Astronomy Lecture Notes PDF

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Astronomy Lecture Notes Chapter 1: Observing the Night Sky Our Place in Space -

Human history achievements: discovery of the incredibly vast Universe around us and understanding our place in it

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Earth is not at the centre of the Universe 

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Earth isn’t particularly special

Discoveries include… 

Exoplanet discoveries: Earth is an ordinary rocky planet

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Earth is the 3rd of 8 planets orbiting a middle-aged, below-average sized star

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In outer suburbs of a huge spiral city of stars called the Milky way

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In small insignificant cluster of galaxies (Local Group)

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One among billions of galaxies spread through observable universe; stretches 98 billion light years across

Arriving at Our Understanding of the Universe -

Through astronomy 

The study of objects beyond earth and the processes that they go through as they evolve and interact with one another

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Astronomy involves attempting to build understanding of history of how everything came to be and how we (earth) have gotten to where we are

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Helps predict future events

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As with other sciences, ideas constantly involving equipment, technology, and theories improve

Astronomy and the Scientific Method -

Astronomy is similar to other sciences in terms of following systematic and logical processes (scientific method) to determine rules that nature follows

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However, different in terms of controlled experimentation not being possible 

Observation leads to a theory explaining what was seen

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Theory leads to predictions consistent with previous observations

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Specific new observations of phenomenon or process carried out

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If data from new observations agrees with the prediction (conclusions), theory gains more support, and more predictions can be made

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If not, a new theory can be made

Laws of Nature and Scientific Understanding -

Good scientific theories must be… 

Testable

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Continually tested

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Simple

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Elegant (symmetry)

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Scientific theories can be proven wrong, but can never be proven with 100% certainty

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The reason we can understand extremely distant, vast, and potentially dangerous objects in the universe are as follows… 

Over history, scientists have discovered many “rules” nature seems to follow

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Scientists have been able to use these rules to develop theories that explain many different phenomena (“Laws”)

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Also discovered that laws of nature are “universal” (apply to large/small, across tiny/large distances, and work the same under all but most extreme conditions)

Scientific Discovery Shaped our Understanding -

Current scientific understanding of cosmos is because of centuries of careful systematic observation, scientific reasoning, and modeling and insight by many astronomers/physicists

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Humans first started to organize their knowledge of the universe through watching the night sky and noticing patterns

Constellations -

At any moment, in dark locations, there are 3000 stars visible above the horizon 

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Over entire sky, 9000 visible to naked eye

Stars are unchanging (in their experience)

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Practical matters taught to use stars… we began to recognize/make patterns (known as constellations) to tell stories, to teach important lessons, to easily keep track of time, to farm, and to navigate

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Early constellations are familiar to daily life; animals, warriors, young maidens, monsters, weapons, strange magical creatures, special people…

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Later ones (southern hemisphere) made for new inventions of things of interest of the time period (i.e. sailing chips, clocks, air pumps…)

Constellations and Mythology: Worldwide Phenomenon -

Constellation patterns existed in many ancient cultures 

Greek/Roman mythology passed on strongly through European/Western culture; acceptance developed internationally through conquest and academia

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Now, 88 recognized internationally. About 67—75 can be seen from Canada/USA (others too far south)

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Constellation patterns aren’t significant in daily life today, but are still used to organize and locate objects

Constellation Stars Not Generally Related -

While stars appear to be grouped close together within constellations, most aren’t physically related to each other 

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Only appear so from earth’s position in space

Constellation figures also change shape over many 1000s of years 

Cause: motion of stars through space

Universe’s Enormous Scale -

As scientists’ understanding of the universe has developed (last 100 years), they realized size beyond comprehension; size of objects/distances between are so large, that numbers in scales we normally use, such as km, are too bulky

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Solutions include… 

Some have used scientific notation to keep the same units; Diameter of S. S. is 9.90 x 109 km

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Some have used astronomical units (AU) to measure distances; Earth is 1 AU from the Sun, and Jupiter’s orbit is 5.2 AU from the Sun

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For much larger distance measurements (i.e. between stars/galaxies), astronomers use the parsec or the light year (ly)

Measuring Distance with Light -

Light year: not measured with time—used to measure distance

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Light travels at the fastest velocity that is possible, 22,792 km/s 

At this velocity, a photon will travel 9,459,922,663,400 km in one year

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So 1 light year is 9.46 trillion kilometers

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This means, as a photon, you would travel… 

7 times around earth in 1 second

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To the moon in 1.282 seconds

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To the sun in 8.5 minutes

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To the nearest star in 4.2 years

Consequences of Light Travel Time -

Because of incredibly enormous distances and limited speed of light photons, even our perception of the Universe is affected; we don’t see objects as now, but as they were in the past 

