What is the Aurora Borealis?

Aurora from 2002 in Poker Flats, Alaska. Credit: Dr. Scott Bounds

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The aurora (plural aurorae) borealis has many other names: northern lights, northern polar lights, polar lights, and more. An aurora borealis is light seen in the sky, nearly always at night, in the northern hemisphere, commonly green but also red and (rarely) other colors; often in the shape of curtains, sheets, or a diffuse glow (when seen from the ground). Northern lights are most often seen at high latitudes – Alaska, Canada, northern Scandinavia, Greenland, Siberia, and Iceland – and during maxima in the solar cycle.

Aurora australis – southern lights – is the corresponding southern hemisphere phenomenon.

Seeing a bright auroral display may be on your list of ‘things to see before I die’! Yep, they are nature’s light show par excellence.

Aurora borealis occur in the Earth’s ionosphere, and result from collisions between energetic electrons (sometimes also protons, and even heavier charged particles) and atoms and molecules in the upper atmosphere. The ultimate origin of the energy which powers the aurora borealis is the Sun – via the solar wind – and the Earth’s magnetic field. Interactions between the solar wind (which carries its own tangled magnetic fields) and the Earth’s magnetic field may cause electrons (and other particles) to be trapped and accelerated; those particles which do not escape ‘downstream’ to the magnetic tail ‘touch down’ in the atmosphere, close to the north magnetic pole.

The different colors come from different atoms or ions; green and red from atomic oxygen, nitrogen ions and molecules make some pinkish-reds and blue-violet; purple is the appearance of combined colors from nitrogen ions and helium; neon produces the very rare orange. The ionosphere is home to most aurorae borealis, with 100-300 km being typical (this is where green is usually seen, with red at the top); however, some particularly energetic particles penetrate much deeper into the atmosphere, down to perhaps 80 km or lower (purple often comes from here).

Viewed from space, when the northern lights are intense they appear as a ring (an oval actually), the auroral zone, with the north magnetic pole near the center.

The University of Alaska Fairbanks’ Geophysical Institute has a good FAQ on the aurora borealis.

Magnetic fields plus solar wind … so you’d expect aurorae on Jupiter and Saturn, right? And auroral displays around the magnetic poles of these planets are now well documented. Aurorae have also been imaged on Venus, Mars, Uranus, Neptune, and even Io.

Some Universe Today stories on aurorae – borealis, australis, … and extra-terrestrial: What are the Northern Lights?, Aurora Reports from Around the World, Behind the Power and Beauty of the Northern Lights, Northern & Southern Aurorae Are Siblings, But Not Twins, Two Rockets Fly Through Auroral Arc, Chandra Looks at the Earth’s Aurora, First Aurora Seen on Mars, and Saturn’s “Dualing” Aurorae.

Greenwich Mean Time

Prime Meridian in Greenwich, England

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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

Evening Star

Venus. Credit: NASA

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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

Why is the Sun Hot?

Plasma on the surface of the Sun. Image credit: Hinode

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The Sun is the hottest place in the Solar System. The surface of the Sun is a mere 5,800 Kelvin, but down at the core of the Sun, the temperatures reach 15 million Kelvin. What’s going on, why is the Sun hot?

The Sun is just a big plasma ball of hydrogen, held together by the mutual gravity of all its mass. This enormous mass pulls inward, trying to compress the Sun down. It’s the same reason why the Earth and the rest of the planets are spheres. As the pull of gravity compresses the gas inside the Sun together, it increases the temperature and pressure in the core.

If you could travel down into the Sun, you’d reach a point where the pressure and temperature are enough that nuclear fusion is able to take place. This is the process where protons are merged together into atoms of helium. It can only happen in hot temperatures, and under incredible pressures. But the process of fusion gives off more energy than it uses. So once it gets going, each fusion reaction gives off gamma radiation. It’s the radiation pressure of this light created in the core of the Sun that actually stops it from compressing any more.

The Sun is actually in perfect balance. Gravity is trying to squeeze it together into a little ball, but this creates the right conditions for fusion. The fusion releases radiation, and it’s this radiation that pushes back against the gravity, keeping the Sun as a sphere.

We have written many articles about the Sun for Universe Today. Here’s an article about how hot the surface of the Sun is, and here’s an article about the parts of the Sun.

