الجمعة، 28 مارس 2008
All About Our Solar System
Our solar system consists of the sun, planets, dwarf planets, moons, an asteroid belt, comets, meteors, and other objects. The sun is the center of our solar system; the planets, over 61 moons, the asteroids, comets, meteoroids and other rocks and gas all orbit the Sun. The Planets
The nine planets that orbit the sun are (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter (the biggest planet in our Solar System), Saturn (with large, orbiting rings), Uranus, Neptune, and Pluto (a dwarf planet). A belt of asteroids (minor planets made of rock and metal) orbits between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic).
The inner planets (those planets that orbit close to the Sun) are quite different from the outer planets (those planets that orbit far from the Sun).
- The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.
- The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto. They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto which is small, rocky, has one moon, and is considered to be a dwarf planet).
Small Bodies
There are other smaller object that orbit the Sun, including asteroids, comets, meteoroids and dwarf planets.
- Asteroids (also called minor planets) are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter.
- Comets are small, icy bodies that orbit the sun. They have very long tails.
- Meteoroids are small bodies that travel through space. They are stony and/or metallic and are smaller than asteroids. Most are very tiny.
The Planets (plus the Dwarf Planet Pluto)
The nine planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (a dwarf planet). A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic). Easy ways to remember the order of the planets (and Pluto) are the mnemonics: "My Very Excellent Mother Just Sent Us Nine Pizzas" and "My Very Easy Method Just Simplifies Us Naming Planets" The first letter of each of these words represents a planet - in the correct order. Our solar system consists of the sun, eight planets, moons, dwarf planets, an asteroid belt, comets, meteors, and others. The sun is the center of our solar system; the planets, their moons, the asteroids, comets, and other rocks and gas all orbit the sun.
The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto (a dwarf planet). Jupiter is so big that all the other planets could fit inside it.
The Inner Planets vs. the Outer Planets
The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun).
Temperatures on the Planets
Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.
Density of the Planets
The outer, gaseous planets are much less dense than the inner, rocky planets.
The Earth is the densest planet. Saturn is the least dense planet; it would float on water.
The Mass of the Planets
Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive.
Gravitational Forces on the Planets
The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared.
A Day on Each of the Planets
A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours.
The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days).
The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change.
The Average Orbital Speed of the Planets
As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion).
The Planets in Our Solar SystemPlanet Distance from the Sun
(Astronomical Units
miles
km)Period of Revolution Around the Sun
(1 planetary year)Period of Rotation
(1 planetary day)Mass
(kg)Diameter
(miles
km)Apparent size
from EarthTemperature
(K
Range or Average)Number of Moons Mercury 0.39 AU, 36 million miles
57.9 million km87.96 Earth days 58.7 Earth days 3.3 x 1023 3,031 miles
4,878 km5-13 arc seconds 100-700 K
mean=452 K0 Venus 0.723 AU
67.2 million miles
108.2 million km224.68 Earth days 243 Earth days 4.87 x 1024 7,521 miles
12,104 km10-64 arc seconds 726 K 0 Earth 1 AU
93 million miles
149.6 million km365.26 days 24 hours 5.98 x 1024 7,926 miles
12,756 kmNot Applicable 260-310 K 1 Mars 1.524 AU
141.6 million miles
227.9 million km686.98 Earth days 24.6 Earth hours
=1.026 Earth days6.42 x 1023 4,222 miles
6,787 km4-25 arc seconds 150-310 K 2 Jupiter 5.203 AU
483.6 million miles
778.3 million km11.862 Earth years 9.84 Earth hours 1.90 x 1027 88,729 miles
142,796 km31-48 arc seconds 120 K
(cloud tops)18 named (plus many smaller ones) Saturn 9.539 AU
886.7 million miles
1,427.0 million km29.456 Earth years 10.2 Earth hours 5.69 x 1026 74,600 miles
120,660 km15-21 arc seconds
excluding rings88 K 18+ Uranus 19.18 AU
1,784.0 million miles
2,871.0 million km84.07 Earth years 17.9 Earth hours 8.68 x 1025 32,600 miles
51,118 km3-4 arc seconds 59 K 15 Neptune 30.06 AU
2,794.4 million miles
4,497.1 million km164.81 Earth years 19.1 Earth hours 1.02 x 1026 30,200 miles
48,600 km2.5 arc seconds 48 K 2 Pluto (a dwarf planet) 39.53 AU
3,674.5 million miles
5,913 million km247.7 years 6.39 Earth days 1.29 x 1022 1,413 miles
2,274 km0.04 arc seconds 37 K 1 large (plus 2 tiny) Planet Distance from the Sun
(Astronomical Units
miles
km)Period of Revolution Around the Sun
(1 planetary year)Period of Rotation
(1 planetary day)Mass
(kg)Diameter
(miles
km)Apparent size
from EarthTemperature
(K
Range or Average)Number of Moons
INTRODUCTION TO VENUS
 | INTRODUCTION TO VENUS |  |
GENERAL INFORMATION
Venus is the second planet from the sun in our solar system. It is the hottest planet in our Solar System. This planet is covered with fast-moving sulphuric acid clouds which trap heat from the Sun. Its thick atmosphere is mostly carbon dioxide. Venus has an iron core but only a very weak magnetic field. This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure. 
