Polaris shares North Star duties with other planets in our solar system

Part-time North Star

Photo: Pixabay

Shakespeare regarded the North Star as a permanent feature of the heavens. He was wrong

"But I am as constant as the Northern Star", says Julius Caesar in the play Shakespeare named after him.

He was making a strong statement that he was reliable and unchanging in his opinions. Shakespeare regarded the North Star as a permanent feature of the heavens. He was wrong. He should have picked something else.

All of us have seen, at some time or another, one of those time exposures of the northern sky, where all the stars leave circular tracks, and in the centre of this circling lies a star that is hardly moving. That is because those circular tracks are due to the rotation of our planet.

If we were at the North Pole, there would be one star overhead not moving because that is where the Earth's axis of rotation meets the sky. We call that star the "North Star.”

That star, named Polaris, is the brightest in the constellation of Ursa Minor, "The Little Bear".

For millennia, Polaris has been used for navigation. If you are sailing the ocean, out of sight of land, the North Star always lies due north, and the angle between the North Star and the northern horizon is equal to your latitude. Therefore, if you are sailing across the Pacific or the Atlantic, you can keep Polaris at the right angle above the horizon and be sure you will reach your destination.

Warning though, that star won't tell you your longitude, so you have to be careful not to arrive at your destination's offshore reefs during the night.

At the moment, our neighbours in the Southern Hemisphere do not have a good, bright "South Star", sitting over the southern end of Earth's rotation axis. The current candidate is an insignificant star named Sigma Octantis. This meant that when ancient sailors crossed the equator, heading south, their navigation aid, the North Star, vanished below the northern horizon, but no usable South star appeared in the south.

In 3000 BCE, well before those classical and mediaeval sailors, the North Star was another star entirely—Thuban, in the constellation of Draco, "The Dragon". Around 1000 BCE, the North Star was Kochab, a red star in the same constellation as Polaris. In 11,250 CE the North Star will be Delta Cygni, in the constellation of Cygnus, "The Swan". Then, in 14,500 CE the bright star Vega, in the constellation of Lyra, will the near the Pole. In 27,800 CE, the North Star will be Polaris again. The end of the Earth's rotation axis describes a circle in the sky, taking about 26,000 years to complete one circuit. What is going on?

Any of us who have played with spinning tops will have seen the answer. If we managed to put the spinning top down completely vertically, it would just sit there spinning until it finally fell over. However, most of the time we would have set it down not quite vertical. When this happens gravity interacts with the spin to make the top lean over, wobbling in little circles. This circular wobble is called precession. Why does the Earth do that?

Although images taken from space suggest our Earth is spherical, it isn't. It is a bit squashed from the poles and bulges at the equator. The gravitational pulls of the Moon and Sun work on that bulge, causing the precession: the 26,000-year parade of North Stars, and intervening periods where there isn't one.

All the major bodies in the Solar System move in the same plane, so we see them moving along a strip of sky that passes through twelve constellations, called "The Zodiac".

The first sign coincides with the position of the Sun at the spring equinox, and is usually assumed to be Aries. However, due to precession, the first sign is now Pisces. Check your horoscope.

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• Venus and Saturn lie close together low in the southeast before dawn.

• In the evening, Mars lies high in the southwest.

• The Moon will reach last quarter on May 20, and be new on May 26.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

Earth relies on its magnetic shield for protection

Earth's magnetic shield

Photo: earth.com

Earth's magnetic shield protects against harmful effects of the Sun.

Our planet is surrounded by a magnetic field that acts as a shield.

It keeps the solar wind away from the top of our atmosphere, preventing it from scrubbing our atmosphere away, and reduces the consequences for us of the Sun's bad behaviour. This magnetic shield is generated by electric currents flowing in our planet's liquid interior.

Mars is an example of what happens when that magnetic field disappears. Being a smaller world, it cooled faster and its interior solidified. The electric currents ceased to be generated and the magnetic field largely vanished, allowing the solar wind to start scrubbing away at the atmosphere.

Today, Mars' surface is a frigid, almost airless desert. Our planet is much larger than Mars, and therefore cooling more slowly, so solidification of its interior lies a long way off. However, recent research suggests a situation that is more complicated.

