Transcript precessold.ppt

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Precession and The Celestial Poles
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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.
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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
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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.
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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
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Polaris
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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)
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Polaris
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In the Southern
Hemisphere there is
no good pole star at
present.
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Precession
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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.
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Precession
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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”
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Precession
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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)
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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)
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Precession
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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???
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Consequences of Precession
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Different Stars are circumpolar at different times.

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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!
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The Sun and the Celestial Sphere
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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.
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The Sun and the Celestial Sphere
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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.
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The Sun and the Celestial Sphere
•
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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
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