Lecture 02 202 Spring 2011 - Understanding the Sky Extra...

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Understanding the Sky Extra topic: Spaceship Earth The motion of Spaceship Earth o Rotation: 0-1,500 km/h around axis o Revolution: 100,000 km/h around Sun o Rotation of Milky Way: 1,000,000 km/h around galactic center o Moving Towards Andromeda Galaxy: 300,000 km/h Understanding the Seasons What causes the seasons? o Tilt of Earth’s axis of rotation relative to Earth’s orbit around the Sun. o Sun’s path and intensity of sunlight changes over a year (one orbit). The Sun’s Daily Path o Sun rises somewhere in the East, sets somewhere in the West. o Reaches highest point around noon (definition of noon) o Consequence of Earth’s daily rotation. o Actual path of Sun varies annually (as Earth orbits the Sun). Finding our way in the Sky o Horizon : Boundary between Sky and Earth. Cannot see below the Horizon. o Zenith : Directly Overhead. o Meridian : Imaginary half-circle: Horizon due South-Zenith-Horizon due North. Azimuth : Direction along the Horizon. Altitude : Angle above the Horizon. The Sun’s Daily Path (Mid-latitudes) o Summer: Rising north of due east, setting north of due west Long (more hours) path high in the sky o Winter: Rising south of due east, setting south of due west Short (less hours) path low in the sky o First day of Spring, first day of Fall Rising exactly due east, setting exactly due west o Mid-latitudes: Sun rises every day, but it never reaches the Zenith.
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Tropics and Polar Circles o Tropics: Latitudes 23.5° North and 23.5° South Between the Tropics (includes equator) Sun can reach the Zenith at latitudes between the tropics Sun’s path high and long all year round. o Polar Circles Latitudes 66.5° North and 66.5° South Between Polar circles and Poles Sun does not rise at least one day Sun does not set at least one day Sun’s path never high in the sky The Reason for Seasons o Earth’s rotation axis tilted with respect to orbit. o Axis points to fixed direction in Space (Polaris, North Star) o Orientation of axis relative to Sun changes over the year Northern Hemisphere tipped towards(away) Sun in June(December) Reverse situation for Southern Hemisphere Opposite Seasons o Summer: Higher, longer path of Sun in sky means More hours of sunlight (more radiation) Higher incidence angle (more intense radiation) o Winter: Lower, shorter path of Sun in the sky means Less hours of sunlight (less radiation)
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