The Universe is all of time and space and its contents. The Universe includes planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy. The majority of matter and energy is m...

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...

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...

Saturn is the sixth planet from the Sun and the second largest. In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root...

The Nine Planets is an overview of the history, mythology, and current scientific knowledge of each planet and the major moons in our solar system. Each page has my text and NASA's images, some have sounds and movies, most provide reference...

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of icy trans-Neptunian objects. It was the first such object to be discovered. It is the largest and second-most massive known dwarf planet in the S...

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...

Venus is the second planet from the Sun and the sixth largest. On June 8 2004, Venus will pass directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun's disk. This event is known as a "transit of Ve...

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...

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...

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...

Claudius Ptolemy was a Greco-Egyptian writer of Alexandria, known as a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in the city of Alexandria in the Roman province of Egypt...

Nicolaus Copernicus was a Renaissance- and Reformation-era mathematician and astronomer who formulated a model of the universe that placed the Sun rather than the Earth at the center of the universe, likely independently of Aristarchus of S...

Tycho Brahe was born in Skane, then in Denmark, now in Sweden. His contributions to astronomy were enormous. He not only designed and built instruments, he also calibrated them and checked their accuracy periodically. He thus revolutionized...

Galileo Galilei was an Italian polymath: astronomer, physicist, engineer, philosopher, and mathematician. He has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and...