Venus is also known as the "morning star" or the "evening star" since it is visible and quite bright at either dawn or dusk. It is only visible at dawn or dusk since it is closer to the sun than we are.
Like the moon, Venus' appearance from Earth changes as it orbits around the Sun. It goes from full to gibbous to crescent to new and back.
SIZE 
Venus is about 7,521 miles (12,104 km) in diameter. This is about 95% of the diameter of the Earth. Venus is the closest to Earth in size and mass of any of the other planets.
MASS AND GRAVITY
Venus' mass is about 4.87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100-pound person would weigh 91 pounds on Venus.
The density of Venus is 5,240 kg/m3, slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).
LENGTH OF A DAY AND YEAR ON VENUS
Venus rotates VERY slowly. Each day on Venus takes 243 Earth days. A year on Venus takes 224.7 Earth days. It takes 224.7 Earth days for Venus to orbit the sun once. The same side of Venus always faces Earth when the Earth and Venus are closest together.
VENUS' ORBIT AND DISTANCE FROM THE SUN
Venus is 67,230,000 miles (108,200,000 km) from the sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67,230,000 miles = 108,200,000 km from the sun.
Venus rotates in the opposite direction of the Earth (and the other planets, except possibly Uranus). Looking from the north, Venus rotates clockwise, while the other planets rotate counterclockwise. From Venus, the Sun would seem to rise in the west and set in the east (the opposite of Earth). No one knows why Venus has this unusual rotation. 
TEMPERATURE ON VENUS
Venus is the hottest planet in our Solar System. Its cloud cover traps the heat of the sun (the greenhouse effect), giving Venus temperatures up to 480°C. The mean temperature on Venus is 726 K (452°C = 870°F).
MOONS
Venus has no moons.
VENUS-EARTH COMPARISONS
FIRST SPACECRAFT TO REACH VENUS
Venera 3 (from the U.S.S.R.) was the first manmade object to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1, 1966 , the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.
VENUS' NAME AND SYMBOL
 This is the symbol of the planet Venus. |
Dogs in Space
| Dogs in Space |  |
 Laika in Sputnik 2 before her launch on November 3, 1957. |
Laika, the First Dog in Earth Orbit
Laika was the first dog sent into orbit around the Earth (other dogs were launched earlier on sub-obital flights). She rode on Sputnik 2, a Russian mission that blasted off on November 3, 1957 (Sputnik 2 was a metal sphere that weighed about 250 pounds = 113 kg). Laika was originally thought to have survived in Earth orbit for four days, dying in space when the batteries to the cabin over-heated. In 2002, it was revealed that Laika died roughly 5 to 7 hours into the flight, from overheating and stress. After orbiting the Earth 2,570 times, Sputnik 2 fell back to Earth on April 14, 1958, burning up during re-entry.
Laika was a female mutt who was part Siberian Husky. Laika means "barker" in Russian.
