Section 3 Mapping the Stars

1. Section 3Mapping the Stars

2. Section 3Mapping the Stars Objectives • Explain how constellations are used to organize the night sky. • Describe how the altitude of a star is measured. • Explain how the celestial sphere is used to describe the location of objects in the sky. • Compare size and scale in the universe, and explain how red shift indicates that the universe is expanding.

3. Section 3Mapping the Stars Patterns in the Sky • Constellations Help Organize the Sky A constellation is a region of the sky. Each constellation shares a border with neighboring constellations. A constellation map is shown on the next slide. • Seasonal Changes As Earth revolves around the sun, the apparent locations of the constellations change from season to season.

4. Section 3Mapping the Stars Spring Constellations in the Northern Hemisphere

5. The sky is divided into 88 constellations Mesopotamia, circa 3000 BC: oldest know constellations Ptolemy, 2ndcentury AD: 48 constellations in northern sky 16th to 18th century AD: unmapped regions of sky filled in

6. Constellations are largely arbitrary Other cultures, other constellations: Example: Ursa Major = bear, dipper, bull's leg, grain scoop, wagon, plow, etc. Stars in a constellation usually are not at the same distance from us. At a different place in our Galaxy, we would see different star patterns.

8. A modern star chart of Ursa Major:

26. Section 3Mapping the Stars Finding Stars in the Night Sky • You can describe the location of a star or planet by using an instrument called an astrolabe and the following points of reference: • Thezenithis the point in the sky directly above on observer on Earth. • Thealtitudeis the angle between an object in the sky and the horizon. • Thehorizonis the line where the sky and the Earth appear to meet.

27. Section 3Mapping the Stars Altitude, Horizon, and Zenith Click below to watch the Visual Concept. You may stop the video at any time by pressing the Esc key. Visual Concept

28. Section 3Mapping the Stars Zenith, Altitude, and Horizon

29. Azimuth – measured in degrees – how far from North an object is.

30. Section 3Mapping the Stars Finding Stars in the Night Sky, continued • Using an astrolabe allows you to describe where a star or planet is relative to you. Scientists need a different method that describes location independently of the observer’s location. • Astronomers describe the location of a star or planet in terms of the celestial sphere.

31. Section 3Mapping the Stars The Celestial Sphere

32. Section 3Mapping the Stars Describing a Star’s Position Click below to watch the Visual Concept. You may stop the video at any time by pressing the Esc key. Visual Concept

33. Stars are “attached” to a celestial sphere Distances to stars are hard to measure. However, we can pretend all stars are at the same distance from us, attached to a large celestial sphere. Position on the celestial sphere is known even when the distance in unknown.

34. Celestial Sphere: A large imaginary sphere centered on Earth

35. Special locations on the celestial sphere North Celestial Pole = point directly above Earth’s North Pole (near the star Polaris) South Celestial Pole = point directly above Earth’s South Pole (no nearby bright star) Celestial Equator = circle directly above Earth’s Equator

36. Distances between points on the celestial sphere are measured in degrees, arcminutes, and arcseconds 360 degrees in a circle 60 arcminutes in a degree 60 arcseconds in an arcminute ½ degree = angular size of Sun & Moon

38. Celestial navigation made simple At Earth’s North Pole: Polaris is directly overhead At Earth’s Equator: Polaris is due north, on the horizon In Earth’s Northern hemisphere: Polaris is due north - height above the horizon (in degrees) is equal to your latitude (in degrees)

39. The celestial sphere appears to rotate about the celestial poles (1 day cycle) Observation: Stars, Sun, Moon and planets move in counterclockwise circles around north (south) celestial pole. Objects near the celestial equator move east to west when above the horizon (“rising” in east, “setting” in west). What causes these circular motions?

42. Section 3Mapping the Stars Circumpolar Stars Click below to watch the Visual Concept. You may stop the video at any time by pressing the Esc key. Visual Concept

49. The Sun appears to move west to east relative to stars (1 year cycle) Today the Sun is “in” Sagittarius, next month in Capricornus, etc. Sun’s path on the celestial sphere = ecliptic Constellations through which the ecliptic runs = zodiac The ecliptic is NOT the same as the celestial equator!