Lecture04 - Welcome to Astronomy 100 Prof Bryan Dunne NASA...

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Welcome’to’Astronomy’100! NASA Prof. Bryan Dunne 1 Monday, October 22, 12 Welcome to Astronomy 100: Lecture 4 - Seasons
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ASTR’100 Lecture’4 Topics for Today What causes the seasons? How are seasons different around the world? 2 Monday, October 22, 12
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Ques8on These images of the Sun were taken 6 months apart. When was the image on the left taken? A.Summer (July) B.Winter (January) January July Big Bear Solar Observatory 3 Monday, October 22, 12 These images of the Sun are taken 6 months apart with the same camera, at the same time of day, from the same location. Dashed lines show Sun in left image appears larger. Why? It must be closer when the picture was taken. Do you think the the image on the left was taken in Summer (July), or Winter (January)? Interestingly, the correct answer is B - the Earth is closer to the Sun in (northern) winter!
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The’Earth’is’closest’to’the’Sun’in’ early January Earth is 5 million km closer to the Sun in early January than it is in early July Sounds like a lot, but it’s only 3% closer! http://commons.wikimedia.org/wiki/File:Aphelion_%28PSF%29.svg July: Aphelion (“far-Sun”) January: Perihelion (“near-Sun”) 147 million km 152 million km 4 Monday, October 22, 12 The Earth's orbit is slightly non-circular, so that the Earth is closer to the Sun at some times, and farther away at others. At perihelion (from Greek for “near-Sun”), Earth is 147.1 Million kilometers from the Sun. This occurs on or about January 4th! At aphelion (from Greek for “far-Sun”), Earth is 152.1 Million kilometers from the Sun. This occurs on or about July 5th! Although a 5 million km variation in distance sounds like a lot, its really only a 3% change from maximum distance to minimum distance. That’s not enough to cause seasons, and remember, the Earth is closer to the Sun in northern winter! The diagram above exaggerates the ellipticity of Earth’s orbit. In reality, Earth’s orbit is nearly circular. So what does cause the seasons?
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So’what’does’cause’seasons? Earth±s’axial’8lt Earth’s rotation axis is tilted to its orbit around the Sun by 23.5° Axis points in the same direction in space as we orbit the Sun - towards Polaris December March June September 23.5º 23.5º 23.5º 23.5º Richard Pogge 5 Monday, October 22, 12 The Earth's rotation axis is tilted relative to the plane of its orbit around the Sun (also called the ecliptic plane). Tilt is about 23.5 degrees from perpendicular relative to the ecliptic plane. Note that the Earth's axis points towards the same direction in space as it orbits around the Sun - toward Polaris. So lets look at how the tilt causes seasons. ..
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Eclip8c’on’the’celes8al’sphere SCP NCP CEq Sun is north of the celestial equator Sun is south of the celestial equator Sun is on the celestial equator Richard Pogge ~23.5º Ecliptic 6 Monday, October 22, 12 To explore the cause of seasons, we need to consider the Sun’s daily path in the sky and how it changes over the course of the year due to the Earth’s axis title.
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Lecture04 - Welcome to Astronomy 100 Prof Bryan Dunne NASA...

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