Posts Tagged ‘observation’

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.



The angle north or south of the celestial equator to a star. It is one  of the two co-ordinates used to describe the position of an astronomical object on the so-called celestial sphere. Together with Right Ascension, declination describes where in the sky an object can be found.

GoTo telescopes are programmed with thousands of celestial co-ordinates and make it extremely easy to find an object. But where is the satisfaction in that? For an amateur astronomer, finding a faint object for yourself is extremely satisfying, users of telescopes with the RA and declination pre-programmed are missing out on half of the fun and can only really be called stargazers, not astronomy enthusiasts. Sadly, the marketing machine of telescope manufacturers suggests otherwise – they just want to get as much of your cash as they can. Cynical, aren’t I?

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


The maximum altitude of a celestial body above the horizon. In other words, when an astronomical body transits the meridian.

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 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.



There are several astronomical meanings. It can be used to describe the hazy looking patch that surrounds the nucleus of a comet or the blurred effect surrounding the images of stars on a photographic plate, or in the observers field of view in a telescope (or binoculars) due to defects in the lenses.

Colour Index

A measure of a star’s colour, which helps astronomers to tell its surface temperature. It is the difference between the magnitude of a star measured in two different areas of the spectrum. The areas are B (blue), V (violet) and U (ultraviolet) regions. The B-V is the most common index used and is close to zero for a white star. It is extremely useful in the classification of stars, it can tell astronomers if the star is a main sequence star, a giant star or a supergiant star.

The intrinsic colour index is a modified form that has been corrected for interstellar extinction.


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.

Chromatic Aberration

A defect of a lens that creates a fringe of colour round an object. It is caused by the fact that different wavelengths of light are refracted by different amounts.

Cassini Division

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

Bok Globules

The Dutch astronomer Bart Bok first drew attention to these small black objects. They appear in gaseous emission nebulae and are thought to be protostars that are still forming but have not yet become hot enough to shine.

Black Drop

An appearance seen at the end of second contact and at the start of third contact of a transit of Venus. As the planet moves across the Sun’s disc it seems to draw a strip of blackness after it.  It makes measurement of the exact time of these contacts difficult to measure accurately.

The black drop has been known since telescopic observation of the transits of Venus and Mercury began with several explanations put forward. It now appears, according to the AAS, that is is a combination of the Sun’s limb darkening and the telescope that produces this effect.


Binary Star

double star in which the components orbit one another. Some binary systems orbit so close to one-another the two stars are distorted by each other’s gravity.

If the two stars eclipse one-another when seen from the Earth, the light changes regularly and predictably over a period of time. This type of binary is an eclipsing binary and is a type of variable star.

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