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Unformatted text preview: 1 Astro 102/104 1 Policy on Laptop Use • Unless you have a disability that requires the use of a computer for taking notes or attending the lecture, we ask that you do not use computers during lecture. Astro 102/104 2 Lecture #4: Earth, Moon, & Sky I • The Reasons for the Seasons: The Reasons for the Seasons: – Role of Role of obliquity. obliquity. – Why your latitude matters. Why your latitude matters. • Timekeeping: Timekeeping: – Length of the day: Solar vs. Sidereal. Length of the day: Solar vs. Sidereal. – Solar Time Solar Time---- many different kinds! many different kinds! • Calendars: Calendars: – Ancient to modern ways of reckoning time. Ancient to modern ways of reckoning time. Astro 102/104 3 The Main Point Earth and other planets have seasons if they spin at a tilted angle relative to their orbit around the Sun. Astro 102/104 4 Seasons: Some Observations • For most inhabitants of Earth, there are substantial variations in the weather during the course of the year. • These variations are more extreme the farther north or south that you live. • The weather is opposite in the northern and southern hemispheres. • WHY? 2 Astro 102/104 5 Seasons • Many people believe that the seasons are caused by changes in the distance between the Earth and Sun. • IS IT TRUE? • Earth's orbital eccentricity is only ~0.02 – There's only about a 3% difference in distance to the Sun between perihelion and aphelion: not enough to explain seasons! • Besides, Earth is closer to the Sun in January! (brrr...) • And why would there be such a difference between the northern and southern hemispheres? Astro 102/104 6 The tilt of the Earth's axis with respect to its orbit, or its obliquity , is the reason for the seasons Pretend the flashlight is the Sun. In summer (left), the light is concentrated into a small spot. In winter (right), the same amount of light is more spread out....
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- Spring '07
- Solar System