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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.

R

RADAR SCATTEROMETER
A radar scatterometer is a device that emits and detects reflected microwave radiation The scatteromenter measures the radar echo (or radar backscattering cross-section, called "sigma-0") of the Earth's surface. Different terrains (like flat land, rough surfaces, and water) scatter microwaves differently. A radar scatterometer has been used to map Earth's polar regions (since it can see through clod and darkness.

RADIAL MOTION

Radial motion is motion towards or away from the observer. Radial velocity can be determined using the doppler shift. Motion towards the observer shifts spectral lines towards the blue; motion away from the observer shifts spectral lines towards the red.
radiant

RADIANT

A radiant is the point in the sky from which a meteor shower seems to be coming. For example, the Geminids meteor shower seems to come from the constellation Gemini.

RADIO ASTRONOMY

Radio astronomy involves exploring space by examining radio waves from outer space. Radio astronomy was pioneered by Karl Gothe Jansky, who in1932 first detected radio waves from a cosmic source - in the central region of the Milky Way Galaxy. Gote Reber (a ham radio operator) made the first true radio telescope (usiing a 32-foot diameter parabolic dish to focus the radio waves) after reading of Jansky's discoveries.

RADIO BUBBLE

The "radio bubble" is a sphere centered on Earth that is expanding at the speed of light (the speed of radio transmissions). The leading edges of the bubble correspond with the first artificial radio signals that escaped the Earth's atmosphere. The radio bubble now extends beyond Alpha Centauri.


RADIO GALAXY

A radio galaxy is a galaxy that emits radio waves. Cygnus A is the most powerful radio galaxy close to Earth.

RADIOSONDE

A radiosonde is a small device that measures many atmospheric conditions high in the atmosphere, like temperature, wind speed and direction, air pressure, and humidity while suspended from a weather balloon. The device has a radio transmitter and sends its data back to ground recorders. A radiosonde can ascend to about 100,000 feet (30,000 m), giving a 3-dimensional model of the weather. The radiosonde was developed in the 1930's.

RADIO STAR

Quasars were called "radio stars" in the early 1960's when they were first detected, because they emit large amounts of radiation, including radio waves.


RADIO TELESCOPE

A radio telescope is a metal dish that gathers radio waves from space. One example is the Very Large Array (VLA) in New Mexico.

RADIOISOTOPE DATING

Radioisotope dating is used to find out how old fossils are.

RADIO WAVES

Radio waves are a form of electromagnetic radiation with long wavelengths and relative low energy.


RADIUS

The radius is the distance from the center of a circle (or a sphere) to the edge.

RALEIGH SCATTERING

Raleigh scattering is the phenomenon in which light is scattered by objects small in relation to the wavelength of the light. For example, the scattering of sunlight by gas molecules in the atmosphere is caused by Raleigh scattering of blue light. Raleigh scattering is named for the British physicist John William Strutt Lord Rayleigh (1842-1919).
REBER, GROTE
Grote Reber (Dec. 11. 1911-Dec. 20. 2002) was a radio engineer and pioneering amateur astronomer who was the first person to follow up Karl Jansky's discovery of radio waves coming from space. Reber built a 9-meter parabolic reflector dish radio antenna in his yard in Illinois - it was tte first radio telescope used for astronomy. He detected the first signals (at a frequency of 160 megahertz, about 2m wavelangth) in 1939, using his third receiver. Reber's work led to many developments in radio astronomy; he made the first radio maps of the sky and showed that the brightest areas corresponded to the center of the Milky Way. Reber started the field of very long-wavelength/low-frequency (1-2 MHz, 150-300 m wavelength) radio astronomy, moving to Tasmania (an island off the southeastern coast of Australia) where these radio waves can be received (because the long-wavelength radio waves can get through the Earth's ionosphere over that part of the globe due to a hole in the ozone layer).

RED DWARF

A red dwarf is a small, cool, very faint, main sequence star whose surface temperature is under about 4,000 K. Red dwarfs are the most common type of star. Proxima Centauri is a red dwarf.

RED GIANT STAR

A red giant is a relatively old star whose diameter has swollen enormously. It's temperature has also cooled appreciably, it's contracting hydrogen core has turned to helium and eventually to carbon. Our Sun will become a red giant star in about 5 billion years.

RED SUPERGIANT STAR

A red supergiant is a relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K). They are frequently orange-red in color. Betelgeuse is a red supergiant.

