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The Celestial Sphere

The Celestial Sphere describes a model of our view of the Universe. You'll learn about constellations, directions in the sky, and how the rotation of the Celestial Sphere looks from here on Earth.

Note: This article was written for observers in Fullerton, California.

What you should know: How to measure angles in degrees. Angles Review from Math is Fun.

Patterns in the Sky

For human observers, the stars appear in fixed patterns in the sky.

You might recognize the pattern of the stars in Vincent van Gogh's Starry Night Over the Rhône, which was painted in 1888:
A nighttime riverside scene with a dark blue sky filled with bright yellow stars and street lamps reflected in the Rhône River.
Source

You might know it today as the Big Dipper, a part of the constellation Ursa Major.
A night sky photograph showing the stars of the big dipper.
Source: Acadia Photo Safari
A part of a sky map showing the stars of the big dipper.
Source: Torsten Bronger

This same pattern appears in texts written by human observers over a thousand years ago.

  • The Dunhuang star map, from around 700 AD:
    A star map on silk, depicting stars as dots connected into patterns with lines, annotated in classical Chinese calligraphy.
    Source

  • The Book of Fixed Stars from around 964 AD:
    A drawing of a bear with stars marked as red and black dots and labeled in Arabic. Source

All around the world, observers notice the same patterns, but tell their own stories:
The same stars as the Greek Orion, Aztec New Fire, Navajo First Slim One, and Hawaiian Cat's Cradle
Sources: Stellarium, NASA/STScI

Constellations

In modern Astronomy, constellations are how we identify and name regions of the sky - NOT the patterns of the stars themselves.

The constellation Orion is everything in the highlighted region in this sky map:
A sky map highlighting the constellation Orion, with labels on its bright stars and on neighboring constellations.
Source: IAU, Sky and Telescope

The Dome of the Sky

Stars in constellations appear to be side-by-side, but they can be at very different distances from the Earth.
The earth, with lines-of-sight through a sideways sky image of Orion to several stars at different distances

The red star Betelgeuse on Orion's "shoulder" is 650 light years away, but the purplish glowing Nebula M42 in the "sword" is 1,344 light years away. So Betelgeuse is closer to us than it is the nebula.
Betelgeuse imaged is a red glowing blob The Orion nebula is a hazy purple glow with bright stars at the core
Sources: Dupree and Gililand, CfA/STScI/NASA/ESA and Filip Loic

How can you figure out how far away things are if you can’t reach them?

  1. Binocular vision (compare view from each eye.)
  2. Known size scaling (cows are really the same size.) Cows grazing in a field. Close cows look bigger Source: RF Vila

  3. Atmospheric perspective (the effect of the air.) A view over rolling hills that look bluer and bluer as they get far away Source: Joaquim Alves Gaspar

  4. Modern astronomers use versions of each of these methods to work out the distances to astronomical objects

  5. None of them work for unassisted human vision
  6. Stars are not small, but are very far away
  7. They have different sizes and distances, BUT appear as points in the sky
    • except for the star closest to us... the Sun!

Part of a panorama of the playa of Death Valley at night. The Milky way is visible as an arc across the sky.
Source

When we look at the night sky they seem to be sprinkled on a dome over our heads.

This is an optical illusion because we lack depth perception for such distant objects.

Directions in the Sky

The Horizon is the edge of the visible sky.

If you look to any side, you see the Horizon. This is where the sky appears to meet the most distant part of the ground you can see.

Horizon examples:

A view of the sunset. The horizon is labeled at the tops of some distant mountains. The Milky way is seen glowing in the sky over many telescopes illuminated in red light. The horizon is labeled where the dark ground meets the sky.

Ground to sky:

North, East, South, and West mark the directions you would travel out toward the horizon. They are the same directions you use to orient a map.

An animation showing a map with directions N E W S turning into the ground of a figure standing with a sky dome above it. The top of the dome is labeled Zenith.

If you look directly overhead, you see the Zenith. This is the "top" of the dome.

Altitude, Azimuth, Meridian

We locate regions in the sky by azimuth (angle around the horizon) and altitude (angle above the horizon).

An observer with their local sky labeled as described below.

Azimuth on the horizon. Which direction do you face? For example: North, East, South, or West.

Up from the horizon to find altitude:

  • On the Horizon (0°)
  • Low in the sky (less than 45° up from the horizon)
  • High in the sky (more than 45° up from the horizon)
  • Zenith (directly overhead, 90° up from the horizon)

Sky Location example

The GPS satellite passes overhead: from far away in the Northwest, to the Zenith, to far away in the Southeast.
An animation showing the path described above

Polaris, the North Star

Polaris is seen directly above the North point of the horizon in the Northern Hemisphere.

