lecture3 - Lecture 3 Discovering the Universe for Yourself...

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Unformatted text preview: Lecture 3 Discovering the Universe for Yourself What are constellations? A constellation is a region of the sky The celestial sphere is divided into 88 constellations The ecliptic is the Sunʼ s path through the celestial sphere The stars appear to lie on a celestial sphere even though they are really at different distances 1 The Milky Way in the night sky is actually our view into the disk of the Milky Way galaxy The constellations along the ecliptic make up the zodiac The north celestial pole is above the North Pole How do we locate objects in the sky? The celestial equator is above Earthʼs equator The south celestial pole is above the South Pole Measuring Angles in the Sky An objectʼs position in the local sky is given by its altitude above the horizon (in degrees) and its direction along the horizon 2 Time Exposure Photograph: Review: Coordinates on the Earth • Latitude: position north (+) or south ( of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) In the northern hemisphere, which way do the stars move in this picture? A. Counterclockwise or B. Clockwise Why can’t everyone see the same constellations? • Your latitude (in degrees) = the altitude of your celestial pole (north or south). • In the northern hemisphere, stars within an angle equal to your latitude of: • the North Celestial pole never set. (circumpolar stars) • the South Celestial pole are never seen by you. The Daily Motion daily circles --- CCW looking north, CW looking south Celestial Equator Your horizon Summary Slide: Depending on your latitude: • Some stars never set (circumpolar stars). • Some stars never rise. • All other stars have daily, circular paths that rise in the east and set in the west. Annual Motion • As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. • The Sun circles the celestial sphere once every year. • Whether those other stars are up during the night or not depends on where they are relative to the Sun on the ecliptic - a position that changes through the year… 3 What causes the seasons on Earth? What causes the seasons? A. The earth’s tilt, which changes the direction of the Sun’s light as the Earth orbits the Sun. When sunlight is the most direct, it is summer. B. The distance from the Earth to the Sun. When the Earth is closer, it is summer. C. The earth’s tilt, which causes some parts of the Earth’ s surface to be closer to the Sun than the rest of the surface. Seasons occur because the directness of sunlight changes over the year The directness of sunlight changes because of the tilt of Earthʼs axis… …NOT the distance of the surface to the Sun. June: December: Sunlight most direct in north (summer) Sunlight least direct in north (winter) Sunlight least direct in south (winter) Sunlight most direct in south (summer) Solstice: Equinox: Time when difference between directness of sunlight between north and south is greatest Time when directness of sunlight in north and south is equal How do we mark the progression of the seasons? 4 Solstices & Equinoxes • • • • Winter solstice: near Dec 21 Spring equinox: near Mar 21 Summer solstice: near June 21 Fall equinox: near Sept 21 The times and dates shift a little because calendar is not precisely synchronized with Earth’s orbit (365.25 days). Seasonal changes are more extreme at high latitudes The Sun is circumpolar above the Arctic Circle at the time of the summer solstice More Questions about Seasons Why does the hottest part of the year come after the summer solstice? – Temperatures continue to increase while the sunlight remains more intense. (Think of heating a pot of water.) Does the orientation of Earth’s axis change with time? Why are seasons more moderate in the southern hemisphere? – There’s more water and less land. Precession causes Earthʼs rotation axis to trace a circle in space over the course of about 26,000 years Because of precession, Earthʼs position in orbit on a particular date slowly shifts 5 ...
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