Lesson 1 - What is astronomy? An introduction into historical and modern astronomy.

Lesson learning outcomes:

By the end of this lesson, you should be able to:

1.    Summarise a basic history of astronomy, including the significance of astronomy in ancient cultures .

2.    Compare the geocentric and heliocentric models of the Solar System.

3.    Describe how Kepler’s laws explain key observable features in our Solar System.

4.    Explain why we observe Venus to have phases.

5.    Demonstrate an understanding of scale in the Universe and consequently, our place in it.

1.1 What is Astronomy?

Astronomy is the scientific study of celestial objects, e.g., planets, stars, galaxies, and the Universe as a whole. It encompasses the study of everything outside the Earth's atmosphere. The name derives from the Greek words astron ("star") and nomos ("law").

Tools of Astronomy

Telescopes

Astronomers use telescopes to observe celestial objects.

How does this actually work? How do astronomers view distant objects? If you’ve ever used a telescope (or seen how a telescope works), you’ll know that you have to point the telescope towards objects to see them. We see objects in space because light leaves distant objects, such as a star or galaxy (or reflected off the surface/atmosphere of a distant planet), and this light eventually makes its way to Earth and into our telescopes.

Objects that are further away are harder to see. You’ll know this from taking careful notice of the lights at your home: lights are brighter (more intense) when they are closer to you and dimmer when they are further away. This is exactly the same for stars and galaxies in the night sky. To see more distant objects, you need to capture as much light from that object as possible. You’ll learn more about the anatomy of telescopes later on.

The quest to study increasingly distant objects and to see them in greater detail has led to larger and larger telescopes, such as the 10-metre aperture Keck telescopes shown here.

Located on Maunakea, Hawaii, the W. M. Keck Observatory works closely with several of NASA's observatories,

including the James Webb Space Telescope, Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope. Credits: Ethan Tweedie Photography/W. M. Keck Observatory

There are also different types of telescopes that are used to observe different types of objects in the Universe. You’ll learn more about these later. The Keck Observatory telescopes (shown above) observe objects in the visible light and infrared wavelength bands.

Spacecraft

Do you know why the sky appears blue? It’s because light particles that arrive in the atmosphere from the Sun are scattered, with blue light being scattered off atmosphere particles more than red light. The atmosphere really makes it difficult for astronomers to observe objects in space. This is why you will see many telescopes on the summits of mountains where the atmosphere is thinner. However, the way around the atmosphere is to go above it! The planets and other objects in our own Solar System can be studied using spacecraft that orbit the Earth, or fly past, orbit or land on the planets.

Computers

Astronomers  use computers to analyse their data and  produce  mathematical  models that can  be compared with observations. Often, these calculations are so complicated that astronomers need to use supercomputers so that the calculations can be done in a human lifetime!

The Internet

Astronomical research is a global endeavour, and many projects involve international collaboration. Astronomers use the internet to both communicate and share data, and even to operate telescopes remotely without having to travel.

The International Centre for Radio Astronomy Research (ICRAR) has a remote observing control room that allows

staff and students to remotely observe the Universe! (https://twitter.com/icrar/status/1070601996858744832)

Laboratories

While astronomical research is largely observational and theoretical (rather than involving laboratory experiments), astronomers use laboratories to develop instruments for use on telescopes and spacecraft, and to measure fundamental properties needed to interpret astronomical data.

Above is a photo of the UK Astronomy Technology Centre (UK ATC). The UK ATC develops scientific instrumentation

and facilities for ground- and space-based astronomy.

Glossary of other "astro" Words

Astrology

Astrology is the notion that the positions of celestial bodies, such as constellations, planets, etc., can provide information on, or influence, events and people on Earth. Astrology, despite being widely accepted in the scientific community as being a pseudoscience, played an important role in the history of science. Read this interesting Conversation article.

Astrophysics

The study of the physics of celestial objects. These days the bulk of astronomy is astrophysics and most professional astronomers would  also  consider  themselves  astrophysicists. Astrophysicists  can  be  broadly  separated  into  two categories based upon their primary area of research, being 1) theoretical and 2) observational.

Astrobiology

The study of the origin, evolution, and distribution of life in the Universe. This is a relatively recent term, and something that we will be studying in this course.

