Encyclopedia
The
aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects. It was discovered and later explained by the third Astronomer Royal,
James Bradley, in 1725, who attributed it to the finite
speed of light and the motion of
Earth in its orbit around the
Sun.
At the instant of any observation of an object, the apparent position of the object is displaced from its true position by an amount which depends upon the velocity of the observer relative to an inertial frame of reference. In the case of an observer on Earth, the direction of its velocity varies during the year as Earth revolves around the Sun , and this in turn causes the apparent position of the object to vary. This particular effect is known as
annual aberration or
stellar aberration, because it causes the apparent position of a star to vary periodically over the course of a year. The maximum amount of the aberrational displacement of a star is approximately 20 arcseconds in
right ascension or
declination. Although this is a relatively small value, it was well within the observational capability of the instruments available in the early eighteenth century.
Aberration should not be confused with
stellar parallax, although it was an initially fruitless search for parallax that first led to its discovery. Parallax is caused by a change in the position of the observer looking at a relatively nearby object, as measured against more distant objects, and is therefore dependent upon the distance between the observer and the object. In contrast, stellar aberration is
independent of the distance of a celestial object from the observer, and depends only on the observer's
instantaneous velocity at the moment of observation, not that this velocity will carry the observer to a different position over time.
Aberration should also be distinguished from light-time correction, which is due to the motion of the observed object, like a
planet, through space during the time taken by its light to reach an observer on Earth. Light-time correction depends upon the motion and distance of the object, but is independent of the velocity of Earth.
Explanation
It has been stated above that aberration causes a displacement of the apparent position of an object from its true position. However, it is important to understand the precise technical definition of these terms.
Apparent and true positions
The
apparent position of a star or other very distant object is the direction in which it is seen by an observer on the moving Earth. The
true position is the direction of the straight line between the observer and star at the instant of observation. The difference between these two positions is caused by aberration.
Aberration occurs when the observer's velocity has a component that is
perpendicular to a line between the star and observer. In Figure 1 to the right,
S represents the position of the star, and
E the position of the observer on Earth. The true direction of the star relative to the observer is thus
ES, whose length represents the speed of light. However, Earth has a velocity in the direction represented by the line
EE’, whose length represents that velocity. The
Law of Aberration states that the star will therefore appear to lie in the direction
ES’, instead of
ES, where
SS’ is parallel and equal in length to
EE’. The star's apparent position is hence displaced from its true position by the angle
SES’.
Moving in the rain
Many find aberration to be counter-intuitive, and a simple thought experiment based on everyday experience can help in its understanding. Imagine you are standing in the rain. There is no wind, so the rain is falling vertically. To protect yourself from the rain you hold an umbrella directly above you.
Now imagine that you start to walk. Although the rain is still falling vertically , you find that you have to hold the umbrella slightly in front of you to keep off the rain. Because of your forward motion relative to the falling rain, the rain now appears to be falling not from directly above you, but from a point in the sky somewhat in front of you.
The deflection of the falling rain is greatly increased at higher speeds. When you drive a car at night through falling rain, the rain drops illuminated by your car's
headlights appear to fall from a position in the sky well in front of your car.
Types of aberration
There are a number of types of aberration, caused by the differing components of the Earth's motion:
- Annual aberration is due to the revolution of the Earth around the Sun
|-
...
.
- Planetary aberration is the combination of aberration and light-time correction.
- Diurnal aberration is due to the rotation of the Earth about its own axis.
- Secular aberration is due to the motion of the Sun and solar system relative to other stars in the galaxy.
Annual aberration
As the Earth revolves around the Sun, it is moving at a velocity of approximately 30 km/s. The speed of light is approximately 300,000 km/s. In the special case where the earth is moving perpendicularly to the direction of the star , the angle of displacement,
SES’, would therefore be the ratio of the two velocities, i.e. 1/10000 or about 20.5 arcseconds.
This quantity is known as the
constant of aberration, and is conventionally represented by
?. Its precise accepted value is 20".49552 .
The plane of the Earth's orbit is known as the
ecliptic. Annual aberration causes stars exactly on the ecliptic to appear to move back and forth along a straight line, varying by
? either side of their true position. A star that is precisely at one of the ecliptic poles will appear to move in a circle of radius
? about its true position, and stars at intermediate ecliptic latitudes will appear to move along a small
ellipse .
