Posted on by Matt Williams

Solar Day

Winter Solstice
Earth as viewed from the cabin of the Apollo 11 spacecraft. Credit: NASA

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Since the dawn of time, human beings have relied on the cycles of the sun, the moon, and the constellations through the zodiac in order to measure time. The most basic of these was the motion of the Sun as it traced an apparent path through the sky, beginning in the East and ending in the West. This procession, by definition, is what is known as a Solar Day. Originally, it was thought that this motion was the result of the Sun moving around the Earth, much like the Moon, celestial objects and stars seemed to do. However, beginning with Copernicus’ heliocentric model, it has since been known that this motion is due to the daily rotation of the earth around the Sun’s polar axis.

Up until the 1950’s, two types of Solar time were used by astronomers to measure the days of the year. The first, known as Apparent Solar Time, is measured in accordance with the observable motion of the Sun as it moves through the sky (hence the term apparent). The length of a solar day varies throughout the year, a result of the Earth’s elliptical orbit and axial tilt. In this model, the length of the day varies and the accumulated effect is a seasonal deviation of up to 16 minutes from the mean. The second type, Solar Mean Time, was devised as a way of resolving this conflict. Conceptually, Mean solar time is based on a fictional Sun that is considered to move at a constant rate of 360° in 24 hours along the celestial meridian. One mean day is 24 hours in length, each hour consisting of 60 minutes, and each minute consisting of 60 seconds. Though the amount of daylight varies significantly throughout the year, the length of a mean solar day is kept constant, unlike that of an apparent solar day.

The measure of time in both of these models depends on the rotation of the Earth. In both models, the time of day is not plotted based on the position of the Sun in the sky, but on the hour angle that it produces – i.e. the angle through which the earth would have to turn to bring the meridian of the point directly under the sun. Nowadays both kinds of solar time stand in contrast to newer kinds of time measurement, introduced from the 1950s and onwards which were designed to be independent of earth rotation.

We have written many articles about Solar Day for Universe Today. Here’s an article about how long a day is on Earth, and here’s an article about the rotation of the Earth.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Solar_time
http://www.tpub.com/content/administration/14220/css/14220_149.htm
http://scienceworld.wolfram.com/astronomy/SolarDay.html
http://www.britannica.com/EBchecked/topic/553052/solar-time?anchor=ref144523
http://en.wikipedia.org/wiki/Hour_angle

Posted on by Matt Williams

Rare Earth Magnets

Permanent Magnet
Super Magnets, the strongest type of permanent magnets

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Magnets are an endless source of fun, not to mention a convenience when it comes to fridge notes and white boards! But when it comes to industrial uses, such as those used by the air force and NASA, only one type of magnet makes the grade. These are called Rare Earth Magnets, a set of strong permanent magnets made from the alloys of particular earth elements. These elements fall into the category of rare earth elements (or metals), which are a collection of seventeen elements in the periodic table; namely scandium, yttrium, and the fifteen lanthanides. Despite their name, rare earth elements are actually quite abundant, but are so named because of their geochemical properties, they are rarely found in economically exploitable concentrations.

Rare earth elements are ferromagnetic, meaning that like iron, they can be magnetized. However, because most rare earth elements have low Curie temperatures (the temperature at which they exhibit magnetic properties), meaning they are only magnetic at low temperatures. However, most form compounds with transition metals like iron, nickel and cobalt, which have higher Curie temperatures, and can therefore be mixed with them to enhance their natural magnetic properties. There are two types: neodymium magnets and samarium-cobalt magnets. The former, invented in the 1980s, are the strongest and most affordable type of rare-earth magnet, is made of neodymium, iron and boron (chemical formula: Nd2Fe14B). On the other hand, Samarium-cobalt magnets (chemical formula: SmCo5), the first family of rare earth magnets invented, are less used than neodymium magnets because of their higher cost and weaker magnetic field strength. However, samarium-cobalt has a higher Curie temperature, creating a niche for these magnets in applications where high field strength is needed at higher operating temperatures.

