Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.
Ever wonder why the periodic table of elements is organized the way it is? Why, for example, does Hydrogen come first? And just what are these numbers that are used to sort them all? They are known as the element’s atomic number, and in the periodic table of elements, the atomic number of an element is the same as the number of protons contained within its nucleus. For example, Hydrogen atoms, which have one proton in their nucleuses, are given an atomic number of one. All carbon atoms contain six protons and therefore have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8, and so on. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same.
Arranging elements based on their atomic weight began with Ernest Rutherford in 1911. It was he who first suggested the model for an atom where the majority of its mass and positive charge was contained in a core. This central charge would be roughly equal to half of the atoms total atomic weight. Antonius van den Broek added to this by formerly suggesting that the central charge and number of electrons were equal. Two years later, Henry Moseley and Niels Bohr made further contributions that helped to confirm this. The Bohr model of the atom had the central charge contained in its core, with its electrons circulating it in orbit, much like how the planet in the solar system orbit the sun. Moseley was able to confirm these two hypotheses through experimentation, measuring the wavelengths of photon transitions of various elements while they were inside an x-ray tube. Working with elements from aluminum (which has an atomic number thirteen) to gold (seventy nine), he was able to show that the frequency of these transitions increased with each element studied.
In short, the higher the atomic number (aka. the higher the number of protons), the heavier the element is and the lower it appears on the periodic table. The atomic number of an element is conventionally represented by the symbol Z in physics and chemistry. This is presumably derived from the German word Atomzahl, which means atomic number in English. It is not to be confused with the mass number, which is represented by A. This corresponds to the combined mass of protons and neutrons in the element.
We have written many articles about the atomic number for Universe Today. Here’s an article about the atomic nucleus, and here’s an article about the Atom Models.
For many people around the world the ability to see the Aurora Borealis or Aurora Australis is a rare treat. Unless you live north of 60° latitude (or south of -60°), or who have made the trip to tip of Chile or the Arctic Circle at least once in their lives, these fantastic light shows are something you’ve likely only read about or seen a video of.
But on occasion, the “northern” and “southern lights” have reached beyond the Arctic and Antarctic Circles and dazzled people with their stunning luminescence. But what exactly are they? To put it simply, auroras are natural light displays that take place in the night sky, particularly in the Polar Regions, and which are the result of interaction in the ionosphere between the sun’s rays and Earth’s magnetic field.
Basically, solar wind is periodically launched by the sun which contains clouds of plasma, charged particles that include electrons and positive ions. When they reach the Earth, they interact with the Earth’s magnetic field, which excites oxygen and nitrogen in the Earth’s upper atmosphere. During this process, ionized nitrogen atoms regain an electron, and oxygen and nitrogen atoms return from an excited state to ground state.
Excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule. Different gases produce different colors of light – light emissions coming from oxygen atoms as they interact with solar radiation appear green or brownish-red, while the interaction of nitrogen atoms cause light to be emitted that appears blue or red.
This dancing display of colors is what gives the Aurora its renowned beauty and sense of mystery. In northern latitudes, the effect is known as the Aurora Borealis, named after the Roman Goddess of the dawn (Aurora) and the Greek name for the north wind (Boreas). It was the French scientist Pierre Gassendi who gave them this name after first seeing them in 1621.
In the southern latitudes, it is known as Aurora Australis, Australis being the Latin word for “of the south”. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red. The auroras are usually best seen in the Arctic and Antarctic because that is the location of the poles of the Earth’s magnetic field.
Names and Cultural Significance:
The northern lights have had a number of names throughout history and a great deal of significance to a number of cultures. The Cree call this phenomenon the “Dance of the Spirits”, believing that the effect signaled the return of their ancestors.
To the Inuit, it was believed that the spirits were those of animals. Some even believed that as the auroras danced closer to those who were watching them, that they would be enveloped and taken away to the heavens. In Europe, in the Middle Ages, the auroras were commonly believed to be a sign from God.
According to the Norwegian chronicle Konungs Skuggsjá (ca. 1230 CE), the first encounter of the norðrljós (Old Norse for “northern light”) amongst the Norsemen came from Vikings returning from Greenland. The chronicler gives three possible explanations for this phenomena, which included the ocean being surrounded by vast fires, that the sun flares reached around the world to its night side, or that the glaciers could store energy so that they eventually glowed a fluorescent color.
