Lesson 2: The Celestial Sphere and circumpolar motion
This lesson will require approximately one 30-minute period.
- draw a conclusion, based on evidence gathered through research and observation, that answers an initial question (e.g., conclude that simulated flour craters are deeper and wider when the marble is heavier or is dropped from greater heights)
- demonstrate how Earth's rotation causes the day and night cycle and how Earth's revolution causes the yearly cycle of seasons
By examining applets on the celestial sphere and circumpolar motion, students will gain an understanding of the Earth's rotation, and how it affects our perception of stellar movement.
In this lesson, students will view the applets, The Celestial Sphere and Circumpolar Motion, in a computer lab, to gain an understanding of the Earth's rotation, and how it affects our perception of stellar movement.
Materials and Resources
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- celestial sphere
- celestial pole
- circumpolar motion
- diurnal motion
- right ascension
Developing the Lesson
Open class by asking students whether the stars' movement in the sky is a product of their physical movement, or of the Earth's movement. Discuss briefly.
In the lab, have students study the Celestial Sphere and Circumpolar Motion applets and write a short paragraph describing their interpretation of them. Discuss some of the ideas presented before going through the applets with the class, explaining the details of the Earth's rotation. Explain that:
The first astronomers believed that the stars were fixed on a celestial sphere surrounding the Earth. Although we now know this to be untrue (the universe is three-dimensional and stars are located at various distances from the Earth), it still helps to use this illustration to better visualize and understand the night sky. Using this representation, the stars do not move with respect to each other, but they do move in our night sky due to diurnal motion, the Earth's rotation on its axis. As the Earth rotates within the celestial sphere, stars will rise in the East and will travel across our sky from East to West. Astronomers use a coordinate system for the celestial sphere much like the coordinate system on the Earth. Right ascension is analogous to longitude, as is declination to latiitude. Right ascension is broken into 24 hours, with smaller divisions of minutes and seconds. While it is not a measurement of time, right ascension is related to time because the entire sky rotates once in about 24 hours. Declination is measured in degrees like on the Earth, but it is generally listed as positive and negative instead of north and south.
Stars appear to circle around the north celestial pole, which happens to be near the star called Polaris (better known as the North Star). Just as the North Pole of the Earth is stationary while the Earth turns, the north celestial pole also appears to be stationary. This phenomenon is clearly visible in time lapse photography of the night sky over a few hours; stars will leave trails that circle around Polaris. This motion is apparent simply by looking at a recognizable or prominent star early in the evening and returning a few hours later to see how the star's position has shifted in the sky. The further north one is from the equator, the higher the North Star will be in the sky, until at the North Pole, where Polaris will be directly overhead at the zenith. As the apparent position of the north celestial pole rises in the sky with the increasing latitude on the Earth, more of the sky becomes circumpolar. Circumpolar stars are ones which do not set below the horizon through the course of a year.
Solicit student questions.
No evaluation is necessary for this lesson.
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