Other Space Dogs
Between 1957 and 1966, the USSR (now Russia) sent 13 dogs into space in preparation for future missions. The dogs included:
- Laika (meaning "Barker") - died during a mission (Sputnik 2, November 1957)
- Lisichka (meaning "Little Fox") and Bars (meaning "Panther" or "Lynx") - died during a test flight on July 28, 1960
- Strelka (meaning "Little Arrow") , Belka (meaning "Squirrel"), 40 mice, 2 rats and a number of plants - safely recovered from Korabl'-Sputnik-2. Launched August 19, 1960, it orbited the Earth 18 times. This was the first successful recovery of living biological specimens after an orbital mission. Strelka later gave birth to a litter of 6 healthy puppies; one was given to President John F. Kennedy as a gift.
A stamp from Bulgaria featuring Strelka, Chernushka, Zvezdochka, and Belka. - Pchelka (meaning "Little Bee") and Mushka (meaning "Little Fly")- died when Korabl'-Sputnik-3 re-entered the Earth's atmosphere at the wrong angle and burned up, (launched December 1, 1960)
- Damka (meaning "Little Lady") and Krasavka (meaning "Beauty") - Launched December 22, 1960, but the third stage of the SL-3 rocket failed, and the orbital launch was aborted; the two dogs survived an unplanned suborbital flight.
- Chernushka (meaning "Blackie"), a dummy cosmonaut (known as "Ivan Ivanovich"), a few mice and a guinea pig - launched March 9, 1961.
- Zvezdochka (meaning "Little Star") and a dummy cosmonaut in a space suit - launched on March 25, 1961 and orbited once in final preparation for the Vostok 1 mission. Zvezdochka was named by cosmonaut Yuri Gagarin.
- Verterok or Veterok (meaning "Breeze") and Ugolyok or Ugolek (meaning "Little Piece of Coal") were launched on February 22, 1966, in the satellite Kosmos 110. This was a 22-day mission.
Web Links about Space Dogs
Laika in Sputnik 2 before her launch on November 3, 1957.  |
Tass News Agency photo of Laika
A newspaper article about Laika from 1957, from the New York Times.
Sputnik missions from the University of Michigan's Windows to the Universe.
A page on Sputnik from NASA.
A page on space dogs from NASA
Animal astronauts from the Univ. of Minnesota
Stamps commemorating the Soviet space dogs.
PLUTO
| PLUTO |  |
GENERAL INFORMATION
Pluto is a dwarf planet that usually orbits past the orbit of Neptune. It was classified as a dwarf planet in 2006; before that it was considered to be a planet, the smallest planet in our solar system. Pluto is smaller than a lot of the other planets' moons, including our moon. Pluto is the only "planet" in our solar system that has not been visited by our spacecraft yet. We only have blurry pictures of its surface; even the Hubble Space Telescope orbiting the Earth can only get grainy photos because Pluto is so far from us. In 2015, a spacecraft called New Horizons (launched by NASA in 2006) will visit Pluto.
SIZE
Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth.
Pluto is smaller than the 8 planets in our Solar System.
MASS AND GRAVITY
Pluto's mass is about 1.29 x 1022 kg. This is about 1/500th of the mass of the Earth. The gravity on Pluto is 8% of the gravity on Earth.
Pluto is the least massive planet in our Solar System (and is now classified as a dwarf planet).
A 100 pound person on Pluto would weigh only 8 pounds.
LENGTH OF A DAY AND YEAR ON PLUTO Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for Pluto to orbit the Sun once).
PLUTO'S ORBIT
Pluto is 39 times farther from than the sun than the Earth is. Pluto ranges from 2.8 to 4.6 billion miles (4.447 billion to 7.38 billion km) from the Sun. From Pluto, the sun would look like a tiny dot in the sky.
Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune. Orbital Eccentricity
Pluto has a very eccentric orbit; that means that its distance from the sun varies a lot during its orbit around the sun. Sometimes it is even closer to the Sun than the planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto also rotates about its axis in the opposite direction from most of the other planets. 
Orbital Inclination
Pluto's orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is the largest inclination of any of the planets. TEMPERATURE ON PLUTO
Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or 35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).
PLANETARY COMPOSITION
Pluto's composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined from density calculations; Pluto's density is about 2,000 kg/m3. There may be methane ice together with frozen nitrogen and carbon dioxide on the cold, rocky surface.
