Module 2 Activity: Part 1

Apparent Diurnal Motion: Northern vs. Southern Hemisphere

Here in Part 1 of the Module 2 Activity, you'll start to become familiar with what the sky looks like as a result of the Earth's rotation - we call this the apparent diurnal motion of the sky.

Due to the Earth's rotation, the stars (and the Sun and the Moon) appear to move, making one apparent trip around the Earth each day. That's not actually what they're doing, but it looks that way.

In our on-campus classes at De Anza College, we use the Planetarium to demonstrate this motion. If you were out under the real sky, you'd have to wait many minutes - or possibly an hour or more - to notice the apparent diurnal motion, because it's so slow. In the Planetarium, we can speed up time and make the stars appear to move faster, thus giving our students a demonstration of the apparent diurnal motion during a single class period.

For our online class, it's a little more difficult, since we're not in the Planetarium. However, I think this is a very important thing for students to see, because the science of astronomy basically began when people started asking "Why do the stars seem to move like that?"

The first time I gave an online class a Module Activity about this, it was a total train wreck! Yikes!

There were two reasons:

1. I had them use some little sky-motion simulators, which are clever, but aren't zoomed-in enough on each of the main points on the horizon. It wasn't clear to the students what they were supposed to be seeing. (I've put these simulators at the end of this assignment, if you're interested in looking at them.)
2. I asked the students to match what they saw in the simulators to written words and phrases, and this was really confusing. It was just a bunch of "word salad", to people who didn't have a lot of experience yet.

I made a video to try and help people out. We're going to use that video, but when you watch it, you'll hear me saying "What word or combination of words..." a lot. That's because I made the video to help people deal with my horrible word salad in the original questions. (You might even want to just watch the video with the sound off - that might make this assignment easier!)

Good news - You won't have to read the horrible word salad! You'll match directions on the horizon to pictures.

How to do Questions 1 and 2:

Question 1 is for the Earth's Northern Hemisphere. Print out this one-page PDF if you can, and watch the first half of the video to do Question 1. (If you can't print it out, the drawings from the one-page PDF are shown below.) Here's the video - the first half is for Question 1 (Northern Hemisphere):

While you're watching the first half of that video, decide which of the pictures best matches the North (N), South (S), East (E), and West (W) points on the horizon. In other words, for each of those directions, which picture best illustrates what a Northern-Hemisphere observer would see?

Question 2 is for the Southern Hemisphere. For Question 2, watch the second half of the video, and decide which picture best matches the apparent star motions at the N, S, E, and W points on the horizon, for a Southern Hemisphere observer.

Before we get to Question 3, here are the pictures, one by one, in case you couldn't print out the one-page version. Instructions for Question 3 are after the pictures.

Note: Pay very close attention to the arrowheads! The stars appear to move in the direction the arrows are pointing.

Picture A:

Picture B:

Picture C:

Picture D:

Picture E:

Picture F:

Picture G:

Picture H:

How to Do Question 3:

Okay, now we've seen the sky from the Earth's northern hemisphere, and from its southern hemisphere. Here's your next challenge:

What if someone took one of your friends, blindfolded them, flew them around in an airplane for many hours, and then dropped them off somewhere under a clear, starry sky, with a phone so they could call you?

Your challenge: How would you help your friend figure out which hemisphere they're in?

Let's assume they have 88 charts with them, showing what each of the 88 constellations looks like. (Here's an example. If you want to see a full set, here they are...scroll down on that page for the charts.) That way, you don't have to try and describe what Orion or Leo (or whatever) looks like.

Write your answer in the text box for Question 3.

Here are some tips to get full points:

Talking about which constellations are visible is helpful, but it's a good idea to be more specific: What about how a constellation appears to move? What about where certain constellations appear in the sky?

The best answers will attack the problem in multiple ways, not just by saying "Can you see such-and-such constellation?" There are many ways to write a good answer!

In Case You Want to Play With Them: The Original Simulators

If you want something you can control yourself, here are two "sky simulators". This first one is for the Northern Hemisphere, as though you were at Lick Obervatory above San Jose:

Instructions: Put your mouse cursor over the picture of the sky and do these two things:

1. Change your direction of view: Use your left mouse button to drag the sky to the left and right. Practice looking towards the South (S), East (E), North (N), and West (W).
2. Speed up time and stop it again: Press the "L" key several times to speed up the flow of time. Press "K" to freeze time. Practice making time flow fast enough for you to see the stars seeming to move, then stopping, then starting again. (I find that if from a stop, it works well if I press "L" about 7 or 8 times to get a good, fast time flow.)

Now let's go to the Earth's southern hemisphere. Here's a map of the sky as seen from a point in the Pacific Ocean, about 4700 miles south of California. We've gone directly south from Lick Observatory, into the southern hemisphere.