Neodymium magnets are typically used in most computer hard drives and a variety of audio speakers. They are also have a number of important medical applications, not the least of which involves magnetic resonance imaging (or MRI) technology. They are also part of the driving mechanisms for electrical and hybrid motors, servomotors, cordless tools, and power steering controls. Samarium-cobalt motors are commonly used in the construction of electrical guitars, high-end Slotcar racing engines, and turbomachinery. In addition, rare earth elements are being used as a catalysts in the petroleum cracking industry and to make auto emissions equipment, and may have many future applications for green technology. Samarium-cobalt magnets may also be used in the making of cryogenic and high-temperature systems for future space travel.

Originally, the high cost of these magnets limited their use to applications requiring compactness together with high field strength, but beginning in the 1990s, rare earth magnets have become steadily less expensive, and the low cost has inspired new uses (such as magnetic toys for children).

We have written many articles about magnets for Universe Today. Here’s an article about where to buy magnets, and here’s an article about what magnets are made of.

If you’d like more info on Rare Earth Magnets, check out Rare Earth Magnetics Homepage, and here’s a link to Wikipedia: Rare Earth Magnets.

We’ve also recorded an entire episode of Astronomy Cast all about Magnetism. Listen here, Episode 42: Magnetism Everywhere.

Sources:
http://en.wikipedia.org/wiki/Rare_earth_element
http://en.wikipedia.org/wiki/Curie_temperature
http://blogs.wsj.com/chinarealtime/2010/11/02/video-how-a-rare-earth-magnet-works/
http://en.wikipedia.org/wiki/Rare-earth_magnet
http://en.wikipedia.org/wiki/Neodymium_magnet
http://en.wikipedia.org/wiki/Samarium-cobalt

Posted on by Matt Williams

Precession of the Equinoxes

Semi Major Axis
Solstice and Equinox - Credit: NASA

When he was first compiling his famous star catalogue in the year 129 BCE the Greek astronomer Hipparchus noticed that the positions of the stars did not match up with the Babylonian measurements that he was consulting. According to these Chaldean records, the stars had shifted in a rather systematic way, which indicated to Hipparchus that it was not the stars themselves that had moved but the frame of reference – i.e. the Earth itself.

Such a motion is called precession and consists of a cyclic wobbling in the orientation of Earth’s axis of rotation. Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative, and takes 25,772 years for a full precession to occur. This has historically been referred to as the Precession of the Equinoxes.

The name arises from the fact that during a precession, the equinoxes could be seen moving westward along the ecliptic relative to the stars that were believed to be “fixed” in place – that is, motionless from the perspective of astronomers – and opposite to the motion of the Sun along the ecliptic.

This precession is often referred to as a Platonic Year in astrological circles because of Plato’s recorded remark in the dialogue of Timaeus that a perfect year could be defined as the return of the celestial bodies (planets) and the fixed stars to their original positions in the night sky. However, it was Hipparchus who is first credited with observing this phenomenon, according to Greek astronomer Ptolemy whose own work was in part attributed to him.

The precession of the Earth’s axis has a number of noticeable effects. First of all , the positions of the south and north celestial poles appear to move in circles against the backdrop of stars, completing one cycle every 25, 772 years. Thus, while today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the “north star”. Second, the position of the Earth in its orbit around the Sun during the solstices, equinoxes, or other seasonal times slowly changes.

The cause of this was first discussed by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica where he described it as a consequence of gravitation. Though his equations were not exact, they have since been revised by scientists and his original theory proven correct.

It is now known that precessions are caused by the gravitational source of the Sun and Moon, in addition to the fact that the Earth is a spheroid and not a perfect sphere, meaning that when tilted, the Sun’s gravitational pull is stronger on the portion that is tilted towards it, thus creating a torque effect on the planet. If the Earth were a perfect sphere, there would be no precession.

Today, the term is still widely used, but generally in astrological circles and not within scientific contexts.

We have written many articles about the equinox for Universe Today. Here’s an article about the astronomical perspective of climate change, and here’s an article about the Vernal Equinox.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.

