Unit 3 : Activity 3 : Earth's Rotation : Daily Motion : Equatorial Coordinates : Meridian Diagrams : Unit Exam

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations

Daily Motion

As Earth turns, in the sense described in the previous sub-unit, we see the sky appearing to revolve around the earth.  Notice that the sky appears to revolve in the clockwise direction or "left-handed" since it appears to move in the opposite direction of the earth's actual rotation (recall your right hand model). This is illustrated below.  Please note that the dashed lines are on the back of the three dimensional sphere.

In this picture, Ames, Iowa is again on top.  So, if you are in Ames, you can imagine that you are standing at the top of this Earth globe watching the sky spinning around you.

You will not actually see all of the sky at once, of course.  You will only see the half of the sky that is above your horizon.   The parts that are below your horizon are parts that you would have to look through the solid Earth to see.

Here is the same diagram with the Ames, Iowa horizon added.

The little circle with an arrow through it is meant to represent the earth as shown in the previous diagram, the earth with Ames, Iowa on top.  So the Ames observer's zenith is straight up, and his or her horizon is horizontal (right to left).

Now, if we zoom in and look at just the parts that the Ames observer can see, you get the following:

In the picture above, we have cut away the parts of the sky that the observer can not see since they are below the local horizon.  This picture thus represents the apparent motion of objects in the sky in Ames, Iowa as a natural result of the planet's rotation.

In the picture below, we have added some arrows that show the direction to the Zenith, the North Celestial Pole, and the north and south points on the horizon.  All of these points lie along the observer's meridian.  The meridian is the imaginary line that passes from one horizon, through the zenith and North Celestial Pole.  We have also added an arrow to show the direction to where the celestial equator and the observer's meridian cross.

The arrows in this picture show the fundamental reference points along your meridian.  These are the basis for the meridian diagram, discussed in detail later.

Diurnal motion and latitude:

At different latitudes the North Celestial Pole (roughly where the North Star is located) will lie at different altitudes above the horizon.  If you are south of the equator, then the North Celestial Pole will be below the local horizon.  For an observer who lives at the earth's equator, the North Celestial Pole and the South Celestial Pole will both lie exactly on the horizon.

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