Abby Cessna, Author at Universe Today

November 25, 2009April 26, 2016 by Abby Cessna

Yellowstone National Park Volcano

Artist's impression of a what lies beneath the Yellowstone volcano. Credit: Hernán Cañellas/National Geographic

Yellowstone National Park is known for its geysers, but did you know that it is also the site of one of the world’s largest volcanoes? The same geologic activity that causes the multitude of geysers in the park is also responsible for the huge volcano located there. Scientists estimate that it is one of the largest volcanoes in the world, and so far it is the largest volcano in North America.

The volcano is 55 km by 72 km in size. Yellowstone’s volcano is in the class of super volcanoes due to its size. There is no exact definition for what qualifies as a super volcano; however, some scientists have defined it as a volcano with an eruption greater than 240 cubic miles.

Additionally, the Yellowstone Volcano does not look like the popular image of a volcano. Instead of being a conical mountain, it is a large depression in the ground like a cauldron. This type of volcanic feature is known as a caldera. It is very difficult to see the actual shape because it is covered with trees and has eroded over thousands of years.

*Yellowstone National Park sits on top of a giant volcanic caldera, or an earthen cap that covers a huge reservoir of superhot liquid rock and poison gasses. Credit: IO9*

The cause of the volcano is the hotspot on which Yellowstone is located. At a hotspot, molten mantle rock tends to rise toward the surface causing different geological activity. Volcanic eruptions from the hotspot also helped form the Snake River Plain.

Not only do volcanic eruptions occur around the Yellowstone region, but the area also experiences many earthquakes. The region experiences around 1000 to 2000 earthquakes each year, although most of them are usually quite small.

Despite the fact that the volcano has not erupted in hundreds of years, it is still active. This is a concern to scientists who have placed sensors around Yellowstone, so the volcano is continually monitored. The possible eruption of the Yellowstone Super volcano is of concern because of the enormous destruction it would cause. The last eruption was believed to be approximately 640,000 years ago.

Geologists estimate that 2 million years ago a cataclysmic series of volcanic eruptions in the Yellowsone Caldera was 2,500 times more powerful than the Mt. St. Helens eruption and perhaps was the largest, most violent volcanic eruption in the history of earth. Credit: Extreme Science — Geologists estimate that 2 million years ago a cataclysmic series of volcanic eruptions in the Yellowstone Caldera was perhaps the most violent volcanic eruption in the history of earth. Credit: Extreme Science

The eruption was on such a scale that it made the 1980 Mount St. Helen’s eruption look like nothing and result in damage and destruction many miles around. Scientists estimate that the other two eruptions from the Yellowstone Super volcano came over one and two million years ago. Since the volcano is still active, there is always a possibility it will once again erupt.

However, scientists do not anticipate that there will be another eruption in the near future, at least one on such a massive scale.

Universe Today has articles on Yellowstone eruption and Yellowstone Super Volcano.

For more information, try Yellowstone Volcano and Supervolcano.

Astronomy Cast has an episode on volcanoes hot and cold.

Sources:
http://en.wikipedia.org/wiki/Yellowstone_Caldera
http://volcanoes.usgs.gov/yvo/

November 15, 2009February 2, 2017 by Abby Cessna

Who were the Space Monkeys?

Albert II in preparation for his historic flight. Image Credit: NASA

The Space Age was an era of unprecedented technological development. In addition to developing the rockets and modules needed to put astronauts into space, considerable resources were also dedicated towards testing the effects spaceflight would have on the human body. In order to do this, test subjects needed to be selected that were physiologically similar enough to human beings.

For NASA, the Russians, and many space programs that have followed, the choice was to send simians (aka. monkeys) into space. While space missions would rely other animals to test the effects spaceflight would have on living organisms (such as dogs, guinea pigs, and even insects), monkeys were the most-widely used since they are more closely related to humans.

Background:

In the late 1940s, both NASA and the Soviet Space Program were working diligently to try and develop space launch capability. However, a going concern at the time was the risks posed by crewed spaceflight. At the time, the effects of weightlessness on the human body were unknown, and whether or not a human could even survive exposure was the subject of much scientific debate.

American Space Monkeys:

Russian Space Monkeys:

Other Space Agencies:

Although men have gone to space, they were not the first ones there. Scientists have sent a number of different animals up into space including monkeys. Both Russia and America sent monkeys into space. This is because scientists wanted to determine what the biological effects of space travel were before they sent humans up. While Russia only used rhesus monkeys, the US used many different species including rhesus monkeys, squirrel monkeys, cynomolgus monkeys, pig-tailed macaques, and chimpanzees. Even France sent up two monkeys into space during the 1960’s. These animals were both recovered alive.