If you’d like more information on the Sun, check out NASA’s Solar System Exploration Guide on the Sun, and here’s a link to the SOHO mission homepage, which has the latest images from the Sun.

We have also recorded an episode of Astronomy Cast about the Sun. Check it out, Episode 30: The Sun, Spots and All.

When Was the Sun Discovered?

Solar flares on the Sun

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When was the Sun discovered? Obviously the Sun is such an important feature in our lives, and the absolute necessity to all life on Earth. It’s kind of impossible to say when the Sun was discovered, since the first life forms on Earth probably relied on its energy. Humans have been well aware of the Sun for tens of thousands of years, and before modern astronomy had no idea what it was.

So perhaps a better question might be, when did we realize that the Sun is a star?

The Sun is incredibly important to our lives. When the Sun is in the sky, we have day. And when the Sun is below the horizon, we have night. Our biological clocks are programmed on it, and we life our lives by this routine. Ancient peoples thought the Sun was some kind of deity, and many civilizations – like the Inca in South America – worshipped it.

The Greek philosopher Anaxagoras first proposed that the Sun was a burning ball of fire, larger than a Greek Island, and not the chariot of a god. And other astronomers were able to calculate the distance to the Sun with surprising accuracy. In the modern scientific era Lord Kelvin proposed that the Sun was ball of hot liquid that was slowly cooling. But it wasn’t until the early 20th century that scientists were finally able to figure out what the source of the Sun’s energy is.

Ernest Rutherford proposed that the Sun’s heat came from radioactive decay, and it was Albert Einstein who used his famous mass-energy equation (E=mc2) to suggest that the Sun was converting mass into energy. And finally, the theoretical concept of fusion was created in the 30s by Subrahmanyan Chandrasekhar and Hans Bethe. They were able to calculate the actual fusion reactions in the Sun that convert hydrogen into helium.

I would say then, that the Sun was really discovered in the 1930s, when astrophysicists finally understood the mechanisms working inside the Sun that gave off so much energy.

We have written many articles about the Sun for Universe Today. Here’s an article about how big the Sun is, and here’s an article about the Sun’s future.

If you’d like more information about the Sun, check out NASA’s website for the SOHO spacecraft mission.

And you should check out an episode of Astronomy Cast where we talk all about the Sun. Listen here, Episode 30: The Sun, Spots and All.

References:
NASA: The Sun, Our Nearest Star
NASA: A History of Our Understanding of the Sun – A Closer Look
NASA: The Life Cycles of Stars

Earth’s Layers For Kids

My son recently came back from a science day camp with one of the coolest things. It was a model of the Earth that he had created out of modeling clay. It showed the internal structure of the Earth, and because he built it, he was able to remember all of the layers of the Earth. Very cool. So here’s a good way to learn the Earth layers for kids.

To make your own, you need some modeling clay of different colors. You start by making a ball about 1.2 cm across. This represents the Earth’s inner core. Then you make a second ball about 3 cm across. This ball represents the Earth’s outer core. Then you make a third ball about 6 cm across. This ball represents the Earth’s mantle. And finally, you make some flattened pieces of clay that will be the Earth’s crust. To make it extra realistic, make some pieces blue and others green.

Take inner core and surround it with the outer core, and then surround that by the mantle. Cover the entire mantle with a thin layer of blue, and then put on some green continents on top of the blue.

If you’ve been really careful, you should be able to take a sharp knife and slice your Earth ball in half. You should be able to see the Earth’s layers inside, just like you’d see the real Earth’s layers. And you can see that the mantle is thicker underneath the Earth’s continents than it is under the oceans.

Here’s a link with more information from Purdue University so you can do the experiment yourself.

If you’re interested in teaching your children Earth science, here’s lots of information about volcanoes for kids.

We have also recorded a whole episode of Astronomy Cast just about Earth. Listen here, Episode 51: Earth.

How Big is Earth?

Blue marble Earth. Image credit: NASA

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Here’s a question: how big is Earth? Let’s take a look at how big our planet is.

First, the equatorial diameter of Earth is 12,756 km. In other words, if you dug a tunnel on the equator that went straight down and went right through the center of the Earth, it would be about 12,756 km long. Just for comparison, that’s about 1.9 times the diameter of Mars. And only .09% the diameter of Jupiter.