RED SHIFT

The red shift is an increase in the wavelength of the light that is emitted from an object that is moving away from us. This increase in wavelength makes the object appear to be redder than it actually is. For example, when a star is travelling away from Earth, its light appears redder (the light waves are elongated, lengthening the wavelength). The expansion of the universe was discovered when E. Hubble observed that the light from almost all other galaxies was red-shifted. Compare with blue shift.


REFLECTING TELESCOPE

A reflecting (or Newtonian) telescope uses two mirrors which magnify what is viewed. The first reflecting telescope was first described by James Gregory in 1663.
Pleiades

REFLECTION NEBULA

A reflection nebula is a nebula that glows as the dust in it reflects the light of nearby stars. These nebulae are frequently bluish in color because blue light is more efficiently reflected than red light. A reflection nebula surrounds the Pleiades Star Cluster.


REFRACTING TELESCOPE

A refracting telescope uses two lenses which magnify what is viewed; the large primary lens does most of the magnification. The first refracting telescope was invented by Hans Lippershey in 1608.

REGIO

A regio is an area which is disctinctive from surrounding regions due to a difference in reflectivity or color.

REGIOMONTANUS

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 obsesrved 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.

REGOLITH

Regoliths are the loose, fragmented mantle rock fragments (of various sizes) and dust on a planet, asteroid, or moon surface. On the Moon's surface, regolith was formed by the meteorite bombardments of the Moon's crust; is the rocky debris which is thrown out of impact craters.


RELATIVITY

Albert Einstein formulated the Theories of Special and General Relativity. Special relativity supplanted Newtonian mechanics, yielding different results for very fast-moving objects. The Theory of Special Relativity is based on the idea that speed has an upper bound; nothing can pass the speed of light. The theory also states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called space-time. Energy and momentum are similarly linked. As a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc2. General Relativity expands the theory of special relativity to include acceleration and gravity, both of which are explained via the curvature of space-time.

REMOTE SENSING

Remote sensing is the gathering of data from a distance. For example, spacecraft are sent into space with devices designed to study objects and phenomena and then send the information back to Earth.

RETROGRADE MOTION

The planets generally move from west to east in the sky. Occasionally, one of the planets seems to slow down, stop, and loop backwards, moving from east to west for a short time. After a short time, all the reverse happens and all is back to normal. The planet did NOT stop or move backwards in its orbit; for a while, the Earth was moving faster than than the planet, overtaking the planet for a short time, making it seem to lag.

RETROGRADE ORBIT

An object is in retrograde orbit around a body when the object orbits in a clockwise direction when viewed from the north pole of the ecliptic. Retrograde orbits are VERY unusual; the moon Triton is in retrograde orbit around the planet Neptune.

RETROGRADE ROTATION

Retrograde rotation means rotating in the opposite direction. All the planets in or Solar System rotate in the same direction except for Venus, Pluto, and Uranus which rotate in the opposite direction (they have retrograde rotation).


REVOLUTION

Revolution is the movement of one object around another. For example, the revolution of the Earth around the Sun takes one year.


REVOLVE

When an object moves in orbit around another object, it revolves around it. The Moon revolves around the Earth. The Earth revolves around the Sun.


RHEA

Rhea is one of the 18 moons of Saturn. It is white with some streaks and is very reflective (because it has an icy surface). It has a diameter of about 800 miles (1300 km). Rhea orbits at about 327,000 miles from Saturn. Its orbital period is 108.5 hours. It was discovered by Cassini in 1672.

Richter, Charles F.

Charles Francis Richter (April 26, 1900- April 30, 1985) was a geologist who developed the Richter scale, a logarithmic scale that measures the intensity of an earthquake. He developed it in 1935 at the California Institute of Technology.

Richter Scale

The Richter scale is a logarithmic scale that measures the intensity of an earthquake. It was developed in 1935 by Charles F. Richter. The magnitude of an earthquake is calculated from the logarithm of the amplitude of waves recorded by seismographs. Beno Gutenberg also contributed to the more general application of the Richter scale. A magnitude 2.0 or less earthquake is called a microearthquake and is not felt by people. A magnitude 4.5 or more earthquake can be measured by seismographs all over the world.
RIDE, SALLY
Sally Kristen Ride (May 26, 1951-2012) was the first American woman in space. Dr. Ride's first trip into space was aboard NASA's space shuttle Challenger (STS-7) in 1983 (June 18-June 24). Her second (and last) space flight was the eight-day Challenger (STS 41-G) mission (in October, 1984).