Sources:
http://en.wikipedia.org/wiki/Axial_precession_%28astronomy%29
http://en.wikipedia.org/wiki/Chaldea
http://en.wikipedia.org/wiki/Ecliptic
http://en.wikipedia.org/wiki/Great_year
http://www.crystalinks.com/precession.html
http://en.wikipedia.org/wiki/Isaac_Newton

Reference:
NASA: Precession

Posted on by Matt Williams

Pompeii Eruption

Pompeii Eruption
mount-vesuvius-naples-bay

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Imagine if you will that it’s bright sunny day in summer. The festival of Vulcanalia, dedicated to the Roman God of Fire, has just passed. Now you’re out looking for some produce to stock up for the coming winter. You’ve just finished a tour of the marketplace and are on your way home when suddenly, the mountain that your town sits at the foot of inexplicably erupts! Fire and ash rain down upon your city, people are baked alive and the town is encased in soot and dirt several meters thick. But, silver lining here, your bodies are so well preserved that when you’re dug up two thousand years later, they’ll have a pretty good idea what life was like at the time of your death. Yes, that’s how the Pompeii Eruption took place. The year was 79 CE; the place, a prosperous town named Pompeii located in the Bay of Naples. It was one of the most significant natural disasters of the ancient world, a major archaeological find in the 18th century, and is now one of the biggest tourist draws in all of Italy.

Based on the letters of Pliny the Younger, historians now believe the eruption to have taken place between the 24th of August and November 23rd, in the year 79 CE. Witnessing the eruption from across the Bay of Naples, Pliny gave a fist-hand account of the destruction. Although it was generally assumed that the people of Pompeii died as a result of suffocation from volcanic ash, a recent multidisciplinary volcanological and bio-anthropological study, merged with numerical simulations and experiments, indicated that heat was the main cause of death. The results of this study show that temperatures would have reached 250 °C up to a distance of 10 kilometers, which would have been sufficient to cause instant death, even if people were sheltered within buildings. The people and buildings of Pompeii were covered in up to twelve different layers of soil which was 25 meters deep and were therefore not discovered for almost two thousand years.

However, rediscovery of the lost city started in 1738, beginning with Pompeii’s sister town of Herculaneum which had also been destroyed in the eruption. At the time, the discovery was the accidental result of workmen digging so that they could build the foundations of a new summer palace for the king of Naples. The discovery of ancient buildings, left largely intact, led to a subsequent intentional excavation of Pompeii itself in 1764 by Francisco la Vega. In addition to intact buildings, many of which contained perfectly preserved Roman frescos, human remains were also uncovered.

For over 20 years now, Pompeii has been one of the most popular tourist destinations in Italy, attracting almost 2.6 million visitors in 2008 alone. In 1997, it was designated a World Heritage Site by UNESCO and attempts are underway to ensure that it can be preserved for future generations. Though the life-blood of the local economy, the pressure exerted by millions of tourists annually is taking its toll on this once-perfectly preserved site.

We have written many articles about Pompeii Eruption for Universe Today. Here’s an article about Mt. Vesuvius, and here are interesting facts about volcanoes.

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

We’ve also recorded related episodes of Astronomy Cast about Volcanoes. Listen here, Episode 141: Volcanoes, Hot and Cold.

Sources:
http://en.wikipedia.org/wiki/Pompeii#Vesuvius_eruption
http://en.wikipedia.org/wiki/Mount_Vesuvius
http://touritaly.org/pompeii/pompeii-main.htm
http://wikitravel.org/en/Pompeii

Posted on by Matt Williams

What is the Multiverse Theory?

Could our Universe be part of a wider Multiverse? And could these other Universes support life? Credit: Jaime Salcido/EAGLE Collaboration

If you’re a fan of science fiction or fantasy then chances are, at some point, you’ve read a book, seen a movie, or watched a series that explored the concept of multiple universes. The idea being that within this thing we call time and space, there are other dimensions where reality differs from our own, sometimes slightly, sometimes radically. Interestingly enough, this idea is not restricted to fiction and fantasy.

In science, this is known as the Multiverse Theory, which states that there may be multiple or even an infinite number of universes (including the universe we consistently experience) that together comprise everything that exists: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. In this context, multiple universes are often referred to as parallel universes because they exist alongside our own.

