Astronomical Concepts – Week 4

This week’s main topic was light.

James Maxwell

James Maxwell was a Scottish scientist who lived from 1831 to 1879. He is well known for the research he did on electromagnetism and light, building on the work of Michael Faraday. He produced a set of equations that explain the properties of magnetic and electric fields which helped to show that light was an electromagnetic wave. He was able to bring together well established laws of electricity and magnetism and with Faraday’s law they could imply that any disturbance in the electric and magnetic fields will travel out together in space at the speed of light.

He also described Saturn’s rings as numerous small particles, and this theory was proved later on in the 20th century by a space probe.

Einstein’s Photoelectric experiment

Einstein’s experiment showed that packets of light, called photons, contained a fixed amount of energy that depends on the light’s frequency. When a metal plate is exposed to light electrons are expelled. This is the photoelectric effect. The effect was discovered by German scientist Heinrich Hertz in 1887. His observations showed there was an interaction between light and matter. But Albert Einstein was needed to explain the theory further in 1905, his theory of light. Einstein said that light is a particle, called a photon. Einstein was awarded the Nobel Prize in physics in 1921 for his experiment.


Spectroscopy is a technique used to measure the light that is emitted or absorbed or scattered by materials. It breaks light into its component parts and this information can be used to identify and quantify those materials.


When light is absorbed or reflected by materials not all light behaves the same way. Only certain wavelengths of light are absorbed other get reflected. When you seperate the light that is passing through a sample you end up with an emission spectrum or absorption line.

An emission spectrum in the visible light range may look like this.


A spectrum like this would be created when material is given extra energy and that extra energy is later emitted as light energy.

An absorption spectrum would look like this.


A spectrum like this is created when light is passed through a gas or liquid or strikes a solid. Certain wavelengths of light will be absorbed by the material and later emitted in random directions. Most wavelengths will pass through the material without being absorbed. sun_spectrum

The image above shows a spectrum of our sun. From this spectra astronomers can tell what elements the sun is made from, for example hydrogen and helium. It is like a rainbow with holes, the holes are coming from the absorption of energy at a particular wavelength, at a particular colour, by the atoms in the cloud. This goes back to the energy levels of the atom, of only taking energy at very particular energies, as electrons move from one excited state to another excited state. So what you’re seeing is the absorption of photons by atoms. When energy is absorbed you are seeing the energy raising the energy level of an electron. So, we see the rainbow because the inside of the sun is hot and it emits a continuous thermal spectrum. The atoms in the outer layer of the sun absorb some of the energy and use it to promote electrons from a low energy level to a high energy level.


Astronomers also take pictures of light, usually through a filter. Astronomers want to see what the light looks like in red light or green light.

Astronomers also do timing with light, which means to measure the brightness or phase changes as things happen in time.

The combination of spectroscopy, imaging and timing can tell us all kinds of information from the thing we are looking at. We can tell hoe fast something is rotating, if it is moving towards or away from us, temperature, density and composition. Light is our spaceship, we can’t travel to stars and planets light years away but light does travel to us, it is the only way to get the information we need.

Doppler effect

Light waves from a moving source experience either a red shift or a blue shift in the lights frequency. A light source moving away from a stationary observer causes a shift towards the red end of the light spectrum, called a red shift. When the light source moves towards an observer the frequency shifts towards the blue end of the spectrum.


Why is the sky blue?

The sky is blue because atoms in our atmosphere scatter blue light more than they scatter red light. When we look towards the sun at sunset we see red and orange because the blue light has been scattered away from the line of sight. Blue is scattered more because it travels as small and short waves.

That’s enough about light, next week is all about the sun.

