Posts Tagged ‘measurement’

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.

 

Declination

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?

Culmination

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

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.

Azimuth

The angle measured from the south point of the horizon toward the west to a point at the foot of a star’s vertical circle. When used as an indication of the position of a star on the imaginary celestial sphere it is referred to as Right Ascension.

Astrometry

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.

Alidade

An alidade is a sighting device that is used to measure angles.

The earliest alidades were simply long sticks with sighting slots. They enabled early astronomers to measure the relative angles between stars and thus to produce the earliest star maps.

Over the years they became more and more sophisticated and incorporated small telescopes, for example, as used in the modern theodolite by terrestrial surveyors.

Angular Distance

There are many ways to measure angular distance using your body …

The apparent distance between two celestial objects. It is measured in degrees, arcminutes (an arcminute is a 60th of a degree) and arcseconds (an arcsecond is a 60th of an arcminute). On average, the distance from your thumb tip to the tip of your little finger of your outstretched hand at arms length is 20 degrees. The width of your palm will be about 12 degrees and the width of the tip of your little finger is about 1 degree. The angular diameter of the Moon (and the Sun) is more or less 1/2 degree.

Angular Distance

Angular distance between two astronomical objects

An observer looks at two different objects. The angle between them can be measured e.g. by using a cross staff. This angle is the angular distance of the two objects in the sky. It is expressed in degrees, arcminutes and arcseconds.

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