Karen Hill

A Mercurial year, or the time it takes for Mercury to revolve around the Sun once, is the equivalent of about 88 Earth days (2,112 hours). A Mercurial solar day, the time it takes for the Sun to go from high noon to high noon as seen from Mercury’s surface, takes 176 Earth days (4,224 … Read more

The planet Mercury is too small for its gravity to retain any significant atmosphere over an extended period of time. However, Mercury does have a “tenuous surface-bounded exosphere” containing hydrogen, helium, oxygen, potassium, and other elements. The hydrogen and helium that make up Mercury’s very thin atmosphere don’t actually originate on the planet. The gases … Read more

Just looking at the planet Mercury doesn’t give astronomers very much information. Fortunately, electromagnetic waves, mostly radio waves, from Mercury are picked up around the world. They have helped establish the topography and composition of Mercury. Also, twice in 1974 and once in 1975, the spacecraft Mariner flew by Mercury and photographed as much of … Read more

The earliest known recorded observations of Mercury are from the Mul.Apin tablets around the 14th century BC. The ancient Greeks called the planet Στίλβων (Stilbon), which means “the gleaming”. The Romans named the planet Mercury after the swift-footed Roman messenger god, Mercury, or Mercurius in Latin. Mercury is the smallest planet in the solar system … Read more

The planet Mercury seems to have been hit by a lot of asteroids. Just under 50 percent of Mercury’s surface has been photographed by spacecraft. The photos show an average of one hundred to one thousand 6-mile (10-km) craters per each 600,000 square miles, or 1 million square kilometers. The appearance of so many craters, … Read more

Mercury orbits the Sun once every 88 days at a speed of 29.8 miles (48 km) per second. Its aphelion (farthest point from the Sun) is at a distance of 43 million miles (69 million km). Mercury’s perihelion (closest distance from the Sun) is 30 million miles (46 million km). Because this orbit is not … Read more

Consider where Mercury is located, an average of 36 million miles from the Sun. Since Mercury never travels far from the Sun, it is not visible at night. But looking for it during the day is dangerous, because of its proximity to the Sun. If you scan the sky, with binoculars, a telescope, or your … Read more

The largest crater photographed so far has been named Beethoven, after the famous composer Ludwig van Beethoven. Mercury’s topographic features are named for famous people, such as Shakespeare and Michelangelo; exploration ships, including the Santa Maria; radar installations, such as the Russian (Soviet) Vostok; and other gods, such as the Scandinavian king of gods, Odin. … Read more

What Is the Most Obvious Physical Feature On the Planet Mercury and How Many Impact Craters Does Mercury Have? The largest topographic feature seen on Mercury is the Carolis Basin, a depression as wide as Texas about 800 miles (1,280 km in diameter). It is bordered on one side by tall mountains, called scarps, which … Read more

Rills are extended crevices on the Moon’s surface. They can run up to at least 150 miles (250 km) and over a mile (1.6 km) wide. The thousands of rills that cut through the Moon’s terrain were probably formed from channels of molten lava. In fluvial geomorphology on earth, a rill is a narrow and … Read more

Being smaller than Earth, the Moon does not have nearly as much gravity. It is enough to cause tides on Earth, but it still is only about 17 percent of Earth’s gravity. Say you weighed 100 pounds (45.4 kg) on Earth. If you went to the Moon for summer vacation, you would weigh only 17 … Read more

A lunar dome is a type of shield volcano that can be found on the surface of the Earth’s Moon. Lunar domes, so named for their shape, may be the remnants of volcanic vents, or openings. They are like volcanic bubbles or blisters dotting the landscape of the mares. Lunar domes are most likely formed … Read more

The Moon’s gravity is not strong enough to hold an atmosphere in place like on earth. Although the moon technically has no atmosphere, some elements such as helium-4 have been detected by Chandrayan-1 and found to vary with latitude. Small traces of water vapor have even been detected and is possibly generated from the sublimation … Read more

In comparison to the outer planets, the four planets nearest the Sun are remarkably similar. The inner planets are all basically made of dense rock; they are all relatively small; and they are all situated fairly close together. Astronomers believe that the cores of the inner planets are made out of metals such as iron … Read more

