The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The 88 official constellations cover the celestial sphere. The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful applet for learning about the sky. When discussing this slide, be sure to explain: North celestial pole South celestial pole Celestial equator Ecliptic It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.

Why do stars rise and set? The answer to the question is very simple if we look at the celestial sphere from the “outside.” But of course, we are looking from our location on Earth, which makes the motions of stars look a little more complex… Earth rotates from west to east, so stars appear to circle from east to west.

Our view from Earth: Stars near the north celestial pole are circumpolar and never set. We cannot see stars near the south celestial pole. All other stars (and Sun, Moon, planets) rise in east and set in west. Now explain the basic motion of the sky seen from Earth.

Thought Question What is the arrow pointing to in the photo below. A Thought Question What is the arrow pointing to in the photo below? A. the zenith B. the north celestial pole C. the celestial equator This question will check whether students understand the pattern they see in this time exposure photograph.

Why do the constellations we see depend on latitude (and time of year)? They depend on latitude because your position on Earth determines which constellations remain below the horizon. (They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.) These are the two basic reasons that the visible constellations vary; next we’ll explore each one.

Review: Coordinates on the Earth Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England) Use this for a brief review of latitude and longitude; it’s also useful to bring in a real globe to class for this purpose. The photo at right is the entrance to the Old Royal Greenwich Observatory (near London); the line emerging from the door marks the Prime Meridian.

The sky varies with latitude but not with longitude. Use this interactive figure to explain the variation in the sky with latitude. Show how the altitude of the NCP equals your latitude (for N. hemisphere)…

Altitude of the celestial pole = your latitude Show students how to locate the NCP and SCP, and how the sky moves around them. (You might wish to repeat the time exposure photo of the sky at this point to re-emphasize what we see.) Can also ask students where they’d find the north celestial pole in their sky tonight…

The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky. Use this interactive figure to explain how the constellations change with the time of year.

Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 50°E. You are at latitude 50°N and longitude 50°E. This question just makes sure the students understand the altitude = latitude idea…