Moon Orbit Project

Check Lab Syllabus for Due Dates

Introduction

This lab is one of the long-terms labs required. Over a two month period you should observe the Moon's position every other day. This lab is worth two regular lab grades and should be taken very seriously.

The goal of this lab is to determine the orbital period and direction of orbit for the Moon via direct observations. You will also gain a fundamental understanding as to why the Moon has phases, and what causes them. During the lab, always try to picture your position on the Earth relative to the Sun and Moon. Develop in your mind a 3-dimensional view of what is happening.

The observations you will be making are very similar to those the ancient peoples made when determining dates of festivals, planting times, etc. The observations only take a few minutes each day and can be done from any location in the Northern Hemisphere.

Follow the directions below and take very accurate measurements. It is very important that you take regular observations. Bad weather may prevent you from taking some observations. If you procrastinate, you may not be able to take the required number of observations (25-30) and you will be graded down. Don't let this happen to you! Make observations whenever possible. Come in early for help if you get confused.

General Instructions

Please read all the instructions before beginning your Moon Orbit project.

1. Weather permitting, make an observation every other day. If you miss a day, then make observations on two consecutive days.

2. The lab TA will tell you when the moon will be up for the first week, but after that you should be able to tell on when you can make an observation.

3. Take VERY careful measurements.

4. Be sure the moon is not more than 45 degrees above the eastern or western horizons when you measure its altitude. This is very important!

5. Make your altitude measurement using a protractor or the fist method described in class, being sure to keep a level base.

6. Record your observations in the table provided - using extra sheets, if necessary.

7. HINT: Go out at different times to make your observations. The moon IS up during the day, too. Spread your observations around, don't go out at the same time every month.

8. After about 5 observations, try plotting your data and see your TA if you think your points are bad, or if you are having problems.

9. All observations should be plotted on the same piece of paper. Work through the directions below for plotting your data. Make sure you understand how to make your drawing. Come in for help right away if you have problems with the plot.

Directions for plotting lunar data:

(Please use a pencil for the plot)

Begin by drawing a small circle (no bigger than an inch across) in the middle of a piece of paper. This will be the Earth. Draw a much larger circle centered on the Earth. This

will represent the orbit of the Moon.

Make a choice for the direction of the Sun (usually off to the right). Indicate near the edge of the sheet the direction that light rays from the Sun would be traveling.

Shade the Earth as it would look if you were in space above the North Pole, looking down at the Earth. Make the night side, the side opposite the Sun, the dark side.

From the view above the North Pole, the circle you drew is the Earth's equator. If someone were standing on the equator, you would see them standing on the circle. The person's location helps tell you what time it is. For example, at noon, the Sun would be directly overhead; it's rays directed straight down on the person. On your Earth circle, indicate 6am, 6pm, 12pm (noon) and 12am. You can now think of the Earth sketch as a clock.

Pick an observation of the moon on a given date. Locate the position on the Earth where you would be at this time. Draw a line from that point of time to the center of the Earth

(radius line).

Draw in a line perpendicular to the radius line you just drew. This will be your horizon. You can see objects above this line, but not below it. Why?

Determine the eastern and western horizons and label them. Remember that the Earth rotates counter-clockwise if you are looking down from the North Pole and that the direction of rotation is east. East is counter-clockwise on the Earth circle and West is clockwise.

Using your observed altitude of the Moon as an angular measure, plot this angle above the eastern or western horizon (as you saw it) using a protractor. Extend the line from

the horizon out to your Moon orbit circle.

Put a small circle at the end of this line (the Moon) and shade it as it should appear from your vantage point in space above the North Pole.

Next to the moon you just drew, sketch the moon showing the phase you actually saw. You should check that the phase makes sense with where the Moon is on its orbit and how one would see the Moon from the Earth's point of view. Write down the date next to the Moon you've just drawn.

Repeat the above for several of your observations. Your final drawing should contain at least 15 moons of all different phases.

Questions

When you have put all your observations on your plot, determine the following from your plot:

a) How long does it take for the Moon to go around the Earth once?

b) In which direction does the Moon orbit the Earth?

c) Do you have any observations which don't fit the general trend? What could have caused these "bad data points?"

Create one final page with observations which cover all of the Moon phases. It is NOT necessary to plot ALL of your data. Strive to make an accurate picture of your observations. All observations however must be in your Moon Data Log.

A Quick Guide to Finding East & West

This page is a guide to help you visualize where East & West are on your Moon diagram.

Above is a map of the Earth viewed as if you were hovering above the North Pole

(the center of all of the circular lines of latitude). Locate on the Earth the North American continent. One can make out the outline of the West coast of the United States as well as the outline of Mexico. Now, let us travel Eastward from Mexico to Florida/Caribbean islands. On this diagram that direction (the short route that is) is COUNTER-CLOCKWISE. Therefore, if one draws an arrow around the globe in a counter-clockwise direction, the tip of the arrow is EAST and the base of the arrow is WEST. Draw this on your map to help remind you which horizon is EAST and which is WEST when you draw your horizon lines.