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Table of Contents Enchanted Learning
All About Astronomy
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Our Solar System Stars Glossary Printables, Worksheets, and Activities
The Sun The Planets The Moon Asteroids Kuiper Belt Comets Meteors Astronomers

Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.
M



M# (MESSIER OBJECTS)

During the late 18th century (1759-1781), the French astronomer Charles Messier made a list of 103 fuzzy objects in space in order not to mistake star clusters, galaxies, and nebulae for comets (for which he was searching). More objects were added later, bringing the total to 110. In it, M1 is the Crab Nebula, M2 is a globular cluster in Aquarius, M3 is a globular cluster in Canes Venatici, etc. For a list of the Messier objects, click here.


M31

M31 (commonly known as Andromeda Galaxy) is the closest major galaxy to us. It is a spiral galaxy (like our galaxy, the Milky Way). It can just be seen with the naked eye.

MACHO

MACHO stands for Massive Compact Halo Object. MACHOs are objects that could account for some (or all) of the dark matter in the halos of galaxies, like white dwarfs, neutron stars, and black holes.


MAGELLANIC CLOUDS

The Magellanic Clouds are irregular-shaped galaxies, congregations of millions of stars. The irregular shape may be the result of a disturbance, perhaps a collision of two galaxies. The Large Magellanic Cloud (LMC) is near the constellation Dorado, and is 163,000 light-years away. The Small Magellanic Cloud (SMC) is near the constellation Tucana, and is under 200,000 light-years away. These two clouds are satellites of our Milky Way Galaxy.


MAGMA

Magma is molten rock (lava) from which igneous rock forms.

MAGNETAR

A magnetar is a highly magnetic star. Magnetar have magnetic fields of about 1015 Gauss, about a thousand trillion times stronger than the Earth's. The strong magnetic field puts the star's surface under enormous stress, perhaps causing "starquakes" and resulting high energy bursts of radiation. These short-lived neutron stars were theorized to exist in 1992 by Robert C. Duncan and Christopher Thompson. Their existence was confirmed in 1998 by Chryssa Kouveliotou et al.
Magnetic Field Size
The Earth at the equator0.32 Gauss
The Earth at the poles0.62 Gauss
Refrigerator magnets102 Gauss
Sunspots103 Gauss
Fields produced in labs105 Gauss
Magnetars8 x 1014 Gauss

MAGNETIC FIELD

A magnetic field is a region near a magnet where other magnets are affected. The Earth's magnetic field is probably caused by its molten iron-nickel core. This field is aligned with the north and south poles, and has reversed many times during geologic history. William Gilbert hypothesized that the Earth was a giant magnet in 1600.

MAGNETIC STORM

A magnetic storm is a temporary perturbation (disruption) of the Earth's magnetic field, caused by solar flares, which eject plasma from the Sun's chromosphere. This solar plasma travels at speeds of roughly 600 to 1,200 miles per second (1,000 to 2,000 km per second), and after about 21 hours, hit the Earth's atmosphere. Since they disturb the ionosphere (the upper atmosphere), magnetic storms can disrupt radio transmissions. Magnetic storms usually last for a few days. During a magnetic storm, auroras increase over polar regions.

MAGNETOPAUSE

A magnetopause is the boundary between the area in which a planet's magnetic field dominates and the magnetic field of the rest of the Solar System dominates.


