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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