Uploaded November 2021 | Updated September 2026, 2 hours ago
If you spend a few hours out under a starry sky, you’ll notice that the universe seems to be circling around us, with stars moving gradually across the sky from east to west.
Many ancient people took this appearance at face value, concluding that we lie at the center of a universe that rotates around us each day. Today we know that the ancients had it backward: It is Earth that rotates daily, not the rest of the universe. We can picture the movement of the sky by imagining the celestial sphere rotating around Earth
From this perspective you can see how the universe seems to turn around us: Every object on the celestial sphere appears to make a simple daily circle around Earth. However, the motion can look a little more complex in the local sky, because the horizon cuts the celestial sphere in half.
This image shows the idea for a typical Northern Hemisphere location (latitude 40°N). If you study this image carefully, you’ll notice the following key facts about the paths of various stars through the local sky:
Stars near the north celestial pole are circumpolar stars, meaning that they remain perpetually above the horizon, circling (counterclockwise) around the north celestial pole each day.
Stars near the south celestial pole never rise above the horizon at all. This means that there are stars in the southern sky that you’ll never see from your house if you live in the United States.
All other stars have daily circles that are partly above the horizon and partly below it, which means they appear to rise in the east and set in the west.
Earth rotates from west to east (black arrow), making the celestial sphere appear to rotate around us from east to west (red arrows).
So for the local sky for a location at latitude 40°N, like California,, Colorado, Pennsylvania, and New Jersey, Italy, Greece, Turkey, China, North Korea, and Japan, The horizon slices through the celestial sphere at an angle to the celestial equator, causing the daily circles of stars to appear tilted in the local sky. This is essentially what we’d see. Some stars stay above the horizon, always in our view and some appear to rise and set.
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
If you spend a few hours out under a starry sky, you’ll notice that the universe seems to be circling around us, with stars moving gradually across the sky from east to west.
Many ancient people took this appearance at face value, concluding that we lie at the center of a universe that rotates around us each day. Today we know that the ancients had it backward: It is Earth that rotates daily, not the rest of the universe. We can picture the movement of the sky by imagining the celestial sphere rotating around Earth
From this perspective you can see how the universe seems to turn around us: Every object on the celestial sphere appears to make a simple daily circle around Earth. However, the motion can look a little more complex in the local sky, because the horizon cuts the celestial sphere in half.
This image shows the idea for a typical Northern Hemisphere location (latitude 40°N). If you study this image carefully, you’ll notice the following key facts about the paths of various stars through the local sky:
Stars near the north celestial pole are circumpolar stars, meaning that they remain perpetually above the horizon, circling (counterclockwise) around the north celestial pole each day.
Stars near the south celestial pole never rise above the horizon at all. This means that there are stars in the southern sky that you’ll never see from your house if you live in the United States.
All other stars have daily circles that are partly above the horizon and partly below it, which means they appear to rise in the east and set in the west.
Earth rotates from west to east (black arrow), making the celestial sphere appear to rotate around us from east to west (red arrows).
So for the local sky for a location at latitude 40°N, like California,, Colorado, Pennsylvania, and New Jersey, Italy, Greece, Turkey, China, North Korea, and Japan, The horizon slices through the celestial sphere at an angle to the celestial equator, causing the daily circles of stars to appear tilted in the local sky. This is essentially what we’d see. Some stars stay above the horizon, always in our view and some appear to rise and set.
If you'd like to support me in this adventure of creating a home-made Masters level course for everyone, please consider donating $1/month at patreon.com/spacebutmessier
