Lab 2: The Moon

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Lab 2: The Moon

Part I: Lighting up the Moon

Setup: Find a small ball somewhere in your house. A good one to use is a ping pong ball, but if you don’t have one of those, any other small ball will work. If you don’t have any kind of ball, you can crumple a piece of paper, clay or aluminum foil into a ball.

Let’s pretend that this ball is the Moon and it will orbit the Earth, which will be repre- sented by your head. Your perspective in this model is the same as your perspective here on Earth when we look into the sky. Next, find a lamp in a darkish room (if there is a lamp shade you will have to remove it to see the direct light from the light bulb). This light will represent the Sun.

Note: If we used the method we learned in Lab 1 to create an Earth-Moon model and the

Moon was the size of a ping pong ball, the Earth would be about the size of a grapefruit. The

distance between the ping pong ball and the grapefruit would be about 3.5 meters (almost 12

feet). The Sun would be nearly 400 times farther from the Earth than the Moon is, so in this

scale model it would be nearly a mile away!

1) How much of the Moon is lit up by the Sun at any given time? Don’t worry yet about what we see from Earth; think only about what fraction of the Moon’s surface is receiving sun light.

2) Under what conditions, if any, could the lit-up fraction of the Moon’s surface change? Explain and make a sketch to illustrate your explanation.

3) Considering your results, do you think these special conditions occur frequently? Why or why not?

Part II: Generating the Moon’s Phases

When standing close to the light, take a close look at the ball (The Moon), rotating it in different directions and orbiting it around your head (of course keep an eye on the ball so you can see how the light on it changes as it goes around).

1) How many different shapes can you make using your half-lit moon? (In other words, how many shapes can you make using the light and shadow side of the ball?) Sketch a few of them.

Do you know the names of any of these shapes (“phases”)? Enter what you and/or your lab mates remember below. What fraction of the lit side is visible in each case? (e.g., “exactly half” or “less than half” or “more than half”). Don’t worry about the order of the phases yet; you’ll work that out later. Try not to use Google for this and just think about what you know. If you don’t get very many, that’s okay! Just fill in what you can.

Table 1: Moon Phases

Name of Phase Amount of Lit Side Visible from Earth

Part III: Ordering the Moon’s Phases

Now we’ll use an online Earth-Moon model simulator to figure out the sequence of Moon phases. Click the following link to take you to the simulator: https://pbslm-contrib. s3.amazonaws.com/WGBH/buac19/buac19-int-earthsunmoon35model/index.html.

Spend a few minutes playing around with the simulator to get a feel for how it works. You should see the Earth/Moon system on the bottom right (*not to scale), the Sun on the left and the view from Earth at the top.

Once you’ve familiarized yourself with the simulator, consider the diagram below of the Earth/Moon system. The view is from far above the Earth’s North Pole.

1) Color in the half of the Moon that is dark in each of the 8 positions (again, the view is from above the Earth, NOT the view from Earth’s surface).

Now consider what the view from Earth is when the Moon is located at each of the positions shown in the diagram. This is going to be the view from Earth (at the top of the simulator). Begin your exploration with the Moon directly between the Sun and the Earth.

What Moon phase do you see?:

2) Write the name of the phase seen from Earth next to each Moon position in the diagram above.

https://pbslm-contrib.s3.amazonaws.com/WGBH/buac19/buac19-int-earthsunmoon35model/index.html

3) In the table below, name all eight phases (see diagram on previous page), starting with New Moon. Then make a sketch of each one in column 2, first by drawing a circle to represent the entire Moon, and then darkening the part that is NOT lit up by the Sun. (Wait on the third column.)

Table 2: Ordered Phases of the Moon

Name of Phase Sketch Approx # of Days Visible

The Moon takes 29.5 days to orbit the Earth and return to the same position relative to the Sun (this is known as the Moon’s “synodic” period).

Do all phases have the same duration?:

4) Which phases last for only one day? (hint: there are four of them! ) Explain.

5) Which phases last several days? (hint: think about the meaning of “waxing” and “waning”) Explain.

Enter your estimates of the duration of each phase in column 3 of Table 2.

Date of the last new moon (use the internet to look this up):

Use this date to make some predictions:

6) What phases should be visible between now and your next lab session?

7) On what date do you expect the next full moon to occur?

Part IV: Angular Sizes

To discuss what we see in the night sky, we need to be able to describe how big objects appear (e.g. the Moon, or M31, the Andromeda galaxy). Astronomers use angles to describe these apparent sizes. To see how this works, hold up the tip of your pointer finger to your eye to see what objects around your room you can cover up with it.

Notice that the closer an object is to your eye, the larger it appears. Your fingertip can have the same angular size as a physically much larger object... if the smaller object is much closer to you than the large one!

The angular size (also called apparent size) of an object can be measured in degrees. Your hand is a very “handy” tool with which to measure the angular sizes of various things in the sky (e.g. the Moon)! Below are the directions on how to use your hands to determine the apparent sizes of objects in the sky:

• Use the image below to make the first hand gesture with your pinky

• Hold your arm stretched out straight out in front of you (don’t let your elbow bend!).

• Close one eye (or cover it with the other hand).

Figure 1: Image Source: http://www.fortworthastro.com/beginner1.html

Try to use these hand gestures to cover up objects in your house for practice. You will need to use this method when completing your SkyWatch assignment.

1) Tape a standard 8.5x11 inch piece of paper to the wall (if you don’t have tape, find a way to make it stand up). How far away do you have to stand for you to cover the paper with three fingers (the 5° hand gesture)? This is your best guess, don’t worry about having perfect measurements. Don’t forget to completely stretch out your arm!

Note: Not all hands are the same size, so they will not all measure the same angular sizes. The guide above is a rough estimate. To check your own hand measurements, you can compare it to the Big Dipper. At some point during the semester, try to find the Big Dipper (we will discuss how to do this later in the semester) and hold up these hand gestures to compare angular sizes.

Figure 2: Image Source: http://www.fortworthastro.com/beginner1.html