ASTR114 Lecture 2 PDF

Motions of the Night Sky, Seasons, & Eclipses Star Trails over the Grand 1Canyon | Image Credit: Babak Tafreshi (TWAN) 2 Review of Previous Lecture… by Neil DeGrasse Tyson… 3  Learning Objectives Understand the modern meaning of the term...

Motions of the Night Sky, Seasons, & Eclipses Star Trails over the Grand 1Canyon | Image Credit: Babak Tafreshi (TWAN) 2 Review of Previous Lecture… by Neil DeGrasse Tyson… 3  Learning Objectives Understand the modern meaning of the term constellation De ne the main features of the celestial sphere Explain the system astronomers use to describe the sky Describe how motions of the stars appear to us on Earth Describe how motions of the Sun, Moon, and planets appear to us on Earth Understand the reasons for the seasons 4 and eclipses fi Constellations LOREN ANDERSON – SHANGHAI – MAY 14, 2013  Constellations A constellation is a loose grouping of stars that appear close together in the night sky. In ancient times, they included only bright stars. Today, there are an agreed-upon set of 88 constellations that together divide the entire sky. Like a country boundary, “the star's address” 6  Clicker Question #1 The brightest stars in a constellation: A. all belong to the same star cluster. B. all lie at about the same distance from Earth. C. may actually be quite far away from each other. 7 Clicker Question #1 Answer The brightest stars in a constellation: A. all belong to the same star cluster. B. all lie at about the same distance from Earth. C. may actually be quite far away from each other. 8  Constellations Are Not Real Groups The stars in constellations are not necessarily physically close to each other. http://astro.unl.edu/ classaction/animations/ coordsmotion/bigdipper.html 9  Constellations & Asterisms Every culture around the world developed their own stories about the shapes in the night sky. Astronomy is one of the oldest sciences! Most of the shapes and stories that we might be most familiar with are from Ancient Greece. 10  Constellations & Asterisms In the modern de nition of a constellation, it is the patch of sky (or sector) that gets the name, rather than the set of stars themselves. Of the 88 of cial constellations de ned by the IAU in 1928, 48 are ancient in origin. 11 fi fi fi  Asterisms An asterism is a less formally de ned grouping of stars, that may be a part of one or more constellations; for example, the Big Dipper in Ursa Major 12 fi  Constellations & Asterisms Orion’s Belt is an asterism within 13 the constellation of Orion.  The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The 88 of cial constellations cover the entire celestial sphere. 14 fi  The Celestial Sphere The ecliptic is the Sun's apparent path through the celestial sphere. 15  The Celestial Sphere North celestial pole is directly above Earth's North Pole. South celestial pole is directly above Earth's South Pole. Celestial equator is a projection of Earth's equator onto sky. 16  What does the Universe look like from Earth? With the naked eye, we can see more than 2,000 stars. We can also see the Milky Way, most of the planets, many comets, a whole bunch of satellites, and some galaxies. 17 Earth Rotates on its Axis There are two main motions that affect what we see in the sky: Motion #1: The Earth rotates on its axis. This causes stars to appear to rotate around the north and south celestial poles. The complete rotation of each star takes 24 hours. 18  Foucault’s Pendulum by Sixty Symbols. — SUPPLEMENTAL How the 19th century physicist directly 19 showed the Earth is rotating…  How do we perceive Earth’s rotation? The Sun appears to rise and set each day. Stars in the night sky appear to rotate around us. Earth rotates from west to east, so stars appear to circle from east to west. 20 Finding the North Star  Our View From Earth Stars near the north celestial pole are circumpolar 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. 21 The Celestial Sphere: Observer- Centered Terms… The north celestial pole is the point where a line extending out of the Earth’s north pole would intersect the celestial sphere. South celestial pole: the projection of the Earth’s geographic south pole into space. 