In the 1960s, magnetometers—devices for measuring the strength and direction of magnetic fields—became sensitive enough to measure the magnetic fields of rocks on the beds of the oceans. Scientists found, particularly in the case of the Atlantic, something really interesting. There were bands of rock of alternating magnetic polarity, showing the Earth's magnetic field has reversed over and over again through history, with the pattern mirror imaged around the Central Atlantic Ridge.

The Atlantic seabed is split at the ridge, with lava coming up forming new seabed. As it cooled, it "remembered" the Earth's magnetic field at the time it solidified. Evidence of these magnetic field reversals has been found all over the world. There have been at least 183 reversals over the last 83 million years. They are global, not local, events.

One consequence of these magnetic field reversals is that for a time in each cycle, the Earth's magnetic field is zero, for thousands of years, allowing the solar wind and other space weather phenomena to plough right into our atmosphere. Looking at the situation on Mars, what did it mean for life on Earth? What has been the impact of these reversals? Reassuringly, the impacts, if any, were slight. One of these periods of zero magnetic field occurred some 750,000 years ago. For maybe as long as 10,000 years, the Earth had no magnetic field. Our planet was teeming with life at the time and our ancestors were prowling around in Africa and elsewhere. They survived. How would we deal with a magnetic field reversal now?

Some scientists have suggested the changes now taking place in the Earth's magnetic field are leading to a reversal event. Others say the magnetic field varies slightly all the time and what we see now is just one of these random variations. At some point we will know which it is.

Magnetic field reversals must have happened many times since life appeared on Earth around 3.5 billion years ago and it is still abundant. However, what might require more consideration is the technical environment we have built for ourselves.

We know that space weather, driven by the Sun's behaviour can severely affect our technical infrastructure (power, communications, transportation etc.), and without the protection of our shielding magnetosphere, some space weather events could produce far more disruption.

We are, however, learning more and more about how solar behaviour affects our technology, and how to mitigate those effects. Canada and other nations are keeping a close eye on the Sun's behaviour and how our planet and technical systems respond to it. Since we know that at some point we will get another magnetic field reversal, we have no excuse for not being ready for it.

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• Venus and Saturn lie close together low in the southeast before dawn.

• Jupiter is low in the west after sunset, with Mars higher, in the southwest.

• The Moon will reach its last quarter on the May 20.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

What was thought to be the planet 'Vulcan' turned out to not exist at all

The planet that wasn’t there

Photo: Contributed

The order of planets by distance from the Sun.

Thousands of years ago, our ancestors noticed that most stars were located in fixed patterns.

Those patterns moved slowly east to west during the night and were visible in particular seasons but did not change. Those ancestors named the patterns, now referred to as "constellations", after mythical heroes, animals and other objects.

Our ancestors also noticed there was a belt of sky, containing the constellations Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus and Pisces, where five "stars" wandered to and fro. Those were called "wandering stars,” or "planets”—Mercury, Venus, Mars, Jupiter and Saturn. When telescopes became important tools in astronomy, another planet, Uranus, was discovered.

When Isaac Newton defined the force now referred to as "gravity", where all bodies in the universe attract one another, he enabled us to calculate how planets move, and in doing so gave us a tool for discovering new ones.

Imagine someone dancing with an invisible partner. By watching how the visible partner is moving, it is easy to realize there is an invisible partner, and maybe also estimate how big they are.

To use this idea to deduce the presence of unknown planets, we measure precisely how a planet moves around the Sun and determine whether that orbit is being perturbed in any way. If so, what size body could account for those perturbations and where it should be located? Then we look for it.

Perturbations of Uranus' orbit led us to the discovery of Neptune.

During the last half of the 19th century, observers found there was something really odd about the orbit of Mercury, the closest planet to the Sun. After the perturbations due to known planets, especially Jupiter, were taken out, there was still an unexplained perturbation too big to be ignored.

Astronomers, such as Urbain Le Verrier in France, assumed them to be caused by a new, unknown planet. The calculations predicted it had to be orbiting the Sun at a distance of 21 million kilometres and taking only 19 days to do so. Mercury orbits the Sun in a very elliptical path, taking it from 46 million kilometres to 70 million kilometres, and it takes 88 days to complete each orbit.

The evidence for the existence of this planet was deemed so strong it was given a name, Vulcan, after the god of fire, with responsibility for volcanoes and such things. The search was on.

