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What does the universe look like from earth?

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• With the naked eye, we can see more than 2,000 stars as well as the Milky way

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Constellations

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• Region of the sky • 88 constellations fill the entire sky • the brightest stars in a constellation may actually be quite far away from each other

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the celestial sphere

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• stars at different distances all appear to lie on the celestial sphere • the 88 official constellations cover the celestial sphere • IMPORTANT: the celestial sphere is an imaginary construct to help us understand the relationship between the earth and the rest of the universe • The sky projects onto a sphere- we measure coordinates/distances in angles • Like navigating the earth

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Ecliptic

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o The sun's apparent path through the celestial sphere o The sun moves relative to the stars o When the earth moves around the sun you see different stars because you go around at different angles around the sun

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North celestial pole

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o Directly above earths north pole

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South celestial pole

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o Directly above earths south pole

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Celestial equator

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o A projection of earths equator onto sky

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The milky way

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• A band of light making a circle around the celestial sphere • Our view into the plane of our galaxy

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Coordinates on the earth

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• Latitude o Position north or south of equator • Longitude o Position east or west of prime meridian o Runs through Greenwich, England • We measure the sky using angles o Pinky 1 degree o Thumb 2 degrees o Fist 10 degrees o Outstretched hand 20 degrees

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Angular measurements

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• Full circle=360 degrees • 1 degree= 60' (arcminutes) • 1' = 60" (arcseconds)

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Angular Size

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• Angular size= physical size x 360 degrees/2pi x distance • An objects angular size appears smaller if it is farther away • Moving an object further away reduces its distance • As long as the angular size is small, we can think of the objects physical size as a small piece of a circle.

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The local sky

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• An objects altitude (above horizon) and direction (or azimuth) (along horizon) specify its location in your local sky o Horizon= altitude 0 degrees o Meridian o Zenith= altitude 90 degrees

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Why do stars rise and set?

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• Earth rotates from west to east, so stars appear to circle from eat to west

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Our view from earth

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• Stars near the north celestial pole are circumpolar (we can see them all the time) and never set • We cannot see stars near the south celestial pole • All other stars (and sun, moon, planets) rise in east and set in west • A circumpolar star never sets • A star below your horizon never rises • There are no circumpolar stars on the equator

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Why do the constellations we see depend on latitude and time of year?

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• They depend on latitude because your position on earth determines which constellations remain below the horizon • They depend on time of year because earths orbit changes the apparent location of the sun among the stars **the altitude of the celestial pole= your latitude the sky varies as earth orbits the sun • seasons are opposite in the N and S hemispheres , so distance is irrelevant • seasons depend on how earths axis affects: o the directness of sunlight o the number of hours of sunlight

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how do we mark the progression of the seasons?

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• We define four special points o Summer (June) solstice o Winter (December) solstice o Spring (march) equinox o Fall (September) equinox

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We can recognize solstices and equinoxes by sun's path across sky

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• Summer (June) solstice o Highest path, rise and set at most extreme north of due east o Longest days • Winter (December) solstice o Lowest path, rise and set at most extreme south of due east • Equinoxes o Sun rises precisely due east and sets precisely due west • Seasonal changes are more extreme at high latitudes

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How does the orientation of earths axis change with time?

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• Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years o Polaris wont always be the north star o Positions of equinoxes shift around orbit • Ex. Spring equinox, once in Ariesm is now in Pisces • Earths axis precesses like the acis of a spinning top

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What causes the seasons?

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o The tilt of the earths axis causes sunlight to hit different parts of the earth more directly during the summer and less directly during the winter o We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon o The summer and winter solstices are when the northern hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight.

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How does the orientation of earths axis change with time?

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o The tilt remains about 23.5 degrees (so the season pattern is not affected), but earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of earths axis

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why do we see phases of the moon?

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o Lunar phases are a consequence of the moons 27.3-day orbit around earth o Average earth-moon distance=380,000 km o Closest to earth= 356,000 km o Farthest from earth= 407,000 km o The full moon appears larger when its closer

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Phases of the moon

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o Half of moon is illuminated by sun and half is dark o We see a changing combination of the bright and dark faces as moon orbits Half moon=third quarter or first quarter • The moon is a sphere so when you look at a "half moon" you only see a quarter of it o You see a full moon at night and a new moon during the day o 29.5 day cycle • New moon, waning crescent, third quarter (last quarter), waning gibbous, full moon, waxing gibbous, first quarter, waxing crescent • waxing moon visible in afternoon/evening gets "fuller" and rises later each day • waning moon visible in late night/morning gets "less full" and sets later each day o we see only one side of moon • synchronous rotation the moon rotates exactly once with each orbit • that is why only one side is visible from earth

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what causes eclipses?

