The Nine Planets Solar System Tour What Is a Planet? While many people can point to a picture of Jupiter or Saturn and call it a "planet," the definition of this word is much more subtle and has changed over time. Many astronomers decided on a new definition in 2006 after the discovery of several worlds at the fringes of the solar system — a decision that remains controversial. The International Astronomical Union defined a planet as an object that: orbits the sun has sufficient mass to be round, or nearly round is not a satellite (moon) of another object has removed debris and small objects from the area around its orbit The IAU also created a new classification, "dwarf planet," which is an object that meets planetary criteria except that it has not cleared debris from its orbital neighborhood. This definition meant that Pluto — considered a planet at the time — was demoted and reclassified as a dwarf planet. Planetary history The term "planet" originally comes from the Greek word for "wanderer." Discovery of more worlds
Phases of the Moon and Percent of the Moon Illuminated Copyright Antonio Cidadao. Used by permission. Click on picture to see large version. From any location on the Earth, the Moon appears to be a circular disk which, at any specific time, is illuminated to some degree by direct sunlight. New Moon - The Moon's unilluminated side is facing the Earth. Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. Full Moon - The Moon's illuminated side is facing the Earth. Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. Lunation Movie Copyright Antonio Cidadao.
Solar System Discovery and exploration Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660) For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth. Structure and composition The orbits of the bodies in the Solar System to scale (clockwise from top left)
The Moon - Zoom Astronomy The moon is Earth's only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles. The same side of the moon always faces the Earth. If you were standing on the moon, the sky would always appear dark, even during the daytime. The moon is about 238,900 miles (384,000 km) from Earth on average. The moon revolves around the Earth in about one month (27 days 8 hours). The Moon's orbit is expanding over time as it slows down (the Earth is also slowing down as it loses energy). SAROS The saros is the roughly 18-year periodic cycle of the Earth-Moon-Sun system. The moon's diameter is 2,159 miles (3,474 km), 27% of the diameter of the Earth (a bit over a quarter of the Earth's diameter). The moon's mass is (7.35 x 10 22 kg), about 1/81 of the Earth's mass. The moon's density is 3340 kg/m 3. MOON OR DOUBLE PLANET?
The Last Shuttle The Solar System - Astronomy For Kids - KidsAstronomy.com Our solar neighborhood is an exciting place. The Solar System is full of planets, moons, asteroids, comets, minor planets, and many other exciting objects. Learn about Io, the explosive moon that orbits the planet Jupiter, or explore the gigantic canyons and deserts on Mars. What Is The Solar System? The Solar System is made up of all the planets that orbit our Sun. In addition to planets, the Solar System also consists of moons, comets, asteroids, minor planets, and dust and gas. Everything in the Solar System orbits or revolves around the Sun. How Did The Solar System form? This is an important question, and one that is difficult for scientists to understand. Scientists believe that the Solar System evolved from a giant cloud of dust and gas. At the center of this spinning cloud, a small star began to form. Further away from the center of this mass where the star was forming, there were smaller clumps of dust and gas that were also collapsing. A Great Storm Beyond The Oort Cloud
Sun Facts - Interesting Facts about the Sun (Sol) The Sun or Sol, is the star at the centre of our solar system and is responsible for the Earth’s climate and weather. The Sun is an almost perfect sphere with a difference of just 10km in diameter between the poles and the equator. The average radius of the Sun is 695,508 km (109.2 x that of the Earth) of which 20–25% is the core. Star Profile Age: 4.6 Billion YearsType: Yellow Dwarf (G2V)Diameter: 1,392,684 kmCircumference at Equator: 4,370,005.6 kmMass: 1,989,100,000,000,000,000,000 billion kg (333,060 x Earth)Surface Temperature: 5500 °C Size of the Sun Facts about the Sun One million Earths could fit inside the Sun: If a hollow Sun was filled up with spherical Earths then around 960,000 would fit inside. Eventually, the Sun will consume the Earth: When all the Hydrogen has been burned, the Sun will continue for about 130 million more years, burning Helium, during which time it will expand to the point that it will engulf Mercury and Venus and the Earth. Satellites
Solar System NASA Planets: The planet count in our solar system has gone as high as 15 before new discoveries prompted a fine tuning of the definition of a planet. The most recent change was in 2006 when scientists reclassified Pluto as a new kind of object - a dwarf planet. Dwarf Planets: This new class of worlds helps us categorize objects that orbit the Sun but aren't quite the same as the rocky planets and gas giants in our solar system. Moons: This count includes only the moons orbiting the eight planets in our solar system. Asteroids: New asteroids are discovered on an almost daily basis. Comets: Orbiting spacecraft such as SOHO have raised this tally in recent years by catching the comets as they plunge toward the Sun - and sometimes vaporize.
Retrograde and prograde motion This article is about retrograde motions of celestial bodies relative to a gravitationally central object. For the apparent motion as seen from a particular vantage point, see Apparent retrograde motion. Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black) Formation of celestial systems When a galaxy or a planetary system forms, its material takes the shape of a disk. Orbital parameters Inclination Axial tilt Planets All eight planets in the Solar System orbit the Sun in the direction that the Sun is rotating, which is counterclockwise when viewed from above the Sun's north pole. Dwarf planets Earth's atmosphere Retrograde motion, or retrogression, within the Earth's atmosphere refers to weather systems which move from east to west through the Westerlies or from west to east through the Trade wind easterlies. Moons and rings The orange moon is orbiting in the opposite direction. 
Mercury as Never Seen Before Mercury as Never Seen Before Date: 6 Oct 2008 The spectacular image shown here is one of the first to be returned from MESSENGER's second flyby of Mercury. The image shows the departing planet taken about 90 minutes after the spacecraft's closest approach. The bright crater just south of the center of the image is Kuiper, identified on images from the Mariner 10 mission in the 1970s. For most of the terrain east of Kuiper, toward the limb (edge) of the planet, the departing images are the first spacecraft views of that portion of Mercury's surface. A striking characteristic of this newly imaged area is the large pattern of rays that extend from the northern region of Mercury to regions south of Kuiper.
Evidence of cosmic inflation expands universe understanding GWEN IFILL: It’s a mind-boggling concept: Our cosmos expanded from almost nothing to its first huge growth spurt in just a trillionth of a trillionth of a trillionth of a second. And that was after the Big Bang. Scientists said they confirmed that theory by using this telescope at the South Pole to look at the oldest light detectable. The light reveals patterns and skewed light waves, shown here in red and blue, that were created by gravitational ripples during the — this incredible expansion known as cosmic inflation. Sean Carroll is a physicist, cosmologist and author at the California Institute of Technology, and he joins us now to explain all of this. And we need your explanation. SEAN CARROLL, California Institute of Technology: Well, it is. The term cosmic inflation was coined around 1980, when the ordinary economic inflation was also very much in the news. For example, it looks similar, it looks smooth all over the place. SEAN CARROLL: Well, it’s very similar.
The Solar System: Space Gallery on Sea and Sky Our Solar System is a place of unimaginable beauty. Home to the Sun, eight planets, and a variety of smaller celestial objects, we have only recently been able to appreciate it fully. Thanks to the marvels of technology, robotic spacecraft have extended our view across vast distances, enabling us to see what was once hidden from our view. Join us now and examine some of the most recent images of our Solar System. Click on an image above to begin a slide show.