We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

What Is the Aberration of Light?

By Phil Riddel
Updated May 21, 2024
Our promise to you
All The Science is dedicated to creating trustworthy, high-quality content that always prioritizes transparency, integrity, and inclusivity above all else. Our ensure that our content creation and review process includes rigorous fact-checking, evidence-based, and continual updates to ensure accuracy and reliability.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

Editorial Standards

At All The Science, we are committed to creating content that you can trust. Our editorial process is designed to ensure that every piece of content we publish is accurate, reliable, and informative.

Our team of experienced writers and editors follows a strict set of guidelines to ensure the highest quality content. We conduct thorough research, fact-check all information, and rely on credible sources to back up our claims. Our content is reviewed by subject-matter experts to ensure accuracy and clarity.

We believe in transparency and maintain editorial independence from our advertisers. Our team does not receive direct compensation from advertisers, allowing us to create unbiased content that prioritizes your interests.

In astronomy, the aberration of light is a shift in the apparent position of an object caused by the relative motion of the object and the observer. Aberration of light is only significant at very large scales and affects the perceived positions of stars and planets for observers on the Earth. The apparent displacement of stars results from the Earth’s motion around the Sun, and from its rotation.

Aberration of light was discovered in the 17th century, when attempts were made to measure the distances from the Earth to various stars using parallax — a concept that describes how an object's position appears to shift when observed from different locations. The idea was that the apparent position of a star 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 thought sufficient to obtain a parallax value and thus calculate the star’s distance using trigonometry.

A number of measurements were made, but the results were puzzling. The greatest apparent displacement of the star being observed should have been found between observations six months apart, when the locations of the observations were furthest apart. The actual displacements, however, followed a completely different pattern and were clearly not due to parallax. The Pole Star, Polaris, for example, was found to follow a roughly circular path, with a diameter of about 40 arc seconds (40”), an arc second being 1/3,600 of a degree. Parallax displacement does occur, but it is very small, even for the nearest stars, and would not have been measureable using the instruments available at that 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 Earth’s velocity, and not to its position, relative to the star. The light from the star takes time to reach the Earth and because the Earth is moving, the starlight appears to come from a point that is displaced slightly from the star’s true position, in the direction of motion. The biggest displacements are observed when the Earth’s motion is perpendicular to the direction of the starlight. The same phenomenon can be seen with rain falling vertically; to a moving observer — for example, in a train or bus — the rain seems 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” to 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 need 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 annual aberration or stellar aberration, and the star’s true position is called its geometric position. Smaller displacements result from the Earth’s rotation; this is known as diurnal aberration. Secular aberration is the term used to describe the astronomical aberration caused by the solar system’s motion within the galaxy; although it has an effect on the apparent positions of very distant stars and other galaxies, it is very small and is not usually taken into account. In calculating stellar aberration, only the Earth’s motion need 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 need to be included to calculate the correct value.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.