Why is the Earth Tilted?

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

Have you ever wondered why the Earth is tilted instead of just perpendicular with its plane of orbit? Scientists have taken a crack at answering that question. The main consensus is that it has to do with Earth’s formation along with the rest of the planets in the Solar system. This time in cosmic history is still a mystery to us but we do have some ideas about what went on. We know that the birth of the Sun created a new source of gravity in the young Solar System. The tidal forces between the young sun and the rest of the nebula the Sun was born from created further instability in the gases and dust left in the nebula. This allowed for the steady formation of the planets.

After millions of years passed enough matter collided to gain mass and its own gravity and become small versions of planets called planetessimals and protoplanets. These pre-planets collided to create even larger planets. This set the stage for how the Earth approached its final form. It looks like it probably collided with a another proto-planet and in the process it was tilted.

All the same the Earth’s tilt is very important. It is perfectly positioned so that it gives us the seasons and on top of that the seasons are near perfectly calibrated for life. When compared with other planets Earth’s tilt allows for season that are not too extreme in temperature but are pretty well balanced. At the same if it had stay in the “perfect” position one side of the Earth would be too hot at time and then too cold.

We have written many articles about the Earth’s tilt for Universe Today. Here’s an article about why Earth has seasons, and here’s an article about the Earth’s axis.

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.

Why is the Center of the Earth Hot

Earth's core.
Earth's core.


It interesting that we have explored further into space than we have explored the depths of the Earth. The main reason for that is the pressure and the heat. We know through seismography that temperatures in the inner parts of the Earth actually exceed the surface temperature of the Sun! That is pretty hot. So why is the center of the Earth Hot. The answer comes from a lot different sources. The first is heat left over from the formation of the Earth. The next source is gravitational pressure put on core by tidal forces and the rotation of the Earth. The last known source of heat is the radioactive decay of elements in the inner part of the Earth.

The Earth is pretty old at 4 billion years old and there are still things we don’t completely understand about its formation. We do know that gravity played a role pulling in more matter and compressing it to form the Earth. When you have matter colliding at high velocities like it did in the early stages of the Solar System’s development all that kinetic energy has to go somewhere. In the case of Earth that energy was turned into heat. This heat is the initial source for the temperatures in the Earth’s interior.

The next source of heat is gravitational pressure. The Earth is under immense pressure due to the tidal forces exerted by the Sun, the Moon, and the other planets in the Solar System. When you include the fact that it is also rotating the Earth’s core is under immense pressure. This pressure basically keeps the core hot in the same way as a pressure cooker. It also helps to minimize the heat it loses.

The last and most important source of heat is nuclear fission of heavly elements in the Earth’s interior. In short the Earth has a nuclear engine inside it. It is thank to the continous nuclear fission of elements in the Earth’s interior that replaces the heat the Earth loses keeping it nice and hot. This fission process occurs in the form of radioactive decay. It also creates the convection currents in the mantle that drive plate tectonics.

We have written many articles about the Earth’s core for Universe Today. Here’s an article about the Earth’s outer core, and here are some interesting facts about 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.


Why Can We See the Moon During the Day?

Crescent Moon
Crescent Moon

We all know the basics of the Diurnal Cycle – day and night, sunrise and sunset. And we are all aware that during the day, the Sun is the most luminous object in the sky, to the point that it completely obscures the stars. And at night, the Moon (when it is visible) is the most luminous object, sometimes to the point that it can make gazing at the Milky Way and Deep-Sky Objects more difficult.

This dichotomy of night and day, darkness and light, are why the Moon and the Sun were often worshiped together by ancient cultures. But at times, the Moon is visible even in the daytime. We’ve all seen it, hanging low in the sky, a pale impression against a background of blue? But just what accounts for this? How is it that we can see the brightest object in the night sky when the Sun is still beaming overhead?

Continue reading “Why Can We See the Moon During the Day?”

Where is Uranium Located

Periodic Table of Elements
Periodic Table of Elements


Uranium is a silvery white metal and is number 92 on the table of periodic elements. It is a well-known element because of its radioactive properties which are used in nuclear reactor powered by nuclear fission. We know that this element is very sought after as source of power by many countries wanting to shift from oil and fossil fuel based economies. So where is Uranium located and how do miners harvest it?

To understand how it is found we need to learn about how it was discovered. Uranium was first discovered by German chemist martin Heinrich Klaproth in 1749 when he was heat treating Minerals. He named the new mineral produced Uranium. The first pure sample of Uranium metal was produced in 1841 by Eugène-Melchior Péligot an analytical chemist who was heat treating Uranium tetrachloride. Demand for Uranium outside its more mundane uses as a window dye was initiated by the discovery of its fissile nuclear properties by Enrico Fermi. Mr. Fermi would go on to lead the Manhattan Project in 1942 that lead to the creation of nuclear weapons and reactors. When the energy it produced was realized the demand for Uranium immediately increased.

