Activity 2: Understanding the Celestial Globe

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Introduction: Although the Earth is traveling around the Sun, it appears to observers on Earth that the Sun, the Stars and all celestial objects are traveling around the Earth. When we observe the sky away from city lights and other distractions, we experience the illusion of a huge hemispherical dome over our heads. Ancient astronomers noticed this as well and modeled the location of celestial objects as if they were located on this dome, a celestial sphere. Originally each location on Earth had its own unique coordinate system, but astronomers realized they needed a universal celestial sphere. The goal of this exercise is to familiarize you with the celestial globe, both local and universal, and give you experience transferring between three-dimensional and two-dimensional models.

The Celestial Globe: In this activity, you will be modeling the celestial sphere in two-dimensions, using a drawing similar to the following.

East North South West The area shaded gray represents the portion of the sphere that is below the horizon, while the clear portion represents the portion above the horizon. In this activity, you will also be provided with a three-dimensional sphere. The three dimensional sphere should be used to help with your modeling of the celestial sphere, but you should record your drawings on the two dimensional spheres provided in this activity. When making a measurement on the celestial sphere, we use angles to represent positions on the Celestial Sphere. In this activity you will be required to measure angles on the Celestial Sphere. 1. What do you think? Why do we measure positions on the Celestial Sphere in angles? (Graded Completion Only)

Any Answer Acceptable

Since you will have both a two-dimensional and a three-dimensional model of the Celestial Sphere, you may want to measure angles on both. You need to know how to do this. There are two possible simple methods for measuring the angle on the two dimensional sphere. The first of these methods only work for measuring angles along the outside circle forming the sphere.

Method 1: Using a Protractor to measure angles in Two-Dimensions · Decide on an angle that you wish to measure · Find the Center of the Sphere · Draw a straight line through the center of the sphere and the point you wish to measure your angle from. (Zenith for Local Coordinates and the Celestial Pole for Celestial Coordinates) · Draw a straight line perpendicular to this first line through the center of your sphere. · Align the center of your protractor on the center of the sphere, and the bottom of your protractor with the line drawn in the previous step. · Read the angle you wish to measure off of the protractor and make a mark at this point. · This is your angular position along the meridian.

2. Why does this measurement only work for points along the outside circle?

The. The angles measured inside the circle are not measured accurately, because the two-dimensional representation flattens the celestial sphere distorted the sphere.

Method 2: Using the Angular Overlay Your instructor will provide you with a transparency with the angles of declination marked on it. You can use this to measure angles on any celestial sphere, local or universal. The overlay is the same size as the two-dimensional spheres shown in this activity. To measure an angle, follow these steps. · Decide on angle that you wish to measure. · Overlay the transparency with your sphere · Align the North Pole of the transparency with the point you wish to measure your angle from. (Zenith for Local Coordinates and the Celestial Pole for Celestial Coordinates) · Measure the appropriate angle.

3. Compare the two types of measurements by marking the position of an object 68 above the north point on the sphere. Make a mark on the sphere for each of your angular measurements.

East North South West

4. Which measurement is more accurate? Why?

The protractor method is more accurate, because it allows you to precisely make a measurement. 5. Which measurement is easier to make? Why?

Using the angular overlay is easier to make, because you can simply line it up with the sphere and make a measurement. 6. Why could the second method be used to measure angles off of the outside circle? The angular overlay has lines of constant declination measured on it showing the distortion due to flattening. Observing on Earth You are now ready to set up the celestial sphere to observe for any location on Earth. In this portion of the activity, we will use both a Local Celestial Sphere (On that only works at your location) and a Universal Celestial Sphere (One that works at all locations on Earth). Let’s start by observing in Edwardsville, IL (40 N). 7. Align the Celestial Globe for Edwardvilles latitude. 8. Set up a local coordinate system for Edwardsville To this you will need a horizon, meridian and a zenith. Mark these on the following sphere.

Zenith

NCP Celestial Equator Meridian

East North Horizon South West

9. Why did you place these coordinates in the positions that you did?

Zenith – Directly overhead Horizon – Plane under your feet Meridian – Imaginary line stretching from the northern horizon, through the zenith to the southern horizon. 10. What coordinates are necessary to pinpoint an objects location on the Local Celestial Sphere?

Direction of observation and altitude above horizon

11. On the same sphere on the previous page, mark the locations of the key features of the Universal Celestial Sphere – the North Celestial Pole and the Celestial Equator. Do not simply estimate their positions, make a measurement on the celestial globe.

12. Why did you place these coordinates in the positions that you did?

North Celestial Pole is 40 above the northern horizon equal to our latitude. Celestial Equator rises due East, crosses the meridian 51 above the Southern horizon (our co-latitude) and sets due west. 13. How accurate is the Celestial Globe?

Measurements on the celestial globe our slightly off of the actual values, 3 14. What coordinates are necessary to pinpoint an objects location on the Universal Celestial Sphere?

Right Ascension and Declination

15. Your Universal Celestial Sphere coordinates are offset with respect to your local coordinate system. How much are they offset by?

Offset equal to our co-latitude (90-latitude) or 50

16. Use the Celestial Globe to set the night sky for two different locations on Earth. One in the southern Hemisphere and one in the Northern Hemisphere. 17. On the Celestial diagrams on the following page, set up both local and universal coordinate systems for your two different locations on Earth. 18. Make sure you mark all relevant points and specify the latitude at which you are observing. Location on Earth:

East North South West

Location on Earth:

East North South West

19. In general, how do the positions of the coordinates change for different locations on Earth? Be sure to discuss both local and universal coordinates.

The local coordinates (horizon, zenith and meridian) remain the same, while the position of the universal coordinates (NCP and Celestial Equator) change positions according to your latitude on earth.