Many of the monkeys sent up into space died either on impact or in space. The US sent four monkeys into space named Albert, all of which died. The first monkey that actually passed the Karman line and made it into space was Albert II. He was sent up in 1949 and died on impact.

Gordo, who was also known as Old Reliable, was sent into space in 1958. A squirrel monkey, he was chosen because of the similarity of the species to the human body. Gordo was lost on impact and neither him nor the shuttle was recovered; however, scientists were heartened by the mission because they believed it helped prove that humans could survive in space. The first monkeys to survive space were Able and Miss Baker who were sent up in 1959.

The Russians sent dogs up into space in addition to monkeys, which is why they did not send nearly as many monkeys into space as America did. Thus the first monkey that they actually sent into space was not until 1983. The monkeys that the Russians sent into space were named according to the letters of the alphabet. One of these monkeys – Dryoma – who went to space in 1987 – was later given to Fidel Castro. The last monkeys the Russians sent up into space were Lapik and Multik whe went up in 1997. Both of them survived the mission, but Multik had a heart attack a day after the flight during medical tests.

One of the most famous monkeys ever sent into space was Ham the Chimp. He was trained to operate the controls of the spaceship becoming the first animal to not just be a passenger. Ham was recovered safe after his capsule crashed in the Atlantic Ocean. Scientists were able to determine that astronauts would then be able to operate instruments in space and Alan Shepard went into space several months after Ham.

We have written many interesting articles about space monkeys and animals sent into space here at Universe Today. Here’s Russia to send monkey to Mars, Who was the First Monkey to go into Space?, 50th Anniversary of Historic Space Monkey Flight, What Animals have been to Space?, Who was the First Animal to go into Space?, Who was the First Dog to go into Space?

For more information, check out animals in space and monkeys in space fifty years later.

Astronomy Cast has an episode on spacesuits.

Sources:

November 15, 2009April 26, 2016 by Abby Cessna

Famous Astronomers

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Throughout the centuries, many astronomers have made incredible discoveries and contributions to science. I cannot do justice to all of them by any means, so I will concentrate on a few of the most famous astronomers throughout history.

Claudius Ptolemy was an astronomer and mathematician in Alexandria. He wrote an extensive treatise on astronomy known as the Almagest. It mapped out complex movements of the stars and planets. His model was geocentric, meaning he placed the Earth at the center of the universe. This geocentric model was widely accepted for more than a thousand years in many cultures. It is often known as the Ptolemaic model.

Galileo Galilei lived between 1564 and 1642 in Italy. He was a physicist and astronomer. Galileo created the first telescope, although his first model was very weak. His next one though was strong enough that he could see craters on the Moon, four of Jupiter’s moons, anda number of stars in the Milky Way.

The Polish astronomer Nicolas Copernicus lived between 1473 and 1543. He is famous for his theory tha the Sun is the center of the universe, not the Earth. His theory is often known as the Copernican model. It was years before his model became widely accepted though.

Johannes Kepler was a famous German astronomer who lived between 1571 and 1630. He was the first person to identify planetary motion. Kepler is probably most famous for his three laws of planetary motion, which describe the motion of two celestial bodies such as a planet and its star.

Edmond Halley lived between 1656 and 1742. He predicted the orbit of the Halley Comet, which was named in his honor. He also published an extensive catalog of stars and created a diving bell, which he improved throughout the years.

Sir Friedrich William Herschel, often known as William Herschel, was a famous astronomer of the late18th to the early 19th century. He is famous for having discovered the plant Uranus and two of its moons. He also made over 400 telescopes during his life. Herschel discovered two of Saturn’s moons – Mimas and Enceladus.

Clyde Tombaugh is an American astronomer who is best known for discovering Pluto in 1930. Pluto was considered a planet for 76 years until it was reclassified as a dwarf planet. He did not actually have any astronomy degrees until after he discovered Pluto when he studied astronomy. He also discovered a number of asteroids.

Universe Today has more articles on astronomers are people too and artist creates portrait gallery of astronomers.

If you are looking for more information, check out famous astronomers and influential astronomers

Astronomy Cast has an episode on building a career in astronomy.

