The_Sky - THE SKY Questions How do we orient objects in the...

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Unformatted text preview: THE SKY Questions How do we orient objects in the sky? How do we find objects on the moving sky? What b t th Wh t about the moving objects? i bj t ? How is the Earth (and humans) affected by the sky? Observations and Theories The Ancient Greeks and Egyptians developed theories to explain their observations. However, they maintained several postulates (e.g., circular motion), which prevented a truly impartial theory. A. Geocentric Cosmology 1. 1 Earth appears to be flat. flat 2. Celestial Sphere appears to be above the Earth. 3. Celestial Sphere moves around the Earth. Stars are fixed, except for meteors. Stars rise in East, set in the West. Observations and Theories B. The Sun’s Motion Sun travels on the ecliptic. There is a 4 minute difference per day of the Sun s position against the stars. The angle between the Sun’s Earth’s orbit and the Earth’s rotation axis is 23o (obliquity). C. C Wandering Stars Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn D. Zodiac along the Ecliptic Constellations are groups of stars. Today, there 88. Distances Here is an example. The distance from my house in Atlanta to my parents’ house north of Nashville is about 17,740,000 inches the number is correct but it is completely meaningless because we do not have a real-world feel for numbers that large. I could state that the distance is about 280 miles. The number 280 is just on the verge of our ability to comprehend its magnitude. An even better way of relating the distance between the houses is to say it takes me 4 hours to drive from one to the other. In expressing it that way, two things have happened: (1) the quantity of 4 hours is easily comprehensible and (2) the measurement units were switched from distance to time. Hopefully by h i i h df di i H f ll b this change in measurement units, you get a better feel for the distance between these two homes. What About Angles on the Sky? Angular Diameters Full Moon = Sun = 0.5o 05 = 30 arcmin = 1,800 arcsec ,800 a csec Measuring Angles Circle 360o in Circumference x -------- = -------360o 2R Angular Measurements Skinny Triangle tan = D / d [Diameter / distance] = D / d (radians) Local Coordinates Relative to the Observer Zenith is Z ith i a point directly over-head. i t di tl h d Meridian is a line through the Poles and Zenith. Star Positions Azimuth is angle from North Altitude is angle from Horizon Celestial Coordinates Celestial Poles are above the North & South Poles. Celestial Equator is directly above the Earth’s Equator. Star Positions Longitude = Right Ascension () Latitude = Declination () Motions Two Motions Rotation The i i R t ti – Th spinning of a body f b d around its axis (one day). Revolution – The orbital motion of a body around another due to Gravity (one year). Diurnal Motion Effects Due to Rotation Gives us Night and Day. Gi Ni ht d D Causes the Sun and Stars to rise in the East and set in the West. If there was no Revolution, then each night sky would be the same. Seasonal Changes Effects Due to Revolution Gives us the Year. Has a role in producing the Seasons, which are not due to a change in distance. Path of the Sun Sun’s Path is the Ecliptic. The Earth’s tilt (obliquity) of 23.5 23 5o is what causes the Sun s Sun’s path not to be on the Celestial Equator. Yearly Solar Motion Yearly Solar Motion The Day Solar Day If there was only the Rotation of the Earth, Earth then the day would be = 24h 00m 00s Think about it – this would be the time from one crossing of the Meridian to the next crossing by the Sun. (am & pm) The Day (cont.) Sidereal Day But the Earth is also Revolving while it is Rotating so with respect to the Rotating, background stars, one day = 23h 56m 04s Think about it – there are 360o in a circle and there are 365.25 days in a year. The Day (Cont.) Mean Solar Day = 24h 00m 00s Think about it – Does a clock really tell accurate time? Is the day really based on the Sun? Time Zones Time Zones 24 zones x 15o/zone = 360o International Dateline The Year Julian Calendar Problem is that 46 BC 1 year = 365 2500 d 365.2500 days 1 year = 365 2422 days 365.2422 1 leap day is added if the year is evenly divisible by 4. Think about it – Should the year and day be nicely linked? (a difference of 11m 14s) Think about it – Should this difference cause a problem? The Year (cont) Gregorian Calendar AD 1582 Easter’s D t h d hift d E t ’ Date had shifted Two Adjustments Removed 10 days Oct 4 Oct 15, 1582 The Leap Year If the year is evenly divisible by 4, Yes, unless If the year is evenly divisible by 100, No, unless If the year is evenly divisible by 400, Yes Key Individuals Pythagoras [ d. 497 BC, Italy] Aristotle [384 - 322 BC] [ Aristarchus of Samos [310 - 230 BC] Eratosthenes [276 - 195 BC, Alexandria, Egypt] Hipparchus [160 - 127 BC, Alexandria, Egypt] Claudius Ptolemy (or Ptolemaeus) [AD 140] Hipparchus [160 - 127 BC] He cataloged 850 stars by position and by magnitude. Hipparchus (cont.) He noted that the North Celestial Pole d h h h l i l l had precessed (changed position) over the previous 150 years. Precession is due to the gravitational pull of the Moon and Sun on the Earth. It takes 26,000 years for one cycle. , y y ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.

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