In Fullerton, we see Polaris at an altitude of 34° above the horizon.

The local sky with North to the back, with polaris above the North point of the horizon and below the Zenith

You can find Polaris from nearby star patterns - it's between the stars of the Big Dipper and the W-shaped Casseopeia.

The W of Casseopeia points to the North Star from one side, the dipping edge of the Big Dipper from the other

The Sun and the Meridian

The sun rises in the East, moves high in the Southern sky, and sets in the West. Its path makes an arc across the sky.
An animation with an observer facing South showing the Sun rising near East, arcing across the sky, and setting near West. A dotted line traces the Sun's path. A second line divides the sky from North to South.

We also divide the sky by the meridian, a line going from the North point of the hoizon to the South point.

This is where we get a.m. (ante meridian, before meridian) and p.m. (post meridian, after meridian).

The Celestial Sphere

Each night, the stars appear to move.

Polaris Time Lapse video: Looking North toward Polaris, the stars rotate around it.

This makes it seem like the stars are all on the surface of a spinning ball around us.

Animation of the Celestial Sphere: Source: Science Sims @ CCNY

Many early astronomers imagined that stars were really fixed on the surface of a Celestial Sphere that surrounded and revolved around the Earth.

A globe-style model of the celestial sphere sitting on a desk.

  • The celestial sphere is a scientific model.

  • We now know it is not how things really are.

  • It’s still a useful way to map the position and motion of the stars in our sky.

Features of the Celestial Sphere Model

The Celestial Sphere surrounds the Earth.

  • The stars are in fixed positions on the sphere.

  • The North Celestial Pole is directly above Earth's North Pole.
    An arrow from Earth's North pole stretched directly up to the North Celestial Sphere, and Polaris.

  • The Celestial Equator is directly above Earth's equator. It's 90° down from Polaris. An line marking Earth's Equator is shown with a red ellipse estending out from it to the Celestial Equator.

The Celestial Sphere and your Local Sky

Your Local Sky is half of the celestial sphere; the half that is above your local horizon

  • Your zenith is directly above your location on Earth. Your horizon is 90° down from your zenith in a full circle. The North Celestial Pole and Celestial Equator are tilted in your sky. An observer stands on Earth. Your Zenith and Your Horizon are directly above and to the side. The North Celestial Pole and the Celestial Equator are tilted.

  • The North point of your horizon is directly beneath the North Celestial Pole. Your meridian runs from North on your horizon, through the Zenith, to South on the opposite side of the horizon. East and West are to the left and the right if facing South. Your meridian connects North on your horizon, the North Celestial pole, the Zenith, and ends on your South.

Example Star Paths in the Local Sky

Below is a diagram of a the local sky facing East, with two example star paths. One path circles Polaris in the sky, and the other arcs from East to West.

A dome of the sky over the ground. East is to the back, West to front, North to left, South to right on the horizon. Polaris is on the upper left edge, above Noth. The Celestial Equator and Star Path A arc from East, up the back of the dome to high in the South, down the front of the dome to West. Star Path B makes a loop on the dome, from low above North, up the back of the dome to the Zenith, down the front of the dome to its original point.

Things to notice:

  • The N/E/S/W directions are on the horizon.
  • Polaris is high in the Northern sky.
  • The Celestial Equator
    • runs from East to West.
    • is tilted compared to the Zenith.
    • from the center of the dome, is 90° away Polaris.
  • Star Path A follows the Celestial Equator. A star on this path rises across the horizon due East, travels high above the Southern horizon, and sets across the horizon due West.
  • Star Path B circles Polaris. A star on this path never crosses the horizon. It moves from low in the Northern sky to the Zenith and back.

Check Your Understanding

#

You walk outside and turn so you are facing due North. Your zenith is:

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You walk outside and turn so you are facing due North. South is:

#

Where in the diagram would you label North?

An observer on Earth faces to the left of the local sky dome. A is directly overhead. B marks Polaris on the upper left edge of the dome. C marks the horizon at the back of the diagram. D is the horizon's the leftmost point. E is the horizon's rightmost point.

#

What direction is the observer looking in the diagram below?

The dome of a local sky with Polaris on the left edge. An observer faces the back of the dome.

#

You look to see the sun in the early evening. It's a few sun-heights above the horizon. You describe it as: (choose two)
A photograph of the evening sun, reflected in the water.

Go Further

What stars can you see? Where Are the Stars? See How Light Pollution Affects Night Skies | Short Film Showcase by National Geographic (YouTube) explains how light pollution affects our view of the stars.

Ideas from this lecture are used in the The Rotating Sky Lab.