Astronomical Concepts – Week 3

The main theme of this week’s session was the outer part of our solar system. This includes:

  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto
  • Asteroids
  • Comets
  • Oort Cloud
  • Kuiper Belt

Oort Cloud

This has never been directly observed but it is believed to exist and it is an area of space on the edge of our solar system between 5,000 to 100,000 AU in distance, so over a vast area. The Oort Cloud consists of millions, perhaps billions of small icy bodies. Every now and then something might disturb one of these bodies and it will become a comet falling towards the Sun. It is named after Dutch astronomer Jan Oort who predicted its existence in 1950.


Kuiper Belt

The Kuiper Belt is another far away region of space that consists of rocky and icy bodies. It extends far beyond Neptune about 30 to 55 AU. It is also predicted to contain over a trillion comets. It takes comets about 200 years to orbit the sun and they travel in a similar plane to the planets. One of the largest and well known objects of the Kuiper Belt is the dwarf planet Pluto. In 2015 the New Horizon’s spacecraft flew past Pluto making it the first mission to a KBO. Another dwarf planet, named Eric, was found in 2005. It is slightly bigger than Pluto and has its own moon. At the time astronomers were considering making Elis the tenth planet, however, in 2006 the International Astronomical Union created a new class of planet called dwarf planets and Pluto and Elis were classified in this new category. The Belt was named after Gerard Kuiper in 1951.

Orbital resonance

A concept described by Paul was orbital resonance. An example to help describe this concept is playing on a push swing. A child can swing by itself at a natural frequency, but the frequency can change by use of an external force, someone else pushing the child on the swing. If the pushes are timed correctly the pushes will build up and the swing gets amplified. With planets and moons when two bodies orbit they exert a regular gravitational influence on each other. This is due to their orbital periods being related by a ratio of two small integers. An example is the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io.


These moons are all in resonance with Jupiter. Io completes exactly 4 orbits and Europa 2 in the same time it takes Ganymede to complete one orbit around Jupiter. During their orbits they sometimes lineup exactly and a gravitational tug is exerted with stretches their orbits into ellipses.

Another example of resonance is the 2:3 resonance of Pluto with Neptune. Pluto completes 2 orbits for every 3 orbits of Neptune around the sun.


Next week … light!

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.


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.


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.


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?


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.


His equation:


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


  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.


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!

Astronomy at Questacon in Canberra

Questacon in Canberra has a pretty cool section on space and astronomy. Check out the pictures I took in the gallery below.

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A few of the highlights from this exhibition.


  • WMAP is the Wilkinson Microwave Anisotropy Probe
  • It launched on June 30, 2001
  • Has mapped fluctuations in the cosmic microwave background (CMB) radiation (the oldest light in the universe) and has produced a full map view of the microwave sky
  • Has determined the universe to be 13.77 billion years old
  • Determined the curvature of space to within 0.4% of flat Euclidean
  • Found that the universe is 24% dark matter
  • Found that dark energy makes 71.4% of the universe, causing the expansion rate of the universe to speed up

Morgan Keenan System (MKK)

  • Classification of stars based on temperature
  • Categories divided into Roman numerals, with sub-categories and classes
  • To completely describe a star the MK luminosity class is appended to the original Harvard classification for the star
  • The Harvard Spectral Classification assigns each star a spectral type with is further divided into 10 sub-classes depending on the absorption features present in the spectrum. E,g, our Sun has a temperature of 5,700 Kelvin and is classified as a G2 star
  • Our Sun is a main sequence star G2 and the full classification is G2V

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Types of orbit

  • The Moon does not orbit the Earth, the Moon and Earth orbit each other around their common centre of mass. The common centre of mass is inside the Earth
  • Kepler showed the orbits of the planets were ellipses with the Sun at one foci
  • Newton showed there were other types of orbit
  • Elliptical and circular orbits are both bound orbits
  • Parabolic and hyperbolic are unbound orbits
  • Comets and asteroids typically have hyperbolic or parabolic orbits, they zip around the Sun once and they go off never to be seen again because they do not have gravitationally bound orbits
  • Look at the circular cone above, how you slice the cone determines what kind of geometric cross-section you get – horizontal makes a circle, an angle creates an ellipse, increase the angle and get to the bottom of the cone you get a parabolic and cut the cone vertically you get a hyperbola
  • So 4 different types of orbits


