One of the very best things about science is that the discipline is self-correcting. A scientist makes a set of observations about nature, and then devises a theory to fit those observations.

Other scientists then test the theory, and if it withstands scrutiny it becomes widely accepted. At any point in the future, if contravening evidence emerges, the original theory is discarded. At its essence, and though in practice it’s more messy, this is how science works.

Needless to say there have been a lot of theories discarded along the way. The following represents my best efforts to select the 10 most spectacularly wrong scientific theories.

To qualify for the list, a large number of scientists at any given time must have subscribed to the particular theory before it was eventually discarded. Thus a long list of pseudoscientific ideas, crackpot though they might be, didn’t make the list.

1. Geocentric universe: The concept that the Earth was at the center of the universe dates back to at least 600 B.C. with Greek philosophers who proposed cosmologies of the Sun, Moon and other heavenly bodies orbiting the Earth. The most famous contortion of the system was Ptolemy’s epicycles to explain the retrograde motion of Mars. This is a prime example of fitting scientific evidence into preconceived notions. The theory was disproven with the publication of Nicholas Copernicus’ De revolutionibus orbium coelestium in 1543.

2. Miasmatic theory of disease: This theory holds that diseases such as cholera, chlamydia or the Black Death were caused by a miasma (ancient Greek: “pollution”), a noxious form of “bad air”. This concept was not disposed of until the late 1800s, with the rise of the germ theory of disease. Miasma was considered to be a poisonous vapor or mist filled with particles from decomposed matter that caused illnesses. It was identifiable by its foul smell.

3. Luminiferous aether: Assumed to exist for much of the 19th century, the theory held that a “medium” of aether pervaded the universe through which light could propagate. The celebrated Michelson-Morley experiment in 1887 was the first to provide hard evidence that aether did not exist, and the theory lost all popularity among scientists by the

1920s. A photo of the aether appears below.

4. Stress theory of ulcers: As peptic ulcers became more common in the 20th century, doctors increasingly linked them to the stress of modern life. Medical advice during the latter half of the 20th century was, essentially, for patients to take antacids and modify their lifestyle. In the 1980s Australian clinical researcher Barry Marshal discovered that the bacterium H. pylori caused peptic ulcer disease, leading him to win a Nobel Prize in 2005.

5. Immovable continents: Prior to the middle of the 20th century scientists believed the Earth’s continents were stable and did not move. This began to change in 1912 with Alfred Wegener’s formulation of the continental drift theory, and later and more properly the elucidation of plate tectonics during the 1950s and 1960s.

6. Phlogiston: Arising in the mid-17th century, physicians conjured up the existence of a fire-like element called “phlogiston”, which was contained within combustible bodies and released during combustion. Charcoal, for example, left little residue upon burning because it is nearly pure phlogiston. Experiments in the mid-1700s led chemists to conclude the theory was false, giving birth to the field of modern chemistry.

Phlogiston in nearly its most pure form?

7. The “four humours” theory of human physiology: From Hippocrates onward, the humoral theory was adopted by Greek, Roman and Islamic physicians, and became the most commonly held view of the human body among European physicians until the advent of modern medical research in the 19th century. The four humours of Hippocratic medicine were black bile, yellow bile, phlegm and blood.

8. Static universe: Prior to the observations made by astronomer Edwin Hubble during 1920s, scientists believed the universe was static, neither expanding nor contracting. Hubble found that distant objects in the universe were moving more quickly away than nearby ones. Very recently, in 1999, scientists unexpectedly found that not only was the universe expanding, but its expansion was accelerating.

9. A young Earth: In the mid-1800s many scientists, including Lord Kelvin, believed the Earth to be just 20 million to 40 million years old. It was around that time that geologists such as Charles Lyell began to believe that the Earth was much older, and this conformed to the views of biologists such as Charles Darwin, who needed a much older Earth for evolution to unfold. It wasn’t until the middle of the 20th century that scientists came to the accepted conclusion today that the Earth is about 4.55 billion years old.

10. The Earth is flat. Actually, this one doesn’t belong on the list but I put it here to prove a point. While there’s a popular belief that “flat earth” was somehow a widely held “scientific” idea, Greeks such as Aristotle knew the Earth was round, as did Thomas Aquinas. In short, most scholarship suggests learned men and women from the dawn of antiquity knew the Earth was round. So science gets a pass on this one.

That’s my list. What did I miss?

NOTE: This blog entry was inspired by a question posed at The Edge and the responses of scientists to it.