22  The Celestial Sphere: Star Motions In the celestial sphere model, we pretend that the Earth is xed and that the stars move across the sky. If we had a camera take a long exposure picture of the night sky, we would see star trails, showing how stars appear to move because the Earth is rotating on its axis. Video: Check out this article for images and diagrams: photopills.com/articles/star- trails-photography-guide 23 fi  Coordinates on the Earth Latitude: Position north or south of equator Longitude: Position east or west of prime meridian (runs through Greenwich, England) 24  Why Do the Constellations We See Depend on Latitude and Time of Year? (Not Longitude) They depend on latitude because your position on Earth determines which constellations remain below the horizon. 25  The Sky Varies as Earth Orbits the Sun Constellations depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars. 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. 26  In what direction is the observer facing? In what direction is the observer Assume they’re in the Northern facing? Hemisphere Celestial Sphere Rotation Star B 2 Star A 1 2 1. 1. toward the South Celestial Sphere Celestial Sphere 3 2. 2. toward the North 4 1 3. 3. toward the East 4. 4. toward the West 3 Horizon 4 Celestial Sphere Rotation Figure 2 27  In what direction is the observer facing? Assume they’re in the Northern Hemisphere Celestial Sphere Rotation Star B 2 Star A 1 2 1. 1. toward the South Celestial Sphere Celestial Sphere 3 2. 2. toward the North 4 1 3. 3. toward the East 4. 4. toward the West 3 Horizon 4 Celestial Sphere Rotation Figure 2 28 Imagine that from your current location you observe a star rising directly in the east. When this star reaches its highest position above the horizon, where will it be? A. A. high in the northern sky B. B. high in the southern sky C. C. high in the western sky D. D. directly overhead 29 Imagine that from your current location you observe a star rising directly in the east. When this star reaches its highest position above the horizon, where will it be? A. A. high in the northern sky B. B. high in the southern sky C. C. high in the western sky D. D. directly overhead 30  Where would the observer look to see the star indicated by the arrow? Assume they’re in the Northern Hemisphere Celestial Sphere Rotation Star B 2 Star A 1 2 A. A. High in the Northeast Celestial Sphere Celestial Sph 3 B. B. High in the Southeast 1 C. C. High in the Northwest 4 D. D. High in the Southwest 3 Horizon 4 Celestial Sphere Rotation Figure 2 31  Where would the observer look to see the star indicated by the arrow? Assume they’re in the Northern Hemisphere Celestial Sphere Rotation Star B 2 Star A 1 2 A. A. High in the Northeast Celestial Sphere Celestial Sph 3 B. B. High in the Southeast 1 C. C. High in the Northwest 4 D. D. High in the Southwest 3 Horizon 4 Celestial Sphere Rotation Figure 2 32  Three Views You should be able to go back and forth between three different views of the sky: 1) Above Earth 2) Above the viewer 3) Viewer’s perspective 33  The Local Sky An object's altitude (above horizon) and direction (along horizon) specify its location in your local sky. 34  Azimuth is the angular distance of an object from the local North, measured along the horizon. An object which is due North has azimuth = 0 degrees; due East is azimuth = 90 degrees; due South is azimuth = 180 degrees; due West is azimuth = 270 degrees. 35 We Measure The Sky Using Angles 36  Back to the title slide… This is an example of star trails… Look at how the stars appear to make big circles. What object is at the “center” of those circle motions? 37  Back to the title slide… This is an example of star trails… Look at how the stars appear to make big circles. What object is at the “center” of those circle motions? —> POLARIS!!! 38 The angle height of the North Star (Polaris) above the horizon is called the altitude. The altitude of Polaris is equal to the latitude that you are at. The Grand Canyon is at 36 degrees North latitude, so Polaris is 36 degrees above the horizon in the Grand Canyon. So what does the sky look like at the Earth’s North Pole? What about at the equator? ALTITUDE: 36 DEGREES 39  How Does the Orientation of Earth’s Axis Change with Time? Although the axis seems xed 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; for example, spring equinox, once in Aries, is now in Pisces! Earth’s axis precesses like the axis of a spinning top 40 fi  The Celestial Sphere: Observer-Centered Terms… Most stars rise above the horizon in the East, follow circular arcs through the sky, and set in the West. This follows a 24 hour cycle. 