Mercury, Venus, Earth, and Mars, the first four planets in order from the Sun to the asteroid belt, are called the inner planets or terrestrial planets. The inner planets all have hard, solid surfaces, terra means “earth” in Latin. They share a few other characteristics of Earth that partially result from their proximity to the … Read more

Sometimes called rays and sometimes called streaks, crater rays are visible lines that radiate from the center or rims of some craters. Rays are made up of material that was ejected from the crater when a meteorite hit the lunar surface. The crater Tycho, located in the southeast quadrant of the Moon, illustrates the most … Read more

Though the era of massive meteorite bombardment has been over for some 3.5 billion years, the Moon is constantly hit by micrometeorites. Micro means tiny, and meteorites are matter from space that crash into the surface of a planet or satellite. Micrometeorites are celestial debris, remnants of asteroids, comets, and planets, that hit the Moon … Read more

Lunar mountains bear little resemblance to mountains on Earth. They were not formed by plate tectonics, as on Earth, but mostly border the mares and craters. About the only thing they have in common is height, and in that category, the Moon beats out Earth. Several peaks in the Moon’s Leibnitz Range are taller than … Read more

Until 1959, when the then Soviet Union sent the satellite Lunik 3 all the way around the Moon, we could only guess at what the far side was like. Scientists wrongly assumed that it was the same as the near side. One nineteenth-century astronomer, however, thought that all the Moon’s air and water had collected … Read more

Galileo first named the dark areas of the Moon mares (or maria for plural) after the Latin word mare, which means “sea.” They looked like large oceans, and that’s what they were thought to be until the twentieth century. We now know that mares are large, relatively flat plains of lava hardened into basalt. Oddly … Read more

One unexpected difference is that there is only one major mare, named Moscoviense, on the far side, but there are many on the near side. Scientists hypothesize that just as the Moon’s gravitational force causes tides on Earth, Earth’s gravitational force probably has an effect on the Moon, especially the side facing Earth. Since there … Read more

The Moon is relatively close to Earth, and so it is the most obvious object in the sky, aside from the Sun. It is also huge for a satellite, being one-quarter the size of Earth. The Moon’s gravity is not strong enough to hold an atmosphere, so there are no winds to fill the craters … Read more

Neither straight, nor a wall, the Straight Wall is a fascinating topographical feature in the Moon’s southwest quadrant on the eastern border of the Mare Nubium. It looks like a straight, precise wall, but it is a fault, or geological crack in the surface, with a 1,000-foot (300-m) drop on its west side. Because of … Read more

The Moon’s surface has highlands and lowlands just like earth. The main feature on the Moon’s surface are the hundreds of thousands of craters, or the remnants of craters, found in both regions, some with crater rays of ejected lunar material emanating out from their edges. Besides craters, there are mountains, mares, rills, and domes. … Read more

The highlands on the moon are areas of mountains and valleys, whereas the lowlands are dominated by mares, or areas of flat plains. The highlands, also known as terrae, have been dated as forming 4.4 billion years ago, are the lighter-colored regions of the Moon. The dark, featureless lunar plains which can clearly be seen … Read more

There are hundred of thousands of craters visible on the Moon, measuring from a few feet across (craterlets) to about 150 miles across (basins or walled plains). The craters seem to be the result of both meteorites and volcanic activity. Most scientists agree that the Moon came under a meteorite bombardment some 4.5 billion years … Read more

A lunar crater is not simply a large hole in the ground. Meteorites build up energy as well as speed. This energy has the force of dynamite upon impact, causing the meteorite and the surrounding crust to vaporize. Material that isn’t instantly incinerated is thrown out of the crater. The ejected debris creates walls around … Read more

The Moon’s gravity creates tides here on Earth. As Earth rotates on its axis, different parts of the world face the Moon. The waters in those areas are attracted to the Moon because of the satellite’s gravitational pull. While the Moon passes overhead, water in oceans and large lakes follows it, creating high tides (higher … Read more

We’ve almost all seen him, the Man in the Moon. His bright, benign face shines down on Earth around the time of the full moon. But he isn’t really there. This demonstration will show you what is going on. Equipment needed approximately seven dominoes (or matchboxes, or any small objects you can make stand upright … Read more

The Moon reflects the light the Sun shines on it. The apparent changes in its shape, from full to gibbous to half to crescent to new, does not mean the Moon is actually getting smaller or larger. The Moon appears to wane (grow smaller) and wax (grow larger) because, as it rotates around Earth, we … Read more

