Skip to content

Daily Motion of the Sky

We'll discuss how Earth's spin causes the motion we see, how the sun and stars appear to move, skies in the Northern and Southern hemises, and the change in sky motion with latitude.

The Rotating Sphere

In the Celestial Sphere model, the Earth is fixed and the Celestial Sphere rotates.

A celectial sphere rotating on the Polar axis through Polaris around the ground of a local sky. Stars are fixed on the sphere. The stars start to make trails as they move, forming circular paths that all parallel each other around Polaris.

  • It rotates around the Polar axis from East to West

    • An axis through the North Celestial Pole (Polaris, the North Star)

  • The Celestial Sphere makes a complete rotation once every day.

  • The stars on the sphere appear to make circular paths.

  • Each star completes a full circle path every 24 hours.

The Celestial Sphere Model describes what we see but isn't how things really are.

Types of Stars

Visible stars at a given location may be circumpolar or rise-and-set stars.

Stars making a full circle in the sky are Circumpolar stars. Circumpolar stars never set below the horizon.

Stars coming up from the horizon are Rise-and-set stars.

In this long-exposure sky photograph, the motion of stars turs them into streaks across the sky.
Stars appear as streaks in a time lapse photo of the sky. Shaded regions show Circumpolar stars and Rise-and-set stars as described in the text.

Polaris is not exactly on the north celestial pole, so it makes a little arc in this photo.

What Causes Daily Motion?

Earth is spinning.

This is a Galileo satellite time-lapse movie of Earth starting 6AM PST, December 11 1990, from 2,000,000 km away. The movie covers 25 hours.

Movie Source: NASA

  • Earth’s motion changes our view of space.

  • When looking at celestial objects, two of our own motions change what we see:

    • The daily rotation of the Earth about its axis
    • The annual revolution (orbit) of the Earth around the Sun

  • Earth’s orbit is slow compared to its daily rotation. We don’t notice it hour-to-hour.

Earth's rotation

An image of earth, with lines extending from North and South Poles. Curved arrows arc around the Earth and its poles showng rotation from west to east

  • The Earth rotates from west to east
  • This makes the celestial sphere appear to rotate around us from east to west
  • It makes a full rotation every 24 hours (23 hours 56 minutes compared to background stars)

Observing from a rotating Earth

  • Most objects in our Solar System move slowly compared to this daily rotation.
  • More distant objects are too far away to see any motion.
  • For most celestial objects, the motion you see in the sky is daily motion from the Earth’s rotation
    • stars, galaxies, the Sun, the Moon, the planets, passing comets…
  • Exceptions: rapidly-moving nearby objects like meteors (shooting stars) and satellites

Check Your Understanding: The Rotating Sky

Choose the best answer!

#

You observe a star rising due East. When the star reaches its highest position above the horizon, where will it be?

#

How much time has passed in this animation?

Stars in a rotating local sky. The most prominent star starts high in the sky, loops under the horizon following its circle path, and returns to its original place.

#

About how much time has passed in this animation?

Stars in a rotating local sky. The most prominent star starts on its path highest in the sky, and moves to the farthest west side of its circle path.

#

Which star is above the horizon for the greatest amount of time?

Two stars follow circular paths around Polaris. Star A is shown to the right of Polaris on a larger circle that dips underneath the horizon. Star B is shown to the left of Polaris on a smaller circle that is entirely above the horizon.

#

Which star is above the horizon for the greatest amount of time?

Two stars follow circular paths around the SCP. Star A is shown up and to the right of Polaris on a smaller circle that is mostly underneath the horizon. Star B is shown to the right of Polaris on a larger circle that arcs above the smaller circle.

#

How much time is there between when a star rises and when the star sets?

The Path of the Sun

This composite image follows the Sun's path through the sky the December solstice day of 2005 in Italy. The view covers about 115 degrees side to side in 43 seperate exposures from sunrise to sunset.

43 images of the sun arc across the blue sky over a sea view from a grassy field with a tree
Source: Danilo Pivato

  • The Sun, Moon, and planets also appear to lie on the Celestial Sphere

  • Over the course of one day, they appear almost fixed with respect to the stars in the Celestial Sphere

  • The Celestial Sphere appears to carry the Sun, Moon, and planets around the Earth once per day (24 hours)

  • The real motion is Earth’s rotation

This timelapse shows the motion of the Sun from sunrise to sunset:

The Sun's path is similar to Star B in our previous Local Sky example:

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.

  • The daily motion of the Sun is just like a rise-and-set star

  • The Sun’s daily motion is from Earth’s rotation

  • The sun is highest around noon. In Fullerton, it is never at the zenith.

The Sun and Stars

When the Sun is above the horizon, our sky changes.

The stars are always there, but we can’t see them during the daytime because of scattered sunlight.

A local sky at Noon, facing South, with atmosphere: Local sky facing south. The Sun is high in the blue sky. Venus is labeled but cannot be seen.

The same local sky at Noon, facing South, without atmosphere: Local sky facing south. The Sun is high in the black night-like sky. Jupiter, Saturn, the Moon, Mercury, Venus, and Mars are all shown, along with bright stars Altair and Fomalhaut, and other dimmer stars.

