Posts Tagged ‘basics’

Double Star

Double star – when you observe a star through a telescope or binoculars, it appears to be two stars as opposed to one when seen with the naked eye.

Sometimes this is a line of sight effect where the two stars are in reality totally unconnected with each other. In other cases the two stars are a genuine pair, orbiting one another, in which case they are referred to as a binary star.

Stars that are only connected by line of site are referred to as optical doubles or visual doubles.

Double Star Programme is also the designation of the fist ever collaboration between the Chinese and the European space agencies. It ran from early 2004 to late 2007. Two Chinese satellites were launched into orbits at 90 degrees to one another in order to study ‘global physical processes in Earth’s magnetic environment and its response to solar disturbances’ (esa). Working in conjunction with the existing Cluster satellites, Double Star Programme has msde several new discoveries, notably ‘Space is fizzy’ (density holes in the Solar wind in the region of the bow shock), Chorus emissions found at a greater distance (areas where high energy particles that can damage electronic equipment) and something called neutral sheet oscillations in the magnetotail.

Doppler Effect

The classic example is the change in tone of the noise of a vehicle engine as it approaches, passes and leaves the observer. As the vehicle approaches, more sound waves per second enter the ears of the observer. The sound appears to be higher pitched that if the vehicle and observer were stationary. This is because more sound waves per second = a higher frequency and higher frequency sound is heard as a higher pitch.

As the vehicle passes the observer and moves away, the engine tone is heard to drop. This is because slightly fewer waves per second enter the ear. Thus, the tone is heard to fall (lower frequency).

The Doppler effect was once used by the emergency services to help them to get through traffic. The first ‘sirens’ were bells, then we had the ‘me-ma’, a two fixed tone siren. Both of these were subject to the Doppler effect and drivers could recognise from which direction an emergency vehicle was approaching. Then some bright spark (read: complete and utter plonker) decided that American sirens were somehow better. These have a continuously changing tone so it is now impossible to tell the direction the emergency vehicle is approaching until it is really close. A brilliant way to make it more difficult to get through traffic and a perfect example of the detrimental way that adopting aspects of the US culture affects us here in the UK. It is as bad as those people who reply ‘I’m good’ when asked the question ‘How are you’ which grammatically makes no sense whatsoever.

Anyway, back to the Doppler effect in connection with astronomy. The same happens to all waves, including light waves and radio waves. In the case of light waves, higher frequencies are bluer and lower frequencies are redder. An object with a negative radial velocity (moving towards the Solar System) will be blue shifted and vice-versa will be red shifted. The amount of red-shift is used to determine the distance of distant galaxies. The Doppler effect causes emission and absorption lines of a spectrum to be shifted from their normal position. The faster an object approaches or receded, the further the lines will be displaced.


Dark Adapted

This is what every astronomer’s eyes should become before they begin observing. On leaving a brightly lit area and entering a dark area, you will notice a rapid increase in visibility over a short period of time. However, it takes about 20 – 30 minutes to become fully dark adapted. In low light levels, the chemicals in the eye increase the ability to see faint objects. See also the Purkinje effect

Counter Glow

The English name for Gegenschein. It is a faint oval patch of light that is very difficult to see, you need a clear and moonless night. It is at the antisolar point i.e. exactly opposite the Sun.

It is apparently best seen when the ecliptic is at its highest above the horizon (midwinter from the northern hemisphere and vice versa for the southern hemisphere). It is not well understood but is thought to be caused by the scattering of sunlight from dust in the main plane of the Solar System. Sometimes it can be seen to be joined to the zodiacal light by a parallel sided beam of light. I think that this beam is called the zodiacal band but cannot be 100% sure. I also believe that it is larger in the tropics than in the temperate zones. Click here to visit the NASA website and view a picture of the gegenschein.

Copernican System

Although this sounds like some far-flung solar system in a science fiction series, it isn’t! It is a description of how the Solar System is arranged.

The Copernican system is in fact the system proposed (in 1543) by Nicholas Copernicus in which the Sun is the central body, with the Earth and the other bodies moving around it. This model superseded the Ptolemaic system which had persisted for nearly 2000 years. Copernicus’ idea was not new, it had been proposed in about 300 BC by Aristarchus of Samos, a Greek philosopher. Both astronomers have Lunar craters named after them.


A pattern of stars named after an object, animal or mythical person. The stars in a constellation have no physical connection, they are a line of sight effect. Sometimes, people confuse the term asterism with constellation.


This is when a planet has the same longitude as the Sun. A planet can only be observed at conjunction if there is a total eclipse or if it transits the Sun. Unlike the other planets, Mercury and Venus have two types of conjunction, inferior when the planet lies between the Sun and the Earth, superior when it lies on the opposite side of the Sun to the Earth. At inferior conjunction, if the inclination of the orbit of Mercury or Venus coincide with that of the Earth, a transit is observed. These are rare phenomena.


