The Celestial Sphere

The Celestial Sphere • From our perspective on Earth the stars appear embedded on a distant 2-dimensional surface – the Celestial Sphere.

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.

A Personal Perspective: Horizon and Zenith Zenith Horizon Horizon

A Merry-Go-Round Perspective • Our observing platform both spins daily and orbits the Sun once a year producing a complicated perspective

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). d

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

Getting Oriented: What Defines the Horizon? Zenith Horizon Horizon

Getting Oriented: What defines the “Horizon” • Each individual observer has their own personal local horizon. • 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.

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. • The observer below is facing south and seeing the southern stars up in the sky.

Altitude and Zenith Altitude

The Sky You Can See Depends Upon... • Your location on the Earth • As the Earth spins on its axis you get a tour of the sky. • Exactly what part of the sky depends on how far North or South of the Equator you are.

The Sky You Can See Depends Upon... • Your location on the Earth • 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.

The Sky You Can See Depends Upon... • Your location on the Earth • If you are standing at the Earth's equator you get a tour of the entire sky as the Earth turns.

The Sky You Can See Depends Upon... • Your location on the Earth • Most observers are somewhere between the pole and the equator. • Portions of the sky are visible to them all of the time • There are parts of the sky that they can never see.

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.

The Sky You Can See Depends Upon... • The time of day or night • Our clocks are referenced to the Sun's position in the sky. • Noon is when your longitude on Earth is pointed at the Sun • Midnight is just the opposite

The Sky You Can See Depends Upon... • The time of day or night • Our clocks are referenced to the Sun's position in the sky. • When you are on the “day” side of the Earth you cannot see the stars. • When you are on the “night” side you can.

Sunset To the Sun Midnight Noon Sunrise

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).

The Sky You Can See Depends Upon... • The time of year • 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.

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.

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.

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

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.

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

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)

Polaris • In the Southern Hemisphere there is no good pole star (at present). • Note that there are some stars (near the pole) that never set below the horizon - “Circumpolar Stars” • For an observer at the North or South pole every star is circumpolar. • At the Equator there are no circumpolar stars

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.

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”

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)

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???

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!

The Celestial Sphere