There were several reports of astronomers seeing Vulcan's black disc crossing the Sun. However, none of the observations could be confirmed. The planet obstinately refused to be conclusively found, no matter how intense the search was and how powerful the telescopes used. Finally, it turned out the explanation of Mercury's weird behaviour was not a planet, it was something much stranger. The scientist whose work was relevant to this particular problem was not Isaac Newton but was Albert Einstein.

In his General Theory of Relativity, Einstein proposed that what we perceive as the force of gravity is really one manifestation of the distortion of spacetime by massive objects. Another is a distortion of the passage of time.

As we approach a massive object that distortion increases faster and faster. As Mercury's highly elliptical orbit takes it between 46 to 70 million kilometres from the Sun, the distortion of spacetime varies by more than a factor of two. That accounted for those additional perturbations of Mercury's orbit.

That strongly supported Einstein's theory. It also removed the evidence for the existence of Vulcan, making it the “planet that never was".

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• Venus and Saturn lie close together low in the southeast just before dawn, with Mercury much lower in the glow.

• Jupiter is low in the west after sunset, with Mars higher, in the southwest.

• The Moon will be full on May 12.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

Seach for alien life on another planet continues

Is alien life out there?

Photo: Castanet file

The James Webb Space Telescope is helping shine new light on possible life on other planets.

The crew of an alien spacecraft entering the Solar System, maybe looking for planets bearing alien life, would be struck by three things about “planet three,” the third planet out from the Sun (Earth).

Firstly, it is radiating a cacophony of obviously artificial radio emissions. However, perhaps fortunately, the explorers would not be able to interpret them.

The second thing is “planet three” is largely covered by water. That is unique in the Solar System.

Finally, the atmosphere consists of about 20% oxygen. The gas is highly reactive and would rapidly disappear from the atmosphere by reacting with iron in rocks and water and with many other materials, especially carbon, unless continually topped up.

The only process we know of by which that can happen is photosynthesis, taking place in plants. In other words, oxygen is an indicator of life.

There are many other chemicals that should not be there in the atmosphere of a lifeless planet. This provides an effective way to search for life on planets orbiting other stars. In fact, the James Webb Space Telescope recently detected evidence of life on an extra-solar planet, that is, a planet orbiting a star other than the Sun.

Not many years ago, it was believed impossible that we would be able to search for planets orbiting other stars. It would be like trying to spot a firefly sitting on the edge of a very powerful searchlight.

It turns out, thanks to modern telescope instrumentation, it is possible to search for the minute dimming of a distant star when one of its planets passes in front of it. Using this method, even advanced amateur astronomers have successfully detected these planets. So far, thousands of extra-solar planets have been found.

We can go much further, thanks to the science of spectroscopy. Centuries ago, Isaac Newton passed sunlight through a prism and broke it down into a spectrum of colours. That is what happens in raindrops, giving us rainbows.

With more sensitive instruments, known as spectrometers, we can spread the spectrum out further, and see that the rainbow of colours is crossed by groups of black lines. That is where elements in the solar atmosphere absorb specific wavelengths of light coming up from the photosphere, the part of the Sun radiating the light and heat. They are imprint their signatures on the light.

If an airless planet passes in front of a star, we just get a dimming of the starlight. However, if that planet has an atmosphere, some of the starlight will pass through it on its way to us. In doing so, it will pick up the signatures of chemicals and elements in that atmosphere. In addition, those signatures will only be there when the planet is passing in front of the star, making them easy to separate from the signatures of elements in that star's atmosphere.

This is what happened for a planet now named, not very poetically, K2-18b.

That planet, around nine times the mass of our world, orbits in the habitable zone of a star lying some 120 light years away. That is, its light takes 120 years to get here.

In this case, the intriguing substance being reported is not oxygen but a chemical called dimethyl sulphide. On Earth it is produced by microbes and phytoplankton (microscopic plants) in the ocean. As far as we know there is no non-biological origin for this chemical.

It is proposed K2-18b is an ocean world with an atmosphere rich in hydrogen, which automatically rules out the presence of oxygen, since it would rapidly combine with hydrogen to form water.

It is likely K2-18b is a young world, with oceans teeming with primitive, microscopic life. Perhaps, fortunately, it is too far away for us to mess with any time soon.

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• Venus now lies very low in the east before dawn.

• After dark, Jupiter shines yellowish-white in the west and red Mars is high in the southwest.

• The Moon will reach first quarter on May 4.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.