Astronomical Concepts – Week 2

The main topics this week were the solar system, gravity and the tidal effect. I have previously written on my blog about the solar system so for this entry I will just write about gravity and the tidal effect.

The two main theories of gravity come from Isaac Newton and Albert Einstein, both are used today, both are brilliant and vastly different. Gravity is one of the 4 main forces of nature, it works on grand scales, the great sculpture of the universe. Our Milky Way galaxy is locked in a gravitational embrace with Andromeda and in a few billion years the two galaxies will collide, just one example of the power of gravity. It holds galaxies together over billions of kilometres.

Gravity is the weakest of the four forces, yet it is so influential. The four fundamental forces of nature are gravity, weak, strong and electromagnetic. Well gravity is by far the weakest, certainly it is very weak here on Earth, but out there in the universe it is quite different. Stand on a planet more massive than ours and you would quickly notice the immense power of gravity. Stand on a neutron star and you would be ripped apart very quickly.

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Newton realised that when objects fall to the Earth their must be a force acting on the object, reaching up and pulling it down. He stated that the force of gravity is always attractive, and affects everything with mass. Newton was also able to show that objects with different masses fall at the same rate because an object’s acceleration due to the force of gravity depends only on the mass of the object pulling it, such as a planet.

Newton’s cannon was a thought experiment that demonstrated his theory further. He imagined firing a cannon ball from the top of a mountain. Without the force of gravity acting on the cannon ball it would simply travel in a straight line. If gravity is present then the cannon ball’s path will depend on its speed. If it is slow moving it will fall down to the surface, if it is travelling fast enough it will go into orbit around the planet and if it reaches the escape velocity it will leave the orbit all togehter.

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Einstein has a different approach. Einstein says that gravity is not a force but rather a property of space-time geometry. Objects in space, such as planets around a star are all attempting to travel in a straight line through space but that the curvature of the fabric of space means objects are constantly falling towards the mass exerting gravity. Einstein says when you are falling around an object you have cancelled out gravity. Astronauts on the International Space Station are weightless because they are continuously falling to Earth. There is gravity where they are, they are travelling at a speed to stay in orbit around the Earth. The astronauts are continually falling to the Earth but they never reach it, that is why they’re weightless. Being weightless means you are in free fall. When you are in free fall you cancel out gravity. Einstein’s elevator thought experiment explains his theory in more detail, read about it here.

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Tidal forces are significant across our solar system. Here on Earth we experience tidal effects thanks to the moon. The Earth experiences two high tides, one on the side of the Earth closest to the moon as the moon pulls the water towards it and on the opposite side as the moon pulls the Earth away from it.

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An extreme case of tidal forces in the solar system is the heating of the moon Io around Jupiter. Jupiter is very massive so the effects on Io are huge,  Jupiter pulls Io inwards and the other moons away from Io pull it the other way, causing Io to distort in shape. This constant change results in lots of friction which in turn drives strong volcanic activity on the surface of Io. Io is the most volcanically active body in the solar system and its surface is constantly changing with large dark spots on the surface caused by collapsed volcanoes.

Our moon is also tidally locked, meaning we see the same side of the moon all the time. It spins once on its axis as long as it takes it to orbit the Earth once, so we always see the same face. The constant tugging from the Earth on the moon has caused this locking to happen.

So what is gravity?

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Can’t wait for week 3 – the outer planets.

Exploring the Heavens – Week 3

The topic for this week was ‘The Characteristics of the Solar System’. The main items covered were the formation of the solar system, the planets and other celestial bodies such as comets and asteroids.

Due to being ill this week I attended the Thursday class instead of my usual Tuesday class, but the format was exactly the same. Paul’s 3D presentation was fantastic as it put us right in the middle of the solar system and we could experience it from many different angles and points of view, which really helped in developing our understanding of how it all works.

One of the main themes throughout this course has been the ‘ecliptic’ – a plane on which all the planets sit and orbit the Sun. Our solar system consists of our star – the Sun – an object which dominates our neighbourhood, consisting of 99% of the mass and also gravitationally. The 8 planets and countless comets and asteroids all belong to the Sun. The unit by which we measure the distance of the planets from the Sun is called an astronomical unit, and the Earth is 1 AU from the Sun (approx. 150m km). It is possible that the distances of all the planets from the Sun can be explained with a mathematical formula that proves they are not just random distances.

A sidereal period is the time it takes a planet to orbit the sun. The further the planet from the Sun the greater the period. Kepler’s third law – the law of periods – found a simple relationship between the distance and the period. The ratio of the average distance from the Sun cubed to the period squared is the same constant value for all planets.