For more information on Ride, click here.
For a cloze activity on Ride, click here



RIGEL

Rigel (beta Orionis) is the brightest star in the constellation called Orion and one of the brightest stars in the sky. It is a blue (very hot) supergiant, over 60 million miles in diameter (almost 100 times bigger than the sun). It is more than 50,000 times more luminous than the Sun. It has an absolute magnitude of -7.1 and an apparent magnitude of +0.12. It is over 900 light-years from Earth.


RIGHT ASCENSION

Right ascension is a celestial coordinate that is used to measure the degrees of longitude on the on the celestial sphere. Zero degrees of right ascension is the position of the Sun during the vernal (spring) equinox (March 21).


RILLE

A rille is a long, narrow valley on the surface of the moon. Hadley Rille (above) is 60 miles (100 km) long, 1300 feet (400m) deep, and almost 1 mile (1500m) wide at its widest point.

RIMA

(plural rimae) A rima is a fissure on the surface of a planet or moon.


RING NEBULA

A ring nebula is a huge cloud of dust and gas that is shaped like a ring. Pictured above is M-57, a ring nebula in Lyra which is 2,000 light-years from Earth and was generated by a dying star (this nebula was discovered by Ch. Messier).


RINGS

Many planets are orbited by rings of rock, ice and/or dust. Saturn, Jupiter, Uranus, and Neptune have rings.
Planet Roche Limit (km) Roche Limit (miles)
Earth 18,470 11,470
Jupiter 175,000 108,000
Saturn 147,000 92,000
Uranus 62,000 39,000
Neptune 59,000 37,000



ROCHE LIMIT

The Roche limit is the distance from the center of a star or other object at which a large orbiting object will break up due to tidal (gravitational) forces. Large planets or moons cannot orbit within the Roche limit; they break up. The Roche Limit was first realized by Edouard Roche in 1848. If a planet and its moon have identical densities, then the Roche Limit is 2.446 times the radius of that planet.


ROCKET

A rocket is a machine that propels itself and a payload into space.

ROMER, OLE

Ole Romer (1644-1710) was a Danish astronomer who, in 1675-1676, was the first person to demonstrate that the speed of light is finite. Romer did this by observing eclipses of Jupiter's moon Io as Jupiter's distance from Earth varied through the year. He noticed that the observed period of Io's orbit differed by about 20 minutes; he concluded that this difference was due to the extra distance that the light had to travel to Earth. His calculations put the speed of light at about 225,000 kilometers per second (it is really a bit faster, at 299,792 kilometers per second).
axis


ROTATE

When an object rotates, it turns around a central point or axis. One planetary day is defined as the time it takes the a planet to rotate around its axis.
Planet Rotational Period
(in earth days or hours)
Mercury 58.65 days
Venus 243.01 days
Earth 23.93 hours
Mars 24.62 hours
Jupiter 0.41 days
Saturn 0.44 days
Uranus 0.72 days
Neptune 0.67 days
Pluto 6.38 days


ROTATIONAL PERIOD
A rotational period is the time that it takes a planet, moon or other orbiting body to turn once around its axis - one "day".
Sojuner rover

ROVER

A rover (or microrover) is a remote-controlled robotic vehicle. Sojuner rover was the first robotic explorer to explore Mars; this 25-pound, six-wheeled microrover explored an ancient Martian flood plain (Ares Vallis) in 1997.

ROYAL GREENWICH OBSERVATORY

The Royal Greenwich Observatory is athe national astronomical observatory of England. It is located in Greenwich, England (near London).

RUNAWAY STAR

A runaway star is a massive, young, and hot star that is moving very, very quickly through space. Runaways are probably propelled through space by the explosion of a companion star undergoing supernova (an idea proposed in 1961 by the Dutch astronomer Adriaan Blaauw). Mu Columba is a runaway star.

RUNOFF

Runoff is water (or other liquids) that drains or flows from the land into streams and rivers, eventually into seas. The water is generally from rain or snowpack melt.

RUPES

Rupes are a line of mountainous cliffs on a planet or lunar surface. For example, the Rupes Altai is a mountain range about 315 miles (50 km) long on our moon.


RUSSELL, HENRY N.

Henry Norris Russell (1873-1967) was an American astronomer who, independently of E. Hertzsprung, realized the relationship between a star's temperature (color) and its brightness, and designed a diagram illustrating this relationship in 1913, later called the Hertzsprung-Russell Diagram.
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.
If the astronomy term you are looking for is not in the dictionary, please e-mail us.

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