Looking at the Night Sky How to find your way around: Position -> where is that object? Distance -> how much space between these two things? Motion -> where will that object be later tonight? Bright/faint objects -> magnitude! Using Angles Distances on the sky are measured in, -, and - 1 degree = arc-min 1 arc-min = arc-sec idth of index finger = º idth of three fingers = º idth of entire hand = º arth s rotation causes the Sun, Planets, Moon and stars to appear to move when viewed from arth Rising and Setting Stars The arth s ward rotation causes stars to appear to move ward. Stars near the move in small circles, and are called Stars far from the pole move in long arcs Nightly Motion of the Stars Imagine looking toward the North. hat do stars appear to do over the course of an evening? hat about stars in the South? ast? est? Directly overhead? Nightly Motion of the Stars For stars (and Moon and planets) that appear in the ern sky: Stars first rise near the horizon, move upward and toward the, and then move down and set near the horizon. 1
Nightly Motion of the Stars Looking North: Stars appear to move counterclockwise around the stationary North Star ( ) we call these stars. Looking North: Circumpolar Stars Circumpolar stars seem to move counterclockwise around the stationary. These constellations and stars are visible any night of the year in the NORTHRN sky because they never rise or set! xamples: Ursa Major, Ursa Minor, Draco, Cepheus, and Cassiopeia How long did it take to get this picture? Motion Lecture Tutorial pg. 3 ork with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the Lecture Tutorial is asking, ask me for help. Measuring Stars: Magnitudes Constellations help us find our way in the sky, but we want to know more Ancient people classified stars into 6 groups: Magnitude stars were the brightest Magnitude stars were the faintest A modified version of this system is still used Magnitudes System ay to group stars by intensity/brightness Brighter star = magnitude Fainter star = magnitude Difference of 1 in magnitude < -- > factor of 2.5 in brightness X: Mag. 3 star vs. mag. 6 star : Mag. 6 star is 2.5 x 2.5 x 2.5 times fainter = times fainter 2
xamples of Magnitudes hat can you see? hat we can see in the sky (constellations) at any given time is determined by three factors: Time of night/day Review: Coordinates on the arth : position north or of equator : position or of prime (runs through Greenwich, ngland) The sky varies with but not. Altitude of the celestial pole = hy do the constellations we see depend on latitude and time of year? They depend on latitude because your position on arth determines. They depend on time of year because arth s. 3
hy can t we see the same constellations all year round? The sky varies as arth orbits the Sun As the arth orbits the Sun, the Sun appears to move along the ecliptic. At midnight, the stars high in the sky are the Sun in the sky. Constellations along the Sun s path The path of the Sun ( ) was an important part of ancient astronomy The planets and Sun represented deities, and their positions were significant Most ancient astronomers were also in the astrology business Zodiac - The 13 Zodiacal constellations that our Sun covers-up (blocks) in the course of one year (used to be only 12) North Star The Zodiacal Constellations that our Sun covers-up (blocks) in the course of one year (only 12 are shown here) 4
North Star North Star hat time is it for the observer? South Figure 2 hat it would look like if you were the observer in hich constellation is the Sun in front of for the situation shown? South Figure 2 hat it would look like if you were the observer in hat is the name of the constellation that would appear on the observers astern Horizon? estern? North Star South Figure 2 hat it would look like if you were the observer in 12 hours later what object will be at the position that is in now? 18 hours later where will the Sun be? here will be? Lecture-Tutorial: Seasonal Stars (page 7) ork with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Come to a consensus answer you all agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the Lecture Tutorial is asking, ask me for help. The changing position of the Sun during the year! How can we describe the motion of the sun in the sky? X Zenith or overhead 5
How can we describe the motion of the sun in the sky? here is the Sun at noon today? LUNCH!! here or there? AM or ante PM or post hat about the path of the Sun? here is the Sun throughout the year? there hat about tomorrow at noon? Mar 21 (Vernal) and Sept 21 (Autumnal) quinox June 21 Summer Solstice JUST A LITTL LOR!! Dec 21 inter Solstice Position of the Sun when photographed during the day of the winter and summer solstice Altitude of the Sun Animation: As we watch the animation, pay attention to here the sun rises and sets Altitude of the sun at mid-day Also, watch how the shadow changes during a day and over the course of the year. 6
The Reason for the Seasons Hypothesis: Seasons are caused by the arth moving closer to or farther from the Sun. 1. Data: hen things are to the a light source (like the Sun) they get. 2. Data: Northern hemisphere has summer in,, and ; Southern hemisphere has summer in, and 3. Data: arth s distance from the Sun hardly changes in the course of a year. (about %) arth is closest to the Sun on Can #2 be true if our hypothesis is correct? Reason for the Seasons The seasons are caused by the tilt of the arth s axis In Summer, the Sun s rays are and the days are In inter, the Sun s rays are and the days are The Seasons do NOT occur because the arth gets closer or farther from the Sun Reason #1: More or Less Intense Sunlight A beam of light shines on a smaller area than a beam. So a beam is more intense Reason #2: Longer or Shorter Days / 7