This activity shows two nearly identical models of the Two-Sphere Universe. The primarily differ by the coordinate system they are drawn in. The first of the two models uses a natural coordinate system for the celestial sphere while the second of the two models uses a natural coordinate system for an observer on Earth.

This applet shows the celestial sphere as part of the two-sphere model of the universe. This version is emphasizes the stars' perspective. Within the model itself, the observer's North Celestial Pole is fixed on the z-axis while the Celestial Equator is fixed in the xy-plane. [br]Several points on the celestial sphere are labeled. NCP and SCP stand for the North and South Celestial Poles. VE, SS, AE, and WS respectively indicate the Vernal Equinox, Summer Solstice, Autumnal Equinox and Winter Solstice. [br]Various "stars" are placed on the sphere. Their placement is arbitrary and does not reflect the location of any actual stars.

The Daily Motion checkbox will cause the celestial sphere to rotate around the North Celestial Pole from East to West. The Annual Motional checkbox will cause the Sun to travel around the Ecliptic from West to East. The Speed slider adjusts how quickly the celestial sphere rotates. When Speed = 1, a sidereal day passes every 10s of real time. The Year Length slider controls the speed of the sun around the ecliptic relative to the speed of the celestial sphere. It's value determines how many sidereal days it takes the sun to complete one path around the ecliptic. [br][br]The Latitude slider is used to tilt the Zenith and Horizon according to the observer's latitude. [br][br]When the Annual Motion checkbox is not checked, the Annual Motion slider will appear. This may be used to place the sun at a specific location along the ecliptic. A value of 0° corresponds to the Vernal Equinox. Values of 90°, 180°, and 270° correspond to the Winter Solstice, Autumnal Equinox and Summer Solstice respectively. [br][br]The Show Horizon checkbox can be unchecked to hide the blue extension of the horizon plane. [br][br]The Obliquity Slider allows the user to adjust the angle between the ecliptic the celestial equator. On Earth, this angle is measured to be 23.4°.

This applet shows the celestial sphere as part of the two-sphere model of the universe. This version is emphasizes the observer's perspective. Within the model itself, the observer's zenith is fixed on the z-axis while their horizon is fixed in the xy-plane. [br]Several points on the celestial sphere are labeled. NCP and SCP stand for the North and South Celestial Poles. VE, SS, AE, and WS respectively indicate the Vernal Equinox, Summer Solstice, Autumnal Equinox and Winter Solstice. [br]The values of Altitude and Azimuth show the position of the Sun as seen by the observer. When the Sun is below the horizon, the value for Altitude may be thought of as negative. [br]Various "stars" are placed on the sphere. Their placement is arbitrary and does not reflect the location of any actual stars.

The Daily Motion checkbox will cause the celestial sphere to rotate around the North Celestial Pole from East to West. The Annual Motional checkbox will cause the Sun to travel around the Ecliptic from West to East. The Speed slider adjusts how quickly the celestial sphere rotates. When Speed = 1, a sidereal day passes every 10s of real time. The Year Length slider controls the speed of the sun around the ecliptic relative to the speed of the celestial sphere. It's value determines how many sidereal days it takes the sun to complete one path around the ecliptic. [br]The Latitude slider is used to tilt the celestial sphere according to the observer's latitude. [br][br]When the Annual Motion checkbox is not checked, the Annual Motion slider will appear. This may be used to place the sun at a specific location along the ecliptic. A value of 0° corresponds to the Vernal Equinox. Values of 90°, 180°, and 270° correspond to the Winter Solstice, Autumnal Equinox and Summer Solstice respectively. [br][br]The Show Horizon checkbox can be unchecked to hide the blue extension of the horizon plane. Likewise, the stars and celestial sphere itself can be hidden.