Auroras on Other Planets:
However, Earth is not the only planet in the Solar System that experiences this phenomena. They have been spotted on other Solar planets, and are most visible closer to the poles due to the longer periods of darkness and the magnetic field.
For example. the Hubble Space Telescope has observed auroras on both Jupiter and Saturn – both of which have magnetic fields much stronger than Earth’s and extensive radiation belts. Uranus and Neptune have also been observed to have auroras which, same as Earth, appear to be powered by solar wind.
Auroras also have been observed on the surfaces of Io, Europa, and Ganymede using the Hubble Space Telescope, not to mention Venus and Mars. Because Venus has no planetary magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc.
An aurora was also detected on Mars on August 14th, 2004, by the SPICAM instrument aboard Mars Express. This aurora was located at Terra Cimmeria, in the region of 177° East, 52° South, and was estimated to be quite sizable – 30 km across and 8 km high (18.5 miles across and 5 miles high).
Though Mars has little magnetosphere to speak of, scientists determined that the region of the emissions corresponded to an area where the strongest magnetic field is localized on the planet. This they concluded by analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor.
More recently, an aurora was observed on Mars by the MAVEN mission, which captured images of the event on March 17th, 2015, just a day after an aurora was observed here on Earth. Nicknamed Mars’ “Christmas lights”, they were observed across the planet’s mid-northern latitudes and (owing to the lack of oxygen and nitrogen in Mars’ atmosphere) were likely a faint glow compared to Earth’s more vibrant display.
In short, it seems that auroras are destined to happen wherever solar winds and magnetic fields coincide. But somehow, knowing this does not make them any less impressive, or diminish the power they have to inspire wonder and amazement in all those that behold them.
There has certainly been a lot of talk over the past few decades about this thing known as the “energy crisis”. In essence, we’re being told that fossil fuels are running low, that we need to start thinking green and about alternative fuels and renewable resources.
However, there’s also been a lot of discussion about places like Alberta Tar Sands and other North American oil deposits, and how these might meet our energy needs for the foreseeable future. One such deposit is the Bakken Formation, a rock unit occupying about 520,000 km² (200773 square miles) of the Williston Basin, which sits beneath parts of Saskatchewan, Manitoba, Montana, and North Dakota.
On the geologic timescale, the rock formation is believed to date from the late Devonian to Early Mississippian age – from roughly 416 to 360 million years ago. It was discovered in 1953 by a geologist named J.W. Nordquist and named after Henry Bakken, owner of the Montana farm where Nordquist first drilled.
This rock formation consists of three members or strata: the lower shale, middle dolomite, and upper shale. Oil was first discovered there in 1951, but pumping it met with difficulties. This is due to the fact that the oil itself is principally found in the middle dolomite member – roughly 3.2 km (two miles) below the surface – where it is trapped in layers of non-porous shale, making the process both difficult and expensive.
While it was postulated as early as 1974 that the Bakken could contain vast amounts of petroleum, it wasn’t until Denver-based geologist Leigh Price did a field assessment for the U.S. Geological Survey (USGS) in 1995 that official estimates were made. Price estimated in 1999 that the Bakken Formation contained between 271 and 503 billion barrels of petroleum.
Impressive, yes? Well, keep in mind that the percentage of this oil that could actually be extracted is debatable. In 1994, estimates ranged from as low as 1% to Price’s estimate of 50%. A more recent report filed in 2008 by the USGS placed the amount at between 3.0 to 4.3 billion barrels (680,000,000 m3), with a mean of 3.65 billion.
By 2011, a senior manager at Continental Resources Inc. (CLR) raised that estimate to an overall at 24 billion barrels, claiming that the “Bakken play in the Williston basin could become the world’s largest discovery in the last 30–40 years”.
But reports issued by both the USGS and the state of North Dakota in April 2013 were more conservative, estimating that up to 7.4 billion barrels of oil could be recovered from the Bakken and Three Forks formations using current technology.