Over one day, the sun and stars move together from East to West on the celestial sphere.

Observing the stars before sunrise, and after sunset, we can determine what constellation (region of the Celestial Sphere) the Sun appears in.

The Sun on the horizon, with half the stars of the constellation Leo above

Check your understanding: The Sun

#

You observe a star the Sun rising due East. When the star Sun reaches its highest position above the horizon, where will it be?

Northern and Southern Skies

Your view of space depends on where you stand.

Two views of earth, one tilted to show North America, one tilted to show South America. Speeeh bubbles from each show images of the constellation Orion, but they are flipped vertically compared to each other.

Think back to the features of our Celestial Sphere Model.

#

Where on Earth would you stand to see Polaris at your Zenith?

An image of earth at the center of the celestial sphere. Earth is oriented with the north pole at the top of the globe. Polaris is marked at the top of the celestial sphere.

#

Where on Earth would you stand to see Polaris on your horizon?

An image of earth at the center of the celestial sphere. Earth is oriented with the north pole at the top of the globe. Polaris is marked at the top of the celestial sphere.

Latitude

Your view of the sky depends on latitude.

Latitude is used to describe locations on Earth.

An image of the earth with three latitudes highlighted. The equator is around the middle and is labeled 0 degrees N. A parallel arc through Fullerton is labeled 34 degrees N. The arctic circle circles closer to the North Pole and is labeled 66 degrees N.

At a given latitude, the celestial pole is that latitude angle above the horizon in your local sky.

Three diagrams show the apparent motion of stars as seen by observers at different Earth latitudes. (a) At the North Pole, the North Celestial Pole is at the Zenith and stars move in circles around it, never rising or setting. (b) At the Equator, the North Celestial Pole lies on the horizon and all stars rise in the east and set in the west. (c) In the Northern Hemisphere, the North Celestial Pole is above the horizon; stars near it circle without setting, while others rise and set.

At a given latitude, you can have a mixture of types of star:

  • Circumpolar stars - never set below the horizon (e.g. Polaris for us)
  • Rise and set stars - rise in the east and set in the west (e.g. Betegeuse for us)
  • Never rise stars - never rise above the horizon (e.g. the Southern Cross for us)

Everyone at a given latitude sees the stars the same way.

Check your understanding: Latitude

#

An image of star trails arcing around the south celestial pole which is close to the horizon. Image: Kwon O Chul

The night-long photo exposure above was taken (near the top of a mountain) in what location?

#

At what location would all visible stars be circumpolar?

Sky Motion and Latitude

How does latitude affect star motion?

  • Stars appear to rotate counter-clockwise around the North Celestial Pole
  • Stars appear to rotate clockwise around the South Celestial Pole.
  • Rise and set stars always rise in the East and set in the West.
  • All these motions come from the rotation of the Celestial Sphere.
    • Really, the rotation of the Earth

In the Northern Hemisphere, the North Celestial pole is seen in the Northern sky.

This video shows how the stars move as seen in the Northern Hemisphere, looking in different directions.

  • Stars appear to rotate counter-clockwise around the North Celestial Pole.
  • Rise and set star paths are tilted to the South in Northern Hemisphere; stars move from East to West.

This next video shows how the stars move as seen in the Southern Hemisphere.

  • The South Celestial pole is seen in the Southern sky.

  • In the Southern Hemisphere, stars appear to rotate clockwise around the South Celestial Pole.

  • Rise and set star paths are tilted to the North in the Southern Hemisphere; stars still move from East to West.

  • All these motions come from the rotation of the Earth, as modeled with a Celestial Sphere. Two observers stand on the top of green spheres. One is labeled Northern Hemisphere and one Southern Hemisphere. A celestial sphere around the northern hemisphere shows counter-clockwise rotation, then flips over to the southern hemisphere showing clockwise rotation

Summary of Star Motion

  • Stars appear in fixed patterns on the Celestial Sphere.
  • The Celestial Sphere appears to rotate around the Earth once every day.
  • Every star completes a full circle every 24 hours.
  • Stars appear to rotate counter-clockwise around North Celestial Pole and clockwise around South Celestial Pole.
  • Rise and set stars always rise in the East and set in the West.

Check your Understanding: Motion and Latitude

#

You look at the sky and see stars moving as shown in the long-exposure star trail photo below. An arrow indicates the direction of an example star.

Star trails make circles high in the sky above a lake. An arrow below the center of the circles points to the right. Kyle Sullivan, BLM

You are in what hemisphere, facing what direction?

#

You look at the sky and see stars moving as shown in the long-exposure star trail photo below. An arrow indicates the direction of an example star.

Star trails make circles around a point to the right of the image above a desert. An arrow to the left of the circles points up from the horizon Credit: R. Wesson/ESO

You are in what hemisphere, facing what direction?

#

You look at the sky and see stars moving as shown in the long-exposure star trail photo below. An arrow indicates the direction of an example star.

Star trails go from upper left to lower right in the image. An arrow points downward and to the right along a trail. Credit: Antoine Lamielle

You are in what hemisphere, facing what direction?

Go Further