Astronomy 110 Lecture 3.
Fall, 2005 Astronomy 110 1 1. The Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun must change as Earth orbits the Sun. 2. Sunlight hits one hemisphere more directly than the other for half the year then the two hemispheres reverse. • AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.
Fall, 2005 Astronomy 110 2 Fall, 2005 Astronomy 110 3 Antarctica
December 21 June 21
Fall, 2005 Astronomy 110 4 Fall, 2005 Astronomy 110 5 How does the tilt of the Earth’s spin axis cause seasons?
What causes the seasons?
Fall, 2005 Astronomy 110 6 The Solstice and Eqinox can be recognized by the Sun’s path across sky:
Summer solstice: highest path, rise and set at most extreme north of due east. Winter solstice: lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west.
Fall, 2005 Astronomy 110 7 Analemmas From APOD
Crimea Delphi Fall, 2005 Astronomy 110 8 Fall, 2005 Astronomy 110 9 Fall, 2005 Astronomy 110 10 Why doesn’t distance matter? • The annual variation of the Earth-Sun distance — about 3%; is so small that it has little effect on temperatures on the Earth. • Note: more ocean and less land mass means, on average, less extreme seasonal temperature variations in the southern hemisphere.
Fall, 2005 Astronomy 110 11 How do we mark the progression of the seasons? • There are four naturally defined annual season markers: Summer Solstice: Maximum northern excursion of the Sun Winter Solstice: Maximum southern excursion of the Sun Vernal Equinox: Sun crosses the Equator moving north. Autumnal Equinox: Sun crosses the Equator moving south.
Fall, 2005 Astronomy 110 12 Seasonal changes are more extreme at high latitudes
The daily motion of the Sun on the day of the summer solstice as viewed by an observer on the Arctic Circle.
Fall, 2005 Astronomy 110 13 What are the indicators of seasonal changes? 1. Changes in the relative lengths of night and day. 2. Changes in which constellations are visible in the night sky 3. Changes in the peak height of the Sun’s path across the sky
Phenomena like its colder, the leaves fall off the trees, football starts etc. are accidents of your latitude and your local climate. Weather and temperature are secondary effects of seasonal changes. It is possible to have seasonal changes without corresponding changes in weather.
Fall, 2005 Astronomy 110 14