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Light aberration is a shift in the apparent position of an object caused by the relative motion of the object and observer. The aberration of light affects the perceived positions of stars and planets for observers on Earth. James Bradley discovered that the observed shifts in the position of a star were due to the speed of the Earth and not its position relative to the star. The seasonal shifts in star positions are known as the annual aberration or stellar aberration.
In astronomy, light aberration is a shift in the apparent position of an object caused by the relative motion of the object and the observer. The aberration of light is only significant on very large scales and affects the perceived positions of stars and planets for observers on Earth. The apparent displacement of the stars results from the motion of the Earth around the Sun and its rotation.
The aberration of light was discovered in the 17th century, when attempts were made to measure distances from Earth to various stars using parallax, a concept that describes how an object’s position appears to change when viewed from different locations. The idea was that a star’s apparent position should change throughout the year as the Earth orbits the Sun. If the star’s exact position in the sky was checked on a given date, then checked again six months later, when the Earth was opposite its position from when the first measurement was taken, this gave two measurements separated by the diameter of the Earth’s orbit – a distance of approximately 186,000,000 miles (300,000,000 km). This was deemed sufficient to obtain a parallax value and then calculate the star’s distance using trigonometry.
A series of measurements were made, but the results were staggering. The largest apparent displacement of the observed star should have been found between observations six months apart, when the positions of the observations were farthest apart. The actual displacements, however, followed an entirely different pattern and were clearly not due to parallax. The North Star, Polaris, for example, has been found to follow a roughly circular path, with a diameter of about 40 arcseconds (40”), one arcsecond being 1/3,600th of a degree. Parallax shift occurs, but it is very small, even for the closest stars, and would not have been measurable using the instruments available at the time.
The mystery was solved by James Bradley, the British Astronomer Royal, in 1729. He discovered that the observed shifts in the position of a star were due to the speed of the Earth and not its position relative to the star. The light from the star takes time to reach the Earth, and since the Earth is moving, the starlight appears to come from a point slightly off the star’s actual position, in the direction of motion. The greatest displacements are observed when the motion of the Earth is perpendicular to the direction of the starlight. The same phenomenon can be observed with vertically falling rain; to a moving observer, such as in a train or bus, the rain appears to fall diagonally from a point of origin ahead of the observer in the direction of motion.
Bradley’s calculation, using the speed of light and the speed of the Earth’s motion around the sun, indicated a maximum displacement of about 20 inches either side of the true position for Polaris. This gave an overall variation of about 40” over the year, in agreement with observations. In calculating the aberration of light, modern astronomers have to take into account the effects of relativity, but in most cases the classical calculation is adequate.
The seasonal shifts in star positions are known as the annual aberration or stellar aberration, and the true position of the star is called its geometric position. Smaller displacements result from the rotation of the Earth; this is known as diurnal aberration. Secular aberration is the term used to describe the astronomical aberration caused by the movement of the solar system within the galaxy; while it has an effect on the apparent positions of very distant stars and other galaxies, it is very small and is usually not taken into account. In calculating stellar aberration, only the motion of the Earth needs to be considered; however, planetary aberration – which affects the apparent positions of the planets – results from the motion of both the Earth and the planets, so both must be included to calculate the correct value.
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