AST 101, Week 2 Notes
AST 101, Week 2 Notes 101
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This 3 page Class Notes was uploaded by Bethany Marsfelder on Tuesday September 20, 2016. The Class Notes belongs to 101 at Syracuse University taught by Professor Walter Freeman in Fall 2016. Since its upload, it has received 185 views. For similar materials see Our Corner of the Universe in Astronomy at Syracuse University.
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Date Created: 09/20/16
September 6, 2016 AST 101 – Professor Freeman Lecture: The daily motion of the sky Textbook Pages: 31-32 Lecture Tutorial: 5-6 (Motion, Part II) Objectives o Consequences of Earth’s rotation Celestial-sphere and its consequences o Consequences of Earth’s revolution What about the Sun? What causes the seasons? What does the Sun do to the Earth? Stars and their Visibility o Depending on where you are in the world, some stars are always visible, some stars are sometimes visible (meaning that they rise and set), and some stars are never visible o For an example: the Australian flag has the Southern Cross on it to symbolize its uniqueness, as the Southern Cross is only visible in the Southern Hemisphere. Which way does the celestial sphere turn? o The Earth rotates from West to East However, the celestial sphere’s apparent rotation is backwards o The celestial sphere rotates from East to West A recap of the celestial sphere model: o We can treat the stars as all rotating together, on an invisible sphere far away o We can expect this to get the stars “right” and the planets and Sun “wrong” o The axis of rotation is the same as the Earth’s, and it rotates once per day o Only half of the sphere is visible, because the Earth is in the way Terms o Circumpolar – a star that remains perpetually above the horizon, circling counterclockwise around the north celestial pole each day. (ie, Polaris in the Northern Hemisphere, hence its nickname, “the pole star”) o Horizon: a plane lying along the Earth at our location o Zenith: the point directly overhead o Celestial pole: the point about which the stars appear to rotate What about the Sun? o Over the course of one day, the Sun doesn’t move very much o This means the celestial sphere model can show us how the Sun moves each day o It rises in the East and sets in the West, just like all the other stars. Variation with Time of Year/Preview for Next Time o The night sky changes throughout the year because of Earth’s changing position in its orbit around the Sun. o As Earth orbits, the Sun appears to move steadily eastward towards the ecliptic (a great circle on the celestial sphere representing the sun’s apparent path during the year, so called because lunar and solar eclipses can occur only when the moon crosses it), with the stars of different constellations in the background at different times of year. o The constellations along the ecliptic make up the zodiac; tradition places 12 constellations along the zodiac, but the official borders include a thirteenth constellation, Ophiuchus. o The Sun’s apparent location along the ecliptic determines which constellations we see at night. o When the Sun is “in” a constellation (ex: the Sun is “in” Leo) we cannot see Leo at this time because it is in our daytime sky, but we can see Aquarius all night long because of its location opposite Leo on the celestial sphere. September 8, 2016 AST 101 – Professor Freeman Lecture: The yearly motion of the sky (The Sun and the stars, the zodiac) Textbook Pages: 32-38 Lecture Tutorial: 7-12, 13-16 Objectives o Consequences of the Earth’s revolution How is the Sun different from other stars? What’s this zodiac business? What does it mean for the Sun to be “in Aries”? We will see how this is only complicated because of how we keep time Brief Tangent: Other Planets? o Are there planets around other stars? Yes! o How many planets (exoplanets, outside our solar system) are there? Tons! o How do we discover planets orbiting stars? When the light of a star gets dimmer, then brighter, it usually means a planet is passing in front of it Planets also make stars “wobble” because of their gravity Doppler Effect/Shift – we can tell what is coming/going o Most likely, most stars have planets; large, Jupiter-like gas giants are easy to tell because of the methods mentioned above, but it is the smaller, Earth-like planets that are harder to detect Why is the celestial sphere model a bit wrong for the Sun? And why do demos, like Stellarium, show that, during the night, the stars spin around, while the Sun moves up and down just a little bit? Answer: there are two kinds of day! o Solar day: judged by the position of the Sun, goes from the highest sun in the sky ot the highest sun in the sky. Is longer than a sidereal day. o Sidereal day: judged only by the rotation of the Earth with respect to the stars, goes by one full rotation. Is shorter than a solar day. Similarities and Differences: Solar Day Sidereal Day The stars move a lot since the Earth is not The stars don’t move at all since earth is pointed pointed in the same direction as them in the same direction as them The Sun moves a bit higher/lower but usually The Sun moves a lot stays in the same position Exactly 24 hours A bit less than 24 hours (to be precise, 23 hours and 56 minutes)