Unit 1 : Activity 1 : Talking About Things in the Sky : "Which Way is North?" : What's Up? : Unit Exam

# What's Up?

Depending on where you are on the planet, you can only see certain stars. The difference between stars seen by one observer and those visible to another observer at a different location on Earth is illustrated here.

Horizon Lines

A person's individual view from the surface of the Earth is best described in terms of horizon lines. In the graphic to the right someone standing at point A will have a horizon A.

A person standing at point B will have horizon B.

As shown in the graphic, the region (dark pink in this case) above where the two horizon lines intersect is the portion of the sky visible to both observers at the same time.

The farther away two people are on the planet, the smaller the area of sky they can both see at the same time.

Let's look at it another way. If we imagine that there is a very large sphere around the Earth, and we zoom out until the Earth appears very small, then it might look like the graphic below. The small white circle is the Earth, and the dark figure represents someone standing on that side of the Earth's surface.

Now we will say that there are three people on the planet with horizon lines A, B, and C as shown.

If there is a star on this very large "Celestial Sphere" as marked in the illustration, which of these observers can see it?

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Answer: The star in the diagram to the left is visible to observer A, but it is below the horizon for observers B and C.

1) Which stars a given observer can see depend on where he or she is located and also on the time of night. Here is observer A again, standing at the North Pole of Earth. Which stars can he see?

2) If there is a star on this very large "Celestial Sphere" as marked in the illustration, which of these observers can see it?

Only observer A.
Observers A and C, but not B.
Observers B and C, but not observer A.

3) As the Earth turns (described in detail in Unit 3), the stars you can see will change.

Could you see the yellow star "now" (if it's dark)? Could you see the same star 12 hours from "now" (if it's dark then)?

Stars that are up in your sky all the time (whether or not you can see them in the daytime) are called "circumpolar."

For an observer at the North Pole, all the stars that are visible at one time are up in the sky all year long.

For an observer at the equator, there are no circumpolar stars.

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4) Stars that are up in your sky all the time (whether or not you can see them in the daytime) are called "circumpolar." For an observer at the North Pole, all the stars that are visible at one time are up in the sky all year long. For an observer at the equator, there are no circumpolar stars.

See if you can draw a diagram like the ones used above to demonstrate that these two statements are true.