The right ascension and declination are defined relative to the plane of the Earth’s equator and to the vernal equinox. Unfortunately, neither of these is fixed in the sky because of precession. The Earth is not quite spherical but is slightly oblate. Because of this, the gravitational force between the Earth and the Sun produces […]
The plane of reference of this system is the equator of the Earth and the poles are at the points where the rotation axis of the Earth cuts the celestial sphere. The poles are called the NORTH AND SOUTH CELESTIAL POLES. The equatorial plane of the Earth cuts the celestial sphere at the CELESTIAL EQUATOR. […]
In this system the plane of reference is not the horizon but is a plane through the observing point parallel to the horizon. (If the observatory is at the top of a mountain, the horizon and the horizon plane defined here are different.) The line perpendicular to this plane, through the observer, passes through the […]
The accurate positions frequently needed in astronomy must be measured relative to a standard reference frame. To understand the coordinate systems used in astronomy, it is helpful to review first the familiar latitude-longitude system. It is convenient to measure positions on Earth in terms of LATITUDE and LONGITUDE. These two coordinates are determined relative to […]
Time It often happens that an astronomer needs to measure the precise position of an object in the sky. Accurate positions are required for many astronomical investigations. To determine proper motions, for example, positions measured over several months or years are needed. It is frequently necessary to compare the position of optically-visible objects with that […]
The theory of stellar structure and evolution gives a fairly good account of the observed properties of stars. Part of the theory is concerned with the generation of nuclear energy inside stars, and it appears that during nuclear fusion processes a substantial flux of NEUTRINOS is produced. Neutrinos have no electrical charge and scarcely any […]
When an electrical charge is accelerated in a straight line or moves around an arc of a circle, the charge radiates an electromagnetic wave, similarly, when a massive object is accelerated general relativity predicts that it will radiate a GRAVITATIONAL WAVE. Although there is no celestial object we can point to as one that does […]
Materials such as silicon and germanium are known as SEMI¬CONDUCTORS. The electrons in these materials are fairly tightly bound to the atoms of which they are composed. Because there are few free electrons in these materials, only a little current flows when an electric field is applied across a specimen of the material. However, photons […]
There is a group of substances that will absorb photons and subsequently emit the electrons excited by them. These materials are said to be PHOTOEMISSIVE and they are used in PHOTOELECTRIC DETECTORS. The emitted electrons are called PHOTOELECTRONS. Materials such as caesium antimonide and gallium arsenide are good photoemitters and can have an efficiency as […]
The most frequently used light detector is still the PHOTOGRAPHIC PLATE. It is easy to get the incorrect impression that the advent of the many modern detectors described below has made the photo¬graphic plate obsolete. The quality and simplicity of use of the modern photographic plate ensures that it will be a long time before […]