For the phases to begin over again, the Moon, Earth, and Sun all have to line up in a row, whether you start a phase at a new moon or a full moon. While one revolution of the Moon around Earth lines up the Moon and Earth again, the Sun is no longer in the … Read more

The troposphere is the lowest section of Earth’s atmosphere and is the atmosphere we breathe. It consists mostly of nitrogen (78 percent) and oxygen (21 percent), with trace amounts of ten other elements. The troposphere contains about 75% of the atmosphere’s mass. Water vapor can account for up to 4 percent of the air, depending … Read more

The gravitational force of Earth is sufficient to trap gases and particles around the planet. Without this gravity, the atmosphere would never have formed, as its components would have gone flying off into space. The Earth’s second atmosphere was formed about 3.4 billion years ago, with nitrogen being the major component. During the same period, … Read more

Seismographic instruments placed on the Moon by the Apollo astronauts determined that the satellite is mostly solid, with a crust, mantle, and core. Scientists are not sure what the core is made of, or whether it is at least partially molten, or melted. We do know, however, that it is made of the Moon’s densest … Read more

When astronauts on the Apollo space missions from 1969 to 1972 brought moon rocks back to Earth, scientists were able to find out what the moon was made of. The moon is not made out of green cheese, contrary to popular belief. The Moon’s crust is made of rock very similar to terrestrial volcanic rock, … Read more

Earth has a natural electromagnetic field of charged particles. These particles essentially allow for the existence of natural electricity. The particles reside in a layer around Earth called the magnetosphere. It is shaped like a teardrop as a result of the solar wind pushing the bulk of it away from the Sun. The area on … Read more

A natural satellite is a celestial body that orbits a planet or smaller body and is also commonly referred to as a moon. They’re both essentially the same thing. A satellite is an object that orbits another, parent, object. For instance, the Moon is Earth’s satellite, and the Earth is a satellite of the Sun. … Read more

The sister theory proposes that the Moon may have formed along with Earth as a result of the formation of the solar system. When astronauts retrieved moon rocks in 1969, however, scientists discovered that they were not made of matter from the solar nebula. If they were, they would have matched Earth rocks more closely. … Read more

There are many theories about how the Moon formed. The current favorite says that a huge asteroid, maybe the same one that is thought to have tilted Earth’s axis, hit Earth, throwing off a mass of debris into a broken ring around the planet. Over time, the debris coalesced, creating the Moon. At first the … Read more

The Moon doesn’t actually get smaller as it rises. It’s an optical illusion, or a trick that’s played on our eyesight. When we see the Moon on Earth’s horizon, our brain registers its size in comparison to objects on Earth: buildings, trees, the ocean. It appears huge and much bigger than when it is high … Read more

A break, or hole, in the ozone layer in Earth’s stratosphere exists over the southern polar region of Earth. This hole allows ultraviolet light from the Sun to reach Earth undiluted by ozone. There has historically been disagreement over whether this hole is a natural phenomenon or the result of man-made pollutants. But we now … Read more

The thermosphere is the biggest layer of the earth’s atmosphere and is located above the mesosphere and below the exosphere. Ultraviolet radiation causes ionization in the thermosphere. The thermosphere is located from about 80 kilometres (50 mi) to 500 kilometres (310 mi) above the earth’s surface. Temperatures rise in the thermosphere because of the presence … Read more

The Mesosphere is located above the stratosphere and directly below the thermosphere. By the time you reach the mesosphere there is only one-thousandth the amount of breathable air as when you are at sea level. The mesosphere is located about 50 to 85 kilometers, or 30 to 50 miles in the atmosphere above the Earth’s … Read more

The ionosphere layer of the atmosphere overlaps the mesosphere and thermosphere and is the uppermost part of the atmosphere. The temperature of the ionosphere ranges from about 200K, or -73 ºC (-99.4 ºF) to 500K, or 227 ºC (440.6 ºF). So basically at the bottom it is 200K. At the top it is 500K. K … Read more

The exosphere is the uppermost layer of the Earth’s atmosphere. Only the lightest gases, mainly hydrogen, with some helium and carbon dioxide find their way into the earth’s exosphere. The molecules in the exosphere are so thinly dispersed that they have as good a chance of escaping the atmosphere as colliding with another molecule. If … Read more