Still, this represents a significant increase from the estimates made back in 1995. Horizontal well and hydraulic fracturing technology have helped, adding about 70 million barrels of production in 7 years in Montana and North Dakota. By 2007, Saskatchewan was also experiencing a boom, producing five million barrels in that year, which was up 278,540 barrels in 2004.
Consistent with the US’ policy of achieving “energy independence”, analyst expect that an additional $16 billion will be spent to further develop the Bakken fields in 2015. The large increase in tight oil production is one of the reasons behind the price drop in late 2014, and keeping prices low is always politically popular.
As more wells are brought online, production will continue to increase in places like North Dakota. While the rate of production per well appears to have peaked at 145 barrels a day since June of 2010, the number of wells has also doubled in the region between then and December of 2011.
The increase in oil and natural gas extraction has also had a profound increase on the economy of North Dakota. In addition to leading a reduction in unemployment, it has given the state a billion-dollar budget surplus and a GDP that is 29% above the national average. However, there has also been the resulting rise in pollution and the strain that industrialization and a population surge has put on the states’ water supply.
Will any of this solve the “energy crisis”? Hard to say. Because of the highly variable nature of shale reservoirs and shale drilling, and the fact that per-well rates seem to have peaked, it seems unlikely that total Bakken production will grow much further or affect the imports of foreign oil.
And given how the price of alternatives like solar, wind, geothermal and tidal energy are dropping all the time, one can expect that a fossil fuel-economy will become something of a fossil itself someday!
We have written many articles about the Bakken Formation for Universe Today. Here’s an article about Alternative Energy Sources, and here’s an article about harvesting solar power from space.
Have you ever noticed how the snow packs on a car windshield after a heavy snowfall? While the temperature is cold, the snow sticks to the surface and doesn’t slide off. After temperatures warm up a little, however, the snow will slide down the front of the windshield, often in small slabs. This is an avalanche on a miniature scale.
On the other hand, a mountain avalanche in North America might release 229,365 cubic meters (300,000 cubic yards) of snow. That’s the equivalent of 20 football fields filled 10 feet deep with snow. However, such large avalanches are often naturally released. They are primarily composed of flowing snow but given their power, they are also capable of carrying rocks, trees, and other forms of debris with them.
In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.
Avalanches are classified based on their form and structure, which are also known as “morphological characteristics”. Some of the characteristics include the type of snow involved, the nature of what caused the structural failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation.
All avalanches are rated by either their destructive potential or the mass they carry. While this varies depending on the geographical region – – all share certain common characteristics, ranging from small slides (or sluffs) that pose a low risk to massive slides that come that pose a significant risk.
An avalanche has three main parts: the starting zone, the avalanche track, and the runout zone. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snowcover and begin to slide. The avalanche track is the path or channel that an avalanche follows as it goes downhill. The runout zone is where the snow and debris finally come to a stop.
Several factors may affect the likelihood of an avalanche, including weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snowpack conditions. However, weather remains the most likely factor in triggering an avalanche.
During the day, as temperatures increase in a mountainous region, the likelihood of an avalanche increases. Regardless of the time of year, an avalanches will only occur when the stress on the snow exceeds the strength either within the snow itself or at the contact point where the snow pack meets the ground or the rock surface.
Although avalanches can occur on any slope given the right conditions, in North America certain times of the year and certain locations are naturally more dangerous than others. Wintertime, particularly from December to April, is when most avalanches will occur with the highest number of fatalities occurs in January, February and March, when the snowfall amounts are highest in most mountain areas.
Deaths Caused by Avalanches:
In the United States, 514 avalanche fatalities have been reported in 15 states from 1950 to 1997. In the 2002–2003 season there were 54 recorded incidents in North America involving 151 people.
In Canada’s mountainous province of British Columbia, a total of 192 avalanche-related deaths were reported between January 1st, 1996 and March 17th, 2014 – an average of roughly ten deaths per year. During the winter of 2014, avalanche concerns also forced the closure of the Trans-Canada highway on a number of occasions.
Avalanches on Other Planets:
Not too surprisingly, Earth is not the only planet in the Solar System to experience avalanches. Wherever their is mountainous terrain and water ice, which is not uncommon, there is the likelihood that material will come loose and cause a cascading slide to take place.
