Presentation on theme: "ASTRONOMY 114 Survey of Astronomy Monday,Tuesday,Wednesday,Thursday 2:30-3:20pm Temple Hall 0001 Dr. Peter Plavchan 626-234-1628"— Presentation transcript:
ASTRONOMY 114 Survey of Astronomy Monday,Tuesday,Wednesday,Thursday 2:30-3:20pm Temple Hall 0001 Dr. Peter Plavchan Office Hours: Mon-Thurs 12:45-2pm Textbook: Sapling Learning + Online textbook
You Please take out a piece of paper, or compose an to me on your phone/computer, and answer: 1.Name & Student ID# (also for attendance) 2.How many semesters have you completed at MSU? 3.What do you know or have you heard about astronomy? 4.What is your dream career? 5.Why are you taking this class? 6.What do you dread doing? What do you love doing? 7.Can you give me tips on how you learn best? 8.What would you like to know about me? 9.What would you like me to know about you? 10.What can I teach you? 11.What can you teach me?
Syllabus & Exams Clicker- you need one. Get it registered (there is help on my course web page) The internet will be used extensively in this class. Lecture notes will be posted after class. Please let me know if you need help accessing the web, or if the web pages are not working.
Extra Credit Options Extra credit is available and can contribute up to 5-10%. The purpose of extra credit is to find a balance between your interests and the subject material of astronomy. Possible extra credit options must be approved by the instructor, but can include for example: Visit Baker Observatory. Bring back proof - photographic - of your visit. To qualify for the extra credit, please write a one page summary of your visit. Astronomy as art. Many of you are non-science majors, and excel in other areas of specialization - art, writing, music, etc. There is a vast history of art inspired by astronomy. See for an example. For another example, consider constructing a scale model of our galaxy or local group of galaxies. In order to qualify for the extra credit, please create an original work of art, music or writing inspired by astronomy and the material you have learned in class. You may team with up to two other people. ALL EXTRA CREDIT REPORTS ARE DUE BY THE FINAL EXAM, DIRECTLY TO ME.
“Group” Projects Sign up for a Thursday to present to the class. Two people will present each week, and there will be a competition for the better presentation. Your classmates will vote! Presentations can be voice only, Powerpoint, Keynote, PDF. Please blog about your presentation here: Presentations are minutes each From each lecture’s topic, pick a subject to go into detail on. Consult with the professor on the topics covered in class well before you prepare your presentation. Will count towards 10% of final grade.
Our Modern View of the Universe Our Place in the Universe
A Modern View of the Universe What is our place in the universe? How did we come to be? Can we know what the universe was like in the past? Can we see the entire universe? Our goals for learning:
Astronomy The branch of science dedicated to the study of everything in the Universe that lies above Earth’s atmosphere
What is our place in the universe? 2500 yr 400 yr 80 yr ago Universe
A large, glowing ball of mostly hydrogen gas that generates heat and light through nuclear fusion at its center Star Our star – the Sun Context: “gas” in astronomy usually means H, or H 2, but could include other atoms and molecules in the gas state.
Planet A moderately large object that orbits (goes around) a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition. Mars Neptune
Moon (or satellite) An object that orbits a planet. Ganymede (orbits Jupiter)
Asteroid rocky A relatively small and rocky object which orbits a star.
Comet A relatively small and icy object that orbits a star.
Solar (Star) System A star and all the material that orbits it, including its planets and moons. Note: planets and orbits are not to scale; planets are tiny compared to their orbits.
Nebula An interstellar cloud of hydrogen gas and/or tiny smoke-like particles called “dust”
Galaxy A great island of stars in space, all held together by gravity and orbiting a common center M31, The Great Galaxy in Andromeda
Universe The sum total of all matter and energy; that is, everything within and between all galaxies
How can we know what the universe was like in the past? The key: light travels at a finite speed – 300,000 km/s, 186,000 miles/s, 670 million miles per hour You can circle Earth 8 times in 1 second More on the Speed of Light when we discuss Relativity DestinationLight travel time Moon1 second Sun8 minutes Sirius8 years Andromeda Galaxy2.5 million years Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time.
Light-year The distance light can travel in one year. About 10 trillion km (6 trillion miles). A light-year is NOT a unit of time! Can also talk about light-seconds. At great distances we see objects as they were when the universe was much younger.