The term was coined in 1895 by the American philosopher and psychologist William James. However, the scientific basis of it arose from the study of cosmological forces like black holes and problems arising out of the Big Bang theory. For example, within black holes it is believed that a singularity exists – a point at which all physical laws cease – and where it becomes impossible to predict physical behavior.

Beyond this point, it is possible that there may be an entirely new set of physical laws, or just slightly different versions of the ones that we know, and that a different universe might exist. Theories like cosmic inflation support this idea, stating that countless universes emerged from the same primordial vacuum after the Big Bang, and that the universe as we know it is just what is observable to us.

Max Tegmark’s taxonomy of universes sums up the different theories on multiple universes. IN this model, there are four levels that classify all major schools on thought on the subject.

In Level One, different universes are arranged one on top of the other in what is called Hubble Volumes, all having the same physical laws and constants. Though each will likely differ from our own in terms of distribution of matter, there will eventually be Hubble volumes with similar, and even identical, configurations to our own.

In Level Two, universes with different physical constants exist and the multiverse as a whole is stretching and will continue to do so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread.

In Level Three, known as the Many Worlds Interpretation of Quantum Mechanics, observations cannot be predicted absolutely but a range of possible observations exist, each one corresponding to a different universe. Level Four, aka.the Ultimate Ensemble devised by Tegmark himself, considers as equally real all universes that can be defined by mathematical structures. In other words, universes with the same or different constants may exist.

We have written many articles about multiverse for Universe Today. Here’s an article about searching life in the multiverse, and here’s an article about parallel universe.

If you’d like more info on the Multiverse, check out some Recent Innovations about the Concept of Universe, and here’s a link to an article about the Size of the Universe.

We’ve also recorded an entire episode of Astronomy Cast all about Multiverses. Listen here, Episode 166: Multiverses.

Sources:
http://en.wikipedia.org/wiki/Multiverse
http://www.sciencedaily.com/releases/2010/01/100112165249.htm
http://www.astronomy.pomona.edu/Projects/moderncosmo/Sean%27s%20mutliverse.html
http://en.wikipedia.org/wiki/William_James
http://en.wikipedia.org/wiki/Big_Bang
http://en.wikipedia.org/wiki/Inflation_%28cosmology%29

Posted on by Matt Williams

Morning Star

Venus Cloud Tops Viewed by Hubble
Venus Cloud Tops Viewed by Hubble

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If you look to the morning sky – to the east that is, as the sun’s rising – you will notice a bright star in the firmament, one that should not be there. Theoretically, stars only come out at night and should be well on their way to bed by the time the sun rises, correct? Well, that’s because the Morning Star, as it’s known, isn’t a star at all, but the planet Venus. It is both the morning and evening star, the former when it appears in the east during sunrise and the latter when it appears in the west during sunset. Because of its unique nature and appearance in the sky, this “star” has figured prominently in the mythologies of many cultures.

In ancient Sumerian mythology, it was named Inanna (Babylonian Ishtar), the name given to the goddess of love and personification of womanhood. The Ancient Egyptians believed Venus to be two separate bodies and knew the morning star as Tioumoutiri and the evening star as Ouaiti. Likewise, believing Venus to be two bodies, the Ancient Greeks called the morning star Phosphoros (or Eosphoros) the “Bringer of Light” (or “Bringer of Dawn”) and the evening star they called Hesperos (“star of the evening”). By Hellenistic times, they had realized the two were the same planet, which they named after their goddess of love, Aphrodite. The Phoenicians, never ones to be left out where astronomy and mythology were concerned, named it Astarte, after their own goddess of fertility. In Iranian mythology, especially in Persian mythology, the planet usually corresponds to the goddess Anahita, and sometimes AredviSura, the goddesses of fertility and rivers respectively. Mirroring the ancient Greeks, they initially believed the planet to be two separate objects, but soon realized they were one.