Astrometry

The science of measuring the positions of celestial objects such as stars and planets. Positions can be measured to great accuracy and provide valuable information about the motions of the objects in the Solar System and Galaxy. Astrometry is a technique used within astronomy and astrophysics. The European Space Agency’s (ESA’s) Hipparcos mission was the first space telescope to map the positions and velocities of, and distances to, more than 100,000 stars. The ESA’s Gaia mission has since superseded Hipparcos in measuring the sky.

Astronautics

This is the study of space flight. It is a branch of engineering (compare with the term Aeronautics).

We will now look at the history of astronomy and the development of models for understanding our place in the Universe. Before that, let’s have a brief look at the history of the Universe. We will talk about this in more detail in lessons to come.

Our cosmic origins. Figure 1.11 in The Cosmic Perspective.

The galaxies in the Universe were closer together in the past than they are today. We will go into the details about how we know this in later chapters, but what it means is that the Universe must have expanded over time. Theoretically then, there must have been an origin to the expansion. Scientists call this point in time the Big Bang.

As the Universe expanded and the space between galaxies increased, gravity (which we will learn about soon!) kept matter together on smaller scales. Expansion of the Universe doesn’t mean that:

1.    The stuff in the Universe expands, it is just the space between stuff.

2.   There is a centre of the expansion. There is no centre of the Universe!

Just like living objects experience the life cycle, so do stars. Thanks to astronomers studying the chemical composition of stars of many ages, we now know that the early Universe had very few heavy elements. In fact, the only elements created in the Big Bang were hydrogen, helium, and a little bit of lithium. So where did all of the other elements come from? As we will learn later on, stars fuse lighter elements into heavier elements and return those elements to the Universe when they die. The death of stars can be very spectacular, the most impressive being that of supernova explosions.

These images, taken between 1994 and 2016 by NASA's Hubble Space Telescope, chronicle the brightening of a ring

of gas around an exploded star. Credits: NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics

and Gordon and Betty Moore Foundation), and P. Challis (Harvard-Smithsonian Center for Astrophysics).

Recycling of stellar material is particularly important for life. By the time that our Solar System formed, around 2% of the hydrogen, helium and lithium created in the Big Bang had been converted to heavier elements. This doesn’t seem like a lot, but it was enough for carbon-rich planets, like the Earth, to form, as well as carbon-based lifeforms (humans!).

1.2 Scale of the Universe

The Solar System

Let us look in more detail at the scale of the Universe.

Let’s begin with the Earth, which is a planet in the Solar System. At the centre of the Solar System is the Sun, and the Earth, while rotating on its axis (tilted at 23.5 degrees), orbits the Sun. The distance from the Earth to the Sun is about 150 million kilometres and is called an astronomical unit (AU). To travel around the Sun at our distance in one year, the Earth must be travelling at over 100,000 km/hr.

How fast are you moving right now? Watch this TED-Ed Animation! Also watch this awesome video about the scale of the Universe.

If we go out to the orbit of the outermost planet Neptune, we are now getting out 4,500 million km. It takes light (travelling at a speed of 300,000 km/s) about 8 minutes 20 seconds to get from the Sun to the Earth, and 4 hours to get from the Earth out to the orbit of Neptune.

You can see that even within our own Solar System, distances are enormous numbers of km. Hence a unit we use to measure distances in astronomy is the light year.

•    A light year is a measure of distance, not time. It is the distance light travels in one year.

•     The speed of light is 300,000 km per second (km/s).

•     One light year is: 9,460,700,000,000 km (i.e., about 9.5 trillion km) and is about 63,241 astronomical units.

•     Even so, we see distant (very bright) objects that are billions of light years away.

The Milky Way Galaxy

Our Solar System belongs to the Milky Way Galaxy, which is a collection of billions of stars all orbiting a common centre. The nearest star to us is about 4.3 light years away. The Sun is about 25,000 light years from the centre of the galaxy. The Milky Way Galaxy is a disk about 100,000 light years across, and it contains over 100 billion stars. At our distance from the centre of the Milky Way, it would take us about 230 million years to make one revolution around the centre. This may seem like a long time, but doing the calculations, you realise that we are travelling around the centre of the Galaxy at around 800,000 km/hr.

