The night sky? With the naked eye, we can see more than 2000 stars as well as the Milky Way. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, the location of any point on a map of the United States is in some state, and the location of any point in the sky is in some constellation.

Constellations A constellation is a region of the sky. Eighty-eight constellations fill the entire sky. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, the location of any point on a map of the United States is in some state, and the location of any point in the sky is in some constellation.

The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The ecliptic is the Sun’s apparent path through 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 tool 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.

The Celestial Sphere The 88 official constellations cover the celestial sphere. If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

Question : The celestial equator is ? A The path of the Sun compared to the stars. B The path of the Moon compared to the stars. C Always directly overhead at the Earth's equator. D The average path of planets on a star chart. E Always along the horizon for people on Earth's equator.

The Milky Way A band of light that makes a circle around the celestial sphere What is it? Our view into the plane of our galaxy On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize: We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy. (2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.

The Milky Way

The Local Sky An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky. Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object: It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north. Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

The Local Sky Zenith: The point directly overhead Horizon: All points 90° away from zenith Meridian: Line passing through zenith and connecting N and S points on the horizon Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object: It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north. Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

Angular Size An object’s angular size appears smaller if it is farther away. Use this slide if you want to review the definitions of arcminutes and arcseconds.

Apparent Brightness A bright star in the sky. A dim star in the sky Is really very bright? Or is it very dim but very close? A dim star in the sky Is it really very dim? Or is it very bright but very far away?

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 west to east, so stars appear to circle from east to west.

Question : In the northern hemisphere, the stars rise in the East, set in the West and revolve counter-clockwise around the North celestial pole. In the southern hemisphere the stars? A. Rise in the East, set in the West and revolve anti-clockwise around the South celestial pole. B. Rise in the East, set in the West and revolve clockwise around the South celestial pole. C. Rise in the West, set in the East and revolve clockwise around the South celestial pole. D. Rise in the West, set in the East and revolve anti-clockwise around the South celestial pole.

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. A circumpolar star never sets. celestial equator Now explain the basic motion of the sky seen from Earth. This star never rises. your horizon

Thought Question What is the arrow pointing to. A. the zenith B Thought Question What is the arrow pointing to? 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.

Question : You are standing at the north pole and you look at the stars. Which statement below describes what you would see? A There are no circumpolar stars. There are only circumpolar stars just after sunset. Some stars will be circumpolar and some will not. D. Every star you see is circumpolar.

Question : You are standing on the Equator and you look at northern pole star on the horizon. Which statement below describes what you would see? A There are no circumpolar stars. There are only circumpolar stars just after sunset. Some stars will be circumpolar and some will not. D. Every star you see is circumpolar.

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 longitude.

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…

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…

The sky varies as Earth orbits the Sun As 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.

Special Topic: How Long Is a Day? Solar day = 24 hours Sidereal day (Earth’s rotation period) = 23 hours, 56 minutes This slide goes with the optional Special Topic box in the text. Note: rather than simply discussing parts (a) and (b), you can do the actual demonstration in class…

Summary Of The Night Sky Now work on Page 1-6 of the tutorial book.