The Romans, who derived much of their religious pantheon from the Greek tradition and near Eastern tradition, maintained this trend by naming the planet Venus after their goddess of love. Later, the name Lucifer, the “bringer of light”, would emerge as a Latinized form of Phosphoros (from which we also get the words phosphorus and phosphorescence). This would prove influential to Christians during the Middle Ages who used it to identify the devil. Medieval Christians thusly came to identify the Morningstar with evil, being somewhat more concerned with sin and vice than fertility and love! However, the identification of the Morningstar as a symbol of fertility and womanhood remains entrenched, best demonstrated by the fact that the astronomical symbol for Venus happens to be the same as the one used in biology for the female sex: a circle with a small cross beneath.

The Morningstar also figures prominently in the mythology of countless other cultures, including the Mayans, Aborigines, and Maasai people of Kenya. To all of these cultures, the Morningstar still serves as an important spiritual, agricultural and astrological role. To the Chinese, Japanese, Koreans and Vietnamese, she is known literally as the “metal star”, based on the Five Elements.

We have written many articles about the Morning Star for Universe Today. Here’s an article about how to find Venus in the sky, and here’s an article about the brightest planet.

If you’d like more information on the Morning Star, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Sources:
http://en.wikipedia.org/wiki/Morning_Star
http://en.wikipedia.org/wiki/Lucifer
http://en.wikipedia.org/wiki/Eosphorus
http://en.wikipedia.org/wiki/Venus
http://en.wikipedia.org/wiki/Isis
http://en.wikipedia.org/wiki/Evening_star

Posted on by Matt Williams

What is Plutonium?

Periodic Table of Elements
Periodic Table of Elements

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The name itself conjures up imagines of mini nukes and sophisticated space-age gadgets doesn’t it? Well for some people it does. For others, Plutonium (Pu, atomic number of 94 on the periodic table of elements) spawns images of nuclear reactors, atomic energy and nuclear waste. All of these are true to an extent, but the reality behind this radioactive element is understandably more complex. For starters, plutonium is a silvery white actinide metal that is radioactive, and hence quite dangerous when exposed to living tissue. It is one of the key ingredients in the making of atomic weapons, but is also produced in nuclear reactors as a result of slow fission. There are also several isotopes of the element, but for our purposes, the most important is Plutonium-239, a fissile isotope that is used for both nuclear power and weapons and has a half-life of 24,100 years.

Plutonium-238 was first discovered as an element on Dec.14th1940, and then chemically identified on February 23rd 1941through the deuteron bombardment of Uranium in a cyclotron by Glenn T. Seaborg and his team of scientists, working out of the University of California in Berkley. The team submitted a paper publishing their findings; however, this paper was retracted when it became clear that Plutonium-239 was a fissile material that could be useful in the construction of an atomic weapon. At this time, the US was deep into the development of an atomic bomb (aka. the Manhattan Project) because it was believed that Germany was doing the same. For this reason, publication of Seaborg’s work was delayed until 1946, a year after the Second World War ended and security surrounding atomic research was no longer a concern. Seaborg decided to name the element after Pluto because of the recent discovery of element 93, Neptunium, and felt that element 94 should accordingly be named after the next planet in the Solar System.

Towards the end of WWII, two nuclear reactors were created which would produce the plutonium used in the construction of “Trinity”, “Fat Man” and other atomic weapons. These were the X-10 Graphite Reactor facility in Oak Ridge (which later became the Oak Ridge National Laboratory) and the Hanford B reactor (built in 1943 and 45 respectively). Large stockpiles were subsequently built up by the US and USSR during the Cold War, and have since become the focus of nuclear proliferation treaty concerns. Today, it is estimated that several tonnes of plutonium isotopes exist in our biosphere, the result of atomic testing during the 1950’s and 60’s.

We have written many articles about Plutonium for Universe Today. Here’s an article about Plutonium shortage in NASA, and here’s an article about Plutonium – 238.

If you’d like more info on Plutonium, check out Wikipedia – Plutonium, and here’s a link to World Nuclear page about Plutonium.

We’ve also recorded an entire episode of Astronomy Cast all about Nuclear Forces. Listen here, Episode 105: The Strong and Weak Nuclear Forces.