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o The earth and moon cast shadows o When either passes through the others shadow, we have an eclipse o Flavors of the shadow • Penumbra Direct shadow Larger shadow • Umbra Indirect shadow

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When can eclipses occur?

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• Lunar eclipses can occur only at full moon • Lunar eclipses can be penumbral, partial, or total • Solar eclipses can occur only at new moon • Solar eclipses can be partial, total, or annular

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Why don't we have an eclipse at every new and full moon?

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• The moons orbit is tilted 5 degrees to ecliptic plane • So we have about two eclipse seasons each year, with a solar eclipse at new moon and lunar eclipse at full moon • We only have 2 solar eclipse seasons

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2 conditions must be met to have an eclipse

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o it must be full moon (for a lunar eclipse) or new moon (for a solar eclipse) o the moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes)

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predicting eclipses

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• eclipses recur with the 18 year, 11/ 1/3-day saros cycle, but type (ex. Partial, total) and location may vary

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why do we see phases of the moon?

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o Half the moon is lit by the sun, half is in shadow, and its appearance to us is determined by the relative positions of sun, moon, and earth

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What causes eclipses?

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o Lunar eclipse: earths shadow on the moon o Solar eclipse: moons shadow on earth o Tilt of moons orbit means eclipses occur each year

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Astronomy incorporates aspects of most major scientific disciplines

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o Physics o Chemistry o Geology o Biology

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The whole universe is our laboratory

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o Examples • Physics- nuclear fusion in stars • Chemistry- chemical reactions in interstellar space • Geology- planetary surfaces • Biology- search for life on mars and our own origins

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Astrobiology

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o A new field that more directly focuses on the formation of life on earth (biology) and beyond (astronomy) o Started by NASA o Try to figure out if life exists elsewhere on the universe

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Science tries to observe, understand, and predict nature

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o We predict so it can be tested and we can determine whether we're right or wrong

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Albert Einstein

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"the most incomprehensible thing about the world is that it is comprehensible"

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Light: electromagnetic radiation

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o Astronomy is an observational science and almost everything we know comes from using light as a messenger o Speed of light: c=3x10^5 km/s (300,000) o 1 light-year= 9.46x10^12 km o examples • sun to earth? 8 light minutes (10^-5 lyr) • earth to Saturn? ~ 1 ½ light-hours • nearest star? 4.4 lyr • orion nebula? 1500 lyr • Andromeda galaxy? 2.5 million lyr

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1 astronomical unit

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• 1 astronomical unit = 1 earth-sun distance = 1500 million km = 93 million miles

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the sun

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o a large, glowing ball of gas that generates heat and light through nuclear fusion o it's a star o fusion happens in the cores of stars and during explosions

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planet

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• a large object, made roughly by its self gravity, which orbits a star and has "cleared out its neighborhood" • it (mostly) shines by reflected lights • may be rocky icy or gaseous in composition • Pluto didn't clear out its neighborhood o Was known as the dwarf planet

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Titan is similar to the earth

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• Has lakes and it rains

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Our cosmic address

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• Earth • The solar system • Galaxy (milky way) • Group of galaxy (amdromita and milky way) • Local group^ • Local Super cluster • Universe

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Venus and mercury

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orbit closer to the sun than earth so they have phases Our sun moves relative to the other stars in the local solar neighborhood Our sun and stars of the local solar neighborhood orbit around the center of the milky way galaxy every 230___

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Galaxy

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• A great island of stars in space, all held together by gravity and orbiting a common center • Our galaxy is the milky way • We infer the structure of our galaxy by mapping out things that are inside of it

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The milky way moves with the expansion of the universe

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• Mostly all galaxies appear to be moving away from us • The farther away they are, the faster they are moving • Just like raisins in a raisin cake: they all move apart from each other as the dough (space itself)expands • Velocity= Distance/Time

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Looking back in time...

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• The farther out we look into the universe, the farther back in time we see! • Andromeda galaxy is 2.5 million lyr away o Light we see now left 2.3 million years ago! • We see one galaxy as it was 7 billion years ago, when the universe was only about half its current age

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Universe

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• The sum total of all matter and energy • It is everything within and between all galaxies

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The laws of nature are universal

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• But is the biology the same?