A special case of annual aberration is the nearly constant deflection of the Sun from its true position by
? towards the
west , opposite to the apparent motion of the Sun along the ecliptic. This constant deflection is often erroneously explained as due to the motion of the Earth during the 8.3 minutes that it takes light to travel from the Sun to Earth. The latter is a type of
parallax, and actually causes the apparent motion of the Sun along the ecliptic towards the
east relative to the fixed stars. Nor is this the Sun's light-time correction because the Sun is almost motionless, moving around the
barycenter of the solar system by usually much less than 0".03 during 8.3 minutes.
Aberration can be resolved into an east-west and north-south component on the
celestial sphere, which therefore produce an apparent displacement of a star's
right ascension and
declination, respectively. The former is larger , but the latter was the first to be detected. This is because very accurate clocks are needed to measure such a small variation in right ascension, but a transit telescope calibrated with a plumb line can detect very small changes in declination.
Figure 3, above, shows how aberration affects the apparent declination of a star at the north ecliptic pole, as seen by an imaginary observer who sees the star transit at the zenith . At the time of the March
equinox, the Earth's orbital velocity is carrying the observer directly south as he or she observes the star at the zenith. The star's apparent declination is therefore displaced to the south by a value equal to
?. Conversely, at the September equinox, the Earth's orbital velocity is carrying the observer nothwards, and the star's position is displaced to the north by an equal and opposite amount. At the June and December
solstices, the displacement is zero.
Note that the effect of aberration is out of phase with any displacement due to parallax. If the latter effect were present, the maximum displacement to the south would occur in December, and the maximum displacement to the north in June. It is this apparently anomalous motion that so mystified Bradley and his contemporaries.
Planetary aberration
Planetary aberration is the combination of the aberration of light and light-time correction . Both are determined at the instant when the moving object's light reaches the moving observer on Earth. It is so called because it is usually applied to planets and other objects in the solar system whose motion and distance are accurately known.
Diurnal aberration
Diurnal aberration is caused by the velocity of the observer on the surface of the rotating Earth. It is therefore dependent not only on the time of the observation, but also the
latitude and longitude of the observer. Its effect is much smaller than that of annual aberration, and is only 0".32 in the case of an observer at the equator, where the rotational velocity is greatest.
Secular aberration
The sun and solar system are revolving around the center of the galaxy, as are other nearby stars. It is therefore possible to conceive of an aberrational effect on the apparent positions of other stars and on extragalactic objects. However, the change in the solar system's velocity relative to the center of the galaxy varies over a very long timescale, and the consequent change in aberration would be extremely difficult to observe. Therefore, this so-called
secular aberration is usually ignored when considering the positions of stars.
However, it is possible to estimate the displacement between the apparent and true position of a nearby star whose distance and motion are known. Newcomb gives the example of 1830 Groombridge, where he estimates that the true position is displaced by approximately 3 arcminutes from the direction in which we observe it. This calculation also includes an allowance for light-time correction, and is therefore analogous to the concept of planetary aberration.
Historical background
The discovery of the aberration of light in 1725 by
James Bradley was one of the most important in astronomy. It was totally unexpected, and it was only by extraordinary perseverance and perspicuity that Bradley was able to explain it in 1727. Its origin is based on attempts made to discover whether the stars possessed appreciable
parallaxes. The
Copernican theory of the
solar system – that the Earth revolved annually about the Sun – had received confirmation by the observations of
Galileo and
Tycho Brahe , and the mathematical investigations of
Kepler and
Newton.
Search for stellar parallax
As early as 1573,
Thomas Digges had suggested that this theory should necessitate a parallactic shifting of the stars, and, consequently, if such stellar parallaxes existed, then the Copernican theory would receive additional confirmation. Many observers claimed to have determined such parallaxes, but Tycho Brahe and Giovanni Battista Riccioli concluded that they existed only in the minds of the observers, and were due to instrumental and personal errors. In 1680
Jean Picard, in his
Voyage d'Uranibourg, stated, as a result of ten years' observations, that
Polaris, or the
Pole Star, exhibited variations in its position amounting to 40" annually. Some astronomers endeavoured to explain this by parallax, but these attempts were futile, for the motion was at variance with that which parallax would produce.
John Flamsteed, from measurements made in 1689 and succeeding years with his mural quadrant, similarly concluded that the declination of the Pole Star was 40" less in July than in September.
Robert Hooke, in 1674, published his observations of ? Draconis, a star of magnitude 2
m which passes practically overhead at the latitude of
London, and whose observations are therefore free from the complex corrections due to astronomical
refraction, and concluded that this star was 23" more northerly in July than in October.