Farther away, light takes longer time to get here, and the farther back in time we see

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The sky is a time machine 

I.e. Eta Cassiopeia is a sun-like star that is 15.4 ly away

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Andromeda Galaxy is 2.5 million ly away; the farthest object the unaided eye can see

Celestial Navigation -

While constellations helped organize the sky for ancients, it wasn’t a practical system for locating objects and measuring motions/changes; also, constellations differ in different cultures

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Astronomers needed a framework (simple, consistent, easy to apply, reliable, global) 

Solution is the coordinate location system (graphing) based on angular measurement

Locating Places on Earth: Latitude and Longitude -

300 BC: Greek philosopher Eratosthenes argues that the earth is round—measures diameter, then develops geometric grid system to locate positions

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160 BC: Greek philosopher Hipparchus was the 1st to use latitude and longitude for estimating distance across earth and locating places

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Great circle: any circle across earth’s surface whose centre passes through Earth’s centre

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Equator: great circle halfway between north and south poles (hemispheres)

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Latitude: a measurement of degree angle above/below the equator as measured from earth’s centre (oN/S)

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Meridian: any great circle path traveling through N/S poles 

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But no easy reference point to start measuring E/W from

Prime meridian: a starting reference point, set internationally at Greenwich, England in 1884

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Longitude: a measure of angle away from the prime meridian as measured from earth’s centre 

Longitudinal lines—meridians traveling pole to pole

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Perpendicular to equator (farthest apart there)

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Measures up to 180o E/W of prime meridian

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180o approximate location of international dateline (12 hr differ from prime meridian)

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Divided into about 24 time zones (slightly more; doesn’t match up with longitudinal lines)

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1o = 60 minutes of arc = 3,600 seconds of arc

We use a coordinate system of latitude/longitude to locate any position on Earth 

Position at UofW is 42o,18’,18.7”N, 83o3’55.8”W (-)

The Celestial Sphere

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It so happens that Polaris (fairly bright “north star”) lies very close to where the north pole projects into space 

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Shows where the North is; no such star in southern celestial hemisphere

In ancient times, people imagined stars were imbedded in a crystal sphere high above that spun once every day

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Useful to think of a celestial sphere in setting up coordinate system for sky (locating objects)

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By projecting N/S poles and equator into space, we establish reference points organizing the coordinate system

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From any point on earth’s surface, only half of the total celestial sphere is visible at any moment

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At the north pole, Polaris stands straight up, at the celestial equator along the horizon

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Zenith: term meaning directly overhead from your location

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Transit: term meaning when an object crosses due south, and is at its highest point in arc across the sky

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As earth rotates toward E horizon, stars appear to rotate westward around the north celestial pole (Polaris)

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Depending on the latitude, the celestial sphere is tilted

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Stars will move in an angled arc to the horizon

As night hours pass, constellations are seen rotating around the polar star 

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Farthest from celestial pole have bigger arc covered

Circumpolar constellations are those that never set below the horizon as the sky rotates (actually Earth rotating) 

Number of them depends on how far north of the equator you are

Celestial Coordinates -

All planets in our solar system orbit the Sun in the same direction, more or less in the same orbital plane 

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“elliptical plane”

The sun and planets always appear to move across the “star-filled” sky along the same path (ecliptic plane projected into sky)

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Because the earth tilts 23o in path around the Sun, celestial equator tilts 23o from perceived path of planets and Sun through space (tilted from ecliptic plane)

Zodiac Constellations -

Because of the ecliptic plane, the Sun and planets (gods to ancients) are always seen to move through the same constellations

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Astrology evolved from the desire to understand and predict how planet movements among these constellations would affect human lives 

The sun spends about 1 month in each constellation, so we have 12 zodiac constellations

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There is a 13th zodiac constellation: Ophiuchus (the healer), but he has been ignored for astrological purposes

Organizing the Sky by Brightness -

Ancient cultures started to keep records of objects they observed within constellations and changes in the sky 

Needed a more convenient system, beyond maps and constellation descriptions, for organizing and locating things

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Middle-Eastern astronomers developed the first method to organize/locate stars; ranking them by brightness in each constellation

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Within constellations, Greeks gave stars letter designations by brightness—limited method, as there are too many stars, however still used today in identifying the brightest stars of each constellation 

Brightest: α , β , γ

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Dimmest: Ω

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I.e. Bellatrix is the 3rd brightest star in Orion, named

γ

Orionis

Celestial Coordinates -

Also started to develop tools to measure things; more precise measurement system needed, in terms of the celestial coordinate system 

Analog to latitude/longitude, except projected into space (virtual, not physical)