How far is a light-year? 1 year = 31.5 million seconds
How far is a light-year? Approximately 10 trillion kilometers; exact value is 9.46 trillion Is this a large distance? Yes! The distance from the Earth to the Sun is 150,000,000 km, also known as 1 Astronomical Unit (AU). So, 1 light-year is over 63,000 AU.
What have we learned? How can we know that the universe was like in the past? – When we look to great distances we are seeing events that happened long ago because light travels at a finite speed Can we see the entire universe? – No, the observable portion of the universe is about 13.7 billion light-years in radius because the universe is about 13.7 billion years old. (We may round this number to 14 billion for convenience but the best modern measurements give 13.7 billion years since the time from the Big Bang.) What is our physical place in the universe? –Earth is part of the Solar System, which is in the Milky Way galaxy, which is a member of the Local Group of galaxies in the Local Supercluster
The Scale of the Universe How big is Earth compared to our solar system? How far away are the stars? How big is the Milky Way Galaxy? How big is the universe? How do our lifetimes compare to the age of the universe? Our goals for learning: It is very important to grasp the huge distances and enormous time spans that we deal with in astronomy. The way to do this is to create a SCALE MODEL.
The easy questions How far away are the Sun and Moon? The Moon is 384 million m from Earth (~250,000 miles). The Sun is 1.5x10 11 m from Earth (150 billion m, or ~93 million miles). This distance is called 1 astronomical unit (AU).
How big is Earth compared to our Solar System? Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter or about 5.6 inches; 2.54 cm = 1 inch). How big is Earth on this scale? A.an atom B.a ball point C.a marble D.a golf ball Radius of the Sun = 700,000 km Diameter Sun = 1.4 x cm Divide by to get 14 cm
Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter). How big is Earth on this scale? A.an atom B.a ball point C.a marble D.a golf ball How far apart would our model Sun and Earth be on this scale?
The scale of the solar system On a 1-to-10 billion scale: – Sun is size of a large grapefruit (14 cm) – Earth is size of a ball point, 15 meters away – 15 meters is about 107 grapefruits – Which means in the real solar system you could fit about107 Suns into the Earth –Sun distance
How far away are the stars? On our 1-to-10 billion scale, it’s just a few minutes walk to Pluto. [See model of Jefferson Ave.] How far would you have to walk to reach the nearest star to the Sun - Alpha Centauri? A.1 mile B.10 miles C.100 miles D.the distance across the U.S. (2500 miles) Answer: D, the distance across the U.S.
Thought Question Suppose you tried to count the more than 100 billion (10 11 ) stars in our galaxy, at a rate of one per second… How long would it take you? A.a few weeks B.a few months C.a few years D.a few thousand years
Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second… How long would it take you? A.a few weeks B.a few months C.a few years D.a few thousand years (100 billion seconds is nearly 3,200 years. Why? Because there are about 30 million seconds in a year, so /3x10 7 = 0.33x10 4 = 3.3 x 10 3 )
How big is the (observable) Universe? The Milky Way is one of about 100 billion galaxies. (10 11 stars/galaxy) x (10 11 galaxies) = stars As many stars as grains of (dry) sand on all Earth’s beaches…
How do human lifetimes compare to the AGE of the Universe? The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year!
How do human lifetimes compare to the age of the Universe? The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year. 1 day represents about 40 million years; 1 second represents about 440 years.
What have we learned? How big is Earth compared to our solar system? – The distances between planets are huge compared to their sizes—on a scale of 1-to-10 billion, Earth is the size of a ball point and the Sun is 15 meters away How far away are the stars? – On the same scale, the stars are thousands of km away How big is the Milky Way galaxy? – It would take more than 3,000 years to count the stars in the Milky Way Galaxy at a rate of one per second, and they are spread across 100,000 light-years
What have we learned? How big is the universe? – The observable universe is almost 14 billion light-years in radius and contains over 100 billion galaxies with a total number of stars comparable to the number of grains of sand on all of Earth’s beaches How do our lifetimes compare to the age of the universe? – On a cosmic calendar that compresses the history of the Universe into one year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second
Spaceship Earth How is Earth moving in our solar system? How is our solar system moving in the Galaxy? How do galaxies move within the Universe? Are we ever sitting still? Our goals for learning:
How is Earth moving in our solar system? Contrary to our perception, we are not “sitting still.” We are moving with the Earth in several ways, and at surprisingly fast speeds… The Earth rotates around its axis once every day. The spin rate at the Equator is ~1000 mph, twice as fast as a commercial airliner.