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Three lines of evidence for a universal biology

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• Organic molecules form easily and naturally o Miller-urey experiment o Murchison meteorite • Earth life can survive under wide range of conditions o Black smokers o Lithophiles • Bacteria inside quartz • Life appeared very early in earths history

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Basic ingredient: atoms

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• Microscopic "building blocks" of all chemical elements • When the universe started it was helium, hydrogen, and lithium • Now there's more • Fusion in the stars created the elements today

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What we know

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• Our star (the sun) has 8 planets with 1 (at least) containing life • We have detected 422 extra-solar planets around nearby stars • 100 billion stars in our galaxy • 100 billion galaxies in the universe

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The science of Astronomy

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• We especially need imagination in science. It is not all mathematics, nor all logic

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A hypothesis only becomes a scientific theory after it has been well tested What is science?

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• Trial and error, specifically organized • Logical • Quantitative

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The scientific method used by the greeks

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• Question • Hypothesis o A tentative explanation • Prediction • Test • Result o Confirm, reject, or modify

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Theory

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o Well established and tested repeatedly with observations and experiments o Quantities involved are well known

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Model

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o Reasonably well established and tested o Not all parameters are determined to high precision

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Hypothesis

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o Plausible explanation, needs testing

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Hallmarks of science

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• Seeks explanations based on natural causes • The simplest models are favored • Models must be predictive and falsifiable by anyone

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A description of reality

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• Self-consistent • Quantitative • Evolving • Predictive

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Moons distance

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3.63x10^5 km to 4.05x10^5 km

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Emergence of astronomy as a science

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• Modern science is rooted in ancient astronomy o Ancient cultures observed the sky o Science emerged as we sought understanding • Ancient astronomical accomplishments o Ancient observatories • Stonehenge, the sun dagger, mayan observatories • Eclipse predictions; time keeping

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Stonehenge

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• Started ~3100 BC; major stones `2100-2400 BC; completed 1550 BC • Astronomical prientation • If you stand in the middle o The directions of sunrise and sunset on the solstices is marked o The directions of extreme moon rise and set

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Mayans

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• Lived in central America • Accurately predicted eclipses

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Greece

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• Western civilization, and for our purposes science and astronomy, blossomed between ~ 500 BC and ~150 AD in the Hellenized regions of Mediterranean

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What did the ancient greeks know

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• The earth is round • The pythagoreans and plato for example taught that the earth was round.

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Eratosthenes (276-195 BC)

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• He measured the circumference of the earth • The sun is at the zenith in the city of syene at noon on the summer solstice • But at the same time in Alexandria, it is 7 degrees from the zenith

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Claudius ptolemy (AD 100-170)

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• Almagest o Star catalogue o Instruments o Motions and model of planets, sun, moon • His model fit the data, made accurate predictions, but was very complicated • He wrote it all down • His geocentric model o Earth at the center o Sun orbits earth

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The dark ages (~500-1400 AD)

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• Astronomy flourishes in the Islamic world- deficiencies in Ptolemaic model explored, but no paradigm shift

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Nicholas Copernicus (1473-1543)

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• He thought Ptolemy's model was contrived • He believed in circular motion • His heliocentric model o Sun is at the center o Earth orbits like any other planet o Retrograde motion occurs when earth overtakes the superior planets • Retrograde motion of mars in the heliocentric model o The greeks rejected the heliocentric model

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Why did the ancient greeks reject the notion that the earth orbits the sun?

answer

o It ran contrary to their senses o If the earth rotated, then there should be a "great wind" as we moved through the air o Greeks knew that we should see stellar parallax if we orbited the sun- but they could not detect it

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Parallax angle

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o Apparent shift of a shars position due to the earths orbiting of the sun • The nearest stars are much farther away than the greeks thought • So the parallax angles of the star are so small, that you need a telescope to observe them

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Tycho Brahe (1546-1601)

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• Greatest observer of his day • Recorded accurate positions of planets • Observed a supernova in 1572 • He measured the stars and planets with high accuracy

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Johannes Kepler (1571-1630)

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• Greatest theorist of his day • Deeply religious • Force makes the planets move