On February 19th, 2008, NASA’s Mars Reconnaissance Orbiter captured the first ever image of active avalanches taking place the Red Planet. The avalanche occurred near the north pole, where water ice exists in abundance, and was captured by the MRO’s HiRISE (High Resolution Imaging Experiment) camera completely by accident.
The images showed material – likely to include fine-grained ice dust and possibly large blocks – detaching from a towering cliff and cascading to the gentler slops below. The occurrence of the avalanches was spectacularly revealed by the accompanying clouds of fine material (visible in the photographs) that continue to settle out of the air.
The largest cloud (shown in the upper images) was about 180 meters (590 feet) across and extended about 190 meters (625 feet) from the base of the steep cliff. Shadows to the lower left of each cloud illustrate further that these are three dimensional features hanging in the air in front of the cliff face, and not markings on the ground.
The photo was unprecedented because it allowed NASA scientists to get a glimpse of a dramatic change on the Martian surface while it was happening. Despite seeing countless pictures that have detailed the planet’s geological features, most appear to have remained unchanged for several million years. It also showed that terrestrial events like avalanches are not confined to planet Earth.
Jupiter is known as the “King of the Planets”, and for good reason. For one, it is the largest planet in the Solar System, and is actually more massive than all the other planets combined. Fittingly, it is named after the king of the Roman pantheon, the latinized version of Zeus (the king of the Olympian gods).
Compare that to Earth, which is the largest of the terrestrial planets, but a tiny marble when compared to the Jovian giant. Because their disparity in size, people often wonder many times over Earth could be squeezed in Jupiter’s massive frame. As it turns out, you could it do many, many times over!
Size and Mass Comparison:
To break the whole size discrepancy down, Jupiter has a mean radius of 69,911 ± 6 km (60217.7 ± 3.7 mi). As already noted, this is roughly 2.5 times the mass of all the planets in the Solar System combined. Compared this to Earth’s mean radius of 6,371.0 km (3,958.8 mi), and you could say that Earth fits into Jupiter almost 11 times over (10.97 to be exact).
And as already noted, Jupiter is more massive than all the other planets in our Solar System – 2.5 times as massive, that is. In fact, Jupiter weighs in at a hefty 1.8986 × 1027 kg (~4.1857 x 1027 lbs), or 1898.6 billion trillion metric tons (2.092 billion trillion US tons).
Compare that to Earth, which has a mass of 5.97 × 1024 kg (13.1668 × 1024 lb) – 5.97 billion trillion metric tons, or 6.5834 billion trillion US tons. Doing the math, we then come to the conclusion that Jupiter is approximately 317.8 times as massive as Earth.
However, figuring for radius is only useful is you are planning on stacking the Earths end to end across the middle of the gas giant. And comparing their masses doesn’t give you a sense of size, seeing as how the planets are widely different in terms of their density.
To know how many Earth’s could truly fit inside in three-dimensions, you have to consider total volume, which you can calculate using the simple formula of 4/3 x Pi x radius2.
Doing the math, we find that Jupiter has a volume of 1.43 x 1015 km³ (1,430 trillion cubic km; 343 trillion cubic mi) while Earth has a volume of 1.08 trillion km3 (259 million mi). Divide the one by the other, and you get a value of 1299, meaning you could fit almost 1300 Earth’s inside Jupiter.
In short, the king of the planets is much, much, MUCH bigger than the planet we call home. Someday, if we ever hope to live around Jupiter (i.e. colonize its moons), we will be able to appreciate just how big it is up close. Until then, these impressive figures will have to suffice!
In our long history of staring up at the stars, human beings have assigned various qualities, names, and symbols for all the objects they have found there. Determined to find patterns in the heavens that might shed light on life here on Earth, many of these designations ascribed behavior to the celestial bodies.
When it comes to assigning signs to the planets, astrologists and astronomers – which were entwined disciplines in the past -made sure that these particular symbols were linked to the planets’ names or their history in some way.
Consider the planet Mercury, named after the Roman god who was himself the messenger of the gods, noted for his speed and swiftness. The name was assigned to this body largely because it is the planet closest to the Sun, and which therefore has the fastest rotation period. Hence, the symbol is meant to represent Mercury’s helmet and caduceus – a herald’s staff with snakes and wings intertwined.