The stratosphere is the second major layer of Earth’s atmosphere, and just below the mesosphere, and just above the troposphere. The upper part of the stratosphere contains ozone, a gas that absorbs ultraviolet rays from the Sun. The temperature of the earth’s stratosphere ranges from about -67°F (-55°C) to 32°F (0°C). With warmer layers higher … Read more

The ozone layer is located in the lower region of the earth’s stratosphere and is a unique form of oxygen. About 90% of the earth’s ozone can be found in the ozone layer. The ozone layer acts as a powerful shield against the Sun’s dangerous high frequency ultraviolet rays. Without the ozone layer, ultraviolet radiation … Read more

Earth began, like the other terrestrial planets, as a very hot mass of volcanic material from the newly formed Sun. The volcanoes released water vapor, which rose, cooled, and formed clouds, which precipitated. The rain hastened the general cooling of the planet until Earth was cool enough that the water began pooling. After billions of … Read more

Volcanic activity led to the creation of land on Earth. Lava that spewed up during eruptions cooled to form first thin layers of the rock called basalt. In time, the layers grew to become the foundations of the landmasses. Chemical reactions between the volcanic materials, the water, and the atmosphere, everything continued to evolve simultaneously, … Read more

The planet Earth has an atmosphere, and it is segmented into layers. The troposphere begins at the planet’s surface and extends between 6 (10 km) and 10 miles (16 km) high. Above that, the stratosphere reaches 30 miles (50 km) from Earth’s surface. The next 20 miles (33 km) is called the mesosphere, and after … Read more

No one knows for sure how life first appeared on earth, but somehow the elements and chemicals on Earth developed into single living cells. These cells ate, reproduced, and died, leaving behind more material from which to make new life. The single plant cells evolved into single animal cells, which not only ate, reproduced, and … Read more

Most of the minor planets in our solar system are located in the asteroid belt between the orbits of the planets Mars and Jupiter. The size of these asteroids varies from tiny particles of dust to the minor planet Ceres, which is about 625 miles (1,000 km) in diameter. Combined, however, the estimated mass of … Read more

Late in the eighteenth century, a mathematical equation, now called the Titius-Bode law, was formalized to figure out the distance between major planets. It worked like this. This equation shows a remarkably accurate prediction of planets’ distance from the Sun. In fact, Uranus was discovered after the equation was published, right where it should have … Read more

The Titius–Bode law, also known as Bode’s law, is a hypothesis that the bodies in some orbital systems, including the Solar System, orbit at semi-major axes in an exponential function of planetary sequence. Johann Bode published Johann Titius’s planetary spacing equation in 1772 8 years after Titius worked it out, as his own. It was … Read more

The Earth is a rocky planet that geologists divide into three layers: the core, the mantle, and the crust. The core, about 4,000 miles (6,700 km) in diameter, is believed to be a solid sphere of iron (90 percent) and nickel (10 percent). Recent observations seem to indicate that the core is made of an … Read more

We are all well acquainted with Earth, but try to think about Earth in the same way you think about the other planets. Earth is the third planet from the Sun, and it orbits in a near-circular ellipse. The planet Earth is about the same size as Venus, and it has the same mass and … Read more

This bizarre-looking word, pronounced sih-zih-gee, is used to describe the alignment of three celestial bodies in a row. For instance, during a lunar or solar eclipse, the Sun, Earth, and Moon are in syzygy. The term syzygy is also applied to each instance of new moon or full moon when Sun and Moon are in … Read more

The terms inferior and superior conjunctions refer specifically to the alignment of the Sun, Earth, and either of the inferior planets, Mercury and Venus. At some point, all three line up in a row, sometimes with the Sun in the middle and sometimes with the inferior planet in the middle. When the Sun is in … Read more

More than 5,000 minor planets, or orbiting asteroids have been cataloged since 1801, when the largest, numbered 1 and named Ceres (1 Ceres), was sighted by Italian astronomer Giuseppe Piazzi. Some 200 more are discovered every year. Astronomers estimate that 50 percent of all asteroids that are 6 miles or larger in diameter have been … Read more