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• Keplers laws

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o Each planets orbit around the sun is an ellipse, with the sun at one focus o A planets orbital motion sweeps out equal areas in equal intervals of time • Key thing- orbiting bodies move faster when closer to the sun • Faster at the perihelion and slower at the aphelion o a^3=P^2 • a= semimajor axis in earth-orbit-radii, or Astronomical units • P= orbital period in years • Applies only to our solar system in this form

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Orbital period

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orbital velocity

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Galileo Galilei (1564-1642)

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• First person to explore the sky with a telescope • Wanted to connect physics on earth with the heavens • Shadows cast by mountains • Craters • Lunar landscape: a "place", not a perfect heavenly body

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Galileo's observations of the phases of venus buried the geocentric model

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o No gibbous or full phases in the geocentric model o All phases seen in the heliocentric model

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Galileo's observations of Jupiter

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o Jupiter has four moons o Jupiter is the center of its own system o Some heavenly bodies do not orbit earth or sun

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Galileo chips away at the perfect, unchangeable universe

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o Observed sunspots, calculated suns rotation period o Resolved milky way into myraids

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dialogue concerning the two chief world systems, 1632

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o this book got him in trouble with the pope o he was condemned to house arrest in 1633

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Astrology

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• Claims that positions of the sun, moon, and planets influence human lives • Drove progress of ancient astronomy • Since kepler and galileo, astronomy became science • Astrology fails scientific tests: it is a pseudoscience • Based on his horoscope, galileo claimed, "the Duke of Tuscany will have a long and fruitful life" o He died shortly after • Don't base serious decisions on astrology

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Displacement

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(distance + direction) o Distance (has no direction)

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Velocity

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(distance traveled/ time taken)o Has a direction! (speed does not)

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Acceleration

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(change in velocity/time taken)o Again, has a direction

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Momentum

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mass x velocity Small mass with a large velocity has same momentum as a large mass with a small velocity

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Force

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• Something that can change momentum Ex. Force of a wind, pushing, gravity, etc • Force causes change in momentum, producing accelteration

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• Mass

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the amount of matter in an object o The quantity of matter

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• Weight

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the force that acts upon an object o You are weightless in free-fall! o The force acting on mass o why are astronauts weightless in space? • There is gravity in space • Weightlessness is due to a constant state of free-fall • Using a rocket to gain enough speed your coul continually "fall" around earth: that is, you be in orbit

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Newtons laws of motion

answer

• 1. An object moves at constant velocity unless a net force acts to change its speed or direction o velocity can be zero o friction is a force that's acting on it o momentum is conserved: mv • 2. Force=mass x acceleration o F=ma • 3. For every force, there is always an equal and opposite reaction force o like pushing on a wall

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why do objects move at constant velocity if no force acts on them?

answer

• Objects continue at constant velocity because of conservation of momentum o The total momentum of interacting objects cannot change unless an external force is acting on them o Interacting objects exchange momentum through equal and opposite forces o Momentum= m x v o Objects exchange momentum through exchange in forces

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conservation of angular momentum

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• angular momentum= mass x velocity x radius • the angular momentum of an object cannot change unless an external twisting force (torque) is acting on it • earth experiences no twisting force as it orbits the sun, so its rotation and orbit will continue indefinitely • the closer the mass is towards the center the faster it will spin • angular momentum conservation also explains why objects rotate faster as they shrink in radius o ice skater spinning o if an astronaut is floating in space, he will spin himself by using a wrench to unscrew a bolt

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where do objects get their energy?

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• Energy can make matter move • Energy is conserved, but it can: o Transfer from one object to another o Change in form

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Kinetic

answer

(motion) • Energy of motion Motion of macroscopic objects Thermal energy (random motion of atoms/microscopic objects) Energy propagated by waves • Sound, light to a certain extent

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Potential

answer

(stored) • Gravitational • Chemical • Mass-energy • Electric • + lots of others

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Radiative

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(light) ** energy can change type, but cannot be created or destroyed

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Conservation of Energy

answer

• Energy can be neither created nor destroyed • It can change form or be exchanged between objects • The total energy content of the universe was determined in the Big Bang and remains the same today

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Potential Gravitational energy=m x g x h

answer

• Ex. In space, an object or gas cloud has more gravitational energy when it is spread out than when it contracts. o A contracting cloud converts gravitational potential energy to thermal energy o Energy is conserved: as the cloud contracts, gravitational potential energy is converted to thermal energy and radiation • Ex. Thermal energy: the collective kinetic energy of many particles (for example, in a rock, in air, in water) o Thermal energy is related to temperature but it is NOT the same o Temperature is the average kinetic energy of the many particles in a substance