Venus’ symbol has more than one meaning. Not only is it the sign for “female”, but it also represents the goddess Venus’ hand mirror. This representation of femininity makes sense considering Venus was the goddess of love and beauty. The symbol is also the chemical sign for copper; since copper was used to make mirrors in ancient times.
Earth’s sign also has a variety of meanings, although it does not refer to a mythological god. The most popular view is that the circle with a cross in the middle represents the four main compass points. It has also been interpreted as the Globus Cruciger, an old Christian symbol for Christ’s reign on Earth.
This symbol is not just limited to Christianity though, and has been used in various culture around the world. These include, but are not limited to, Norse mythology (where it appears as the Solar or Odin’s Cross), Native American cultures (where it typically represented the four spirits of direction and the four sacred elements), the Celtic Cross, the Greek Cross, and the Egyptian Ankh.
In fact, perhaps owing to the simplicity of the design, cross-shaped incisions have made appearances as petroglyphs in European cult caves dating all the way back to the beginning of the Upper Paleolithic, and throughout prehistory to the Iron Age.
Mars is named after the Roman god of war, owing perhaps to the planet’s reddish hue, which gives it the color of blood. For this reason, the symbol associated with Mars represents the god of wars’ shield and spear. Additionally, it is the same sign as the one used to represent “male”, and hence is associated with self-assertion, aggression, sexuality, energy, strength, ambition and impulsiveness.
Jupiter’s sign, which looks like an ornate, oddly shaped “four,” also stands for a number of symbols. It has been said to represent an eagle, which is Jupiter’s bird. Additionally, the symbol can stand for a “Z,” which is the first letter of Zeus – who was Jupiter’s Greek counterpart.
The line through the symbol is consistent with this, since it would indicate that it was an abbreviation for Zeus’ name. And last, but not least, there is the addition of the swirled line which is believed to represent a lighting bolt – which just happens to Jupiter’s (and Zeus’) weapon of choice.
Like Jupiter, Saturn resembles another recognizable character – this time, it’s an “h.” However, this symbol is actually supposed to represent Saturn’s scythe or sickle, because Saturn is named after the Roman god of agriculture.
The sign for Uranus is a combination of two other signs – Mars’ sign and the symbol of the Sun – because the planet is connected to these two in mythology. Uranus represented heaven in Roman mythology, and this ancient civilization believed that the Sun’s light and Mars’ power ruled the heavens.
Neptune’s sign is linked to the sea god Neptune, who the planet was named after. Appropriately, the symbol represents this planet is in the shape of the sea god’s trident.
Although Pluto was demoted to a dwarf planet, it still has a symbol. Pluto’s sign is a combination of a “P” and a “L,” which are the first two letters in Pluto as well as the initials of Percival Lowell, the astronomer who discovered the planet.
The Moon is represented by a crescent shape, which is a clear allusion to how the Moon appears in the night sky more often than not. Since the Moon is also tied to people’s perceptions, moods, and emotional make-up, the symbol has also come to represents the mind’s receptivity.
And then there’s the sun, which is represented by a circle with a dot in the middle. In the case of the Sun, this symbol represents the divine spirit (circle) surrounding the seed of potential, which is a direct association with ancient Sun worship and the central role Sun god’s played in ancient pantheons.
The planets have played an important role in the culture and astrological systems of every human culture. Because of this, the symbols, names, and terms that denote them continue to hold special significance in our hearts and minds.
When you look up into the night sky, it seems like you can see a lot of stars. There are about 2,500 stars visible to the naked eye at any one point in time on the Earth, and 5,800-8,000 total visible stars (i.e. that can be spotted with the aid of binoculars or a telescope). But this is a very tiny fraction of the stars the Milky Way is thought to have!
So the question is, then, exactly how many stars are in the Milky Way Galaxy? Astronomers estimate that there are 100 billion to 400 billion stars contained within our galaxy, though some estimate claim there may be as many as a trillion. The reason for the disparity is because we have a hard time viewing the galaxy, and there’s only so many stars we can be sure are there.
Structure of the Milky Way:
Why can we only see so few of these stars? Well, for starters, our Solar System is located within the disk of the Milky Way, which is a barred spiral galaxy approximately 100,000 light years across. In addition, we are about 30,000 light years from the galactic center, which means there is a lot of distance – and a LOT of stars – between us and the other side of the galaxy.