MAGNETOSPHERE

The magnetosphere is the Earth's magnetic environment. The Earth is a huge dipole (2-pole) magnet. The Earth's magnetic field is probably cause by its molten iron-nickel core. This magnetic field is aligned with the north and south poles, and has reversed many times during geologic history. Charged particles become trapped on the field lines of the magnetosphere. William Gilbert hypothesized that the Earth was a giant magnet in 1600. Thomas Gold proposed the name "magnetosphere" in 1959. The Earth's magnetosphere extends far into space and is influenced by the solar wind (ions and electrons emitted from the sun). It extends into space from 60 to 37,280 miles (100 to 60,000 km) towards the Sun, and over 186,500 miles (300,000 km) away from the Sun, forming the Earth's magnetotail.
Star Absolute Magnitude Apparent Magnitude Distance from Earth
(light-years)
The Sun +4.8 -26.72 .
Sirius +1.4 -1.46 8.6
Canopus -2.5 -0.72 74
Rigel Kentaurus +4.4 -0.27 4.3
Arcturus +0.2 -0.04 34
Vega +0.6 0.03 25
Capella +0.4 +0.08 41
Rigel -8.1 +0.12 900
Betelgeuse -7.2 +0.7 1,500
Altair +2.3 +0.77 16
Deneb -7.2 +1.25 1,500
Proxima Centauri +15.5 +11.05 (var.) 4.3

MAGNITUDE

Magnitude is a measure of brightness of celestial objects. Lower numbers represent brighter objects than higher numbers; very bright star are 1st magnitude, less bright stars are 2nd magnitude, etc. The magnitude scale is logarithmic; a difference in magnitude of 5 is a 100-fold increase in brightness (the difference in each successive magnitude is a factor of about 2.512 times). This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C. The current system was developed in 1850 by the English astronomer Norman Robert Pogson.

Apparent magnitude is the visible brightness of an object from Earth. Absolute magnitude is the brightness the object would have if seen from a distance of 10 parsecs (32.6 light-years) from Earth. Bolometric magnitude includes a star's entire spectrum of radiation, not just the visible light.



MAIASAURA

The first dinosaur in space was Maiasaura peeblesorum (a duck-billed dinosaur). A piece of bone from a baby Maiasaura and a Maiasaura eggshell were taken into space by astronaut Loren Acton on an 8-day NASA mission (Spacelab 2) in 1985. The historic Maiasaura fossils now reside at the Museum of the Rockies in Bozeman, Montana, USA.


MAIN SEQUENCE STARS

Main sequence stars are the central band of stars on the Hertzsprung-Russell Diagram. These stars' energy comes from nuclear fusion, as they convert Hydrogen to Helium. Most stars are Main Sequence Stars. For these stars, the hotter they are the brighter. The sun is a typical Main Sequence star.


MAIN SEQUENCE TURNOFF

The main sequence turnoff is the place on the Hertzsprung-Russell Diagram where an aging (evolving) star begins to run out of hydrogen in the core and expand, thus moving off the main sequence.


MAKSUTOV-CASSEGRAIN TELESCOPE

A Maksutov-Cassegrain telescope (MCT) is a wide-angle reflecting telescope with a curved correcting lens (called a Meniscus Corrector Shell.) that minimizes spherical aberration and a concave mirror that receives light and focuses an image. A second mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube. The Cassegrain telescope (named for the French sculptor Sieur Guillaume Cassegrain) was developed in 1672; the Meniscus Corrector Shell (a curved lens that corrected spherical aberration and was easier to produce than the Schmidt's correction lens) was added in 1941 by Dmitry Maksutov from Russia and A. Bouwers of Holland.


MANTLE

The mantle is the layer of the Earth (or other planet) located between the crust and the molten core.


MAP

MAP (Microwave Anisotropy Probe) is a NASA satellite that will be launched in April, 2001, and will orbit for 27 months. MAP was designed to help answer the cosmological questions about the early universe: "What are the values of the cosmological parameters of the Big Bang theory?", "When did the first structures of galaxies form?", and "How did structures of galaxies form in the universe?". MAP will measure minute temperature fluctuations (anisotropy) in the cosmic microwave background radiation (the radiant heat left over from the Big Bang) over the entire sky (these were originally detected by the COBE satellite). MAP will orbit at the unstable Sun-Earth Lagrange point L2 (see the diagram above); the Sun, Earth, and MAP will always be in a straight line.