The volume of Earth is 1.08 x 1012 km3. Written another way, that’s 1.08 trillion cubic kilometers of rock and metal. Again, it’s about 6.6 times more volume than Mars.

The surface area of Earth is 510,072,000 square kilometers. Of that, 29.2% is covered by land and 70.8% is covered by water. Just for comparison, that’s 3.5 times as much surface area as Mars.

The mass of Earth is 5.97 x 1024 kg. Here that is written out: 5,970,000,000,000,000,000,000,000 kg. Yeah, that’s a really big number. And yet, it’s only 0.3% the mass of Jupiter (and Jupiter is mostly lightweight hydrogen).

We have written many articles about Earth for Universe Today. Here’s an article about how fast Earth rotates, and here’s an article about Earth’s magnetic field.

You can learn more about Earth from NASA’s Earth Observatory, as well as NASA’s Solar System Exploration Guide.

We have also recorded an entire episode of Astronomy Cast that’s just about Earth. Listen here, Episode 51: Earth.

Nebula Pictures

Omega Nebula

Here are some cool nebula pictures taken by the Hubble Space Telescope.

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This is a cool picture of the Omega Nebula, which is also known as the Swan Nebula, or M 17. It’s located in the constellation Sagittarius, located about 5,000 light-years from Earth.


Cat's Eye Nebula
Cat's Eye Nebula

This is picture of the Cat’s Eye Nebula, a planetary nebula in the constellation Draco. This used to be a star similar to our Sun, but then it died and became a white dwarf, puffing off its outer layers into space.


Carina Nebula
Carina Nebula

This is the Carina Nebula, a star forming nebula in the Carina Constellation. It holds Eta Carinae, one of the most massive stars ever discovered, which is expected to detonate as a supernova in the next few hundred thousand years.


Bubble Nebula. Image credit: Hubble
Bubble Nebula. Image credit: Hubble

This is the Bubble Nebula, also known as NGC 7635. This nebula glows because of a hot central star that’s providing radiation and exciting the nebula atoms.


Full view of the Trifid Nebula.  Credit: ESO
Full view of the Trifid Nebula. Credit: ESO

This is a picture of the Trifid Nebula taken by the European Southern Observatory. This nebula was cataloged M 20 by Charles Messier as part of his famous catalog. It’s called the “Trifid Nebula”, because it appears to be broken up into three parts.

We have written many stories about nebulae for Universe Today. Here’s an article with more details about the Trifid Nebula, and here’s an article about planetary nebulae found around heavy stars.

If you want more cool pictures of nebula, you should check out the source. Go to the Hubble Space Telescope page on nebulae.

We have recorded an entire episode of Astronomy Cast that’s just about nebulae. Listen here, Episode 111: Nebulae.

International Space Station Viewing

The ISS. Credit: NASA

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Now that it’s mostly complete, the International Space Station is the brightest human-built object in space. It’s easy to see with your own eyes, the trick is knowing when to step outside and look up to see the station go overhead. If you do get your timing right, you’ll see the station as a bright star moving quickly in the sky. It only take a couple of minutes to pass through the sky above your house. Want to see the station for yourself? Here are some resources for International Space Station viewing.

The best place to go is NASA’s Human Spaceflight tracking page. This shows you the current location of the International Space Station, the Hubble Space Telescope, and any space shuttles currently in orbit.

So that shows you where the space station and shuttles are right now, but how will you know when they’re going to be passing over your part of the Earth?

NASA has a page for sighting opportunities. You can either choose your location from a list of common locations around the world, or you download an application that lets you pick your specific spot on Earth. It will then tell you the exact times ISS will be passing overhead.

If you’ve got an iPhone, check out the ISS Visibility App. This tool will calculate the next times you’ll be able to see the ISS pass overhead.

You can also use a great service called Heavens Above. This will also show you the current location of satellites, as give you times when ISS will be passing overhead.

We have written many articles about the International Space Station for Universe Today. Here’s an article about how ISS is now visible in the daytime.

We have recorded an episode of Astronomy Cast about the space shuttle. Listen to it here, Episode 127: The US Space Shuttle.