To complicate matter further, when astronomers look out at all of these stars, even closer ones that are relatively bright can be washed out by the light of brighter stars behind them. And then there are the faint stars that are at a significant distance from us, but which elude conventional detection because their light source is drowned out by brighter stars or star clusters in their vicinity.
The furthest stars that you can see with your naked eye (with a couple of exceptions) are about 1000 light years away. There are quite a few bright stars in the Milky Way, but clouds of dust and gas – especially those that lie at the galactic center – block visible light. This cloud, which appears as a dim glowing band arching across the night sky – is where our galaxy gets the “milky” in its name from.
It is also the reason why we can only really see the stars in our vicinity, and why those on the other side of the galaxy are hidden from us. To put it all in perspective, imagine you are standing in a very large, very crowded room, and are stuck in the far corner. If someone were to ask you, “how many people are there in here?”, you would have a hard time giving them an accurate figure.
Now imagine that someone brings in a smoke machine and begins filling the center of the room with a thick haze. Not only does it become difficult to see clearly more than a few meters in front of you, but objects on the other side of the room are entirely obscured. Basically, your inability to rise above the crowd and count heads means that you are stuck either making guesses, or estimating based on those that you can see.
All of these telescopes have been deployed over the past few years for the purpose of examining the universe in the infrared wavelength, so that astronomers will be able to detect stars that might have otherwise gone unnoticed. To give you a sense of what this might look like, check out the infrared image below, which was taken by COBE on Jan. 30th, 2000.
However, given that we still can’t seem them all, astronomers are forced to calculate the likely number of stars in the Milky Way based on a number of observable phenomena. They begin by observing the orbit of stars in the Milky Way’s disk to obtain the orbital velocity and rotational period of the Milky Way itself.
From what they have observed, astronomers have estimated that the galaxy’s rotational period (i.e. how long it takes to complete a single rotation) is apparently 225-250 million years at the position of the Sun. This means that the Milky Way as a whole is moving at a velocity of approximately 600 km per second, with respect to extragalactic frames of reference.
Then, after determining the mass (and subtracting out the halo of dark matter that makes up over 90% of the mass of the Milky Way), astronomers use surveys of the masses and types of stars in the galaxy to come up with an average mass. From all of this, they have obtained the estimate of 200-400 billion stars, though (as stated already) some believe there’s more.
Someday, our imaging techniques may become sophisticated enough that are able to spot every single star through the dust and particles that permeate our galaxy. Or perhaps will be able to send out space probes that will be able to take pictures of the Milky Way from Galactic north – i.e. the spot directly above the center of the Milky Way.
Until that time, estimates and a great deal of math are our only recourse for knowing exactly how crowded our local neighborhood is!
We have written many great articles on the Milky Way here at Universe Today. For example, here are 10 Facts About the Milky Way, as well as articles that answer other important questions.
Human beings have been observing the Moon for as long as they have walked the Earth. Throughout recorded and pre-recorded history, they have paid close attention to its phases and accorded them particular significance. This has played a major role in shaping the mythological and astrological traditions of every known culture.
With the birth of astronomy as a scientific discipline, how the Moon appears in the night sky (and sometimes during the day) has also gone long way towards helping us to understand how our Solar System works. It all comes down to the Lunar Cycle, the two key parts of this cycle involve the “waxing and waning” of the Moon. But what exactly does this mean?-day
First, we need to consider the orbital parameters of the Earth’s only satellite. For starters, since the Moon orbits Earth, and Earth orbits the Sun, the Moon is always half illuminated by the latter. But from our perspective here on Earth, which part of the Moon is illuminated – and the amount to which it is illuminated – changes over time.
When the Sun, the Moon and Earth are perfectly lined up, the angle between the Sun and the Moon is 0-degrees. At this point, the side of the Moon facing the Sun is fully illuminated, and the side facing the Earth is enshrouded in darkness. We call this a New Moon.
After this, the phase of the Moon changes, because the angle between the Moon and the Sun is increasing from our perspective. A week after a New Moon, and the Moon and Sun are separated by 90-degrees, which effects what we will see. And then, when the Moon and Sun are on opposite sides of the Earth, they’re at 180-degrees – which corresponds to a Full Moon.
