Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 0 Charting the Heavens Lecture Presentation
0.0 Astronmy a why is that subject! Q. What rare astronomical event happened in late summer 2017 in the US? A. A total solar eclipse! (but what does: Total and Solar and Eclipse mean?)
0.1 The Obvious View Universe: Totality of all space, time, matter, and energy Astronomy: Study of the universe But be warned: you will probably find that the universe is more amazing than almost anything else you know! So let s get started!
0.1 The Obvious View About 3000 stars are visible at any one time. Humans see patterns that we call constellations. Constellations have meaning to those doing the grouping but differ from group to group! Hmmm! Useful: Polaris, which is almost due north, is called the north star. Astrology makes predictions about individuals based on the star patterns at their birth: what do you think?
0.1 The Obvious View Time lapse photo looking north. What does this tell us? What are the streaks in the sky? Why are some streaks longer than others?
0.1 The Obvious View Stars, eg in constellation Orion, that appear close in the sky may not actually be close in space. Why is that!? Because they are so very far away!
0.1 The Obvious View The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth. They aren t, but we can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects. The Earth s rotation makes the stars appear to move: 15 degrees/hour
0.1 The Obvious View Declination (like latitude): Degrees north or south of celestial equator. Right ascension (like longitude): Measured in hours, minutes and seconds with 1 hour = 15 degrees.
More Precisely 0.1: Angular Measure Full circle contains 360º (degrees). Each degree contains 60 (arc minutes). Each arc minute contains 60 (arc seconds). Angular size of an object depends on its actual size and distance from Earth.
0.1 The Obvious View Measuring angles in the sky: eg the angular size of the Moon is about ½ degree!
0.2 Earth s Orbital Motion Earth s daily cycle (due to Earth s rotation on its axis): Sun overhead to Sun overhead, is diurnal motion solar day. Stars aren t in quite the same place 24 hours later, though, due to Earth s revolution around the Sun; when stars are in the same place again, one sidereal day has passed: Is this a shorter or longer period of time? Only astronomers care about this!
0.2 Earth s Orbital Motion Seasonal changes to the night sky are due to Earth s revolution around Sun! Why do we see different constellations in the summer VS in the winter?
0.2 Earth s Orbital Motion As the Earth revolves around the Sun the part of the celestial sphere that is in night time changes! As stars are only visible at night then different stars are visible at different times of the year. As radio telescopes (Chapt 3) do not care if it is day or night they can observe eg Orion every day: but sometimes it is in daytime and sometimes it is when it is night time!
0.2 Earth s Orbital Motion The entire sky is divided into 88 (modern day) constellations: see figure. Today this is mostly for convenience: think how we use the fact that the continental US is divided into 48 states! The Sun s path is the ecliptic (red dotted line), which traces through 12 of the constellations.
0.2 Earth s Orbital Motion The 12 constellations the Sun appears to move through are called the zodiac.
0.2 Earth s Orbital Motion Ecliptic is (also) plane of Earth s path around the Sun. It is tilted by 23.5º to celestial equator because Earth s rotation axis is tilted by 23.5 o. Sun s northern-most point (above celestial equator) is summer solstice; southern-most is winter solstice; points where path crosses celestial equator are vernal and autumnal equinoxes. Combination of day length and sunlight angle gives seasons. Time from one vernal equinox to next is tropical year.
0.2 Earth s Orbital Motion The Earth s rotation axis points in a fixed direction, BUT because it is not at 90 degrees to the plane of the Earth s orbit, sometimes the northern hemisphere has the Sun more overhead and sometimes the southern hemisphere has the Sun more overhead (see previous page)!
0.2 Earth s Orbital Motion Time from one vernal equinox to next, i.e. from one beginning of spring to the next is set by the position of the Sun as viewed from the Earth, is called a tropical year. Time for Earth to orbit once around the Sun, but set by the position of the Sun with respect to the fixed stars, is called a sidereal year. Tropical year follows seasons (so we care about this!); sidereal year follows distant stars (only astronomers care about this!).
0.3 The Motion of the Moon The Moon takes about 29.5 days to go through whole cycle of phases the synodic month. Phases are due to different amounts of sunlit portion of the Moon being visible from Earth. To see tonight s phase go to: http:// www.moongiant.com/ phase/today Note: for the Moon to revolve around the Earth, aka sidereal month, the time is ~2 days shorter than synodic month only astronomers care about this!
0.3 The Motion of the Moon For those super nerdy: the Moon s sidereal and synodic months are analogous to Earth s sidereal and solar days. As we care about Moon phases we care about the longer 29.5 day synodic month!
0.3 Motion of the Moon Eclipses occur when Earth, Moon, and Sun form a straight line but most importantly when one (Earth or Moon) is in the shadow cast by the other!
0.3 The Motion of the Moon Lunar eclipse: Occurs when Earth is between the Moon and Sun. A partial eclipse occurs when only part of the Moon is in shadow. A total eclipse occurs when all the Moon is in shadow.
0.3 The Motion of the Moon Solar eclipse: The Moon is between Earth and Sun. Because the umbra shadow is tapered and the moon s orbit is not round: some eclipses are annular!
0.3 The Motion of the Moon A solar eclipse is partial when only part of the Sun is blocked, total when all is blocked, and annular when the Moon is too far from Earth for total.
0.3 The Motion of the Moon Eclipses don t occur every month because Earth s and the Moon s orbits are not in the same plane. Thus only two points on the Earth s orbit are favorable for eclipses! Can you find these in this figure?
0.3 The Motion of the Moon (Predicted and actual) Solar Eclipse tracks (locations on Earth for viewing solar eclipses), 2010 2030. See the Aug 21 eclipse: https://www.youtube.com/watch?v=g10m2zzrh4u
0.5 Science and the Scientific Method Scientific method: Observations à theory à predictions then make new observations. Do new observations agree with predictions or not? Then what do you do? Scientific theories: Must be testable (i.e. allow predictions to be made for observable(s) that can be tested by observations). Should be continually tested: particularly as the quality of observations improve. Should be simple and elegant not essential but deep understanding usually finds simple, elegant underlying truths Scientific theories can be proven wrong, but they can never be proven right with 100% certainty Is that a reason to 2017 Pearson discount Education, Inc. (or ignore) them?
0.5 Science and the Scientific Method So: observations naturally lead to a theory to explaining them. Theory leads to predictions consistent with previous observations. Predictions of new phenomena are then observed. If the observations agree with the prediction, more predictions should be made. If not, a new theory should be made. Sounds simple; also profound! Try it!
Summary of Chapter 0 Astronomy: Study of the universe Stars can be imagined to be on inside of celestial sphere; useful for describing location. Plane of Earth s orbit around Sun is ecliptic, at 23.5º to celestial equator. Angle of Earth s axis causes seasons. Moon shines by reflected light, has phases. Solar day sidereal day, due to Earth s revolution around Sun.
Summary of Chapter 0 (con t) Synodic (i.e. calendar) month sidereal month, also due to Earth s revolution (in orbit) around Sun. Because the Earth moves (revolves around the Sun) the Sun appears to be in different zodiac constellations at different times of the year. Eclipses of Sun and Moon only occur occasionally as orbits are not in same plane. Scientific method: Observation, theory, prediction, observation