MARE

(plural maria) Mare means "sea," but maria on the moon are plains on the moon. They are called maria because very early astronomers thought that these areas on the moon were great seas. The first moon landing was in the Mare Tranquillitatis (the Sea of Tranquility).
Mariner Mission Planet Explored Date of Launch
Mariner 1 None (attempted Venus flyby) 1962
Mariner 2 Venus 1962
Mariner 3 None (attempted Mars flyby) 1964
Mariner 4 Mars flyby November 28, 1964
Mariner 5 Venus flyby 1969
Mariner 6 Mars February 24, 1969
Mariner 7 Mars March 27, 1969
Mariner 8 None (attempted Mars orbiter) 1971
Mariner 9 Mars orbiter May 30, 1971
Mariner 10 Venus/Mercury flyby 1973

MARINER

NASA's series of Mariner missions were designed to gather information from the planets Venus, Mercury, and Mars. They were sent to map these planets and take atmospheric measurements (like temperature, composition, pressure, and density).


MARIUS, SIMON

Simon (Mayr) Marius (1570-1624), was a German astronomer and physician who studied with Kepler and attended Galileo's lectures. He claimed to have discovered the four largest moons of Jupiter in 1610, the same year that Galileo discovered them (independently).


MARS

Mars, the "red planet," is the fourth planet from the sun.


MARS FACE

This photograph of the Cydonia Mense region of Mars was taken by NASA's Mars Global Surveyor in 1998. It is a coincidental alignment of rocks and other geologic formations that happens to look like a human face from this angle.
Planet Mass
(kg)
Mercury 3.3 x1023
Venus 4.87 x1024
Earth 5.98 x1024
Mars 6.42 x1023
Jupiter 1.90 x1027
Saturn 5.69 x1026
Uranus 8.68 x1025
Neptune 1.02 x1026
Pluto 1.29 x1022

MASS

Mass is a measure of the quantity of matter.

MASS DEFECT

The mass of an atomic nucleus is less than the sum of the masses of the individual protons and neutrons that constitute it. This difference in mass is called the mass defect. When protons and neutrons come together to form an atomic nucleus, the extra mass is converted into energy (E=mc2), which is released. Stars are powered by this mechanism.

MASS EXTINCTION

Mass extinction is the process in which huge numbers of species die out suddenly. The dinosaurs (and many other species) went extinct during the K-T extinction, which was probably caused by an asteroid colliding with the Earth.

MASS LUMINOSITY RELATION

The mass luminosity relation states that more massive a star is, the more luminous it is.

MATTER

Physical objects are made of matter. Matter can exist in four phases (solid, liquid, gas, and plasma) and a few other extreme phases, like critical fluids and degenerate gases.

MCAULIFFE, CHRISTA

Sharon Christa Corrigan McAuliffe (1948-1986) was an American schoolteacher who chosen to be first teacher in space. She was killed, along with her six fellow astronauts (Francis R. Scobee, Michael J. Smith, Judith A. Resnik, Ellison S. Onizuka, Ronald E. McNair, and Gregory B. Jarvis), when the NASA's Space Shuttle Challenger Mission 51-L exploded only 73 seconds after its launch on the morning of January 28, 1986. McAuliffe was born on September 2, 1948, in Boston, Massachusetts. She taught at Concord High School in New Hampshire before being chosen for the Space Shuttle mission (she was chosen from over 11,000 applicants). McAuliffe was married and had two children.

MEGAPARSEC

A megaparsec (Mpc) is a unit of distance that is equal to one million parsecs, 3.26 x 106 light-years or 3.085678 x1019 kilometers. The Local Group of galaxies is roughly a megaparsec in diameter.


MERCURY

Mercury is the planet closest to the sun. It a small, dense, rocky planet with almost no atmosphere.

MERIDIAN

The meridian is an imaginary north-south line in the sky that passes through the observer's zenith.