In the asteroid belt, between the planets Mars and Jupiter, a band of asteroids, relatively small, irregular, rocky chunks of matter, orbits the Sun. They are also called minor planets. The term “minor planet” has been used since the 19th century to describe astronomical objects in direct orbit around the Sun that is neither a … Read more

The materials found in the minor planets are similar to those of the major planets. Most meteorites found on Earth are asteroids, composed of the same rocky-metallic materials as planets. Their existence supports eighteenth-century astronomer Johann Titius’s theory that there should have been another major planet. Titius developed a remarkably accurate mathematical calculation to explain … Read more

There are three basic types of these asteroids, based on their physical composition. The rather common C-type asteroids are made mostly of carbon-based matter. They do not reflect much sunlight. S-type asteroids contain metals and silica minerals. They are brighter than C-type asteroids, but not as bright as M-type asteroids. M-type asteroids seem to be … Read more

There are three basic solar orbits: elliptical, parabolic, and hyperbolic. Planets have elliptical orbits, bound by the force of the Sun’s gravity. Elliptical orbits can be anywhere between almost circular and almost flat ovals. If the speed of an object is equal to the Sun’s escape velocity, 380 miles, or 630 km per second, it … Read more

Although these terms sound judgmental, the terms Inferior and Superior Planets are not meant that way. Relative to a particular planet, inferior planets are those that are closer to the Sun and superior planets are ones that are farther away from the Sun. For Earth, Mercury and Venus are inferior planets and all the others … Read more

Every planet orbits the Sun on a different plane. The differences between orbital planes are measured by their angles to Earth’s orbital plane. The difference is known as orbital tilt or incline. For instance, Mars’s orbit is tilted at an angle of 1.85° to Earth’s orbital plane. Orbital plane inclines range from 0.77° (Uranus) to … Read more

Opposition is the term for syzygy, or alignment of the Sun, Earth, and a superior planet when Earth is in the middle. This places the superior planet in full sunlight, making it best for viewing. A favorable opposition occurs when the superior planet is at its closest to the Sun (perihelion). An unfavorable opposition is … Read more

An occultation occurs when any celestial body obscures the sight of another by passing in front of it. The word can be applied to stars, planets, asteroids, comets, and all other space objects. Astronomical events which cause occultation include transits and eclipses. Every time an occultation occurs, an eclipse also occurs. During an “eclipse” of … Read more

Astronomical transit occurs when a smaller celestial object passes in front of a larger one. For instance, all inferior planets will cross the face of the Sun at some point in their orbits. When Mercury transits the Sun, as seen from Earth, the planet may be seen as a dark spot traveling across the Sun’s … Read more

The gravitational pull of the Sun keeps the planets in regular orbit. Even before the planets were born, the matter that created them rotated and spun around the Sun, then kept spinning as a result of the Sun’s explosion into existence. If it weren’t for the Sun’s gravity, the material would have gone sailing out … Read more

We are currently able to absolutely confirm if there are, in fact, planets in other solar systems. Some scientists believe that other planets exist in solar systems around all other stars. If you take the view that planets are simply leftover spinning matter from the formation of a star, why not? But we do not … Read more

When the sun began its nuclear energy production, the explosion sent gases, dust, and particles flying outward. The lightest matter, such as the gases hydrogen and helium, simply flew farther away. The farther reaches from the Sun were not effected as much by the Sun’s heat, so the gases remained relatively inert, they didn’t change … Read more

A planet’s orbital plane is the flat surface along which it orbits the Sun. If you sliced an orange in half and put the halves back together with an oval piece of cardboard in between, the orange could represent the Sun and the cardboard could represent a planet’s orbit. The planet would follow the edge … Read more

Ellipses are oval shapes. In more technical terms, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. All planets orbit the Sun in ellipses, not perfect circles. This is because their trajectories are affected by the gravitational forces of … Read more

The elements that make up stars, mostly hydrogen and helium, are also found on planets. Jupiter, Saturn, and Uranus’s atmospheres, for example, are also mostly hydrogen and helium. Earth’s matter is mostly carbon-based, and carbon is found in stars, particularly old-age white dwarfs. Stars, however, grow big enough to spark nuclear fusion and planets don’t. … Read more

We know that Earth came into existence about 4.6 billion years ago, and it is most likely that the other planets formed at about the same time, just after the Sun started shining some 5 billion years ago. Almost all meteorites date to around this same time, which substantiates this theory of when the planets … Read more

