## Astronomy with a Stick

AWS home### AWS Unit 3: Making and Using Models

**Introduction :: Activity 1 :: Activity 2**

### Activity 1

**Objective **

To construct a longitude-latitude grid on a sphere.

**Time Allotment**

It will take 45 minutes to an hour to construct the miniature globes.

**Procedure**

- Spin the sphere in a counterclockwise direction until a nearly motionless spot appears at its top. Stick a pin in this spot and flag it NP for north pole.
- Locate the point on the sphere opposite the North Pole and stick a pin in it. Flag it SP for South Pole. If you have trouble locating the South Pole, you can press a flat thumbtack flush into the North Pole spot and then turn the sphere over so that the thumbtack acts as a bearing to spin the Earth on. This will reveal the South Pole as another motionless spot. This time, because the North Pole is down, you should spin the Earth clockwise.
- Find the greatest circumference of the sphere, equidistant from the two poles and at right angles to the axis of rotation. Encircle the sphere at this point with a thin colored strip of paper or ribbon. Label it Equator.
- At places that measure one quarter, one half; and three quarters of the distance between the equator and the pole, pin three more strips of colored paper or ribbon above the equator to represent northern latitudes and three strips below the equator to represent southern latitudes. The most northern and most southern strips represent, respectively, the Arctic Circle and the Antarctic Circle. The two strips nearest the equator represent the Tropic of Cancer in the north and the Tropic of Capricorn in the south. The two strips in the middle represent 45° north and 45° south, respectively.
- Pin twelve strips of another colored paper or ribbon around the sphere vertically (at right angles to the equator) to form lines of longitude. All the lines of longitude should intersect at the poles and be equidistant from each other at the equator.

**Discussion **

- Discuss the fact that a rotating sphere turns on an axis. The North and South Poles are designations given to the points at the ends of the mathematical axis around which the Earth rotates. There is no pole through the Earth around which it turns-as seen in a globe, for example. The axis comes into being when a sphere rotates-it is a mathematical concept only.
- Great circles are drawn on a sphere at its greatest circumference. This is because a great circle must always have its center at the center of the sphere. Great circles can go around a sphere at any angle. On the Earth, a great circle perpendicular to the axis and equidistant from each pole is called the equator. None of the other lines of latitude are great circles because their planes do not go through the center of the sphere. They are parallel to the equator and smaller in circumference. We call them parallels.
- The longitude lines are all great circles. Each of them has a plane that passes through the center of the sphere. Longitude lines are also called meridians. Meridians spaced 15° apart at the equator starting at the prime meridian mark the centers of the time zones. Every 15° equals one hour of time (divide 360° by 24 hours). The prime meridian, which passes through Greenwich, England, is the one we designate as 0°. All other meridians are designated with a plus number or the letter W if they are west of Greenwich and a minus number or the letter E if they are east of Greenwich.

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