Waxing vs. Waning:
The period in which a Moon will go from a New Moon to a Full Moon and back again is known as “Lunar Month”. One of these lasts 28 days, and encompasses what are known as “waxing” and “waning” Moons. During the former period, the Moon brightens and its angle relative to the Sun and Earth increases.
When the Moon starts to decrease its angle again, going from 180-degrees back down to 0-degrees, astronomers say that it’s a waning moon. In other words, when the Moon is waning, it will have less and less illumination every night until it’s a New Moon.
When the Moon is no longer full, but it hasn’t reached a quarter moon – i.e. when it’s half illuminated from our perspective – we say that it’s a Waning Gibbous Moon. This is the exact reverse of a Waxing Gibbous Moon, when the Moon is increasing in brightness from a New Moon to a Full Moon.
This is followed by a Third Quarter (or last quarter) Moon. During this period, 50% of the Moon’s disc will be illuminated (left side in the northern hemisphere, and the right in the southern), which is the opposite of how it would appear during a First Quarter. These phases are often referred to as a “Half Moon”, since half the disc is illuminated at the time.
Finally, a Waning Crescent is when the Moon appears as a sliver in the night sky, where between 49–1% of one side is illuminated after a Full Moon (again, left in the northern hemisphere, right in the southern). This is the opposite of a Waxing Crescent, when 1-49% of the other wide is illuminated before it reaches a Full Moon.
Even today, thousands of years later, human beings still look up at the Moon and are inspired by what they see. Not only have we explored Earth’s only satellite with robotic missions, but even crewed missions have been there and taken samples directly from the surface. And yet, it still possesses enough mystery to keep us inspired and guessing.
The gas (and ice) giant known as Uranus is a fascinating place. The seventh planet from out Sun, Uranus is the third-largest in terms of size, the fourth-largest in terms of mass, and one of the least dense objects in our Solar System. And interestingly enough, it is the only planet in the Solar System that takes it name from Greek (rather than Roman) mythology.
But these basic facts really only begin to scratch the surface. When you get right down to it, Uranus is chock full of interesting and surprising details – from its many moons, to its ring system, and the composition of its aqua atmosphere. Here are just ten things about this gas/ice giant, and we guarantee that at least one of them will surprise you.
Many people think that the answer to ‘what is the largest moon in the Solar System’ is our Moon. It is not. Our Moon is the fifth largest natural satellite. Ganymede, a moon of Jupiter, is the largest in this solar system. At 5,268 km at the equator, it is larger than Mercury, the dwarf planet Pluto, and three times larger than the Moon orbiting Earth. According to information from NASA,if Ganymede were to break free of Jupiter’s gravitational pull it would be classified as a planet.
Ganymede has an iron-metallic core that generates a magnetic field. The core is surrounded by a mantle of rock, which is, in turn, covered by a thick shell of ice and rock. The outer shell is up to 800 km thick. On top of the outer shell is a thin layer of material(accreted?) that is a mixture of ice and rock. In images taken by the Hubble Space Telescope in 1996, astronomers discovered a tenuous atmosphere of oxygen. The atmosphere would not support known life forms, but its very existence is interesting to science.
Through spacecraft observation, there are several facts known about the surface of Ganymede. Its surface shows two types of terrain. Forty percent of the surface is covered with highly cratered dark regions, while the remainder is covered by light grooved terrain. The light grooving forms intricate patterns across surface of the moon, some of which are thousands of km long. Sulcus, a term that means a groove or burrow, is often used to describe the grooving. This portion of the terrain was most likely formed by tensional faulting or the release of water from beneath the surface. Ridges as high as 700 m have been observed. The dark regions are heavily cratered, old and rough, and are believed to be the original crust of the moon.
Ganymede was discovered by Galileo on January 7, 1610. He made the discovery, along with three other Jovian moons. It marked the first time a moon was discovered orbiting a planet other than Earth. The discovery contributed to the acceptance of the heliocentric viewpoint over the geocentric that held sway prior to that.
Now you know the answer to ‘what is the largest moon in the Solar System’ and a few interesting facts about Ganymede. Jupiter has 63 moons, so there are plenty more facts for you to discover.