Scientists have come up with many different theories about planet formation. The protoplanet theory, or planet formation theory, seems to answer the most questions without raising too many others. When a star bursts into existence, its released energy shoots some gases, dust, and particles outward into rotation around itself. While the matter spins in eddies, … Read more

There have been several reports of falling meteorites killing both people and livestock, but only a few of these seem credible. The most well known documented fatality from a meteorite impact is that of an Egyptian dog that was killed in 1911, although this report is highly disputed. The first known modern case of a … Read more

Meteorites are rocks from outer space come in three basic groups, although there are many subgroups: stony, iron, and stony-iron. In general, stony meteorites resemble material from Earth’s crust. Iron meteorites are almost pure iron and nickel-90 percent iron and 10 percent nickel, the same metals that are believed to make up Earth’s core. Stony-iron … Read more

Meteorites hit Earth every day, but are so small that only a handful each year are noticed. Chances are about a million to one that a substantial meteorite will reach Earth in any given year, but, statistically, it is inevitable. The Vitim event on September 25, 2002 is believed to be an impact by a … Read more

Meteorites can fall anywhere on Earth. Depending on where they fall and their size, they could cause considerable damage. But this has rarely happened. Most meteorites weigh only 2.2 pounds, or about 1 kg, and most break into smaller pieces or vaporize on impact. The most destructive meteorite of all time is purely hypothetical. Many … Read more

Many qualities define a planet, but not all planets meet all criteria. A planet is a body that revolves around a star. It does not generate light, but reflects the light of a star. While stars appear as points of light through a telescope, planets look like disks. The word planet comes from the Greek … Read more

Earth passes annually through cometary orbits, which are littered with leftover particles. The consequent meteor showers are plentiful and heavy. The showers known as the Perseids display up to sixty-eight meteors an hour at its peak around August 12. The Eta Aquarids peak around May 5, average about ten an hour, up to a maximum … Read more

Meteorites small and large have probably fallen unnoticed almost everywhere on Earth. Thinly populated areas, such as Australia and the Antarctic, are popular hunting grounds for meteorites because the meteorites remain undisturbed. The impact of a large meteorite generally makes a deep impression, called a crater, on Earth’s surface. The most recent large meteorite, with … Read more

Meteors in a meteor shower seem to radiate from a single point in the sky, like water gushing from a shower head as you look straight up at it. Meteor showers got their name because the meteors are observed to radiate from one point in the night sky. The particles of debris streak across the … Read more

Not all meteors come from comets. The word “meteor” specifies a space object that enters Earth’s atmosphere, but burns up before hitting the planet’s surface. They can come from anywhere in space, since meteors are simply debris. The solar system is full of chunks of rock and metal, left over from the creation of the … Read more

Meteor showers are given the name of the constellation in which the radiant appears. The Perseids, for example, are seen in the constellation Perseus; the Geminids in Gemini; and the Orionids in Orion. The most visible meteor shower for many years are the Perseids, which peak on August 12 of each year at over 1 … Read more

Meteorites are not the same as meteors, but meteors can become meteorites. Not all Meteors are Meteorites, as some Meteors will burn up in the atmosphere. Meteorites are generally small chunks of rock, stone or metal matter, many from asteroids, that enter Earth’s atmosphere and hit the planet’s surface. In September 1969, a meteorite fell … Read more

Comets’ orbits around the Sun are not reliable. While the paths of more than 100 short-period comets have been mapped or tracked, astronomers can suddenly “lose” a comet. Sometimes it is simply gone. Other times, when it is found, it isn’t always as expected. The Wilson-Harrington comet, discovered in 1949, was expected to return in … Read more

Earth is more likely to pass through the tail of a comet than to collide with a comet head on. After all, a tail can be millions of times larger than the nucleus. Passing through a tail doesn’t have any great effect, beyond a good meteor shower, since comets’ tails are severely lacking in density. … Read more

Cometary orbits are not steady and orderly like planetary orbits. Comets can come from anyhere on any trajectory. The chances of a planet getting in the way are pretty good, especially since a planet’s gravity will draw a nearby comet toward it. On June 30, 1908, a comet was reported to have crashed in Russia’s … Read more