ATMOSPHERE
Not much is known about Pluto's atmosphere. It is probably mostly nitrogen with a little carbon monoxide and methane - definitely not breatheable by humans. The atmospheric pressure is probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen methane is vaporized by the solar heat. When it is farther from the Sun, the methane freezes again. From Pluto, the sky would appear black, even when the Sun (the size of a star) is up.
PLUTO'S MOONS
Pluto has one large moon, named Charon; two minscule moons were discovered in 2005.
Although Charon is small, about 1,172 km (728 miles) in diameter, it about half of the size of Pluto itself. Charon orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon's orbit takes exactly one Pluto day.
Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades.
The two tiny moons are from 30 and 100 miles (45 to 160 km) in diameter, and orbit Pluto about 27,000 miles (44,000 km) from Pluto, more than twice as far as the orbit of Charon.
DISCOVERY OF PLUTO
Pluto was discovered after the 8 planets and was originally considered a planet itself (until 2006). In the early 1900s, Planet "X" was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto. He did his observations at the Lowell Observatory in Arizona.
PLUTO-EARTH COMPARISON
PLUTO'S NAME
 This is the symbol of Pluto. |
Its symbol is the combined letters "P" and "L," either for Percival Lowell or for Pluto.
The name Pluto was suggested by Venetia Burney of England, who was 11 years old at the time. She suggested the name to her grandfather, who was Librarian at Oxford. He passed her idea to the astronomers who were trying to name the newly-discovered planet.
PLUTO: PLANET, ASTEROID, OR TNO
Pluto's unusual orbit makes some scientists think that Pluto is not a regular planet. but a "dwarf planet," "minor planet" or a Trans Neptunian Object (TNO) [Kuiper Belt objects left over from the formation of the solar system]. In 2006 Pluto was re-classified as a dwarf planet. Alternatively, Pluto could have been be listed as an asteroid (it would have been given the asteroid number 10,000). It also may be the first TNO - it will also still be considered a dwarf planet.
Jupiter::ATMOSPHERE AND PLANETARY COMPOSITION
Atmosphere: Jupiter is a gaseous planet; it does not have a solid surface like the Earth does (but probably has a solid, rocky core 10 to 15 times the mass of the Earth). When we look at Jupiter, we are seeing icy clouds of gases moving at high speeds in the atmosphere. Jupiter's atmosphere is composed of about 90% hydrogen and 10 % helium. There are only minute traces (0.07%) of methane (CH3), water, ammonia, and rock dust.Mantle: Pressurized hydrogen in the mantle may generate electric currents which generate Jupiter's powerful magnetic field. The outer mantle is liquid hydrogen; the inner mantle is liquid metallic hydrogen. The layers of extraordinarily-compressed hydrogen are in a state so extreme that it has never been produced on Earth. The pressure is so great that the hydrogen molecules inside Jupiter conduct heat and electricity very well, in a metal-like fashion (they do not do this under Earth-like condition). Inside Jupiter, electrons from hydrogen molecules move freely from molecule to molecule (like the electrons of a metal); this is what allows the electrical and heat conductivity.
Core: At the center of the planet is a molten rock core which is many times bigger and more massive than the entire Earth. It is 20,000 °C, about three times hotter than the Earth's core.
Internal Heat: Jupiter is a heat source; it radiates 1.6 times a much energy as it receives from the Sun. This energy is produced by Jupiter's shrinking due to gravity, and this produces heat. Also, it is still cooling down, losing its initial energy (the energy it received as the Solar System formed).
Does Jupiter produce energy by nuclear fusion -- NO. Jupiter, the biggest of the gas giants, is too small to produce a core temperature that is hot enough to undergo fusion (you need about 3 million degrees to start the fusion of hydrogen). You'd need a body that was many times the mass of Jupiter to get nuclear fusion (the theoretical limit is about 8 percent of the mass of the Sun).
Magnetic Field: Jupiter has a very strong magnetic field. The magnetic field is probably generated as the planet spins its deep metallic-hydrogen layer with electrical currents .
Jupiter's magnetic field (Jupiter's magnetosphere) extends for millions of miles into space. The tail of this magnetic field (which is extended by the solar wind), extends into the orbit of Saturn! A tremendous amount of charged particles are trapped within this magnetosphere, especially in the inner parts of this field. This makes Jupiter the most deadly radiation environment of any of the planets.