Sources:
NASA: Cosmology
NASA: Kepler
NASA: Edmond Halley
SEDS.org
NASA: Clyde Tombaugh

November 15, 2009December 24, 2015 by Abby Cessna

When Was Saturn Discovered

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Saturn was one of the five planets visible to the naked eye that have been known to exist for thousands of years. The Ancient Greeks knew about the planet and named it after their god of agriculture – Kronos. That may be due to the planet’s golden color, which is similar to wheat. The planet later became known as Saturnus or Saturn, which was the Roman equivalent of Kronos. Saturn was the most distant of the five planets that can be seen with the naked eye. The Romans and Greeks were not the only ones who knew about Saturn. In Hindu culture, it was one of the nine Navagrahas, which are the main celestial bodies that are supposed to have influence over people’s lives. Saturn was known as Shani and was the Judge of the planets. The Chinese and Japanese termed it the earth star; their classification was based on the five elements. To the Ancient Hebrews, Saturn was known as Shabbathai.

Although people have known about Saturn for thousands of years, discoveries have still been made about the planet more recently. No one knew that Saturn had rings until the 1600’s. Galileo discovered them with his telescope in 1610, but he did not know what these were either. Thus they remained a mystery until 1655 when the astronomer Christian Huygen figured out that they were planetary rings.

Additionally, Saturn’s moons were discovered over a period of time. Christian Huygen discovered Titan, which is Saturn’s largest moon. Giovanni Domenico Cassini discovered four moons – Iapetus, Rhea, Tethys, and Dione. In 1789, the astronomer William Herschel discovered two more moons – Mimas and Enceladus. In 1848, British scientists discovered a moon called Hyperion.

We have learned much more about Saturn with the use of probes. To date, scientists have discovered 60 moons around Saturn. In 1979, Pioneer 11 flew by the planet and took photos. In 1980, the Voyager 1 probe took images of the planet and its largest moon, Titan. Voyager 2 also gathered information and showed scientists changes in the planet’s ring system. They also discovered gaps in the rings.

In 2004, Cassini-Huygens extensively studied the Saturn system. It brought back detailed data about both Titan and Saturn. Scientists believe that the information that Cassini gathered has shown them proof of geysers on Enceladus that have eruptions of liquid water. Scientists were also able to discovere another ring of Saturn in 2006 due to photos that Cassini had taken.

Universe Today has articles on how long does it take Saturn to orbit the Sun and interesting facts about Saturn.

If you are looking for more information, try discover Saturn or Saturn.

Astronomy Cast has an episode on Saturn.

Reference:
NASA

November 2, 2009April 26, 2016 by Abby Cessna

What is a Subduction Zone?

Transform Plate Boundary — Tectonic Plate Boundaries. Credit:

IF you don’t know anything about plate tectonics you might be wondering about what is a subduction zone. A subduction zone is a region of the Earth’s crust where tectonic plates meet. Tectonic plates are massive pieces of the Earth’s crust that interact with each other. The places where these plates meet are called plate boundaries. Plate boundaries occur where plates separate, slide alongside each other or collide into each other. Subduction zones happen where plates collide.

When two tectonic plates meet it is like the immovable object meeting the unstoppable force. However tectonic plates decide it by mass. The more massive plate, normally a continental will force the other plate, an oceanic plate down beneath it. This is the subduction zone. When the other plate is forced down the process is called subduction. The plate enters into the magma and eventually it is completely melted. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.

Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. A good example is the subduction zone near Chile. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench.

There are some interesting theories about why Subduction occurs in the Earth’s crust. One common theory is that subduction was initiated by major impacts by asteroids or comets early in Earth’s history. This makes a lot of sense due to the geologic evidence of large impacts scattered around the world.

Understanding how subduction zones work is important because it helps scientist to identify areas of high volcanic and seismic activity. Monitoring these areas can help them warn people who live near them of imminent events and also people who could be affected by the side effects of such events such as ash clouds or tsunamis.

Subduction continues to be one of the most powerful and dynamic processes on planet Earth and as technology improves we can come to understand more about this amazing process.

We have written many articles about the subduction zone for Universe Today. For example, here is one on the Ring of Fire and plate boundaries.

You should also check out plate tectonics and subduction.

If you’d like more info on the subduction zone, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
http://en.wikipedia.org/wiki/Subduction
http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm

Subduction is a process in geology where one tectonic plates slides underneath another one and merges into the Earth’s mantle. The denser plate is the one that slips under the less dense plate; the younger plate is the less dense one. The process is not a smooth one. The tectonic plates grate against each other, which often causes earthquakes. The plate that slips under does not stay that way. Due to the heat caused by it rubbing against the other plate as well as the natural heat of the mantle, the plate melts and turns into magma. The area where subduction occurs is known as the subduction zone.