Why don’t we see stars whizzing past us in the night sky? It’s because of the incredible distances that these stars are from us.

The Sun is located in the Orion Arm of the Milky Way. Image credit.

Galaxies and Clusters

Galaxies come in different types and sizes. There are dwarf galaxies containing only 100 million stars, and giant galaxies containing as many as 1 trillion stars.

They also come in different shapes. There are elliptical galaxies, which are just an elliptically shaped cloud of stars. There are spiral galaxies (our Milky Way Galaxy is one), and these have a pattern of spiral arms. The spiral arms are the regions where new stars are forming. There are barred spirals where the spiral arms appear to originate from a bar spanning across the centre. There are irregular galaxies that have no particular shape. The Large Magellanic Cloud is one of these. It is one of the nearest galaxies to our own and can be seen in the southern hemisphere as what looks like a piece of the Milky Way that is detached from the main band of the Milky Way.

M87 is an enormous elliptical galaxy visible in the constellation Virgo. The above image was captured by the

European Space Agency’s Very Large Telescope.

NGC4414 is classified as an unbarred spiral galaxy. It is also called a flocculent spiral galaxy since the spirals are not

well defined. The above was observed using the Hubble Space Telescope.

NGC1300 is a barred spiral galaxy in the Eridanus constellation. Image credit.

There are billions of galaxies in the Universe, and they are not distributed randomly. They are clumped together in groups called clusters of galaxies. These clusters of galaxies themselves group together into superclusters. Our own Milky Way Galaxy is part of a small group of over 50 galaxies called the Local Group, and this is part of a much larger group of galaxies called the Local Supercluster. Our Local Supercluster is also called Laniakea, Hawaiian for “immense heaven” .

The Andromeda galaxy (M31) at 2.9 million light years away, is one of our nearest neighbours and part of the Local Group. The Milky Way Galaxy is moving towards Andromeda at around 300,000 km/hr. A largish cluster of galaxies called the Virgo cluster is 59 million light years away. A more distant cluster of galaxies, Abell S0740, is 450 million light years away.

M31 (Andromeda).

The Virgo Cluster. The above image is a mosaic of telescope images.

The Abell S0740 Cluster of galaxies captured by the Hubble Space Telescope. Image credit: NASA/ESA/Hubble

Heritage Team.

Beyond galaxies

Over the last decade, techniques have been developed to observe the positions and distances of large numbers of galaxies and make large scale maps of their distribution. One of these projects was carried out with the largest optical telescope  in  Australia, the  Anglo-Australian  Telescope  (AAT)  located  near  Coonabarabran  in  NSW.  It  used  an instrument called the Two-degree Field (or 2dF). 2dF was able to measure properties of 400 galaxies at one time. From these observations it is possible to determine the distance to each galaxy, and consequently plot these galaxies on a three-dimensional map.

The map of the galaxy distribution from the completed survey.

The map above includes 200,000 galaxies. Each blue dot is a galaxy, and the clustering of these galaxies is clearly visible. There are gaps where there are few galaxies and dense regions with many galaxies. This map extends out to about 2 billion light years from us.

Even more distant galaxies can be seen in deep exposures with the Hubble Space Telescope. This telescope has been used to take a series of deep images of the sky — the Hubble Deep Field, the Hubble Ultra Deep Field, and the Hubble Extreme Deep Field. The most recent image of the most distant galaxies is the image below, released in 2018. Each of the 15,000 specks in the image is a galaxy. Some of the light left these galaxies 11 billion years ago.

Here we can introduce the concept of lookback time. For galaxies in the map above, we know (through reasons that we will explain in later weeks) that they are 2 billion light years away from us; meaning that it took light leaving those galaxies 2 billion years to reach us. That means the light that is now arriving at us from those distant galaxies left those galaxies 2 billion years ago; we are seeing the light as it was 2 billion years in the past! Therefore, the farther away we look in distance, the further back in time we look. Yes, there is a limit to this, called the observable Universe.

The age of the Universe puts a limit on the size of the observable Universe. Figure 1.4 in The Cosmic Perspective.

You might be looking at the survey data and wondering what redshift means? Redshift is a way of measuring lookback time and distance and puts the values into something more reasonable (e.g., a redshift of 0.05 corresponds to 0.50 billion years).