  • Newton’s laws of gravity showed that Kepler’s first 2 laws, ellipses and equal area in equal time are a consequence of the conservation of angular momentum (Mass x speed x distance = constant)
  • At perihelion the distance to the Sun is smaller so to keep the mass times the speed times the distance the same the conservation of angular momentum the plane must move faster
  • At aphelion, further away from the Sun, conversely the speed must decrease
  • P squared is a cubed (P^2 = a^3)
  • P is the period the time it takes to go around and the semi-major axis of a of the ellipse, a is the average orbital distance

Thank you F.X. Times at Arizona State University for some of the information included.

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.


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.


“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 (


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


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.



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.



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!

Exploring the Heavens – Week 1

On Tuesday 3 May I attended the first week of my new course on astronomy at Sydney Observatory. The course is titled ‘Exploring the Heavens’ and is led by Dr Paul Payne. The structure of the course is as follows:

  1. History of Astronomy
  2. Celestial rhythms
  3. The solar system
  4. The stars
  5. Telescopes

So the course started with Dr Payne’s two hour version of the history of astronomy in the Sydney Observatory 3D theatre. Some of the main figures covered included:

  • Aristotle (384 – 322 BC)
  • Claudius Ptolemy (~140 BC)
  • Nicolas Copernicus (1473 – 1543)
  • Tycho Brahe (1546 – 1601)
  • Johannes Kepler (1571 – 1630)
  • Giordano Bruno (1547 – 1600)
  • Galileo Galilee (1564 – 1642)
  • Isaac Newton (1642 – 1727)
  • Edmund Halley (1656 – 1742)
  • Charles Messier (1730 – 1817)
  • William Herschel (1738 – 1822)
  • James Bradley (1693 – 1762)
  • Friedrich Bessel (1784 – 1846)
  • John Adams (1819 – 1892)
  • Jean Le Verrier (1811 – 1877)
  • Albert Einstein (1879 – 1955)

Dr Payne kept the pace moving pretty quickly to cover all of these historical and important figures and more about the ancient Greeks, Romans, Babylonians, Renaissance as well as advancements in mathematics, engineering and technology.

Dr Payne also used his homemade 3D graphics to help demonstrate certain themes and concepts, including retrograde motion of planets, constellations and the movement of the moon and planets from Earth’s point of view. The graphics were great and they certainly enhanced the presentation putting us firmly in the cosmic realms.

I was surprised at how important astrology was in ancient times and how seriously it was taken to predict future events. People were also very superstitous and heavenly objects played an important role in how people lived their lives, including Roman emperors. It is also amazing at how much people knew about the solar system hundreds and thousands of years ago without even the aid of a telescope.


As well as learning about the history of astronomy Dr Payne gave us a tour of the night sky on what was a beautiful and clear evening in Sydney. Some of the notable objects we spotted were: Mars, Jupiter, Alpha Centauri, Sirius, Betelgeuse, as well as some notable constellations.


The final part of the evening involved us moving to the south dome of the observatory to use the telescope to view the night sky. We were lucky enough that the sky was clear from clouds and we were treated to an amazing view of Jupiter and the Galilean moons Ganymede, Io, Europa and Callisto.


Image of the telescope in the south dome of Sydney Observatory.

This telescope is the oldest working one in Australia and was built in 1874 by Hugo Schroder in Hamburg, Germany. An interesting article about the telescope can be found here.

This was actually my first time looking through a telescope and the view did not disappoint. I was amazed by how clear Jupiter appeared, being able to clearly identify its white zones and brown belts, both of which are cloud systems with winds that blow in opposite directions. The Galilean moons, although tiny, were very bright and also so clear to see. This is exactly what I had been hoping to see and has just added more fuel to my growing love for astronomy.

Looking forward to week 2!