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Temperature vs. Heat

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• Temperature is the average kinetic energy per particle. Heat is the total kinetic energy

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• Newton's Extensions of Kepler's Laws

answer

o Drop a cannon ball from a mountain top. o Now, fire the cannon ball with increasing power o At some point, the cannon ball will go far enough to enter orbit o With enough initial velocity, the orbit becomes more elliptical o Finally, with enough kinetic energy in the cannon ball, you break the earth's orbit altogether. o Kepler's first 2 laws apply to all orbiting objects, not just planets. o Ellipses are not the only orbital paths. o 1. Objects other than planets can have elliptical orbits o 2. Ellipses are not the only possible orbit o 3. Objects orbit around their center-of-mass along the line that connects them gravitationally o 4. The mass of the system enters into kepler's third law: P^2=a^3 becomes (M+m)(P^2=a^3)

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• What determines the strength of gravity?

answer

o The universal law of gravitation • 1. Every mass attracts every other mass • 2. Attraction is directly proportional to the product of their masses • 3. Attraction is inversely proportional to the square of the distance between their centers o Fg= G ((M1 x M2)/ d^2) o Accelteration of M2= Fg/M2 = GM1/d^2 • All objects fall the same- independent of mass of M2

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How do gravity and energy together allow us to understand orbits?

answer

o Total orbital energy (gravitational + kinetic) stays constant if there is no external force o Angular momentum is constant o Orbits cannot change spontaneously o More gravitational energy, less kinetic energy • Less gravitational energy, more kinetic energy o ****Total orbital energy stays constant

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changing an orbit

answer

o so what can make an object gain or lose orbital energy? • Friction or atmospheric drag (Mir example) • A gravitational encounter

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Center of mass

answer

• Because of momentum conservation, orbiting objects orbit around their center of mass • Like a teater tauter or sea saw • The earth and the sun go around eachother • The center of gravity is at the pivot and then there is blance o Center of gravity is not in the middle in the case of the sun and planets

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The earth moon system

answer

• Pivot/center of gravity is called the barycenter • Keplers extenstions to keplers law 4. The mass of the system enters into kepler's third law: P^2=a^3 becomes (M+m)(P^2=a^3) o Where the masses are measured with respect to the mass of the sun

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Newton and Keplers third law

answer

• Newtons laws of gravity and motion showed that the relationshio between the orbital period and average orbital distance of a system tell sus the total mass of the system o Ex. Earths orbital period (1year) and average distance (1 AU) tell us the sun's mass

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How does gravity cause tides?

answer

• Moon's gravity pulls harder on near side of earth than on far side • Difference in moon's gravitational pull stretches earth • Tidal effects of moon vs. sun?

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Tides and phases

answer

• Size of tides depends on phase of moon

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wave

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o a Wave is a pattern of motion that can carry energy without carrying matter along with it

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properties of waves

answer

• amplitude is the strength of the wave • wavelength is the distance between two wave peaks • frequency is the number of times persecond that a wave vibrates up and down • wave speed(constant)= wavelength x frequency

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speed of light

answer

o speed of light(C)= wavelength x frequency

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Light: Electromagnetic waves

answer

• Electrons move when light passes by, showing that light carries a vibrating electric field o A light wave is a vibration of electric and magnetic fields o Light interacts with charged particles through these electric and magnetic fields

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Wavelength and frequency

answer

• Wavelength= 1 cm • Frequency= 30 GHz • 30 billion vibrations a second, caused by the speed of light being so large

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Particles of light

answer

• Particles of light are called photons • Each photon has a wavelength and a frequency (like a wave) • The energy of a photon depends on its frequency • They sometimes act like waves, and sometimes act like particles in different circumstances

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Wavelength, Frequency, and Energy

answer

• Wavelength x frequency=Constant (c)= speed of light • C=3.00x10^8 m/s= speed of light • E= h x f= photon energy • h= 6.626 x 10^-34 joule x s =photon energy

question

how do light and matter interact?

answer

• Emission • Absorption • Transmission o Transparent objects transmit light o Opaque objects block (absorb, scatter) light • Reflection/scattering • **Objects often do all for of these!

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Why is the sky blue?

answer

• Air molecules are small, smaller than light waves • Scatter blue light especially well, red light a lot less well • Sunset red → long path length

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