There are two ways of viewing the night sky. The view from Earth where the sky appears to rotate over head and the position of the Sun moves in a circle during the year. Or the view one would have from space if you were looking at the Solar System from outside. Then you would see the Earth rotating while the stars remained fixed and you would see the Earth go around the Sun in a year. Neither of these perspectives are right or wrong, they are just different views of the same thing.
Coordinate Systems
The two different views can be expressed as two different coordinate systems. That is two ways to address the location of a star. The view from Earth is called horizon coordinates. The point where the sky meets the ground is called the horizon, we assume there are no mountains, buildings or trees blocking your view and you can see down to the ground all around you. The zenith is the name for the point directly above your head. In this system, locations are given by two angels, azimuth and altitude. Azimuth is an angle from 0 to 360 staring with 0 pointing north going around the horizon. It is the usual north/south/east/west but in degrees. Altitude is the angle starting at 0 at the horizon and going up to 90 at the zenith.
The celestial coordinate system is very similar to the longitude-latitude system we use for locations on Earth. The names are instead called right ascension and declination, but they measure the same things going around the celestial equator and the angle from the celestial equator. The only weird thing about the system is that right ascension is measured in hours instead of degrees. Going around the circle is 24 hours instead of 360 degrees, so each hour equals 15 degrees. The reasoning behind this weird unit is that the Earth rotates in the direction of right ascension so for example a star whose right ascension is 6 hours greater that another star will also rise 6 hours later.
Laboratory Tools
For this lab we will be using the Rotating Sky Explorer developed at the University of Nebraska-Lincoln. Click on the link to open it, you will need Java to run for it to work. If you have trouble try to enable Java or use a different browser.
The Rotating Sky Explorer shows the celestial sphere view and the horizon view side by side. You can change the perspective you have on either view by click-dragging the mouse on that view. Note this only change how you are looking at the view, note the location of the observer in the view.
At the bottom are four controls for the explorer.
- Observer’s Location – allows you to set the location (longitude, latitude) of the observer. You can drag the circle on the map if you don’t know the numbers. New York City has a longitude of 74° W and a latitude of 40.7° N.
- Animation Controls – allows you to start and stop the rotation of the Earth and control the speed at which it rotates.
- Appearance Settings – allows you to turn on/off various lines and labels.
- Star Controls – lets you add/remove stars and turn on/off star trials. You can also add stars by shift-clicking with the mouse and delete them with delete-clicking over either of the views. You can move any star by dragging it in either view.
Clicking on a star will make it active showing you its coordinates in both views. Click on a blank region to make no star active.
Assignment
Observer on the North Pole. The connection between the two coordinate systems is easiest to understand for an observer who is at the North Pole.
Set your observer to the North Pole. Add 3 random stars and start the animation. Notice how the stars move in the horizon view. Record the coordinates of your 3 stars in a table like this:
| Star | right ascension | declination | azimuth | altitude | |
| A | |||||
| B | |||||
| C |
Add the constellations of The Big Dipper, Orion and the Southern Cross in the explorer. Notice how they move as the Earth rotates. Do the stars rise and set for this observer? How do stars move in the sky for this observer?
Observer on the Equator. Move your observer to the equator. Let the animation run and observe how the stars and constellations move now. Record the coordinates of your 3 stars for your new location in another table. Do the stars rise and set for this observer? How do they move in the sky?
Observer in New York City. Move your observer to New York City. Again let the animation run and observe how the stars and constellations move. Record the coordinates of your 3 stars for your new location in another table. Read this about Do the stars rise and set for this observer. Read this about star paths. Determine the declinations that divide rise and set stars, circumpolar and never-rise stars.
Questions
- When your observer is at the North Pole, what is the relationship between declination and altitude? Do you think the Sun and Moon would rise and set?
- From New York City can the big dipper always, sometimes or never be seen? How about Orion and the Southern Cross?
- From what you’ve learned is there a way to determine your latitude from the paths of stars in the sky. If so, how would you try and do this?