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His equation:

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Our best guess at how our solar system was formed is called the Solar Nebula theory. Any theory has to explain the characteristics of our system:

  1. The order of the orbits of planets
  2. The categories of planets – terrestrial and jovian
  3. The amount of comets and asteroids
  4. Anomalies

formation

  1. Our solar system formed from the gravitational collapse of a large cloud of gas and dust
  2. As the cloud collapses, conservation of energy, momentum and angular momentum flatten it out into a disk
  3. The diffuse clouds end up as a spinning disk of gas and dust with the young protosun at the middle
  4. The spinning disk is hotter at the centre and colder on the outside, so closer in are more material of rock and iron, further out more hydrogen compounds such as methane, explaining the makeup of our planets
  5. Finally the Sun ignites and releases a strong solar wind to clear away the remaining dust other material. We are left with the planets we have now

We talked about all of the planets and their characteristics. When we were outside looking up we could clearly see Jupiter and Mars on the ecliptic. The signs of the Zodiac pass through the ecliptic. This is a good way to find a planet!

The synodic period is the time it takes for a planet to return to the same angle with respect to the Sun. The synodic period for Mars is 780 days for it to move from opposition to opposition. Prior to opposition is when the planets move in retrograde motion.

We had an amazing view of the Moon tonight through a telescope on the balcony at the observatory. This was my first time viewing the Moon through a telescope and it looked amazing, we could see so much detail. I can only imagine the view from the Moon looking back at Earth.

Moon

The Moon has a sidereal period (orbit) of 27.3 days and appears to go through its phases every 29.5 days (synodic period). The basis for our month. The phases of the Moon are due to the changing appearance relative to the Sun, however, we only ever see one side of the Moon as it has been locked in its orbit, due to the tidal effect from Earth.

The Moon is the major force behind tides on Earth. The gravity of the moon pulls the water  up towards it, creating an uneven distribution. The Earth and moon orbit a common centre of mass, located close to the surface of the Earth. As the Earth rotates on its axis each point on the surface is subjected to a sequence of high and low tides every 6 hours. These tides are also causing the moon to slowly drift further away from Earth at 3.7 cm per century.

Finally, we talked about eclipses, both solar and lunar. Paul said that we should all try to make it to a total solar eclipse, when the moon obscures the Sun. This happens when the 3 bodies are in exact alignment, which happens every 6 months or so. This phenomenon is possible because the Sun and Moon look the same size in the sky. The Sun is approx. 400x larger than the moon but is is 400x times further away, hence a total eclipse of the Sun is possible. Very few places on Earth will see a total eclipse due to the precision needed to make this happen.

Next week… The Stars!

Exploring the Heavens – Week 2

The focus of this week’s class was Celestial Rhythms and the development of the constellations.

We started the class upstairs in the observatory and outside on the balcony with Paul describing our session and then identifying a few of the objects on display in the night sky, including a crescent Moon. Paul described how the Moon is moving around the Earth and how the position of the Sun in relation to the Earth and Moon affects the different phases of the Moon. He explained that next week the moon will have moved across the sky in counter-clockwise direction and showed us approximately where it would be.

We then headed downstairs and outside to a large model of the solar system painted on the ground, as seen in the image below during the daytime. We sat around the model and Paul described the motion of the Earth and other planets around the Sun. This demonstration was a great way to visualise our solar system and how various planets move in relation to the Sun. Paul also described how our view on Earth is affected by the Sun and how the Sun blocks various stars at certain parts of the year.

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This session was about the importance of understanding how the sky moves during a night and throughout a whole year.

“The sky is on a continual march that presents us with different constellations at different times of night, and different times of year.”

After our outside demonstration and further observing of the night sky we moved back inside for a much needed tea break and to escape the chilly Sydney evening weather, should have taken a jacket!

After our break we headed downstairs to the 3D theatre and enjoyed another presentation by Paul. The focus of the inside session was to demonstrate the motion of the Earth travelling around the Sun and its rotation on its own axis, what causes the seasons as well as examine closely what makes a day and a year, which is not quite what we thought.

I learnt so much over our 2 and a half hour class. As we spin around our axis once per day we actually move about 1 degree around the Sun. We rotate and orbit in a counterclockwise direction and in a year there are 365.25 days. So, the Earth, rotating at 1360 km/h, must spin on its own axis 360 + 1 degrees to have the Sun reappear in the same position. A day as we know it is 24 hours long, this is actually a solar day as our clocks have been tuned to the combined motion of our rotation and orbit around the Sun.

A different kind of day is called a sidereal day, which is 23 hours 56 minutes and 4 seconds long. This is the time astronomers use to predict the location of stars on the night sky. All of this means that we will see a slightly different sky each night.