Sources:
http://en.wikipedia.org/wiki/Plutonium
http://www.world-nuclear.org/info/inf15.html
http://periodic.lanl.gov/elements/94.html
http://en.wikipedia.org/wiki/Nuclear_proliferation
http://en.wikipedia.org/wiki/Actinide
http://en.wikipedia.org/wiki/Cyclotron

Posted on by Matt Williams

What is a Plutoid?

About Dwarf Planets

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Pluto, we hardly knew ya! Don’t worry, she’s not going anywhere. However, this once happy planet will no longer be listed amongst the “planets” in our solar system. According to International Astronomical Union (IAU), which began meeting in August of 2006, the term Plutoid now applies to Pluto, as well as any other small stellar body that exist beyond the range of Neptune. Arriving at this working definition in 2008, two years after first meeting, the IAU defines the term Plutoids thusly: “Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.”

The reason the IAU began meeting in the first place was to iron out some ambiguities that exist in the terminology of astronomy. For example, thought some might find it shocking, astronomers had never actually come up with a definition of “planet”. Originally, a planet meant a “wandering star” – ie. a star that appeared to move from constellation to constellation. This was the definition used by ancient astronomers, and it applied to the sun and moon as well. However, Copernicus’s heliocentric model changed all that; now it was clear that the Earth was a planet itself and moved around the Sun with the rest of them. In addition, more and planets were being discovered beyond Jupiter, such as Uranus and Neptune, and then between Jupiter and Mars. This included Ceres, Pallas, Vesta, and Juno, but astronomers soon realized that these bodies were far too small to fit with the rest of the planets.

Then came Pluto’s discovery. At the time, scientists thought it to be several times larger than it actually was; accordingly they placed it on the list of planets. Eventually, its true size was realized and other bodies similar to Pluto in size and composition were found far beyond Neptune, in what is known as the Kuiper Belt. Pluto was to these stellar objects what Ceres was to large objects in the asteroid belt – that is to say, comparable in size. Astronomers proposed several names for these objects, but matters did not come to a head until Eris was discovered. This dwarf planet was actually larger than Pluto, 2500 km in diameter, making it twenty-seven percent larger than Pluto.

In the end, the IAU could only resolve this matter by removing Pluto from the list of planets and devising a new category for dwarf planets that could no longer be considered true planets. Plutoid was the result, and now applies to the trans-Neptunian objects of Pluto, Haumea, Makemake, and Eris.

We have written many articles about Plutoid for Universe Today. Here are some facts about Pluto, and here’s an article about why Pluto is no longer a planet.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded an episode of Astronomy Cast dedicated to Pluto. Listen here, Episode 64: Pluto and the Icy Outer Solar System.

Sources:
http://en.wikipedia.org/wiki/Plutoid
http://astroprofspage.com/archives/1685
http://www.sciencedaily.com/releases/2008/06/080611094136.htm
http://en.wikipedia.org/wiki/Eris_%28dwarf_planet%29

Posted on by Matt Williams

Last Day of Summer

Winter Solstice
Earth as viewed from the cabin of the Apollo 11 spacecraft. Credit: NASA

Summertime is a joyous time for so many reasons. There’s the sense of vacation, that feeling of freedom we remember so fondly from our childhoods. There’s the warmth weather, the sunshine, the early mornings and cool, late evenings. Seriously, there’s nothing wrong with summer, except the unfortunate fact that sooner or later, it has to end.

But when exactly is the very last day of summer? Well, it differs from place to place, depending on your location, whether you are north or south of the equator and by how much. But in the Northern Hemisphere, the change in seasons occurred on September 22nd for the year of 2010. In the Southern Hemisphere, it took place on February 28th.

In order to understand why this date was pegged as the end of the season, we need to understand exactly how the season itself is measured. These have to do with the equinoxes and solstices, seasonal markers that occur twice a year respectively. From an astronomical point of view, the equinoxes and solstices are in the middle of the respective seasons, but a variable seasonal lag means that the meteorological start of the season, which is based on average temperature patterns, occurs several weeks later than the start of the astronomical season.