MAP OF MERCURY
Mercury is a heavily cratered planet. Its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. The largest crater on Mercury is the Caloris Basin which is 800 miles (1,300 km) across. It may have been formed by as asteroid hitting Mercury. Many of the craters on Mercury have been named for writers, musicians, painters, and other artists.
There are also areas of relatively flat planes on Mercury; these are called planitas.
Lines of mountainous cliffs, rupes, scar the surface of Mercury. These long lines were probably formed when the planet cooled about 4 billion years ago, causing these lon cracks to form.
THE MOON
The lunar crater Aristarchus ( on the NW edge of the Oceanus Procellarum). This huge, circular crater is 25 miles (40 km) in diameter and 2.2 miles (3.6 km) deep (from rim to floor). There is a lot of ejecta (material thrown from the crater at impact) surrounding the crater.The surface of the moon is scarred by millions of (mostly circular) impact craters, caused by asteroids, comets, and meteorites. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in our atmosphere). Also, there is no erosion (wind or precipitation) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it. These craters range in size up to many hundreds of kilometers, but the most enormous craters have been flooded by lava, and only parts of the outline are visible. The low elevation maria (seas) have fewer craters than other areas. This is because these areas formed more recently, and have had less time to be hit. The biggest intact lunar crater is Clavius which is 100 miles (160 km) in diameter. A rille is a long, narrow valley on the surface of the moon. Hadley Rille is a long valley on the surface of the moon. This rille is 75 miles (125 km) long, 1300 feet (400 m) deep, and almost 1 mile (1500 m) wide at its widest point. It was formed by molten basaltic lava that carved out a steep channel along the base of the Apennine Front (which was explored by the Apollo 15 astronauts in 1971). MOON OR DOUBLE PLANET?The Earth and the Moon are relatively close in size (4:1 in diameter, 81:1 in mass), unlike most planet/moon systems. Many people consider the Earth and Moon to be a double planet system (rather than a planet/moon system). The moon does not actually revolve around the Earth; it revolves around the Sun in concert with the Earth (like a double planet system). LIBRATIONLibration is a rocking movement of the Moon. Librations cause us to view the Moon from different angles at different times, enabling us to see about 59 percent of the Moon's surface from Earth, even though the same side always faces us. There are librations due to variations in the rate of the Moon's orbital motion (longitudinal libration) and to the inclination of the Moon's equator with respect to its orbital plane (latitudinal libration). There is also an apparent libration due to an observer on Earth viewing the Moon from different angles as the Earth rotates (diurnal libration, which occurs each day).
TWO LUNAR MONTHSThe sidereal and synodic lunar months have different lengths. The sidereal month is the amount of time it takes the Moon to return to the same position in the sky with respect to the stars; the sidereal month is 27.321 days long. The synodic month is the time between similar lunar phases (e.g., between two full moons); the synodic month is 29.530 days long. LUNAR EXPLORATION
Astronaut Buzz Aldrin's footprint on the moon's Sea of Tranquility, from the Apollo 11 mission in 1969.There have been many missions to the moon, including orbiters missions and moon landings. NASA's Apollo missions sent people to the moon for the first time. Apollo 11's LEM (Lunar Excursion Module) landed on the moon on July 20, 1969 with Neil Armstrong and Edwin "Buzz" Aldrin (Michael Collins was in the orbiter). Neil Armstrong was the first person to set foot on the moon. His first words upon stepping down the Lunar Module's ladder onto the lunar surface were, "That's one small step for man, one giant leap for mankind." Aldrin described the lunar scenery as "magnificent desolation." Apollo 12-17 continued lunar exploration. MOON ROCKS NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks. Using radioisotope dating, it has been found that moon rocks are about 4.3 billion years old. THE ORIGIN OF THE MOONMost scientists believe that the moon was formed from the ejected material after the Earth collided with a Mars-sized object. This ejected material coalesced into the moon that went into orbit around th Earth. This catastrophic collision occurred about 60 million years after Earth itself formed (about 4.3 billion years ago). This is determined by the radioisotope dating of moon rocks BLUE MOONWhen two full moons occur in a single month, the second full moon is called a "Blue Moon." Another definition of the blue moon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)