So where is Uranium located? In space Uranium is formed naturally occurring in supernovas. However since we can’t even travel to the nearest star it is just a minor fact. On Earth Uranium is surprisingly plentiful for a heavy metal. In fact estimate place the Earth’s supply of Uranium at 30 times that of Silver. This is because Uranium can be found in topsoil anywhere on the planet as well as in the mantle. Scientist even theorize that the natural decay of Uranium and other radioactive elements is what heats the Earth’s core and mantle causing convection currents in the magma and creating plate tectonics.

Uranium can be found as part of a lot of different minerals such as uranite. The element rarely occurs in its pure form. Even then the more fissile kinds of isotopes aren’t plentiful in nature. Uranium ore is the main source of uranium even though with the discovery of how wide spread it is in the Earth’s crust and scientist are looking for inexpensive ways to process it from the soil. In the meanwhile Uranium ore can be found in mines in Canada, Russia, and in Sub-Saharan Africa.

We have written many articles about Uranium for Universe Today. Here’s an article about the lunar Uranium, and here’s an article about nuclear fission.

If you’d like more info on Uranium, check out Wikipedia, and here’s a link to the Encyclopedia of Earth.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Encyclopedia of Earth
World Nuclear Association

Where is the Ozone Layer Located

Ozone layer hole. Image credit: NASA
Ozone layer hole. Image credit: NASA


The Ozone Layer is the portion of the atmosphere that contains high levels of the oxygen molecule ozone. This molecule plays an important role acting as a natural UV shield for the Earth. You may wonder where is the ozone layer located to play such a vital role so effectively. The Ozone layer is actually located in the stratosphere in a region that is 10 to 50 km above the Earth.

So why is the Ozone layer so important? As mention before the secret lies in oxygen molecules. Normal oxygen in its natural molecular state is made up of only two atoms. However this changes when oxygen in the thermosphere is exposed the Sun’s ultraviolet rays. The rays separate oxygen molecules the free oxygen joins with the remaining two atom oxygen molecules to create ozone. This process might seem simple but it helps to screen out 99.5 percent of the ultraviolet radiation that the Sun sends towards earth. The times that the ozone layer didn’t screen out this type of radiation at such levels life was almost wiped out according to the geologic record.

You might think that this is an exaggeration until you observe the biological damage UV rays can do. We have already seen the harm caused when people don’t take the proper precautions when going to the beach. The least harm comes in the form of sun burn. People overexposed to the UV rays that do make it to earth have their skin damaged by the UV energy that penetrates their skin. However it gets more serious the longer a person is exposed to UV rays. The reason is because the damage gets to the cellular level causing cancers and genetic damage. Essentially it’s like being exposed to a nuclear reactor in melt down. The high energy radiation over time would accumulate harm in living tissue until it killed the organism exposed to it.

Despite its importance industry produced and released chemicals into the air that interfered with the ozone cycle. The main problem chemical CFC’s prevented oxygen molecules from complete the bonding process that is important for the completion of the ozone cycle this caused a major depletion of ozone in key areas of the Earth’s atmosphere. This is huge when the natural concentration of ozone was already quite low. This just goes to show the delicate balance that was upset. Fortunately nations upon hearing the harm caused started bans on CFC’s while industry tried to find alternatives to use in products. The result started to show with ozone depletion actually slowing down and reversing with scientist predicting recovery within the next century.

We have written many articles about the ozone layer for Universe Today. Here’s an article about the depletion of the ozone layer, and here’s an article about the ozone layer.

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.

Where is Helium Found



Helium is the second lightest element in the known universe. It is also the second most abundant. According to some estimates helium accounts for as much as 24 percent of the Universe’s mass. This element is also plentiful since it is a prime product of fusion nuclear reactions involving hydrogen. So if it is so plentiful where is Helium found?

The problem is that just because an element is common in the universe at large does not mean that it is common on Earth. Helium is an element that fits this scenario. Helium only accounts for 0.00052% of the Earth’s atmosphere and the majority of the helium harvested comes from beneath the ground being extracted from minerals or tapped gas deposits. This makes it one of the rarest elements of any form on the planet.

Like mentioned before Helium is rare on Earth but there are places where it is readily found. If you look at space the majority of helium is in stars and the interstellar medium. This is due to the fusion reaction that powers most stars fusing single hydrogen atoms to create helium atoms. This process balanced with a star’s gravity is what helps it to stay stable for billions of years. On Earth the majority of helium found comes from radioactive decay. This is the opposite nuclear reaction called fission that splits atoms. For this reason radioactive minerals in the lithosphere like uranium are prime sources for helium.