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Declination (Dec): analogous to latitude, it projects an angular measure north (+) of the celestial equator, or south (-) of the celestial equator (from +90o to -90o)

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Right ascension (RA): analogous to longitude, it projects grid systems into space (wrapping earth) and is divided into 24 hours 

Each hour of angular measurement is further divided into 60 minutes of arc; each minute has 60 seconds of arc

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With a system projecting a grid onto the sky, we can give a very accurate location of any object in the sky 

(y,x)—(lat, long)—(Dec, RA)

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Similar to lat/long system, Dec/RA must be anchored somehow (to the sky in this case)

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Declination is anchored to celestial equator

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As with lat/long, astronomers can quickly locate objects knowing RA/Dec coordinates

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Having an object’s specific coordinate is particularly important when it is moving and/or changing velocity in space (any given day different position)

Celestial Cycles of Motion & Tracking of Time -

From earliest times, ancients understand concepts of past/present/future and could see different repeating patterns of motions of objects in the sky

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Sun/Moon each day

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Planets moving back and forth among stars (cycles of years)

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Stars/Constellations rising and setting at the same places on a seasonal schedule

Many societies developed sundials, clocks, and astronomical calendars 

Used these patterns to track time and use this time-keeping ability to better understand and control their world

Importance of the Night Sky to the Ancients -

Early people of Earth had other very practical reasons for studying the sky 

Some stars were circumpolar; navigational aids (sailing and traveling to unknown regions during night travel)

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Constellations and moon used to pass along oral history, important lessons, and entertainment

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Using stars, the moon, and the sun to track seasons; better predict correct time to plant and harvest crops

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Observing repeating patterns in the sky and associating with regular events on earth, astronomers began establishing concrete connections between celestial events and everyday life

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Studying sky began our 1st steps toward a true scientific understanding of cosmos (observer techniques, record-keeping, etc…)

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In a real way, power within human society, or even at times just survival, depended on astronomical knowledge

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Ability to predict and explain astronomy events highly prized and closely guarded skill in many societies

Earth’s Orbital Motion – Keeping Time -

Apparent daily motion of Sun, Moon, planets, and stars across the sky which is due to Earth’s rotation is called Diurnal Motion

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But positions of stars in the sky do not appear exactly the same after 24 hours… slight shift each night (easily noticeable in a week or two) 

Reason: Earth has moved in its orbit

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Because of the shift, 1 day as measured by position of background stars (sidereal day) is 4 minutes shorter than 1 day measured by the position of the sun (solar day)

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A solar day takes 24 hours, but a sidereal day takes 23 hours and 56 minutes

Sidereal vs Solar Day -

Sidereal Day: Time for one complete earth rotation around distant stars is 23 hours and 56 minutes 

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They end up in exactly the same place

Because earth has moved around the sun a small amount in that time, earth must rotate for another 4 minutes to place the sun back in the same position in the sky as before

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A solar day is 24 hours long

Noting Earth’s Cycle Around the Sun: 1 Earth Year -

As earth moves around its orbit, the Sun appears to move through the ecliptic plane, visiting different constellations of the zodiac

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However, on the night-time side of Earth, we also see different constellations at different times of the year 

Constellations change with seasons

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During each season, earth’s night sky points to different parts of space (different constellations)

Explaining the Seasons -

Seasons on Earth are caused by axial tilt (angle at which sunlight hits earth changes as earth travels around its orbit) 

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Earth is closer to the sun in the winter

Heating of ground and air are different due to the angle of sunlight and duration of solar heating each day 

Higher arc means longer days, longer period of heating, and more efficient heating (ground gets warmer faster… warmer conditions)

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Lower arc across the sky means shorter period of heating (ground not efficiently heating nor for long… colder weather)

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Curvature of earth means southern hemisphere has opposite conditions of northern hemisphere—opposite seasons

Precession of Earth’s Spin -

Earth spins on its axis continuously 

North polar spin axis points at north star (Polaris) from every point in orbit around the Sun; creates seasons by changing the angle of sunlight at different points in the solar orbit

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However, rotation of earth is similar to a spinning top; wobbles in its spin with time (“precession”)

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Wobble is very slow but cyclical

Pole star changes with time over 26,000-year cycle 

14,000 AD – Vega pole star

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20,000 AD – star Thuban

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Cause: gravity torques exerted by the moon and sun on earth’s spin, slightly oblate the earth

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Over a long time, frame, it effects how we see the sky, use the coordinate system, and how to navigate with stars (ancestors)

Precession: Seasonal Effect -

Seasons defined by when the Sun appears travels across ecliptic plane (start of spring and fall) 

“Vernal/Autumnal Equinox” and by highest (in the summer) and by lowest (in the winter) point in constellations during the year – “Summer/Winter Solstice”
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