The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for l...

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea. After the discovery o...

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a...

Uranus is the 7th planet from the Sun. It was discovered in 1781 by William Herschel and named for the Greek god personifying heaven. A blue-green gas giant, it has almost 15 times the mass of Earth and over 50 times its volume. It is less...

Imhotep was an Egyptian polymath who served under the Third Dynasty king Djoser as chancellor to the pharaoh and high priest of the sun god Ra (or Re) at Heliopolis. He is considered by some to be the earliest known architect and engineer a...

The zodiac is an area of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon a...

Hesiod was a Greek oral poet and is often identified as the first economist. His date is uncertain but leading scholars, favor the the eighth century BC for when Hesiod lived. Since at least Herodotus's time, Hesiod and Homer have generally...

Anaximander was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia (in modern-day Turkey). He belonged to the Milesian school and learned the teachings of his master Thales. He succeeded Thales and became the second mast...

Anaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae in the Persian Empire (modern-day Urla, Turkey) Anaxagoras was the first to bring philosophy to Athens. According to Diogenes Laertius and Plutarch, in later life he was ch...

Aristarchus of Samos was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the known universe with the Earth revolving around it. He was influenced by Phi...

Eratosthenes of Cyrene was a Greek mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. He invented the discipline of geography, including the...

The Antikythera mechanism is an ancient mechanical computer designed to calculate astronomical positions. It was recovered in 1900–1901 from the Antikythera wreck. Its significance and complexity were not understood until decades later. Its...

Posidonius of Apameia or of Rhodes was a Greek Stoic philosopher, politician, astronomer, geographer, historian and teacher native to Apamea, Syria. He was acclaimed as the greatest polymath of his age. None of his vast body of work can be...

The Julian calendar was introduced by Julius Caesar in 45 BC. It was in common use until the 1500s, when countries started changing to the Gregorian Calendar. However, some countries (for example, Greece and Russia) used it into this centur...

Zhang Heng was a Chinese astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman, and literary scholar from Nanyang, Henan. He lived during the Eastern Han Dynasty (AD 25–220) of China. He was educated in the...