41  The Celestial Sphere: Observer-Centered Terms… Stars near enough to the celestial pole in your hemisphere never set and are said to be circumpolar. They still follow a 24 hour cycle. 42 A Different View The zenith and horizon move with the observer The celestial poles and equator are oriented relative to the Earth The observed altitude of the celestial poles changes as the observer moves 43  The Celestial Sphere at different locations on Earth… 44  The Celestial Sphere: star motions If you are having dif culty picturing star trails or how the view would change at different latitudes, I strongly suggest you spend time using the Rotating Sky Explorer: http://astro.unl.edu/naap/motion2/animations/ce_hc.html 45 fi  True North vs Magnetic North… The north magnetic pole is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic eld points vertically downwards (in other words, if a magnetic compass needle is allowed to rotate in three dimensions, it will point straight down). 46 fi  True North vs Magnetic North… As a rst-order approximation, Earth's magnetic eld can be modeled as a simple dipole (like a bar magnet), tilted about 10° with respect to Earth's rotation axis (which de nes the geographic north and geographic south poles) and centered at Earth's center. 47 fi fi fi  True North vs Magnetic North… — SUPPLEMENTAL Earth’s geographic north pole – and magnetic north pole – were rst recognized as two different places in 1831. Until the early 1990s, the magnetic North Pole was known to lie some 1,000 miles south of true north, in Canada. Yet, as scientists realized, the location of magnetic north was not xed. Magnetic north was drifting at a rate of up to about 9 miles (15 km) a year. Since the 1990s, however, the drift of Earth’s magnetic north pole has turned into “more of a sprint,” scientists say. Its present speed is about 30 to nearly 40 miles a year (50-60 km a year) toward Siberia. 48 fi fi  Magnetic pole reversals… — SUPPLEMENTAL Over the life of Earth, the orientation of Earth's magnetic eld has reversed many times, with magnetic north becoming magnetic south and vice versa – an event known as a geomagnetic reversal. Evidence of geomagnetic reversals can be seen at mid-ocean ridges where tectonic plates move apart and the seabed is lled in with magma. As the magma seeps out of the mantle, cools, and solidi es into igneous rock, it is imprinted with a record of the direction of the magnetic eld at the time that the magma cooled 49 fi fi fi fi  Magnetic pole reversals… — SUPPLEMENTAL A reversal happens over hundreds or thousands of years, and it is not exactly a clean back ip. Magnetic elds morph and push and pull at one another, with multiple poles emerging at odd latitudes throughout the process. Scientists estimate reversals have happened at least hundreds of times over the past three billion years. 50 fl fi  Magnetic pole reversals… — SUPPLEMENTAL It is believed that the movement of molten iron in Earth’s core is responsible for the creation of Earth’s magnetic elds, and the reversals of the magnetic poles result from the complicated dynamics of this molten material — however the exact cause/ triggers of these reversals is an active area of research… 51 fi  Looking North (from the Northern Hemisphere). Some of these stars are circumpolar, never rising or setting. 52  Looking East (from the Northern Hemisphere) Looking East (from the Northern Hemisphere). Stars rise in the East…. 53  Looking South (from the Northern Hemisphere) Looking South (from the Northern Hemisphere). Stars are high in the Southern sky, moving from East to West. 54 What direction is the camera facing in this picture? (Taken from the northern hemisphere) 55 WEST 56  Which direction is the observer facing in this picture (photo was taken from the northern hemisphere)? a) North b) East c) South d) West 57  Which direction is the observer facing in this picture (photo was taken from the northern hemisphere)? a) North b) East c) South d) West 58 How long did it take to get this picture? A: 6 hours B: 12 hours C: 24 hours D: Impossible to determine 59 How long did it take to get this picture? A: 6 hours B: 12 hours C: 24 hours D: Impossible to determine 60 What causes the motion of the celestial sphere throughout the night? a) The orbit of the Earth around the Sun b) The motion of the stars orbiting the Earth c) The