A body of our Solar System. Comets are composed of rocks, dust and ices. This composition was first recognised by the astronomer Fred Whipple (Lawrence Frederick Whipple: November 5, 1906 – August 30, 2004) who described them as an ‘icy conglomerate’, translated for the press as being like a ‘dirty snowball’. As they approach the Sun, the ices begin to evaporate forming a coma and one or more tails. The particles frozen in with the ices are carried off as the ices blast their way off the surface and it is the Sun’s light reflecting off these that enable the tail to be seen. The dust tail is often curved whilst the second tail, if present, is straight  and usually much fainter; it it made up from plasma – charged particles of gas.

Long period comets are believed to come from the Oort cloud and possibly contain material from the earliest days of the Solar System.

Short period comets are believed to originate from the Kuiper Belt.

The first spacecraft to visit a comet was the European Space Agency’s firt deep space mission – Giotto. This was launched to take a closer look at Halley’s comet. It collected samples from the tail and managed to take some images of the nucleus, showing clearly that the gas and dust erupted from the surface of comets as jets. Despite being damaged, the craft survived to visit a second comet – Grigg-Skjellerup.



A cluster is a group of stars whose members are genuinely associated. A cluster of stars is formed from the same gas/dust cloud. There are two main types: open and globular.

Cluster is also the name given to a group of four spacecraft that have been placed in Earth orbit in order to study the interactions between particles ejected from the Sun and the Earth’s magnetic field. The first attempt to put Cluster into space in 1996 failed when the Ariane rocket exploded. Oops.

Circumpolar Stars

Cirumpolar stars lie within a region of the sky that is always visible round the celestial pole closest to the observer. An object in this area will therefore never set, at any time of the night (or day of course) and can be observed at any time of the year e.g. the Plough asterism is in the circumpolar region from the UK and can be seen in all four seasons, Orion is not circumpolar and so can only be observed for part of the year.

If you want to know if a star will be circumpolar, you need to know your latitude. Subtract that from 90 and all stars within that distance from the pole will be circumpolar.

On the other hand, all stars within that distance from the opposite pole will never rise at your latitude.

Sun Grazer

A description applied to a comet that will inevitably disappoint the astronomical world – it is a synonym of ‘the comet of the century’! A Sun Grazer comet is one which passes close to the Sun and therefore offers great potential to become extremely bright with a massive tail. The problem is that as a sun grazer approaches perihelion, the temperatures and gravitational pull will destroy it. Every once in a while, a Sun grazer actually survives and dos indeed become a ‘comet of the century’.

Sun grazers were recognised in the 18th century by the astronomer Heinrich Kreutz working in the last two decades of the 19th century. He noticed that the sun grazers had very similar orbits and postulated they came from a large one that broke up. Very few sun grazers have been discovered from the surface of the Earth. Since the end of the 1970s, satellites using coronographs have been used to discover large numbers of this type of object. In fact, well over 700 have been discovered in thae last 30 years, about 500 being attributed to the ‘Kreutz Group’ of comets, following the orbit he determined all those years ago. The remaining comets seem to belong to 3 separate groups but those of the Kreutz group are approach the Sun much closer than the others. That is probably the reason he saw them as they become brighter before meeting their evaporative end.

An astrophysics chappie, probably an American I guess, has said that a big sun grazer could create a massive solar flare that knocks out electronics on earth.


Cassini Division

The principal division in Saturn’s ring system, separating ring A from ring B.

Blue Shift

If an astronomical body is moving towards the observer, the light will seem to be shifted to the blue end of the spectrum. The faster the movement, the greater the blue shift. It occurs because the wavelength of light is slightly compressed by the Doppler effect as the body moves towards the observer. Blue shift is measured by looking at the key spectral lines. For an object moving towards the Solar System, they will appear closer to the blue end than normal. The faster the object is approaching, the greater the blue shift will be.

Black Body

A body that absorbs all the radiation that it receives, giving it an albedo (reflecting power) of zero i.e. it is 100% efficient at absorbing radiation.  A black body also radiates with 100% efficiency, it is therefore a theoretical object although many stars come very close to this theoretical position.

The Aurora

glow in the northern (aurora borealis) or the southern (aurora australis) sky caused by particles from the solar wind interacting with the atmosphere. The particles are held in the Van Allen belts. When there is a solar flare, there is a huge increase in the density of the Solar wind. This overloads the Van Allen belts and causes high energy particles to spill into the atmosphere. As these interact with the gases of the atmosphere, light of different wavelengths is emitted and we see this as an aurora.

Book: The nature of the Aurora was worked out by the Norwegian Scientist Kristian Berkeland. I was riveted by the book Northern Lights, an account of how the mysteries of the Aurora were unravelled and learn about the man who invented a weapon developed in the USA’s SDI (Star Wars) early in the twentieth century.

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