Posts Tagged ‘movement’

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.


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.

Astronomical Unit

The mean (average) distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU). It is a convenient way of describing distances within our Solar System.

Nominally, 93 million miles or 150 million km. It is therefore a lot more convenient to use but even this mind-bogglingly enormous distance is inadequate to express the distance to galaxies so that is when parsecs are used.


The branch of astronomy dealing with the movements and positions of celestial bodies.

Astrometry dates back to the earliest days of astronomy when the first star catalogues were being produced, e.g. that of Hipparchus in 190BC. Early measurements were probably made using cross-staffs to measure the relative positions of stars from one-another and from features on the horizon. As time progressed, more sophisticated instruments were used, such as the astrolabe.

The same principles are still used in modern astronomy but have become very precise and can measure the wobbles in the movements of stars that could indicate the presence of extrasolar planets or to find astrometric binary star systems.

Astrometry also includes the measurement of parallax. If you observe an object from two widely spaced locations, you will measure a slight difference in position. From the annular differences in position and knowing the distance between the two locations of observation, the distance of stars can be determined. That was taken to a new level with the Hipparcos satellite launched by the ESA. Early parallax measurements were limited to relatively close stars, however, the distances of several cepheid variable stars was measured. Thes can then be used as ‘standard candles’ when seen in distant galaxies to give a reasonable estimate of their distances. Further refinement to astrometric determination of distant objects came with the advent of interferometry.

As with all measurements of the extremely large and extremely small, astrometric measurements require careful error correction. As knowledge and instrumentation improves, distances and speeds of star movements are constantly being refined and it is believed that it is now possible to see the peturbations in stellar motion caused by planets not much more massive than the Earth.


Aberration of light is the apparent displacement of a star from it’s true position in the sky. It is caused by a combination of the motion of the Earth in orbit round the Sun (about 30 km per sec) and the finite velocity of light (299,792.5 km per sec or , if you prefer imperial units, 186,252.5 miles per second). The rotation of the Earth also gives rise to the aberration of starlight.

To understand aberration, we need to start off with a simple easy to understand example from the familiar world around us. Imagine you are in a parked car and you look out of the window and the falling rain. Imagine that there is no wind so the rain is falling vertically. As the driver pulls away and picks up speed to say 30mph, you notice that the rain is no longer falling vertically. Actually it is, but you are moving forwards, past the raindrops, thus greating the illusion that the rain is falling diagonally, slanting towards the back of the car.

OK, so back to the starlight. The Earth is moving forwards through space and, despite the high speed at which light travels, the starlight we see effectively is slanting backwards compared to the direction of movement of the Earth in its orbit. But the Earth moves in an ellipse round the sun so the direction of ‘slant’ of the light changes too. The net result is that if the precise position of a star is recorded throughout the year, it will be seen to describe a small ellipse around its ‘true’ position … the ‘true’ position being where the star would have been seen had the Earth been stationary.

There is also a very much smaller daily effect caused by the rotation of the Earth. This is called diurnal aberration.

The maximum displacement is 20.5 seconds of arc. This number is called the constant of aberration.  For a much more thorough treatment, including a discussion of relativity and aberation, click here.

Switch to our mobile site