When one plate begins to slip underneath another one a trench is formed. The earthquakes that result due to the plates grinding against each other often cause magma to spill out through the trench in submarine volcanoes. Various formations such as mountain ranges, islands, and trenches are caused by subduction and the volcanoes and earthquakes it triggers. In addition to causing earthquakes, subduction can also trigger tsunamis.

When the older plate is holding a continent however, it does not sink, which is reassuring. Instead, the less dense material slips into a trench behind the denser oceanic crust where it gets stuck. The pressure continues to build until the trench flips over and the less dense plate slips underneath the one with the continent.

It is possible for a whole tectonic plate to disappear. This happens when the plate goes through subduction faster than new material can be added to the plate through seafloor spreading. The spreading pushes the plate slowly toward the subduction zone until the whole thing disappears. When this happens, the other tectonic plates rearrange to cover the area.

Subduction zones are mainly located in the Pacific Ocean. This is because seafloor spreading – the process by which new oceanic crust is created – occurs mostly in the Pacific. Thus the new material pushes the older plates outward and then they need to undergo subduction. This also explains why so many earthquakes originate in the Pacific Ocean near the Ring of Fire. That is where the subduction zones are concentrated.

Continental plates also converge, but this is not considered subduction because these plates do not have different densities and thicknesses to subduct. Landforms such as the Himalayas are formed from these convergences though.

November 2, 2009December 24, 2015 by Abby Cessna

Geomagnetic Reversal

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Geomagnetic reversal is when the orientation of the Earth’s magnetic field becomes reversed. Thus, magnetic north and south switch places. The process is a gradual one though that can take thousands of years. The possibility that the magnetic field could reverse was first brought up in the early 1900’s. However, at this time scientists did not understand the Earth’s magnetic field very well so they were not interested in the concept of geomagnetic reversal. It was not until the 1950’s that scientists began a more in-depth study of geomagnetic reversal.

Scientists have not reached a consensus on what causes pole reversal. Some believe that it is simply an effect of the nature of the planet’s magnetic field. They base this hypothesis on the magnetic field lines’ tendency to move around and think that it becomes agitated enough to flip. Other scientists propose that external influences cause the shift. For example, a tectonic plate that undergoes subduction and goes into the Earth’s mantle may disturb the magnetic field enough to make it turn off. When the field restarts, it randomly chooses orientation, so it could shift.

In order to better understand the process, scientists study past geomagnetic reversals. This is possible because the reversals have been recorded in minerals found in sedimentary deposits or hardened magma. Scientists have discovered that the magnetic field has actually reversed thousands of times. Scientists also discovered a record of reversals on the ocean floor.

The time between geomagnetic eruptions is not constant. One time, five reversals occurred over a period of a million years. Sometimes however, none happen for a very long time. These periods are known as superchrons. The last time a geomagnetic reversal occurred was 780,000 years ago and is referred to as the Brunhes-Matumaya reversal.

Geomagnetic reversal has also been linked to 2012. Some people believe that in 2012 when the Mayan calendar runs out we will experience some cataclysmic event that will destroy our world or life as we know it. There are various theories for exactly what this event is. One theory says that geomagnetic reversal will occur during 2012. Since the magnetic field is weaker at first when it switches, some claim that the Earth will be ravaged by solar rays. Scientists still have not determined what effects a geomagnetic reversal will have on humans; however, humans did survive the last reversal 780,000 years ago. One hypothesis is that the solar winds actually create a magnetic field sufficient enough to protect us while Earth’s magnetic field restarts.

Universe Today has articles on no geomagnetic reversal in 2012 and field reversal may take 7000 years.

For more information, you should check out geomagnetic flip may not be random and magnetic storm.

Astronomy Cast has an episode on magnetism everywhere.