MESON

Mesons are a type of hadron (strongly interacting particles) that contain an even number of quarks. Most mesons are composed of one quark and one antiquark. Examples include the pion, B-meson and the kaon.


MESOSPHERE

The mesosphere is the atmospheric layer between the stratosphere and the ionosphere. The mesosphere is characterized by temperatures that quickly decrease as height increases. The mesosphere extends from between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.


MESSIER, CHARLES

Charles Messier (1730-1817), was a French astronomer who searched the skies for comets. He compiled a list of 103 fuzzy objects (nébuleuse sans étoile, or starless nebulosities) in space in order not to mistake star clusters, galaxies, and nebulae for comets (for which he was searching). The Messier list has been added to and now consisted of 35 galaxies, 30 open clusters, 29 globular clusters, 4 planetary nebulae, 7 diffuse nebulae, and two unconfirmed objects (which were mistaken for nebulae by Messier). For a list of the Messier objects, click here.


MESSIER OBJECTS

During the late 18th century (1759-1781), the French astronomer Charles Messier made a list of 103 fuzzy objects in space in order not to mistake star clusters, galaxies, and nebulae for comets (for which he was searching). More objects were added later, bringing the total to 110. In it, M1 is the Crab Nebula, M2 is a globular cluster in Aquarius, M3 is a globular cluster in Canes Venatici, etc.


METAMORPHIC ROCK

Metamorphic rocks are compacted by pressure and heat from deep inside the earth.


METEOR

A meteor is a meteoroid that has entered the Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star. Most burn up before hitting the Earth.


METEORITE

A meteorite is a meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.


METEORITES FROM MARS

Many meteorites from Mars have landed on Earth, one with a suspected microbial fossil.


METEOROID

Meteoroids are tiny stones or pieces of metal that travel through space.
Meteor Shower Approximate Dates Date of Maximum Approximate Hourly Rate of Meteors Velocity
km/sec
Parent Comet
Quadrantids (visible by Boötes) Dec. 8-Jan. 7 Jan. 3 40-60 42 unknown
Lyrids Apr. 16-25 Apr. 22 10-15 48 Thatcher 18611
Eta Aquarids April 21-May 12 May 5-6 20-50 66 Halley (periodic)
Delta Aquarids July 14-Aug.18 July 28-29 20 41 unknown
Perseids July 23-Aug. 22 Aug. 12 50-75 60 Swift-Tuttle (periodic)
Orionids Oct. 15-29 Oct. 21-22 20-25 66 Halley (periodic)
Southern Taurids Sept 17-Nov. 27 Oct. 30- Nov. 7 10-15 - Encke (periodic)
Leonids Nov. 14-20 Nov. 17-18 15-80+++ 71 Tempel-Tuttle (periodic)
Geminids Dec. 6-19 Dec. 13-14 50-85 35 Asteroid #3200 Phaeton
Ursids Dec. 17-25 Dec. 22 15 34 Tuttle 1790

METEOR SHOWER
A meteor shower is a phenomenon in which many meteors fall through the atmosphere in a relatively short time and in approximately parallel trajectories. They occur when the Earth passes through a comet's orbit, and left-over comet debris (rocks, etc.) bombards the Earth. Each meteor shower occurs at a predictable time each year. Showers are named after the constellation they seem to originate from (their radiant). A huge meteor shower is called a meteor storm; it can produce up to thousands of meteorite each hour.


METEOR STORM

A meteor storm is a very intense meteor shower.

METHANE

Methane (CH4) is an odorless, colorless, flammable gas.


METIS

Metis is the closest of Jupiter's 16 moons . Metis is 25 miles (40 km) in diameter and orbits 79,500 miles (128,000 km) from Jupiter, within Jupiter's main ring. Metis and the next moon, Adrastea, are probably the source of the dust in this ring. Metis has a mass of 9 x 1016kg. It orbits Jupiter in 0.294780 (Earth) days; this is faster than Jupiter rotates on its axis. Metis was discovered by Stephen Synnott (Voyager 2) in 1980.

