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 […]
The main bodies of the Solar System are the bun and, in orbit round it, the nine major planets. In order of increasing distance from the Sun these are Mercury, Venus, Earth, Mars, Jupiter, Saturn (all known since antiquity), Uranus, Neptune and Pluto. Sir William Herschel discovered Uranus in 1781 during a review of the […]
Centrifugal forces due to the rotation of the Sun may cause the solar wind and the interplanetary magnetic field to vary away from the solar equatorial plane. Consequently, space probes that journey out of the ecliptic are important for filling in the details that cannot be provided by the usual interplanetary missions. The motion of […]
The AURORAE BOREALIS and AUSTRALIS have been associated with magnetic activity since the eighteenth century. Most of the light in these displays is produced by excitation and ionization of the upper atmosphere at high latitudes by an influx of electrons with energies about l0keV. Disturbances of the magnetic field con¬figuration within the magnetotail energize these […]
SOLAR STORMS, described above, are spectacular outbursts due to the sudden transformation of a considerable amount of energy (about 1026 joules) into electromagnetic and charged-particle radiation within an hour or so. A typical event is a solar flare, which, apart from the burst of radio, optical and X-radiation, releases electrons and protons with energies that […]
The absorption of solar Lyman-a (121.6 nm) and X-radiation by the upper atmosphere of the Earth results in the ionized regions known as the ionosphere. The ions and electrons are here present to such an extent that radio-wave propagation is seriously affected and indeed reflection occurs at long wavelengths. The D-REGION lying between about 60 […]
The continuous expansive motion of the coronal gas gives rise to the SOLAR WIND. At the distance of the Earth its velocity is normally about 400 km s”1, the particle density about 107nr3, and the temperature approximately 105 K. Even though the total outflow of material from the Sun is about one million tonnes per […]