Reference:
NASA: Earth’s Inconstant Magnetic Field

October 26, 2009December 24, 2015 by Abby Cessna

2003 ub313

The mysterious Eris and moons. Credit: NASA

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In 2003, a celestial object was discovered, but little did astronomers know that this object, which was designated 2003ub313, was going to change astronomy forever. Although the object was first photographed in 2003 by Mike Brown and other astronomers, it was not until 2005 that astronomers announced their discovery. You may better know 2003 ub 313, which was its designation given when it was believed to be a minor planet, as Eris. Eris made such a fuss because it is larger than Pluto – 27% more massive. Some people labeled it as the tenth planet while others did not think it should join the ranks of the nine planets we had. Finally, the International Astronomical Union (IAU) met to decide on a definition of a planet. Eventually, they decided on a definition in 2006, and 2003ub313 was not classified as a planet but rather a dwarf planet. In addition to Eris, Pluto was reclassified as a dwarf planet, and several other celestial bodies – including Ceres, Haumea, and Makemake – were classified as dwarf planets. Astronomers are evaluating dozens more celestial bodies to see whether they fall under the classification of dwarf planets.

Eris is the ninth largest celestial body in our Solar System that orbits the Sun and the most distant object orbiting the Sun. It takes the dwarf planet 556.7 years to orbit our star. Eris is located in the scattered disc, which is a region beyond the Kuiper Belt. In addition to being a dwarf planet, Eris is also classified as a Trans-Neptunian Object (TNO). The surface of the dwarf planet is grey, and astronomers believe that the surface is covered with methane ice, which is what causes it to appear grey. Methane is the same substance that makes Uranus and Neptune blue. Scientists think that Eris’ composition is similar to that of Pluto. Eris also has a very eccentric orbit, and it is also highly inclined. At some point in its orbit, Eris will actually be closer to the Sun than Pluto will be.

Like most celestial bodies, Eris was named after a figure in mythology. Eris was the Greek goddess of strife and discourse. Many believe this is a very fitting name for the dwarf planet, which caused so much division over the definition of a planet and the fate of Pluto. The dwarf planet Eris also has a moon, which was named Dysnomia. Dysnomia was Eris’ daughter in Greek mythology and the demon of lawlessness.

Universe Today has articles on Eris including dwarf planet Eris and plutoid Eris is changing.

For more information, check out the discovery of Eris and former 10th planet officially named Eris.

Astronomy Cast has an episode on Pluto’s planetary crisis you will want to hear.

Source: NASA

October 13, 2009April 26, 2016 by Abby Cessna

Greenwich Mean Time

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Greenwich Mean Time was formulated in the 1800’s in order to deal with the confusion that existed when each country decided its own solar time. This was especially important with the increase in traveling and train schedules. In 1884, at the Meridian Conference, 27 countries decided to implement a system that is practically the same one that we have today. They based their system on one formulated by a Canadian railway planner and engineer, Sir Sanford Fleming.

In this system, there are 24 lines that go from the North to the South called meridians of longitude. They are spaced 15° apart. The first line, known as the prime meridian, is located in Greenwich, England. It is 0° longitude. Greenwich Mean Time (GMT) refers to solar mean time at the Royal Observatory in Greenwich, England. Solar mean time is calculated using the angle of the mean Sun – a fictional Sun used to compute time. Since Britain was an advanced maritime nation, they had long used Greenwich as a point of longitude to calculate their longitude in relation to the Greenwich meridian. Due to its long history, the world decided to accept it as the prime meridian as well.

Different countries have legally defined their local time in reference to Greenwich Mean Time. These countries include the United Kingdom, Belgium, the Republic of Ireland, and Canada. Although GMT is used all year round in Iceland, it is only used during the winter in Ireland and the UK. During the summer, British Summer Time (BST) is used. This is also known as Greenwich Daylight Savings Time (GDT), which corresponds to daylights savings time in the United States. The Greenwich Mean Time is also known as the Universal Time (UT).

The issue of time zones is made more complicated because the zones are not always split along the longitudinal lines. They are also divided along political boundaries. This means that although it may be one time in the one country, if you head north or south while staying in the same longitude, the time may change.

GMT is used for a variety of things. For example, many emails between people from different time zones include a reference to GMT, explaining how many hours that person’s time zone is from GMT. On the International Space Station, they use GMT. The US Government National Weather Service and the Weather Channel both use weather maps that use GMT. These are just a few of many ways that the GMT is used today.

Universe Today has a number of articles on interesting facts about Earth and how long a day on Earth is.

If you are looking for more information, you should check out what is GMT and Greenwich Mean Time.

Astronomy Cast has an episode on Earth.