MICROGRAVITY

Microgravity is a state in which gravity is reduced to virtually negligible levels. For example, when an object is in free fall, it experiences microgravity.

MICROWAVE RADIATION

Microwave radiation is a type of electromagnetic radiation that has a wavelength between 1 mm and 30 cm.

MILANKOVITCH THEORY

The Milankovitch Theory attempts to explain major temperatures changes on Earth, especially ice ages, by slight variations in the amount of sunlight reaching the Earth caused by the eccentricity of the Earth's orbit around the Sun. The Earth's orbital eccentricity changes the Earth's average distance from the sun and therefore slightly changes the amount of sunlight reaching the Earth. Milankovitch looked over the past 600,000 years and correlated summer temperature mimnima with four major ice ages in this time. Eccentricity cycles last over 100,000 years. This theory was proposed by Milutin Milankovitch in 1938. Recently, scientists (Richard A. Muller and Gordon J. MacDonald, July 11, 1997, Science) have found that changes in the axal tilt of the Earth's orbit more closely match glacial cycles for the past million years.


MILKY WAY

The Milky Way is a bright line of stars stretching across the night sky. It is easier to see when you are far from bright city lights.


MILKY WAY GALAXY

The Milky Way Galaxy is a spiral galaxy; our sun and solar system are a small part of it. Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-years in diameter and 1,000 light-years thick. There are about 2 x 10 11 stars in the Milky Way. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-years from the center of the Milky Way Galaxy. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur.

MILLIBAR

A millibar is a unit of pressure equal to a thousandth of a bar.

MILLIKAN

Robert Andrews Millikan (Mar. 22, 1868-Dec. 19, 1953) was an American physicist who measured the charge of the electron (in his oil-drop experiment, 1911). Millikan named "cosmic rays" in 1928, while studying the radiation from space. He won the Nobel Prize for Physics in 1923 for his work on the electron's charge and his work on the photoelectric effect. Millikan wrote many books.
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MILLION

A million is a thousand thousand. The dinosaurs lived millions of years ago.


MIMAS

Mimas is one of larger of the 18 moons of Saturn. It has a diameter of 235 miles (390 km) and it orbits at about 116,000 miles from Saturn. It has a very cratered surface, including one huge crater, Herschel, that is about one quarter of the moon's diameter (80 miles = 130 km wide). Herschel's central mountain is 4 miles (6 km) tall, about 2/3 the height of Mount Everest. Mimas has very low temperatures (-200° C = -328°F). Its orbital period is about 22.5 Earth days. Mimas was discovered by William Herschel in 1789.

MINERAL

A mineral is a naturally-occurring solid of definite chemical composition whose atoms usually form a regular pattern.


MINOR PLANET

A minor planet is another name for an asteroid.


MINTAKA

Mintaka (Delta Orionis) is the uppermost and faintest of the three major stars that form belt of Orion, a constellation in the Northern Hemisphere (the other two stars in the belt ate Alnitak and Alnilam). Mintaka (which means "belt") is located quite near the celestial equator. It is the seventh brightest star in Orion; it is a variable star of magnitude 2.22-2.5; it is also a double star. It is about approximately 915 light-years from Earth. Its spectral type is O9.5II .


MIR

Mir (which means "peace" in Russian) was a Russian space station, the first semi-permanent human habitat in orbit around the Earth. The first part of Mir (the Mir module) was launched on February 20, 1986; it orbited the Earth for over 15 years. Mir is composed of many of different modules that have been attached to it through the years; the modules perform different functions. Cosmonauts and astronauts from many countries have lived and performed scientific experiments on Mir. Mir fell back to Earth on March 23, 2001.