Bradley's observations
When James Bradley and Samuel Molyneux entered this sphere of astronomical research in 1725, there consequently prevailed much uncertainty whether stellar parallaxes had been observed or not; and it was with the intention of definitely answering this question that these astronomers erected a large telescope at the house of the latter at Kew. They determined to reinvestigate the motion of ? Draconis; the telescope, constructed by George Graham , a celebrated instrument-maker, was affixed to a vertical chimney stack, in such manner as to permit a small oscillation of the eyepiece, the amount of which was regulated and measured by the introduction of a screw and a plumb line.
The instrument was set up in November 1725, and observations on ? Draconis were made on the 3rd, 5th, 11th, and 12th of December. There was apparently no shifting of the star, which was therefore thought to be at its most southerly point. On December 17, however, Bradley observed that the star was moving southwards, a motion further shown by observations on the 20th. These results were unexpected and inexplicable by existing theories. However, an examination of the telescope showed that the observed anomalies were not due to instrumental errors.
The observations were continued, and the star was seen to continue its southerly course until March, when it took up a position some 20" more southerly than its December position. After March it began to pass northwards, a motion quite apparent by the middle of April; in June it passed at the same distance from the zenith as it did in December; and in September it passed through its most northerly position, the extreme range from north to south, i.e. the angle between the March and September positions, being 40".
Aberration vs nutation
This motion was evidently not due to parallax, for the reasons given in the discussion of Figure 2, and neither was it due to observational errors. Bradley and Molyneux discussed several hypotheses in the hope of finding the solution. The idea that immediately suggested itself was that the star's declination varied because of short-term changes in the orientation of the Earth's axis relative to the celestial sphere – a phenomenon known as
nutation. Because this is a change to the observer's frame of reference , it would therefore affect all stars equally. For instance, a change in the declination of ? Draconis would be mirrored by an equal and opposite change to the declination of a star 180 degrees opposite in right ascension.
Observations of such a star were made difficult by the limited field of view of Bradley and Molyneux's telescope, and the lack of suitable stars of sufficient brightness. One such star, however, with a right ascension nearly equal to that of ? Draconis, but in the opposite sense, was selected and kept under observation. This star was seen to possess an apparent motion similar to that which would be a consequence of the nutation of the Earth's axis; but since its declination varied only one half as much as in the case of ? Draconis, it was obvious that nutation did not supply the requisite solution. Whether the motion was due to an irregular distribution of the
Earth's atmosphere, thus involving abnormal variations in the refractive index, was also investigated; here, again, negative results were obtained.
On August 19, 1727, Bradley then embarked upon a further series of observations using a telescope of his own erected at the Rectory, Wanstead. This instrument had the advantage of a larger field of view and he was able to obtain precise positions of a large number of stars that transited close to the zenith over the course of about two years. This established the existence of the phenomenon of aberration beyond all doubt, and also allowed Bradley to formulate a set of rules that would allow the calculation of the effect on any given star at a specified date. However, he was no closer to finding an explanation of why aberration occurred.
Development of the theory of aberration
Bradley eventually developed the explanation of aberration in about September 1728 and his theory was presented to the
Royal Society a year later. One well-known story was that he saw the change of direction of a wind vane on a boat on the
Thames, caused not by an alteration of the wind itself, but by a change of course of the boat relative to the wind direction. However, there is no record of this incident in Bradley's own account of the discovery, and it may therefore be apocryphal.
The discovery and elucidation of aberration is now regarded as a classic case of the application of
scientific method, in which observations are made to test a theory, but unexpected results are sometimes obtained that in turn lead to new discoveries. It is also worth noting that part of the original motivation of the search for stellar parallax was to test the Copernican theory that the Earth revolves around the Sun, but of course the existence of aberration also establishes the truth of that theory.
In a final twist, Bradley later went on to discover the existence of the nutation of the Earth's axis – the effect that he had originally considered to be the cause of aberration.
References
- P. Kenneth Seidelmann , Explanatory Supplement to the Astronomical Almanac , 127-135, 700.
- Simon Newcomb, A Compendium of Spherical Astronomy , 160-172.
- Arthur Berry, A Short History of Astronomy , 258-265.
- S. Rigaud, Memoirs of Bradley
- Charles Hutton, Mathematical and Philosophical Dictionary .
- H. H. Turner, Astronomical Discovery .
See also