Earth orbits the Sun (revolves) once every year: at an average distance of 1 AU ≈ 150 million km with Earth’s axis tilted by 23.5º (pointing to Polaris) and rotating in the same direction it orbits, counter- clockwise as viewed from above the North Pole.
Our Sun moves relative to the other stars in the local Solar neighborhood… typical relative speeds of more than 70,000 km/hr but stars are so far away that we cannot easily notice their motion … And orbits the center of the Milky Way galaxy every 230 million years.
More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in modern astronomy Most of Milky Way’s light comes from disk and bulge … …. but most of the mass is in a huge and DARK halo.
How do galaxies move within the universe? The Universe is expanding. In the 1920s Edwin Hubble discovered that galaxies are carried along with the expansion of the Universe. But how did Hubble figure out that the universe is expanding?
Hubble discovered that: All galaxies outside our Local Group are moving away from us. The more distant the galaxy, the faster it is racing away. Conclusion: We live in an expanding universe.
What have we learned? How is Earth moving in our solar system? – It rotates on its axis once a day and orbits the Sun at a distance of 1 A.U. = 150 million km How is our solar system moving in the Milky Way galaxy? – Stars in the Local Neighborhood move randomly relative to one another and orbit the center of the Milky Way in about 230 million years
What have we learned? How do galaxies move within the universe? – All galaxies beyond the Local Group are moving away from us with expansion of the Universe: the more distant they are, the faster they’re moving Are we ever sitting still? – No! OKAY, NOW WE HAVE A GOOD OVERALL PERSPECTIVE. NEXT WE NEED SOME MORE DETAILS
Discovering the Universe for Yourself
Patterns in the Night Sky What does the universe look like from Earth? Why do stars rise and set? Why do the constellations we see depend on latitude and time of year? Our goals for learning:
The Celestial Sphere Stars at different distances all appear to lie on the Celestial Sphere. Ecliptic is Sun ’ s apparent path through the celestial sphere.
The Celestial Sphere The 88 official constellations cover the celestial sphere. It is important to realize that these named patterns have no relation to life on Earth and the stars in a given constellation are often not connected with each other physically.
The Milky Way A band of faint light making a circle around the celestial sphere. What is it? Our view into the “ plane ” of our spiral galaxy. Fish Eye lens view
The Local Sky Zenith: The point directly overhead Horizon: All points 90° away from zenith Meridian: Line passing through zenith from N to S points Altitude (above horizon) Azimuth (along horizon) specifies location
We measure the sky using angles Full circle = 360º 1º = 60 (arcminutes) 1 = 60 (arcseconds)
Angular Size An object ’ s angular size appears smaller if it is farther away Aside: We can define a new unit of angular measure called a radian such that 1 radian = 360/2π = 57.3 degrees
Why do stars rise and set? Earth rotates west to east, so stars appear to circle from east to west.
The sky varies with latitude but not longitude
The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky.
The Reason for Seasons Our goals for learning: What causes the seasons? How do we mark the progression of the seasons? How does the orientation of Earth ’ s axis change with time?
What causes the seasons? Seasons depend on how Earth ’ s axis affects the directness of sunlight
Axis tilt changes directness of sunlight during the year
Sun ’ s altitude in the sky also changes with seasons Sun ’ s position at noon in summer: higher altitude means more direct sunlight. Sun ’ s position at noon in winter: lower altitude means less direct sunlight.
How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox
We can recognize solstices and equinoxes by Sun ’ s path across sky Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west.
How does the orientation of Earth ’ s axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. Polaris won ’ t always be the North Star. Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces! Earth ’ s axis precesses like the axis of a spinning top
The Moon, Our Constant Companion Why do we see phases of the Moon? What causes eclipses? Our goals for learning:
Phases of Moon Half of Moon is illuminated by Sun and half is dark. NOT caused by Earth’s shadow! We see a changing combination of the bright and dark faces as Moon orbits
We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit of ~28 days That is why only one side is visible from Earth
What causes eclipses? The Earth and Moon cast shadows. When either passes through the other ’ s shadow, we have an eclipse.
When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. August 21 st, 2017
When can eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular.
Why don ’ t we have an eclipse at every new and full moon? – The Moon ’ s orbit is tilted 5° to ecliptic plane… – So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.
Planets Known in Ancient Times Mercury –difficult to see; always close to Sun in sky Venus –very bright when visible; morning or evening “ star ” Mars –noticeably red Jupiter –very bright Saturn –moderately bright
What was once so mysterious about planetary motion in our sky? Planets usually move slightly eastward from night to night relative to the stars. But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion
We see apparent retrograde motion when we pass by a planet in its orbit.
Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we “ lap ” another planet (or when Mercury or Venus laps us) But very difficult to explain if you think that Earth is the center of the universe! In fact, ancients considered but rejected the correct explanation
Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallax was a major factor. Not to scale 1 AU p d tan p = 1AU/d (AU) For small angles: p = 1/d If the angle p = 1 second of arc then d is defined as 1 parsec (= AU).
The Greeks knew that the lack of observable parallax could mean one of two things: 1.Stars are so far away that stellar parallax is too small to notice with the naked eye 2.Earth does not orbit the Sun; the Earth is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.
Length of a Day Sidereal day: Earth rotates once on its axis relative to the distant stars in 23 hrs, 56 min, and 4.07 sec.
Solar day: The Sun makes one circuit around the sky in 24 hours (by definition) Length of a Day
Why are the two different? Solar day is longer than a sidereal day by about 1/360 because Earth moves about 1° in orbit each day
Length of a Month Sidereal month: Moon orbits Earth in 27.3 days. Earth & Moon travel 30° around Sun during that time (30°/360° = 1/12) Synodic month: A cycle of lunar phases; therefore takes about 29.5 days, 1/12 longer than a sidereal month Synodic means “ meeting ”
Length of a Year Sidereal year: Time for Earth to complete one orbit of Sun Tropical year: Time for Earth to complete one cycle of seasons Tropical year is about 20 minutes (1/26,000) shorter than a sidereal year because of Earth ’ s precession.
Planetary Periods Planetary periods can be measured with respect to stars (sidereal) or to apparent position of Sun (synodic).
Planetary Periods Difference between a planet ’ s orbital (sidereal) and synodic period depends on how far planet moves in one Earth year for outer planets
Planetary Periods Difference between a planet ’ s orbital (sidereal) and synodic period depends on how far planet moves in one Earth year for inner planets
How do we tell the time of day? Apparent solar time depends on the position of the Sun in the local sky A sundial gives apparent solar time
Mean Solar Time Length of an apparent solar day changes during the year because Earth ’ s orbit is slightly elliptical. Mean solar time is based on the average length of a day. Noon is average time at which Sun crosses meridian It is a local definition of time
Standard Time & Time Zones Rapid train travel required time to be standardized into time zones (time no longer local)
Universal Time Universal time (UT) is defined to be the mean solar time at 0° longitude. It is also known as Greenwich Mean Time (GMT) because 0° longitude is defined to pass through Greenwich, England It is the standard time used for astronomy and navigation around the world
Time by the Stars Sidereal time is equal to right ascension that is passing through the meridian Thus, the local siderial time is 0 h 0 m when the spring equinox passes through the meridian A star ’ s hour angle is the time since it last passed through the meridian Local Sidereal Time = RA + Hour Angle
When and why do we have leap years? The length of a tropical year is about days. In order to keep the calendar year synchronized with the seasons, we must add one day every four years (February 29). For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years unless they are divisible by 400 (e.g., 2000).
Celestial Coordinates Right ascension: Like longitude on celestial sphere (measured in hours with respect to spring equinox). Declination: Like latitude on celestial sphere (measured in degrees above celestial equator)
Celestial Coordinates of Vega Right ascension: Vega ’ s RA of 18 h 35.2 m (out of 24 h ) places most of the way around celestial sphere from spring equinox. Declination: Vega ’ s dec of +38°44 ’ puts it almost 39° north of celestial equator (negative dec would be south of equator)
Celestial Coordinates of Sun The Sun ’ s RA and dec change along the ecliptic during the course of a year Sun ’ s declination is negative in fall and winter and positive in spring and summer
How can you determine your latitude? Latitude equals altitude of celestial pole Altitude and declination of star crossing meridian also gives latitude. You can determine Sun ’ s declination from the day of the year Measuring the Sun ’ s altitude when it crosses meridian can tell you latitude
How can you determine your longitude? In order to determine your longitude from the sky, you need to know time of day because of Earth ’ s rotation You also need to know day of year because of Earth ’ s orbit Accurate measurement of longitude requires an accurate clock.
GPS Navigation The Global Positioning System (GPS) uses a set of satellites in Earth orbit as artificial stars GPS devices use radio signals to determine your position relative to those satellites GPS satellites correct for General Relativity!