According to meteorologists, summer extends for the whole months of June, July and August in the northern hemisphere and the whole months of December, January and February in the southern hemisphere. Interestingly enough, in this hemisphere, the end of the summer season is also dependent on whether or not it is a leap year (during leap years, an extra day is added).

In North America, summer is often fixed as the period from the summer solstice (June 20 or 21, depending on the year) to the fall equinox (September 22 or 23, again depending on the year). Therefore, Sept. 22 was the last day of summer and the beginning of the 2010 autumnal equinox, which officially began at 11:09 p.m. EST., the full moon having peaked the following morning at 5:17 a.m. EST which marked it as the first day of fall in the Northern Hemisphere.

The moon closest to the September equinox is considered the “Harvest Moon.” Its name stems from when farmers would rely on the light to work in the fields as the days grew shorter. For the first time since 1991, the full moon fell on the equinox, creating a “Super Harvest Moon.” In the Southern Hemisphere, the last day of summer was February 28th since 2010 was not a leap year.

We have written many articles about Summer for Universe Today. Here’s an article about the summer solstice, and here’s an article about the Earth seasons.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Summer
http://www.tonic.com/article/last-day-of-summer-first-night-of-fall-super-harvest-moon/
http://en.wikipedia.org/wiki/Equinox
http://en.wikipedia.org/wiki/Solstice
http://wiki.answers.com/Q/What_is_the_last_day_of_summer_in_Southern_Hemisphere

Posted on by Matt Williams

What is Interstellar Space?

Glittering Metropolis of Stars
Glittering Metropolis of Stars

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The boundary of what is known, that place known as the great frontier, has always intrigued and enticed us. The mystery of the unknown, the potential for discovery, the fear, the uncertainty; that place that exists just beyond the edge has got it all! At one time, planet Earth contained many such places for explorers, vagabonds and conquerors. But unfortunately, we’ve run out of spaces to label “here be dragons” here at home. Now, humanity must look to the stars to find such places again. These areas, the vast stretches of space that fall between the illuminated regions where stars sit, is what is known as Interstellar Space. It can be the space between stars but also can refer to the space between galaxies.

On the whole, this area of space is defined by its emptiness. That is, there are no stars or planetary bodies in these regions that we know of. That does not mean, however, that there is absolutely nothing there. In fact, interstellar areas do contain quantities of gas, dust, and radiation. In the first two cases, this is what is known as interstellar medium (or ISM), the matter that fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is known as the interstellar radiation field. On the whole, the ISM is thought to be made up primarily of plasma (aka. ionized hydrogen gas) because its temperature appears to be high by terrestrial standards.

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. The term first appeared in print in the 17th century in the works of Sir Francis Bacon and Robert Boyle, both of whom were referring to the spaces that fell between stars. Before the development of electromagnetic theory, early physicists believed that space must be filled with an invisible “aether” in order for light to pass through it. It was not until the 20th century though that deep photographic imaging and spectroscopy that scientists were able to postulate that matter and gas existed in these regions. The discovery of cosmic waves in 1912 was a further boon, leading to the theory that interstellar space was pervaded by them. With the advent of ultraviolet, x-ray, microwave, and gamma ray detectors, scientists have been able to “see” these kinds of energy at work in interstellar space and confirm their existence.

Many satellites have been launched with the intention of sending back information from interstellar space. These include the Voyager 1 and 2 spacecraft which have cleared the known boundaries of the Solar System and passed into the heliopause. They are expected to continue to operate for the next 25 to 30 years, sending back data on magnetic fields and interstellar particles.

We have written many articles about interstellar space for Universe Today. Here’s an article about deep space, and here’s an article about interstellar space travel.

If you’d like more information on the Interstellar Space, here’s a link to Voyager’s Interstellar Mission Page, and here’s the homepage for Interstellar Science.

We’ve recorded an episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel.

Sources:
http://en.wikipedia.org/wiki/Interstellar_space#Interstellar
http://en.wikipedia.org/wiki/Interstellar_medium
http://www.seasky.org/solar-system/interstellar-space.html
http://en.wikipedia.org/wiki/Electromagnetic_radiation
http://en.wikipedia.org/wiki/Heliopause#Heliopause