Randy L. Phelps.

The following Applet was written and provided by Walter Fendt and is copyrighted by him. The right of commercial use remains with that author.

The purpose of this exercise is to illustrate the daily motions of stars/objects on the celestial sphere, as seen from the Earth's Equator. Be sure to complete all three exercises as described on the previous page.

Exploration

Anticipate the Results

Instructions/Sugestions to Help Answer These Questions

1. The Appearance of the Sky at the Earth's Equator

Where is the North Celestial Pole as seen by the observer?
Where is the Celestial Equator as seen by the observer?
What particular star, if any, is located near the zenith?

Set the latitude in the applet for the Earth's Equator and hit the "Enter" key - Note: you must know what the latitude is at the Equator and you must hit the "Enter" key after inputing into the applet!
Note the positions of the observer (O), the North Celestial Pole (NP), the celestial equator (the cirle on which the letter V is marked), the zenith (Ze), and a specified star (ST).

2. Circumpolar Stars

At the Equator, what stars are circumpolar?
At the Equator, are all objects north of the Celestial Equator visible at some time during the year?
At the Equator, are all objects south of the Celestial Equator visible at some time during the year?
What fraction of the stars in the sky are visible over the course of the year at the Equator?
If a camera were pointed toward the zenith, and a time exposure photograph were take, what would the resulting picture look like?

Recall what circumpolar stars are...
Set the applet as described in Exploration 1 above.
Position the cursor over the "Rotation/Pause" icon, and click on it.
Note the motion of the indicated star (ST).
Stop the simulation by clicking "Rotation/Pause"
Change the "Declination" of the star from the default value (20^o) to a value from 0^o to 90^o. A value of 0^o corresponds to a star located on the Celestial Equator, while a value of 90^o corresponds to a star at the North Celestial Pole. Restart the simulation.
Repeat the simulation for stars with various "Declinations" between 0^o to 90^o, inclusive.
Repeat the exercise for "Declination" values from 0^o to -90^o. The minus indicates stars located south of the Celestial Equator.
In all cases, note how large the circle the star makes on the sky is at various distances from the celestial pole, and see if the star dips below the horizon.

Now try to explore the following. First think about the question, and then determine what parameters in the applet need to be changed to help you explore the concept.