The The Celestial Sphere

Using Stellarium to understand the Celestial Sphere

The following is from the Stellarium "Manual" Right click to Download the Manual as PDF File.

You can download a pdf of Stellarium Keyboard shortcuts here

The Celestial Sphere is a concept which helps us think about the positions of objects in the sky. Looking up at the sky, you might imagine that it is a huge dome or top half of a sphere, and the stars are points of light on that sphere. Visualizing the sky in such a manner, it appears that the sphere moves, taking all the stars with it—it seems to rotate. If watch the movement of the stars we can see that they seem to rotate around a static point about once a day. Stellarium is the perfect tool to demonstrate this!

1. Open the configuration window, select the location tab. Set the location to be some- where in mid-Northern latitudes. Vancouver, WA is perfect! (Latitude:N 45° 38' 7.5182" Longitude:W 122° 39' 5.6944")
2. Turn off atmospheric rendering and ensure cardinal points are turned on. This will keep the sky dark so the Sun doesn’t prevent us from seeing the motion of the stars when it is above the horizon.
3. Pan round to point North, and make sure the field of view is about 90◦.
4. Pan up so the ‘N’ cardinal point on the horizon is at the bottom of the screen.
5. Now increase the time rate. Press k, l, l, l, l - this should set the time rate so the stars can be seen to rotate around a point in the sky about once every ten seconds If you watch Stellarium’s clock you’ll see this is the time it takes for one day to pass as this accelerated rate.

The point which the stars appear to move around is one of the Celestial Poles. The apparent movement of the stars is due to the rotation of the Earth. The location of the observer on the surface of the Earth affects how she perceives the motion of the stars. To an observer standing at Earth’s North Pole, the stars all seem to rotate around the zenith (the point directly upward). As the observer moves South towards the equator, the location of the celestial pole moves down towards the horizon. At the Earth’s equator, the North celestial pole appears to be on the Northern horizon.

Similarly, observers in the Southern hemisphere see the Southern celestial pole at the zenith when they are at the South pole, and it moves to the horizon as the observer travels towards the equator.

1. Leave time moving on nice and fast, and open the configuration window. Go to the location tab and click on the map right at the top - i.e. set your location to the North pole. See how the stars rotate around a point right at the top of the screen. With the field of view set to 90◦ and the horizon at the bottom of the screen, the top of the screen is the zenith.
2. Now click on the map again, this time a little further South, You should see the positions of the stars jump, and the centre of rotation has moved a little further down the screen.
3. Click on the map even further towards and equator. You should see the centre of rotation have moved down again. To help with the visualization of the celestial sphere, turn on the equatorial grid by clicking the button on the main tool-bar or pressing the on the e key. Now you can see grid lines drawn on the sky. These lines are like lines of longitude and latitude on the Earth, but drawn for the celestial sphere.

The Celestial Equator is the line around the celestial sphere that is half way between the celestial poles - just as the Earth’s equator is the line half way between the Earth’s poles.

Coordinate Systems

Altitude/Azimuth

The Altitude/Azimuth coordinate system can be used to describe a direction of view (the azimuth angle) and a height in the sky (the altitude angle).

The azimuth angle is measured clockwise round from due North. Hence North itself is 0◦, East 90◦, Southwest is 135◦ and so on.

The altitude angle is measured up from the horizon. Looking directly up (at the zenith) would be 90◦, half way between the zenith and the horizon is 45◦ and so on. The point opposite the zenith is called the nadir.

The Altitude/Azimuth coordinate system is attractive in that it is intuitive - most people are familiar with azimuth angles from bearings in the context of navigation, and the altitude angle is something most people can visualize pretty easily. However, the altitude/azimuth coordinate system is not suitable for describing the general position of stars and other objects in the sky - the altitude and azimuth values for an object in the sky change with time and the location of the observer.

Stellarium can draw grid lines for altitude/azimuth coordinates. Use the button on the main tool-bar to activate this grid, or press the z key.

Note

In order to transmit useful astronomical information using Alt/Az you need to know your latitude and the date and time of the observation.
With Azimuth, altitude, latitude and time, you can convert the position of the celestial observation to the Equatorial System

Right Ascension / Declination Coordinates

Like the Altitude/Azimuth system, the Right Ascension/Declination (RA/Dec) coordinate system uses two angles to describe positions in the sky. These angles are measured from standard points on the celestial sphere. Right ascension and declination are to the celestial sphere what longitude and latitude are to terrestrial map makers.

The Northern celestial pole has a declination of 90◦, the celestial equator has a declination of 0◦, and the Southern celestial pole has a declination of -90◦.

Right ascension is measured as an angle round from a point in the sky known as the first point of Aries, in the same way that longitude is measured around the Earth from Greenwich. Figure F.2 illustrates RA/Dec coordinates.

Unlike Altitude/Azimuth coordinates, RA/Dec coordinates of a star do not change if the observer changes latitude, and do not change over the course of the day due to the rotation of the Earth (the story is complicated a little by precession and parallax - see sections F.4 and F.5 respectively for details).

RA/Dec coordinates are frequently used in star catalogues such as the Hipparcos catalogue.

Stellarium can draw grid lines for RA/Dec coordinates. Use the button on the main tool-bar to activate this grid, or press the e key.