Planets, Stars, Galaxies, and the Universe

Day and Night


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The most obvious change in the sky is that for about half of the day the sky is brightly lit and, for the other half, it is dark. If the sky is clear, we can tell that the Sun is “up” during the day and “down” at night. Even though this is obvious to most observers, there is one question about day and night that took many centuries to solve:

Is the Earth stationary and is the Sun orbiting it, or vice versa?

In this animation, above, we first see the motion of the Sun from east to west that we are familiar with seeing over the course of the daylight hours on one day.  In the second stage of this animation, we see a stationary observer on a stationary Earth watching a moving Sun.  This would reproduce what we see of the Sun appearing to move from east to west across our sky.  The last step of the animation shows another alternative; the Sun is at a stationary location from our perspective, and the Earth is rotating, causing the Sun to appear to move.

We will discuss some of the history of this question (which is a very interesting discussion of the history of astronomy) in Lesson 2, but for now I will directly answer the question.

The Earth rotates around an imaginary line that goes through the North and South Poles, which is called the axis of rotation. 

This animation shows the rotation of the Earth along the axis that goes through the North and South poles.  Here is an example of how this rotation of the Earth makes the Sun appear to move across the sky: If an object is at a fixed location on a wall (for example, a clock) and I am facing it, then I can see the clock. If I rotate so that I'm facing away from the clock, I can no longer see it. While I am rotating, the clock will, from my point of view, appear to be moving. You can try this out wherever you are right now!

If the Sun is in a "fixed" location on the sky and the Earth is facing this location, then we can see it (daytime!). When the Earth has rotated by 180 degrees, we are no longer facing the Sun, so we can't see the Sun (nighttime!). From our point of view fixed to a specific location on the Earth, as the Earth rotates it is the Sun that appears to move, just like the clock in my example above. This is our modern understanding of day and night.

This has consequences beyond day and night. The Earth's rotation causes every object in the sky to appear to trace out a path on the sky from East to West. So if you pick out a star in the sky, it will appear to make an arc across the sky. This is because, for our purposes, we can consider the star to be fixed in space, and the Earth is rotating underneath it. This happens slowly, so from minute to minute you don't notice the star's motion. However, over the course of a few hours you will be able to tell that the stars have moved a substantial distance on the sky. Since, like the Sun, the stars will (for the most part) appear to rise in the east and set in the west, so the apparent motion of a star will depend on which direction you face. An example follows.  In the top left panel, you see stars appear to rotate counterclockwise around the North Celestial Pole, which is what we would see if facing north.  In the top right panel, you see stars rising in the east, moving along circular, clockwise paths, and setting in the west, which is the behavior we would see if facing south.  The lower panel shows the stars rising diagonally from the east on the first part of this circular path, which is what we would see if facing east.

Reproducing the above animation with real observations of the stars is a popular type of astrophotography. An example is an image following of the apparent motion of the stars across the sky as seen from Mt. Kilimanjaro.

Time-lapse photo of apparent motion of stars across the night sky as seen from Mt. Kilimanjaro
The Star Trails of Kilimanjaro

The picture was created by leaving the camera shutter open for more than three hours (Note: can you tell how long the exposure was? How might you figure this out?), so it shows the path the stars in the sky follow over a small fraction of the night. This image also illustrates another important point—the Sun and the stars all appear to rotate around one point, which is the point directly above the North Pole of the Earth for observers in the northern hemisphere (called the North Celestial Pole or NCP), or the point directly above the South Pole of the Earth (the South Celestial Pole or SCP) for observers in the southern hemisphere. There is a star that is positioned very close to the NCP, which is called Polaris, the Pole Star. It is NOT the brightest star in the sky; it isn't even in the top 25. The importance of Polaris is that it roughly marks the location of the NCP, so all stars visible to observers in the northern hemisphere appear to rotate around Polaris.

Test this with Starry Night!

  1. Open up Starry Night.
  2. Using the bar at the top, set the time to a time after Sunset, say 9:00 p.m.
  3. Set the time flow rate to 300x.
  4. Face South (if you aren't already, type "S").
  5. Press the play button, and watch the stars move on the sky from 9:00 p.m. - 11:00 p.m.
  6. Reset to 9:00 p.m. and then do the same, but facing East and then West. How did the motion of the stars differ?

We can prove to ourselves that the apparent motion of objects in the sky depends on your location on the Earth! For example, consider the following cases:

  • If you are standing at the north pole, Polaris will be directly overhead. From your point of view, all stars will appear to make horizontal, circular paths around your zenith.
  • If you were standing on the Earth’s equator (say near Mt. Kilimanjaro, as in the image linked above), you would have to look due North towards your horizon to find Polaris, so the stars would appear to rise almost vertically from the eastern horizon, and the point around which they appear to rotate is near or at the northern horizon.
  • If you are in between the equator and the north or south poles (say in State College, PA), the point around which all the objects appear to rotate is about midway between your horizon and the zenith (at an altitude equal to your latitude, approximately 40 degrees).

Note that for observers at most locations above or below the equator, there are some stars that are near the NCP, and these trace out small circular paths that are never below the horizon. These stars that never rise or set are called Circumpolar Stars. The lower the altitude Polaris appears from your viewing location (that is, the closer your latitude is to that of the equator), the fewer the number of stars that are circumpolar from your point of view.

Test this with Starry Night!

  1. Open up Starry Night.
  2. Change your location to the North Pole.
  3. Set the time to 9:00 p.m.
  4. Set the time flow to 300x.
  5. Press the play button and watch the stars move on the sky.
  6. Do the same for the equator by choosing a location near or on the equator, such as most places in Ecuador.
  7. Face North and watch the stars move on the sky. How did the motion of the stars differ? Can you visualize why the stars behave in this way?

There are many tools to help you visualize the celestial sphere and the relationship between your location on Earth and the apparent motion of astronomical objects.  One good one is the Rotating Sky Explorer from the University of Nebraska.  It should default to showing you a space-based view of Earth and a ground-based view of the sky from a location in the mid-northern latitudes.  You can turn on a few familiar constellations and start the animation to see why the sky appears to change depending on your location.