On Earth there are key locations where concentrated helium can be harvested. The United States produces the majority of the world’s helium supply at 78%. The rest of the world’s helium is harvested in North Africa, The Middle East, and Russia. The interesting thing is that thanks to these deposits the world’s demand for helium is being met regularly. Also unlike petroleum which can decades to form from organic material, 3000 metric tons of Hydrogen is produced yearly. Until helium demand reaches at least the same level of demand as petroleum there it little chance of that demand outpacing supply.

Helium is looking to be a major player in the near future. Governments are looking into using the gas as source of hydrogen for fuel cells and other transportation technologies. At the moment the promise is still tentative but at least with better surveying and knowledge of gas deposits there will be a supply waiting if becomes the next major element to power human civilization. In the meanwhile ours is still a planet beholden to carbon.

We have written many articles about Helium for Universe Today. Here’s an article about the discovery of Helium, and here’s an article about composition of the Sun.

If you’d like more info about helium 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.

Source: Wikipedia

Where Does Geothermal Energy Come From

Earth's core.
Earth's core.


You may not have heard much about geothermal energy but it is one of the hottest alternative energy commodities around. It is a renewable and clean energy source that will be around for a long time. However where does geothermal energy come from? The answer is the earth itself. The world geothermal comes from the Greek words geo which means earth and therme which means heat. Basically geothermal energy is heat energy harvested from the Earth itself.

Geothermal heat is produced by the core of the Earth itself. You may not think that energy to be much when compared to the sun but you would be wrong in some aspects. The Earth’s core alone is 11,000 degrees. That is hotter than the surface of the sun. The Earth’s geothermal energy is created by the decay of radioactie materials in the core and in the surrounding layers of rock.

However this still doesnt tell us how this energy becomes accessible. The deepest mankind can even go with the best technology is around 11 km. The answer is plate tectonics. bounadaries and faults are cracks in the Earth’s crust where magma rises near or to the surface. Geothermal plants take advantage of this fact using water heated by this volcanic activity to produce electric power.

The main place where geothermal energy can be used have not only volcanic activity but also enough ground water to be used to power the turbines that generate power. Prime areas are near volcanoes, hot springs, and geysers. Large volcanic islands like Greenland have vast resources in terms of geothermal energy. In the end the most common location for geothermal reservoirs will be where ever there are major plate boundaries with a lot of seismic and volcanic activity.

The benefits of geothermal energy is already being discussed in nations like Iceland as way to reduce reliance on foreign oil. Geothermal is abundant where it can be accessed and can easily produce energy on par with the output of other types of energy production such as nuclear reactors. The best part is that it is clean energy. There is no way it can produce pollution that can harm the environment. The only risk is that drilling in active volcanic area can make them vulnerable to earthquakes.

In the end Geothermal still one of the best possible sources of clean energy on the planet. As the technology improves for accessing it more homes around the world will have the opportunity to be powered by this renewable energy resource.

We have written many articles about geothermal energy for Universe Today. Here’s an article about Geothermal Energy, and here’s an article about how geothermal energy works.

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.


Where are Tornadoes likely to Occur

Tornado at Union City, Oklahoma Credit, NOAA Photo Library
Tornado at Union City, Oklahoma. Credit: NOAA Photo Library


Everyone knows about tornadoes. You may have seen them in movies or heard about them in the news. However one of the most important facts for a person to know is where Tornadoes are likely to occur. This makes simple sense. If you want to avoid hurricanes you know that you should likely not live in the Gulf Coast or Florida. If you want to avoid the chance of mudslides you wouldn’t live in Oregon. Knowing where and how tornadoes can appear can help you stay safer and better prepared in case such a storm happens.

For the most part we know that Tornadoes as they are known in the United States are largely a North American phenomenon. The unique position and composition of North America’s topography gives thunderstorms enough space, time, and energy to form tornadoes. The traditional red zone for tornadoes is the Great Plains region of the United States called Tornado Alley. This region is known for spawning several tornadoes a year and in this region tracking storms and preparing for tornadoes is a way of life. The flat grasslands are perfect place for pressure systems to collide, creating powerful storms and in turn powerful tornadoes.

Interesting enough Tornado Alley is not the only area where tornadoes can happen. Tornadoes can occur anywhere in continental United States if the conditions for tornado formation are met. That means if you have a particularly strong thunderstorm system in your area with high winds there is a strong possibility of a Tornado happening.

The frequency of tornadoes happening outside the Tornado alley have increased with powerful storms ripping up areas that would be by conventional wisdom considered safe such as the Southeast or the Atlantic Seaboard.

