to define) the length of […]
The path of a planet in the sky as seen from the Earth results from a combination of the effects of the. orbital motion of both the Earth and the planet. The form of the observed motion depends primarily on whether the planet is nearer the Sun than the Earth (an INFERIOR PLANET), or further […]