Sources: NASA, Wikipedia

October 13, 2009December 24, 2015 by Abby Cessna

Evening Star

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Venus is also known as the evening star. It was given that name by ancient civilizations, such as the Greeks and Egyptians, who saw it in the sky. The planet was eventually named after the Roman goddess of love because of its beauty. Many ancient cultures have his planet with love and womanhood. Venus has been an important object in a number of different cultures including to the Babylonians and Mayans. The Mayans even used the movement of the planet to help create their complex calendar.

Venus is close to the Earth as well as the Sun. As soon as the Sun sets and it gets dark enough, Venus can often be seen in the sky. Because it seems In addition to being known as the evening star, Venus was also called the morning star because it could be seen for a few hours before the Sun grew too bright. The planet actually becomes brightest before the Sun rises or just after sunset. The ancient civilizations thought that the morning star and the evening star were separate celestial bodies. Pythagoras, the famous Greek mathematician, is believed to be the first person to realize that the morning and evening stars were actually the same object – Venus.

The Egyptians had two names for the planets because they thought it was actually two stars. The morning star was called Tioumoutri, and the evening star was known as Ouaiti. The Greeks called the evenings star Hesperos, the “star of the evening.” The Greeks called the morning star, Phosphoros “the Bringer of Light,” or Eosphoros, “the Bringer of Dawn.”

Besides the Sun and Earth’s Moon, Venus is the brightest object in our Solar System. Its brightness is caused in part by the clouds of toxic gases that comprise its atmosphere. The sulfur dioxide and other elements in these clouds reflect light from the Sun causing the planet to shine.

Long after astronomers discovered that Venus was no longer the evening or morning star it has captivated the imagination of many. The swirling clouds that hid the surface of this shining planet from view were thought to shield a tropical paradise. Ironically, what many considered to be the most beautiful planet turned out to be a burning wasteland – the hottest planet in our Solar System. Another one of Venus’ many names is Earth’s twin because it is similar in size and mass to our own planet.

Universe Today has articles on the morning and evening star and the history of Venus.

For more information, you should take a look at Venus and an introduction to Venus.

Astronomy Cast has an episode on Venus.

References:
NASA History: Earth’s Sister and the Twilight Planet
NASA: Planets

September 27, 2009December 24, 2015 by Abby Cessna

The Arctic Circle

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The Arctic Circle is a region on Earth that is marked as one of the five major circles of latitude on maps of our planet. It is located between the Arctic to its north and the Northern Temperate Zone to its south. Its counterpart in the Southern Hemisphere is known as the Antarctic Circle. In addition to the Antarctic Circle, the other three major latitude lines are the Tropic of Cancer, the Tropic of Capricorn, and the Equator.

The Arctic Circle is the farthest southern region that experiences polar day and polar night. A polar day is where it is 24 hours of continuous daylight, and a polar night is 24 continuous hours of darkness. North of the Arctic Circle more than one polar day and night occur per year. However, the Arctic Circle only experiences a polar day and night once a year on the June and December solstices respectively. Usually, those days fall on the 21st of each month. Polar day is also known as midnight sun and polar-summer while polar night is also known as darkness at noon or midwinter darkness. The Antarctic Circle also experiences a polar day and a polar night once a year.

Due to the Earth’s shifting axial tilt – a fluctuation of 2° during 40,000 years – which is especially a result of the Moon’s orbit, the Arctic Circle is moving. It is drifting north about 15 kilometers per year. The Earth’s axial tilt is the same thing that causes the different seasons on Earth.

The Arctic Circle passes through seven countries that have a considerable portion of land within the Arctic Circle. The countries are the United States of America, Greenland, Canada, Russia, Norway, Sweden, and Finland. Iceland has a tiny region – less than one square km – inside the Arctic Circle. Some areas within the Arctic Circle include Lapland Province in Finland; Yukon, the Northwest Territories, and part of Nunavut in Canada; Davis Strait in the Atlantic Ocean; the Island of Grimsey, which is part of Iceland; and the Greenland and Norwegian Seas in the Atlantic Ocean. The Arctic Circle is also a popular tourist site. People travel from different parts of the world to experience the region. Many places in the Arctic Circle also offer tours; Alaska has quite a few tours, although you can also find them in other countries. Some of these involve hiking around the Arctic Circle region during the summer.

Universe Today has articles on why Earth has seasons and 10 interesting facts about Earth in case you want to learn more about our planet.

If you are looking for more information, check out what is the Arctic and the Arctic.

Astronomy Cast has an episode on Earth.

Sources:
http://nsidc.org/arcticmet/basics/arctic_definition.html
http://en.wikipedia.org/wiki/Arctic_Circle