The Celestial Sphere. The Milky Way. Astronomy 110 Announcements: How do we locate objects in the sky?

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1 Astronomy 110 Announcements: The Celestial Sphere No class next Monday Memorial Day Homework #1 due Tuesday come see me with questions. Reading for Tuesday is very light: ECP pp There will not be a reading quiz on Tuesday since the homework is due. The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy. How do we locate objects in the sky? (1) Know your reference points. (2) Locate an object by its altitude (above horizon) and direction (along horizon)

2 Why do stars rise and set? We measure the sky in angles... Earth rotates according to the right hand rule counter clockwise. Thus, the stars appear to move in the opposite direction: from East to West. From the Northern Hemisphere: 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. Celestial Equator Your horizon Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator

3 What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator Why don t we see the same constellations throughout the year? 1. Depends on whether you stay home: Constellations vary with latitude. 2. Depends on time of year: Constellations vary as Earth orbits the Sun. Review: Coordinates on the Earth Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England) The sky varies with latitude but not longitude.

4 altitude of the celestial pole = your latitude 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 the in the sky. What have we learned? What are constellations? A region of the sky; every position on the sky belongs to one of 88 constellations. How do we locate objects in the sky? By its altitude above the horizon and its direction along the horizon. What have we learned? Why do stars rise and set? Because of Earth s rotation. Why don t we see the same constellations throughout the year? The sky varies with latitude. The night sky changes as Earth orbits the Sun.

5 What was once so mysterious about planetary motion in our sky? We see apparent retrograde motion when we pass by a planet in its orbit. Planets usually move eastward from night to night relative to the stars. You cannot see this motion on a single night; rather, planets rise in the east and set in the west. But sometimes they go westward for a few weeks or months: apparent retrograde motion Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we lap another planet (or when Mercury or Venus lap 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.

6 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 Sun; it 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. Polynesian Voyaging Non-Instrument Navigation How do you navigate without the use of a compass, watch, GPS, etc.?! By the stars, of course! What kinds of things do we need to determine in order to navigate? Direction (N,S,E,W) Location (Latitude, Longitude) Speed IDEA: sail until you reach the proper latitude (easy to determine from the Sun and stars), then sail E-W along that latitude until you reach land. Other important points of reference: ocean swells winds seamarks seabirds

7 Sun Determine direction from Sun most useful when sun is near the horizon (harder to trace its path back as it gets higher in the sky). Knowledge of the sun s movement through the ecliptic and how its path across the sky changes with the seasons can help you identify direction precisely. Align rising and setting sun with marks on rails of the canoe. Stars Observations of the stars can give you both direction and latitude. Stars - Direction North Star - Hokupa a: fixed in the sky (NCP). Always know where north is as long as you re above the equator. Direction of rising and setting stars tells you where E and W are Meridian Pairs pair of stars that align N-S along the meridian 10 pairs visible in N 11 pairs visible in S (example Southern Cross) Stars - Latitude Zenith Stars each island has its own zenith star, the star that is directly overhead when it crosses the meridian (depends on latitude of island) Hawaii s star is Hokule a (Arcturus) at 19 N Tahiti s star is Sirius at 17.5 N Angle (with respect to the zenith) that stars make in their rise and set patterns gives latitude. Setting pairs pair of stars that set together only at a certain latitude (ex: Sirius and Pollux in Tahiti) Special stars Hokupa a (at NCP) tells you what latitude you re at by determining its altitude; Southern Cross when the distance between the top and bottom stars equals the distance between the bottom star and the horizon, you ve reached the latitude of Hawaii

8 Planets (Hoku hele traveling stars ) Move along ecliptic (like the sun) Good for finding direction at night bright Other methods of navigation What if it s cloudy? Need something that doesn t depend on your ability to see the sky Ocean swells (generally stable for long periods, as long as no major storm systems are moving in and competing with them) Wind not as stable as the ocean swells Seamarks schools of fish, whales, etc. Seabirds observe their flight direction: go out to sea in the morning to hunt, return to land at night. Finally determine speed by timing bubbles passing the canoe. Keep track of time by Sun (ex: 6 hrs from noon to sunset) and stars (move 15 /hr). Distance = speed * time