Transcript precess.ppt
1 Precession and The Celestial Poles The “North Celestial Pole” lies overhead for an observer at the North Pole and on the horizon for an observer on the Equator The altitude of the pole equals your latitude. 2 Precession and The Celestial Poles The “North Celestial Pole” lies overhead for an observer at the North Pole and on the horizon for an observer on the Equator The altitude of the pole equals your latitude. To Pole 3 Precession and The Celestial Poles The “North Celestial Pole” lies overhead for an observer at the North Pole and on the horizon for an observer on the Equator The altitude of the pole equals your latitude. 4 The Celestial Poles The “North Celestial Pole” lies overhead for an observer at the North Pole and on the horizon for an observer on the Equator The altitude of the pole equals your latitude. 5 Precession and The Celestial Poles The rotating Earth makes it look like the Celestial Sphere is spinning about the celestial poles. http://www.atscope.com.au/BRO/warpedsky.html 6 Polaris In the Northern Hemisphere there is a star, not all that bright, near the North Celestial Pole. It resides at the end of the handle of the “Little Dipper” and is called Polaris (for good reason – at least for now) 7 Polaris In the Southern Hemisphere there is no good pole star at present. 8 Precession Because the Earth is a spinning top, the direction of its pole in the sky is fixed (at least from the perspective of a human lifetime). Of course, this consistency of tilt is related to the cause of the seasons. 9 Precession Over long periods of time, the direction of the Earth's pole drifts in the same way it does for a child's top or gyroscope. The drifting of the axis of a top is called “precession” 10 Precession It takes 26,000 years for the Earths pole to trace out a full circle on the sky. That circle is 47 degrees in diameter (2 x 23 ½) 11 Precession It takes 26,000 years for the Earths pole to trace out a full circle on the sky. That circle is 47 degrees in diameter (2 x 23 ½) 12 Precession Different stars occupy different positions above the Earth's pole over time. Polaris is currently getting further from the pole every year. Just how long will we hang on to it as our pole star??? 13 Consequences of Precession Different Stars are circumpolar at different times. 3000 years ago the Big Dipper was circumpolar at our latitude. Stars that currently never rise above our Southern horizon will be visible. The Southern Cross will be visible from Charlottesville in 10,000 years. Go home and prove it for yourself with Starry Night! 14 The Sun and the Celestial Sphere • • As the Earth orbits the Sun we seen the Sun in different locations against the backdrop of stars. The Earth reaches the same location in its orbit on the same calendar date each year. 15 The Sun and the Celestial Sphere • • As the Earth orbits the Sun we seen the Sun in different locations against the backdrop of stars. The path the Sun follows amongst the background of stars is nearly identical from year to year and is called the Ecliptic. 16 The Sun and the Celestial Sphere • • As the Earth orbits the Sun we seen the Sun in different locations against the backdrop of stars. The set of constellations through which the Sun passes is called the Zodiac. The Sun lies in front of your “birthsign” constellation on your 17 The Visible Constellations Change Throughout the Year • What we see of the Universe at night depends on where the Earth is relative to the Sun. Our midnight sky is the direction in space opposite the direction to the Sun 18 The Visible Constellations Change Throughout the Year • What we see of the Universe at night depends on where the Earth is relative to the Sun. As the Earth orbits the Sun our perspective on the night sky changes throughout the year. 19 The Visible Constellations Change Throughout the Year • What we see of the Universe at night depends on where the Earth is relative to the Sun. The Earth moves enough in one day to advance the setting time of a particular star by about 4 minutes - 2 hours a month. 20 The Visible Constellations Change Throughout the Year • What we see of the Universe at night depends on where the Earth is relative to the Sun. At 8pm one month from now the sky will look as it does at 10 pm today. 21 Constellations Change with Time • The stars are all moving relative to one another These motions are perceptible to the eye only over thousands of years. The constellations/asterisms known to us will be obsolete 100,000 years from now. Movie of the Changing Big Dipper