Transcript Solar Day
Info for the Test Bring a #2 pencil. No electronic devices: No cell phones, headphones, etc. No books, notes, etc. No hats. Grades will be posted on the course website. Studying Study questions on website highlight important topics Know definitions of BOLD terms in chapter summaries Questions from lectures will give you some idea of what questions I may ask - I may reuse or slightly modify some of these! This review will focus on the more complex topics we have gone over This review will not cover everything from the lectures and text The "Solar Day" and the "Sidereal Day" Solar Day How long it takes for the Sun to return to the same position in the sky (24 hours). Sidereal Day How long it takes for the Earth to rotate 360o on its axis. These are not the same! Which is longer? Why? One solar day later, the Earth has rotated slightly more than 360o A solar day is longer than a sidereal day by 3.9 minutes. (24 hours vs. 23 hours 56 minutes 4.091 seconds) Difference due to rotation and revolution of Earth. The Earth's Seasons Why does the Earth have seasons? How is this related to how high the Sun rises in the sky in Summer? In Winter? Seasons: Due to tilt of Earth's Axis of Rotation Summer Winter In winter, the sun never gets very high in the sky => each bit of ground receives less radiation => cooler Seasons caused by tilt of Earth’s axis of rotation. Earth is farthest from sun during summer. What motion of the Earth acts to slowly change the orientation of the Earth's axis of rotation? How long does one cycle of this motion take? Precession Earth Moon Vega * * Polaris Spin axis Precession Period 26,000 years! Parallax How does the parallax angle vary with the distance to the foreground object? How does the parallax angle vary with the length of the baseline? Parallax angle: Decreases with distance to the foreground object. Increases with the length of the baseline. Eclipses During which phase(s) can a lunar eclipse occur? What about a solar eclipse? How do the angular diameters of the moon and the Sun compare and how do we know? Geocentric vs. Heliocentric Models The geocentric model explained the retrograde motion of the planets by introducing _______? The geocentric model was finally abandoned because it was unable to explain the observed ______ of Venus. Apparent motion of Mars against "fixed" stars. Mars July 7 * Earth 7 * 6 6 5 3 4 4 5 3 2 2 * 1 1 January * * * Geocentric model fails to account for phases of Venus Heliocentric model easily accounts for phases of Venus Review: Kepler's Laws 1. Planets travel aound the sun in elliptical orbits with the sun at one focus of the ellipse. 2. A line connecting the Sun and a planet sweeps out equal areas in equal times. 3. The square of a planet's orbital period is proportional to the cube of its semi-major axis. (P2 α a3) Review: Newton's Laws of Motion 1. Every object continues in a state of rest or a state of uniform motion in a straight line unless acted on by a force. (Inertia) 2. F = m*a 3. For every action there is an equal and opposite reaction. G m1 Newton's Law of Gravity: F = m2 R2 Orbit of Earth around Sun Gravity and Orbits • Throwing an object fast enough will put the object into orbit! (Neglecting air resistance) • Moon is continually “falling” towards the Earth in its orbit (Gravity vs. inertia) Correction to Kepler’s Third Law Earth and sun actually rotate about their common center of mass Corresponds to a point inside sun Used to detect extrasolar planets Question The acceleration experienced by a falling object due to Earth's gravity _____ the mass of the object. A) increases with B) decreases with C) is independent of That one was complicated, because we need to remember: Fg = G m mE R2 AND Fg a = m So, a = G m mE m R2 = G mE R2 Chapter 2: EM Waves What is a wave? What are the main properties of waves? What two things do all waves transport? Waves are a type of disturbance that can propagate or travel. Waves carry information and energy. Properties of a wave wavelength () crest Equilibrium position amplitude (A) trough is a distance, so its units are m, cm, or mm, etc. Period (T): time between crest (or trough) passages Frequency (f): rate of passage of crests (or troughs), f 1 T (units: Hertz or cycles/sec) velocity (v) v = f All radiation (including visible light) travels as Electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: What are some examples of objects with electric fields: All radiation (including visible light) travels as Electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: a bar magnet the Earth the Sun What are some examples of objects with electric fields: Electrical appliances Lightning Protons "charged" particles that make up atoms. electrons } Anything with too many or to few electrons! Electric Force opposites attract, likes repel Oscillating charges radiate All objects have temperatures greater than absolute zero random thermal motion All objects radiate! Why don’t we see the radiation coming from many ordinary objects? What is the radiation spectrum displayed by most astronomical objects called? What property of the spectrum tells us the temperature of the object? Overall shape of blackbody curve is the SAME for objects at ALL temperatures! "cold" dust "cool" star Sun "hot" stars frequency increases, wavelength decreases Emission and Absorption Spectra How are the emission and absorption spectra of a certain element related? How do the emission (or absorption) spectra of two different elements compare? For a given element, emission and absorption lines occur at the same wavelengths! Each contains same information and serves as a unique fingerprint for that element. Sodium emission and absorption spectra When an atom absorbs a photon, it moves to a higher energy state briefly. When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system. Doppler Shift How can the doppler shift be used to detect extrasolar planets? What type of velocity information does this effect give us? • Star with extrasolar planet wobbles around common center of mass. • Causes small Doppler shift of its absorption lines. • Only gives information about velocity along line of sight! Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them? Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them? Light (see) Infrared (heat) Ultraviolet (sunburn) Microwaves (cooking, communication) AM radio (communication) FM radio (communication) TV signals (communication) Cell phone signals (communication) X-rays (medical applications)