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The Celestial Sphere
From our perspective on Earth the stars appear embedded on a distant 2­
dimensional surface – the Celestial Sphere.
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The Celestial Sphere
Although we know better, it is helpful to use this construct to think about how we see the night sky from Earth.
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A Personal Perspective: Horizon and Zenith
Zenith
Horizon
Horizon
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A Merry­Go­Round Perspective
Our observing platform both spins daily and orbits the Sun once a year producing a complicated perspective
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The Key to Understanding the Night Sky
For a given observer the Earth blocks ½ of the sky at any instant.
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The key is understanding which half.... which depends on
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The observer's location on the Earth
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The time of day/night (which way the Earth is turned relative to the Sun)
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The time of year (which part of the Sky is washed out by daylight).
d
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Getting Oriented: Which Way is “Up”?
Leaving the “flat Earth” of everyday perspective behind, ➔
“up” is the direction away from the center of the Earth
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Getting Oriented: What Defines the Horizon?
Zenith
Horizon
Horizon
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Getting Oriented: What defines the “Horizon”
Each individual observer has their own personal local horizon.
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In simplest terms this horizon is a flat plane tangent to the Earth at the observer's location.
The giant observer below is misleading. For an observer of proper size the Earth would block ½ of the sky, defining the horizon.
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Getting Oriented: Which way is “North”?
Point in the direction of the North Pole. As you move your finger up into the sky from the horizon you are defining the northern sky.
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The observer below is facing south and seeing the southern stars up in the sky.
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Altitude and Zenith
Altitude
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The Sky You Can See Depends Upon...
Your location on the Earth
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As the Earth spins on its axis you get a tour of the sky.
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Exactly what part of the sky depends on how far North or South of the Equator you are.
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The Sky You Can See Depends Upon...
Your location on the Earth
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If you are standing at the North pole you will only see the Northern half of the sky all of the time.
The turning Earth does not give you access to the southern sky.
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The Sky You Can See Depends Upon...
Your location on the Earth
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If you are standing at the Earth's equator you get a tour of the entire sky as the Earth turns.
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The Sky You Can See Depends Upon...
Your location on the Earth
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Most observers are somewhere between the pole and the equator.
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Portions of the sky are visible to them all of the time
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There are parts of the sky that they can never see.
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The Key to Understanding the Night Sky
For a given observer the Earth blocks ½ of the sky at any instant.
●
The key is understanding which half.... which depends on
➔
The observer's location on the Earth
➔
The time of day/night (which way the Earth is turned relative to the Sun)
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The time of year (which part of the sky is washed out by daylight.
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The Sky You Can See Depends Upon...
The time of day or night
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Our clocks are referenced to the Sun's position in the sky.
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Noon is when your longitude on Earth is pointed at the Sun
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Midnight is just the opposite
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The Sky You Can See Depends Upon...
The time of day or night
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Our clocks are referenced to the Sun's position in the sky.
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When you are on the “day” side of the Earth you cannot see the stars.
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When you are on the “night” side you can.
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Sunset
To the Sun
Midnight
Noon
Sunrise
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The Key to Understanding the Night Sky
For a given observer the Earth blocks ½ of the sky at any instant.
●
The key is understanding which half.... which depends on
➔
The observer's location on the Earth
➔
The time of day/night (which way the Earth is turned relative to the Sun)
➔
The time of year (which part of the sky is washed out by daylight).
20
The Sky You Can See Depends Upon...
The time of year
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The Sun washes out the stars in the daytime sky, so the Sun effectively hides a good fraction of the sky from view.
The Earth shifts position relative to the Sun throughout the year, reaching the same point in its orbit on the same calendar date.
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Reference Points on the Celestial Sphere
Extend the Earth's poles and equator onto the sky and you have defined the celestial poles and celestial equator.
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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
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The altitude of the pole equals your latitude.
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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
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The altitude of the pole equals your latitude.
To Pole
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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
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The altitude of the pole equals your latitude.
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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
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The altitude of the pole equals your latitude.
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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
In the Northern Hemisphere there is a star, not all that bright, near the North Celestial Pole.
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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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In the Southern Hemisphere there is no good pole star (at present).
Polaris
Note that there are some stars (near the pole) that never set below the horizon ­ “Circumpolar Stars”
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For an observer at the North or South pole every star is circumpolar.
At the Equator there are no circumpolar stars
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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).
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Of course, this consistency of tilt is related to the cause of the seasons.
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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.
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The drifting of the axis of a top is called “precession”
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Precession
It takes 26,000 years for the Earths pole to trace out a full circle on the sky.
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That circle is 47 degrees in diameter (2 x 23 ½) 32
Precession
It takes 26,000 years for the Earths pole to trace out a full circle on the sky.
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That circle is 47 degrees in diameter (2 x 23 ½) 33
Precession
Different stars occupy different positions above the Earth's pole over time.
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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
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.
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The Southern Cross will be visible from Charlottesville in 10,000 years.
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Go home and prove it for yourself with Starry Night!