A combined effort from the Hubble Space telescope, and other ground- and space-based telescopes produced the

above image that contains around 15,000 galaxies. Image credit: NASA, ESA, P. Oesch of the University of Geneva,

and M. Montes of the University of New South Wales.

The farthest away galaxy detected to date: GN-z11. Image credit: NASA, ESA, P. Oesch (Yale University, Geneva

University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa

Cruz).

The farthest away galaxy to date is GN-z11. It is seen in the photo as it was 13.4 billion years in the past (400 million years after the Big Bang). From the number of galaxies visible in this small region of sky we can estimate that there are over 200 billion galaxies in the observable Universe.

1.3 Human History and Astronomy

Astronomy is an ancient science.

The study of astronomy goes back at least to the ancient civilisations of Mesopotamia (located in modern day Iraq). The Sumerians developed the first form of writing (cuneiform) sometime before 3000 BC.

They studied the motions of celestial bodies and developed the sexagesimal (base 60) system of numbers that we still use today for time and angles (e.g., dividing the hour into 60 minutes, the minute into 60 seconds etc.).

Astronomy played a significant role in many ancient cultures for a variety of reasons. Indeed, the ways in which ancient cultures learned and used astronomical phenomena depends on the location on Earth of the culture.

In central Nigeria, the orientation of a waxing crescent Moon correlates with the average amount of rainfall at different times of the year, with October-February being the dry season, and March-September being the rainy season.

Figure 3.1 from The Cosmic Perspective.

Some cultures built structures with astronomical alignments that still exist today, e.g., pointing to the rising and setting Sun on specific dates, because many cultures used the Sun to gauge the time of day and year. The modern clock can be traced back 4,000 years ago to ancient Egypt, where daytime and night-time were divided into 12 equal parts. By using the positions of particular stars in the sky, the Egyptians created star clocks to estimate the time of night. These were abandoned in 1500 BC for water clocks (for more information, read here).

Stonehenge, in southern England, is a famous example. Another less well-known example includes the Abu Simbel temples in Egypt in which the Sun illuminates the entire length of the temple on only two days of the year.

In the absence of written records, it can be difficult to know what purpose these particular structures served. It is likely that they served a range of purposes, including ancient observatories, the keeping of time, or religious and/or ceremonial roles.

1.4 Ancient Greek Astronomy

The real beginnings of astronomy as a science can be traced to the philosophers of Ancient Greece. The Greeks developed ideas about the structure of the Universe that were to remain influential for almost two thousand years.

One of the earliest astronomers we know of is Pythagoras (c. 580 — 500 BC). You might recognise Pythagoras from his formula for calculating the side lengths of a right-angled triangle. Pythagoras and his contemporaries postulated many ideas that we know are true today, including that the Earth is spherical.

Geocentric Model

The Greek philosophers developed what is known as the geocentric model: a model of the Universe with the Earth at its centre. The easiest way to remember this is that geo means “Earth”, and centric means “centre”!

A comparison of the geocentric and heliocentric models of the universe. Credit: history.ucsb.edu.

One of the early models was proposed by Eudoxus, a pupil of Plato (427 - 347 BC). In his model, the Earth was at the centre and there were a series of concentric spheres carrying the Sun, Moon and planets, and an outer sphere carrying the fixed stars.

Possibly the most beautiful representation of the geocentric model is the armillary sphere. Armillary spheres range from the very basic to the exceedingly complicated. They feature the Earth at the centre, with the planets and constellations on surrounding rings. Some armillary spheres even feature the Christian Cross, God, and angels.

Begun on March 4, 1588, and completed on May 6, 1593, this large armillary sphere was built under the supervision of Antonio Santucci at the request of Ferdinand I de' Medici. This sphere is at the Museo Galileo in Florence, Italy.

Fun fact: many scientific concepts inspired beautiful artwork, and in ancient times, art, religion, and science were intricately linked, much more than what they are today! Many scientists believed that God created a perfect Universe and used art to express their scientific ideas.