MIRA

Mira (Omicron Ceti) is a well-known variable red giant star in the constellation called Cetus. It was discovered in 1596 by David Fabricus, an amateur Dutch astronomer. Mira (meaning "wonderful") was named by Johannes Hevelius in 1662. Its mass is about the same as our Sun but it varies in size and brightness over a period of 332 days (about 11 months). During this period, its magnitude varies from 3.4 to 9.3.


MIRANDA

Miranda is one of the 18 moons of Uranus. Its surface is marked by huge grooves, and appears to have shattered many times. It has a bright, V-shaped groove on its surface, the Inverness Corona, which has been nicknamed the chevron. Miranda's diameter is about 290 miles (470 km) and its mass is 6.3 x10+19 kg. It orbits at a distance of about 80,000 miles (129,780 km) from Uranus, and orbits once every 33.8 hours. It was discovered by Gerard Kuiper in 1948.
Some Mira Variable Stars Magnitude Range Period (days)
R Carinae 3.9-10.5 308.7
R Centauri 5.3-11.8 546.2
Mira
(Omicron Ceti)
3.4-9.3 332.0

MIRA VARIABLE STAR

A Mira variable star is a variable star whose brightness and size cycle over a very long time period, in the order of many months. Miras are pulsating red giants that vary in magnitude as much as a factor of many hundred (by 6 or 8 magnitudes). Mira variables were named after the star Mira, whose variations were discovered in 1596.
MITCHELL, MARIA
Maria Mitchell (August 1, 1818 -June 28, 1889 ) was the first woman Professor of Astronomy of the United States. In 1865, Maria Mitchell became a professor of astronomy at Vassar College in Poughkeepsie, New York (she had previously been a librarian) . She discovered a comet (in 1847), studied the planets Jupiter and Saturn, and photographed many stars. Despite her accomplishments, when she visited the Vatican Observatory in Italy, she was only allowed to enter the observatory during the day. Maria Mitchell was the first woman accepted into the American Academy of Arts and Sciences (1848), the Association for the Advancement of Science (1850), and the American Philosophical Society (1869). Mitchell was one of the founders of the American Association for the Advancement of Women (1873).


Mohorovicic, Andrija

Andrija Mohorovicic (1857 - 1936) was a Yugoslavian geophysicist. After examining seismic waves from the 1909 Kulpa Valley earthquake, Mohorovicic theorized that a boundary between the Earth's crust and the upper mantle existed (about 50 km beneath the surface) in which the speed of earthquake waves became very rapid. This region is now called the Mohorovicic discontinuity. A crater on the far side of the moon was also named for Mohorovicic.


MOHOROVICIC DISCONTINUITY

The Mohorovicic discontinuity separates the crust and the upper mantle of the Earth.


MOLDAVITE

Moldavite is a rare, glassy, translucent, dark green gemstone. Moldavite is a silica-based tektite, a mineral formed when a meteorite (a rock from space) struck the Earth's surface and melted and fused the surrounding rock. Moldavite is only found in Bohemia (the Czech Republic) in the Ries Crater in the Moldau River valley (which it was named for). Moldavite was discovered in the late 1800's; the meteorite from which it formed hit the Earth about 14.7 million years ago. Moldavite has a Mohs hardness of 5.5-6.6. Inclusions of gas bubbles and iron/nickel spherules are common. This natural glass has been used for jewelry, religious articles, and decorative objects since prehistoric times.

MOMENTUM

Momentum is an object's mass times its velocity.

MONOCHROMATIC

Monochromatic means having light of only one wavelength.


MONS

A mons is a mountain. For example, the Olympus Mons is a huge mountain (an extinct volcano) on Mars.


MOON

A moon is a natural satellite orbiting a planet. (Not all planets have moons.)

The moon is Earth's only natural satellite. It travels around the Earth in about one month (27 days 8 hours). It is about 240,000 miles (384,000 km) from Earth. The temperature on the Moon ranges from -320°F to 248°F (-160°C to 120°C). The moon has no atmosphere.