We all know we have four seasons on Earth and the seasons are caused by our axis of rotation being tilted by 23.5 degrees to the plane of our orbit, called the ecliptic. The axis points in the same direction as we orbit the Sun, and this is what causes the seasons and the variation in the length of a day throughout the year. Paul gave us examples of the equinoxes and the solstices. The equinoxes occur approx. half way between the solstices on March 21 and 21 September, the solstices on 21 June and 21 December. In June the axis is pointed towards the Sun so the northern hemisphere will see the Sun high in the sky and will receive many hours of sunbathed daylight. In the southern hemisphere people will see the Sun much lower in the sky and will experience less daylight compared to the northern hemisphere. The reverse is true for December. At equinox the day and night are the same length for both hemispheres – equal.

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“We spend our days and nights humbly on planet Earth. We suffer from the illusion that we feel stationary on the surface of the Earth and that the Sun and stars seem to revolve around us.”

Infant, our axis is slowly precessing, like a spinning top over a period of 26,000 years. This means that the celestial poles on the celestial sphere sweep out a circle every 26,000 years carrying our coordinate system of the stars with it. This small drift was actually measured by Hipparchus around 140 BC. Paul has recommended we purchase a planisphere, seen below, a dynamic map that can portray the night sky for a given location at any time of night throughout the year. These are only designed for one latitude and do not contain much information.

There are 88 constellations in the night sky. Each constellation is an area of the sky and are recognised and classified by the International Astronomy Union (IAU). We learnt heaps about the constellations:

  • Each culture developed its own constellations
  • Constellations often symbolised mythological creatures
  • Constellations are a reminder of a lesson or a seasonal event
  • Many originated in Babylonia
  • The Greeks adopted many and developed their own mythologies to explain them
  • The constellations the Sun passes through along the ecliptic are called the zodiacs
  • There are actually 13 zodiacs, including Ophiuchus, although the Sun does not spend too much time in this sign
  • The Babylonians set up the zodiacs and their counting system was based on base 12, so they stuck with just the 12 constellations and that is why we have 12 hours of day, 12 hours of night and 12 months a year
  • The Sun now spends more time in Ophiuchus than in some neighbouring constellations

I am a Pisces and here is my constellation from the IAU (http://www.iau.org/static/public/constellations/gif/PSC.gif)

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A few facts about Pisces:

  • One of the most ancient constellations
  • Depicts 2 fishes swimming in opposite directions with their tails joined
  • In mythology the two fishes represent Aphrodite (Venus) and Eros (Cupid)
  • One day they hide to hide in rushes along the Euphrates to escape a monster called Typhon. The two fishes swam away to safety
  • Pisces is watery and faint, it lies in an undistinguished part of the sky
  • Its brightest stars are only magnitude 4
  • One way to locate Pisces is by reference to the square of Pegasus
  • The Sun passes through Pisces between Feb 19 and March 20
  • The 12th sign
  • Famous Pisces include Steph Curry (Mar 14), Einstein (Mar 14), Steve Jobs (Feb 24), Daniel Craig (Mar 2), Jon Hamm (Mar 10)

Pisces stars

Pisces

Eta Piscium is the brightest star at 316x that of the Sun. It is 294 light years from Earth and is a G class bright giant star.

Pisces contains a Messier object called Messier 74, a spiral galaxy located between the stars alpha Arietis and eta Piscium. It is also known as the Phantom Galaxy, shown below.

M74

Messier-74

The grand-design spiral galaxy Messier 74 as photographed by the Hubble Space Telescope in 2007. Image: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)

The red areas indicate pockets of hydrogen gas. They glow due to the radiation from hot, young stars. Astronomers call these areas H2 regions. The brighter stars are not part of the galaxy and are actually located a lot nearer to us. The galaxy appears face-on and is approx. 30 million light years from Earth. It is roughly the same size as the Milky Way with a diameter of 95,000 light years. Two supernovae have been seen exploding in recent years in this galaxy. It contains about 100 billion stars. It is not easy to observe due to low surface brightness and requires clear skies. The only other object with a lower surface brightness is M101, the Pinwheel Galaxy, shown below.

M101

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The Phantom is an example of a grand design spiral galaxy with 2 clearly defined arms which extend for about 1,000 light years. The arms contain clusters of young blue stars and star forming nebulae. It is receding at a speed of 793 km/s.

Zoom into M74:

M74 was first observed in 1780 by French astronomer Pierre Méchain who told his good friend Charles Messier about it, Charles added it to his famous catalogue.

More about M74 here.

Thank you Hubble!