rotation of the Earth on its axis d) The Moon 61  Length of a Day Solar Day —> rotation period of Earth with respect to the Sun (24 hours on average, varies by +/- 25 seconds) Siderial Day —> rotation period of Earth with respect to the stars (Earth’s precise rotation period) A solar day is 4 minutes longer than a sidereal day because Earth not only turns, but also moves along its path around the Sun 62  Revolution versus Rotation Motion #1: The daily motion of the stars in the sky is caused by the rotation of the Earth on its axis. This causes the daily motion of the star across the sky. Mo on #2: The yearly mo on of the Earth around the Sun is called revolu on. This causes the change in which stars are visible at night throughout the year. 63 ti ti ti Earth Revolves around the Sun Motion #1 – Earth rotating (daily) Mo on #2 – Earth revolving around the Sun (yearly) Takes 365.242 days We have a “leap year” every four years - 365.25 days on average - Unless if year divisible by 100 - > 365.240 - Unless divisible by 40 -> 365.242 64 ti  The Size of Earth’s Orbit Earth’s orbit is nearly circular (within a couple percent). The average distance to the Sun is called the astronomical unit, or AU. 1.5 x 1011m 67 fi The Ecliptic The Sun’s position is always found on the ecliptic The signs of the zodiac are along the ecliptic All planets and even the moon are found near ecliptic – the plane of the Solar system The Earth’s axis is tilted 23.5 degrees w.r.t its orbital path, so the ecliptic is inclined 23.5 degrees w.r.t the celestial equator 68  Signs of the Zodiac Do you know your astrological sign? (a.k.a. sign of the zodiac) Astrology is pseudoscience, but it has roots in astronomical observation. SUPPLEMENTAL 69  Signs of the Zodiac The Babilonians rst identi ed the signs of the zodiac around 700 B.C. Important: astrology, which uses the signs of the zodiac, is not science! Do not confuse it with astronomy! SUPPLEMENTAL 70 fi fi The Ecliptic The astrological signs represent the constellations that the Sun “appears” in throughout the year. But that also means that our night time constellations change during year. 71  The Zodiac As Earth revolves about the Sun, the Sun is seen against different constellations. These constellations are called the signs of the zodiac. The constellations (and stars) we see at night depends on the season (motion #2). “Winter” constellations are up during the day in summer. 72  Sun and the Celestial Sphere Sun xed to celestial sphere during one day (almost). The Sun follows the eclip c throughout the year, not the day (Stars and constellations shown here in their correct locations, but they cannot be viewed with the naked eye during the day, as shown) 73 fi ti  VISUALIZATION PRACTICE http://www.solarsystemscope.com/ 1 day/frame for ecliptic Notice: 1) Sun’s position with respect to the signs of the zodiac and how this changes throughout the year 2) Sun’s location with respect to the signs of the zodiac does not change throughout one day 74 Seasons? 75  What Causes The Seasons? Seasons depend on how Earth's axis affects the directness of sunlight & how concentrated light energy is 76 Why Doesn't Distance Matter? Variation of Earth–Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt. Variation in any season of each hemisphere— Sun distance is even smaller! 77 Solstices and Equinoxes ◦ Summer solstice Sun farthest north (highest in the sky, more direct, longest day), ~June 21. ◦ Fall (autumnal) equinox Sun on the equator (roughly equal night and day), ~Sept 21. ◦ Winter solstice: Sun farthest south (lowest in the sky, less direct, shortest day), ~Dec 21. ◦ Spring (vernal) equinox Sun on the equator (roughly equal night and day), ~Mar 21. ◦ Equinoxes are the only two days when the Sun precisely rises East and sets West. 78 Path of the Sun Throughout the Year 1) The Sun is higher in the summer than in the winter 2) The days are longer in the summer than in the winter 3) The Sun will rise further to the north in summer These are all interrelated! SUPPLEMENTAL  SUPPLEMENTAL Position of Sun with Latitude For the Northern hemisphere, the highest and lowest point of the Sun at noon are: Highest: 90–latitude+23.5 Lowest: 90–latitude-23.5 (In Southern hemisphere, the latitudes are negative, and are added to 90 deg.)  