Circles on Circles

The Greek philosophers believed that celestial objects should move in circular paths, because, according to Plato, the circle was the most perfect figure. However, it was becoming increasingly clear that planetary motion could not be modelled with a single circular motion about the Earth. Such motion includes the apparent backwards motion of the inner planets, called retrograde motion.

To explain these effects, the Greek astronomers developed increasingly complex systems of spheres carried on other spheres, or circles carried on larger circles.

1.5 The Ptolemaic System

This system of explaining the planetary motions reached its culmination with the work of Claudius Ptolemy (c. 100 – 170 AD). Ptolemy wrote a treatise on astronomy known as the Almagest. It is the only comprehensive work on ancient astronomy to be preserved to the present day. It includes:

•     A catalogue of stars believed to be based on the earlier work of Hipparchus.

•     His model for the planetary motions which is based on the geocentric model, and an even more elaborate system of circles on circles (or "epicycles").

•    Tables predicting the positions of the Sun, Moon and Planets, which, at that time, were the best available.

The Ptolemaic system required the use of epicycles. An epicycle is a small circle that carried the planet, with the centre of the epicycle itself moving around the Earth on larger circle called the deferent.

This type of model was necessary to explain features such as the retrograde motion of the planet Mars. This is the fact that there are times when Mars appears to move backwards relative to the stars compared with the normal east to west motion seen most of the time.

As we will see, retrograde motion can be explained much more directly using the heliocentric (or Copernican) model, combined with the knowledge that planets move on elliptical (not circular) orbits.

1.6 The Copernican System

It wasn't until 1543 that there was a challenge to the Ptolemaic system. This was when Polish astronomer Nicolaus Copernicus published his book De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres).

The Copernican System places the Sun rather than the Earth at the centre.

Here he proposed what we now know as the Copernican (or Heliocentric) system: a model of the Solar System with the Sun, rather than the Earth, at the centre.

The Copernican system provided a simpler explanation of the retrograde motion of Mars. It occurred because of Earth catching up with and passing Mars due to its faster orbital motion.

Copernicus still used the platonic idea of circular motions, therefore he still needed systems of multiple circles to explain the actual motions of the planets (which move on elliptical, rather than circular orbits). The ideas that Copernicus put forward would take some time to become accepted.

1.7 Kepler's Laws

The final solution to the problem of planetary motions was provided by German astronomer Johannes Kepler (1571 – 1630). Kepler used the best astronomical observations of planets then available (acquired by Danish astronomer Tyco Brahe (1546 – 1601)) to determine that the orbits of the planets were ellipses, not circles.

In case you have forgotten some geometry about ellipses, check out the image below.

Figure 3.16 from The Cosmic Perspective. An ellipse is a special type of oval. These diagrams show how an ellipse differs from a circle and how different ellipses vary in their eccentricity.

In the figure above, F1 and F2 are the focal points (or foci) of the ellipse (in red). The semi-major axis is the distance from the centre of the ellipse to the furthest point around the edge of the ellipse.

Figure 3.17 from The Cosmic Perspective. Kepler’s first law: The orbit of each planet about the Sun is an ellipse with the Sun at one focus. (The eccentricity shown here is exaggerated compared to the actual eccentricities of the planets.)

The elliptical orbit of a planet is shown above. The ellipticity of the orbits of the planets in our Solar System only depart from a circle by a very small amount.

With elliptical orbits, the motion of the planets could be explained in a much simpler manner than the models of either

Ptolemy or Copernicus. The need for the complex system of epicycles was completely eliminated. Kepler was able to outline three laws of planetary motion.

Kepler's 1st law:  The planets move in elliptical paths with the Sun at one focus.

Kepler's 2nd law:  (The law of areas)  The line between the planet and the Sun sweeps out equal areas in equal times. This means that, in the figure above, for the areas to be equal, the distance that the planet travels around the arc (outline) of the ellipse much be larger for when the planet is closer.

Kepler's 3rd law:  The square of the orbital period of the planet (in years) is proportional to the cube of the semi-major axis of the orbit (in astronomical units).

Kepler's 2nd law has the implication that the planet moves fastest when near the Sun. Kepler's 3rd law implies that the planets furthest from the Sun move slower than those closer in. We will look at these laws in more detail in future lessons.

Figure 3.19 from The Cosmic Perspective. Graphs based on Kepler’s third la