One type of location that is generally safe from Tornadoes is the city. However recent events have proven that not likely doesn’t mean never. Two years ago a powerful tornado ripped through downtown Atlanta and doing major damage to the CNN headquarters. The other major tornado in a major city happened recently in New York City. A twister touched down in the Bronx in September of this year in the early morning hours also did serious property damage.

The danger of tornadoes in unlikely locations is that they are harder to spot. The tornado that struck Piedmont, Alabama became one of the deadliest on record because the area was hilly and full of trees. This made it impossible for residents to see the storm funnel approaching. This is why it is important for local news to have good weather tracking systems to properly warn residents in case of unusual weather conditions.

We have written many articles about tornadoes for Universe Today. Here’s an article about the Tornado Alley, and here’s an article about how tornadoes are formed.

If you’d like more info on tornadoes, check out the National Oceanic & Atmospheric Administration (NOAA) Homepage. 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.

Where are Stars Born?

Spitzer Uncovers Star Hatchery
Spitzer Uncovers Star Hatchery

Have you ever wondered where stars are born? Stars are formed in nebulas, interstellar clouds of dust and gas. Nebulas are either remnants of matter from the original big bang or the result of stars either collapsing or going supernova. Nebulas have long been noted and observed by astronomers but very little was known about them until the 21st century.

Galaxies because of their similar appearance were once thought of as nebulas. It was later determined that they were actually larger grouping of stars a great distance away from the Earth. So how are Nebulas star forming regions? The answers lie in the gravitational force and nuclear fusion.

Most nebulas are disparate clouds of gas and cosmic dust floating in the interstellar medium. Nebulas are the more dense parts of the gas and dust that exist in the space between stars and galaxies. We know due to the law of universal gravitation that every particle in the universe exerts an attractive force on every other particle. This happens over times with nebulas as the particles that make up the interstellar medium start to gather together.

Since gases have mass it is inevitable that the process will continue as great mass will create a stronger gravitational field. At some undefined point in time a tipping point between the gas pressure and the gravity of the nebula is crossed and the nebula collapses under its own gravity. Since molecular hydrogen is the most abundant element in the nebula the pressure from the collapse causes the nebula to undergo nuclear fusion. This starts the birth of a star.

As evidenced by how many stars and galaxies are in the universe you can see that is process that happens just about everywhere. More recently scientists have started become interested in how common it is for stars to from planets, especially those that are likely to support life. Scientists have recently discovered one such planet Gliese 581-g. This planet while closer to it star than Earth is well with in habitable zone necessary for liquid water and the right temperatures for life to occur.

The study of nebulas and the interstellar medium have yielded a lot important information about the formation and stars. As better telescopes and probes are created we will get a clearer picture about our universe and how it was formed and continues to grow over time.

We have written many articles about the birth of stars for Universe Today. Here’s an article about the star birth myth, and here’s an article about the birth of the biggest stars.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?


What Galaxy Do We Live In?

Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA

If you are not an astronomy enthusiast you not have thought much about what galaxy do we live in. So depending on that the answer may surprise you. If you know anything about galaxies you know that they are groupings of stars that number in the hundreds of billions. The most famous is the Milky Way. It is from this galaxy that we even have the term. The simple point is that the Earth is part of the Milky Way even though if we see it in the sky it looks like we are observing it from the outside. Why is that? To understand you need to know exactly where we live in neighborhood of the Milky Way Galaxy.

As we are part of the solar system Earth pretty much follows the path of the sun as it goes through its own orbit around the galaxy. The Milky Way is a spiral galaxy type so it has arms sort of like an octopus. The Sun is located near the outward tip of the Sagittarius arm of the Milky Way. This makes Earth about 28,000 light years from the galactic core of our home galaxy.

The Solar System also has a galactic year that it follows. It takes around 200 million to 250 million years for the solar system to orbit the Sun. Another indicator of our position is where the galactic equator. While our star system is considered to be on the outskirts of the Milky Way this is only an estimate. It is believed that the Milky Way is larger than first estimated. There is also suspicion that our galaxy is in the process of absorbing other smaller galaxies. However, there is not enough empirical evidence available to support the claim.

So what would be so important about knowing what part of the galaxy we live in? One reason is space exploration. Some time in the future mankind may find a way to achieve faster than light space travel. This can provide a new set of challenges for engineers and astronomers to tackle. For example how would an astronaut keep from getting lost in space? Detailed mapping and computer programming in the future could help galactic wayfarers know where they are going and more importantly how to get home.

The other reason is that it never hurts to know our place in the scheme of things. Just thinking of the challenge of finding earth if we were so far way helps us to understand how truly vast the universe is.

We have written many articles about the Milky Way galaxy for Universe Today. Here are some facts about the Milky Way, and here’s an article about the closest galaxy to the Milky Way.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

Sources: SEDS, Daily Galaxy