SUPPLEMENTAL Position of Sun with Latitude So from Binghamton (latitude = 42 deg.), these are: 90 – 42 + 23.5 = 71.5 deg. 90 – 42 - 23.5 = 24.5 deg. (In Southern hemisphere, the latitudes are negative, and are added to 90 deg.) Position of Sun with Latitude The Sun can be directly overhead at the Tropic of Capricorn (latitude = +23.5 deg.) and the Tropic of Cancer (latitude = -23.5 deg), but is never directly overhead north or south of the tropics. Highest (tropic of cancer at noon): 90 – 23.5 + 23.5 = 90 deg Position of Sun with Latitude Above the Arctic Circle (latitude > 66.5) and below the Antarctic Circle), the Celestial equator is within 23.5 degrees of the horizon – Endless day (in summer) and night (in winter) Lowest (arctic circle at noon): 90–66.5-23.5 = 0 deg.  Analemma In astronomy, an analemma is a diagram showing the position of the Sun in the sky as seen from a xed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a gure eight. Afternoon analemma photo taken in 1998–99 in Murray Hill, New Jersey, USA, by Jack Fishburn. The Bell Laboratories building is in the foreground. 84 fi fi  Note the height of the analemma is controlled by axial tilt, width by the elliptical orbit of the earth, and angular skew of gure 8 by the latitude of the observer. 85 fi If the ecliptic were aligned with the celestial equator, what would happen to the seasons? a) Nothing. They would be the same as now. b) Each season would last longer. c) We would not have seasons at all. d) Can’t tell from the information given. If the ecliptic were aligned with the celestial equator, what would happen to the seasons? a) Nothing. They would be the same as now. b) Each season would last longer. c) We would not have seasons at all. d) Can’t tell from the information given. The Moon & Eclipses 88  SUPPLEMENTAL Collision That Formed The Moon https://www.youtube.com/watch?v=o2lRpiediP8  Apollo 8 Apollo 8 (December 21–27, 1968) was the rst crewed spacecraft to leave low Earth orbit, and also the rst human space ight to reach another astronomical object, namely the Moon, which the crew orbited without landing, and then departed safely back to Earth. These three astronauts—Frank Borman, James Lovell, and William Anders—were the rst humans to witness 90 and photograph an Earthrise. fi fi fl fi  Apollo 11 Apollo 11 (July 16–24, 1969) was the space ight that rst landed humans on the Moon. Commander Neil Armstrong and lunar module pilot Buzz Aldrin formed the American crew that landed the Apollo Lunar Module Eagle on July 20, 1969, at 20:17 UTC (14:17 CST) See full list of Apollo missions here: https://en.wikipedia.org/wiki/List_of_Apollo_missions 91 fl fi  The Moon (the 2nd brightest object in the sky) Revolves around the Earth every 29.5 days. The word moon is the origin of the word month. – Some cultures still follow a moon month (kinda) It rotates about its axis every 29.5 days. Therefore, the same side always faces the Earth. It doesn’t shine under its own power, but reflects the sun’s light The Motion of the Moon Moon’s cycle can be measured in solar or “synodic” months (~29.5 solar days, one twelfth of a solar year) or slightly shorter sidereal months (~27.3 solar days, the time it takes the moon to orbit once around the earth with respect to the stars). The phases of the moon cycle through once per solar month, but the Moon completes a full orbit of Earth once per sidereal month. The Moon rotates about its own axis once per sidereal month! The Moon is said to be “tidally locked” to Earth or “synchronously rotating”. Tidal locking ensures that we always see the same face of the Moon from Earth  SUPPLEMENTAL The Witching Hour In folklore, the “witching hour” or devil's hour is a time of night, typically between midnight and 1 am, that is associated with supernatural events, whereby witches, demons and ghosts are thought to appear and be at their most powerful. Since the earliest days of recorded history, the Moon has been believed to have a powerful in uence over human and animal behavior. To the Romans, staring at a full Moon was thought to drive a person crazy – hence the term “lunatic”. Farmers in the past would plant their crops “by the moon”, which meant sowing their seeds in accordance with the Moon’s phases in 94 the hopes of getting a better harvest. fl Phase of the Moon: Order & Length  The Moon We can only see the other side of The same side always faces Earth. the Moon by sending spacecraft It’s orbit is “tidally locked” out to look at it. http://antwrp.gsfc.nasa.gov/apod/image/9911/lunation_ajc.gif “Dark” vs. “Light” Side Near Side Far Side The differences in the Moon’s appearance from one night to the next are due to changing illumination by the Sun, not to its own rotation. The back side is dark no more frequently than the front side. Since the Moon rotates, the Sun rises and sets on all sides of the Moon. 99 Question 100 Answer 101  While half of the Moon is always lit by the Sun, we see different amounts of the lit half from Earth depending on where the Moon is located in its orbit. Phases of the Moon Waxing Gets “fuller” Waning Gets “less full”  Moon Phases Not to scale! The Moon is actually 30 Earth-diameters away from us. The Moon’s orbit is also tilted relative to the path of the Sun in the sky  The Moon orbits Earth counterclockwise (as seen from above) once a month. It is mostly in the plane of the ecliptic, so to see it you need to look in the southern sky. Remember that the Earth is rotating! It rotates ~28 times for every one Moon orbit. Therefore… One half of a the Moon’s orbit represents about 2 weeks. One quarter of the Moon’s orbit represents about 1 week. There are about 3.5 days between each phase shown. The “Meridian Times” are when the Moon is at its highest point in the sky (i.e. above due South). Examples: The meridian time for the First Quarter moon is 6pm. The meridian time for the Full Moon is midnight. The meridian time for the New Moon is Noon. The Meridian time for the Waxing Crescent Moon is 3pm. 107 @6PM Eclipses of the Sun & Moon 110 Eclipse Requirements: Orbital Alignment 111 Eclipse Requirements: Orbital Alignment 112 Eclipse Requirements: Orbital Alignment 113 Eclipse Requirements: Shadows 2,3,4 are in the penumbra 114 What causes eclipses? The Earth and Moon both cast shadows. When either passes through the other’s shadow, we have an eclipse. In a lunar eclipse, the Earth’s shadow obscures the Moon This is always the geometry of a lunar eclipse: Earth in between Sun and Moon Why don’t eclipses occur all the time? The Moon’s orbit is inclined 5 degrees with respect to the ecliptic. Eclipses can occur only when the plane of the Moon’s orbit crosses the ecliptic. Total solar eclipses only occur when Moon is closest to the Earth.  Lunar Eclipses When there is a Full Moon and the Moon intersects the ecliptic, you get a lunar eclipse.  Lunar Eclipses Occur when the shadow of the Earth falls on the Moon. Occur only when the Moon is Full.  Solar Eclipses: Moon’s Shadow When there is a New Moon and the Moon intersects the ecliptic, you get a solar eclipse. The Sun and Moon have very nearly the same angular size in the sky because though the Moon is ~400 times smaller than the Sun it is ~400 times closer to Earth than the Sun. The distance between the Moon and Earth varies a little bit, and if the Moon is far enough away during a solar eclipse, you can get an “annular eclipse”  Not All New Moons Cause Solar Eclipses The Moon’s orbit is inclined by about 5 degrees relative to the ecliptic so most new moons are not perfectly between the Earth and Sun.  Solar Corona A faint outer layer of the Sun’s atmosphere called the corona becomes visible when the Moon blocks out the main part of the Sun during a solar eclipse. Solar Eclipses Occur when the shadow of the Moon falls on the Earth Geometry of Solar eclipse: Very rare! always Moon in between Occur only when the Moon is in its Earth and Sun. New phase. Only get total eclipses because Moon and Sun have similar apparent sizes. Partial Eclipse Annular Eclipse Total Eclipse When can a Solar Eclipse Occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. SUPPLEMENTAL 125 The gravitational tug of the Moon produces two “tidal bulges” on the Earth, one on the side of Earth facing the Moon and one on the opposite side. The surface of the Earth rotates under each of these tidal bulges every day, leading to two high tides and two low tides daily. 126  Tides Tides are strongest when gravitational forces line up, i.e., the earth-sun-moon system is in a straight line 127 Spring and Neap Tides Spring and Neap Tides Spring and Neap Tides Spring and Neap Tides SUPPLEMENTAL  Reading/HW Assignment Read Chapters 2 & 3 by Tuesday, August 27 First homework assignment on mastering astronomy will be assigned in the next few days to be due next Friday, August 30, at 5PM I will upload an announcement to Brightspace and send an email when HW 1 is available 133