MOON BUGGY

The Moon buggy (also known as the lunar rover or the Lunar Roving Vehicle) was a Jeep-like vehicle that the astronauts used to drive on the moon during the Apollo 15, 16, and 17 missions. It weighed 450 pounds, (or 75 pounds in the Moon's gravity). It could carry 1000 Earth-pounds. On the flight from the Earth to the moon, the lunar rover was folded up in the lunar module. There are three abandoned lunar rovers on the Moon.


MOON LANDING

NASA's Apollo missions sent people to the moon for the first time. Apollo 11's LEM (Lunar Excursion Module) was the first to land on the moon, on July 20, 1969, with Neil Armstrong and Buzz Aldrin (Michael Collins was in the orbiter). Apollo missions 12 through 17 continued lunar exploration.

MOONRISE

The moon rises and sets every day, appearing on the horizon just like the sun. The time depends on the phase of the moon. It rises about 30 to 70 minutes later each day than the previous day, so the moon is out during daytime as often is it's out at night. At the time of the new moon, the moon rises at about the same time the sun rises, and it sets at about the same time the sun sets. As the days go by (as it waxes to become a crescent moon, a half moon, and a gibbous moon, on the way to a full moon), the moon rises during daytime (after the sun rises), rising later each day, and it sets at nighttime, setting later and later each night. At the full moon, the times of moonrise and moonset have advanced so that the moon rises about the same time the sun sets, and the moon sets at about the same time the sun rises. As the moon wanes (becoming a half moon and a crescent moon, on the way to a new moon), the moon rises during the night, after sunset, rising later each night. It then sets in the daytime, after the sun rises. Eventually, the moon rises so late at night that it's actually rising around sunrise, and it's setting around sunset. That's when it's a new moon once again.
Planet Number of Moons
Mercury 0
Venus 0
Earth 1
Mars 2
Jupiter 16
Saturn 18
Uranus 15
Neptune 1
Pluto 1

MOONS

A moon is a large body that orbits around a planet. Many planets have moons.
MOORE, PATRICK
Patrick Moore (1923- ) is an English astronomer who has written over 60 books on astronomy and made regular BBC television appearances popularizing astronomy. He has done work on lunar mapping.
MT. WILSON OBSERVATORY
The Mt. Wilson Observatory is an astronomical observatory located in Pasadena, California, California. It was founded in 1904 by the astronomer George E. Hale.

MULLER, JOHANN

Johann Müller, also known as Johann Regiomontanus (1436-1476) was a German astronomer and mathematician. He studied trigonometry, translating Ptolemy's Almagest, from the original Greek. Ironically, his translation helped overthrow the Ptolemaic view of the universe (in which the Earth was thought to be at the center of the universe). He also did work on plane and spherical trigonometry. Muller also observed the motion of the moon, planets, and comets. A 108 km diameter lunar crater, called Regiomontanus (Latitude: -28.3 degrees, Longitude: 1.0 degrees), was named for Muller.


MUSES-CN

MUSES-CN is a Japanese nanorover, a small, book-sized rover designed to be sent to explore the Near-Earth asteroid 4660 Nereus. MUSES stands for the Mu Space Engineering Spacecraft; C indicates that it is third in a series, N is for NASA. It will be launched from Japan in Jan. 2002; it will be the smallest rover ever to fly on a space mission. It will analyse rock samples, send color video to Earth, collect rock samples, and will eventually return to Earth (in 2006).


MUTTNIK

Muttnik is a term that many people used for the dogs who were sent into space on Sputnik missions. Between 1958 and 1961, the USSR (now Russia) sent 13 dogs into space in preparation for future missions. Laika (meaning "barker" in Russian) was the first dog in space; she was launched in Sputnik 2 on November 3, 1957.
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MYA

"mya" stands for millions of years ago.
Astronomy Dictionary
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