Moon Tracking Exercise

Due date

April 29

Procedure

Waxing gibbous Moon

Waxing gibbous moon

During the next several months it will be your task to observe the Moon at every available opportunity and record enough information to track its motion and changing appearance. Because the weather is so unpredictable, it is advisable to take advantage of every clear evening and not wait until later in the semester to start your observations. I've seen far too many students over the years who decide there is plenty of time to complete the assignment only to find that poor weather and cloudy skies conspire against them. You can always take it easy if you start the assignment early and have plenty of observations, but once you pass up the opportunity to make observations, you risk losing all of your points on this assignment.

Having said that, your assignment is to observe the Moon's appearance every clear night and measure its position with respect to at least three surrounding stars. In the process you will become familiar with the stars and the constellations and notice the regularity of the Moon's motion and pattern of phases. In order to begin your observations, you will need the calibrated string made in class January 20. If you do not have the calibrated string, or need to replace the one you had, it is possible to make one at home using some simple instructions.

Making a Measurement of the Moon

  1. Hold one of the pen marks between your left thumb and a finger.
  2. Keeping your elbows locked, shut one eye, place your left thumb and finger over the Moon, and stretch the string with your right thumb and finger until they are at the position of a nearby star. At this point the string should be stretched tight at arm's length between the Moon (covered by your left thumb and finger) and a star near the Moon (covered by your right thumb and finger).
  3. Being careful to not move your hands, count the number of segments between your thumbs. For example, if the distance between your thumbs is three segments, the distance between the star and the Moon is 12° (3 segments x 4°). In most cases, the number of segments will not be exact and you'll have to approximate a little. For example, if the number of segments between your thumbs is 4 1/4, the distance between the star and the Moon is 17° (4 segments x 4° + 1/4 segment x 4°).
  4. Measure a minimum of three different stars and record your data.

I'll be making observations right along with you for the rest of the semester, too. When I was an undergraduate, Professor Van Allen did the exercise right along with the class and claimed that he always had more observations than even the best students even though he occasionally traveled and had to miss classes. While making my observations, I always find it useful to record each observation on a notepad or small piece of paper. Specifically, I record the

Plotting the Data

Eventually you will plot your observations on the SC001 star chart. Use the vertical scale (marked in one degree increments) on the star chart to determine how far you should open your compass. Then, make an arc around the appropriate star. For clarity, I made arrows in three different colors and labelled the number of degrees in a corresponding color.

Of course I realize the hardest part of the exercise will be learning the stars, but if you do the exercise several nights in a row, you'll be able to use the same stars in most cases. It's only when you do the exercise occasionally that you will almost certainly have to use different stars each night and thus have a harder time learning the pattern of the Moon's motion and the stars it moves past.

Finding the Moon

To find the time of moonrise or moonset for any day of a particular month (in addition to the Sunrise and Sunset times), click here or complete the table provided below:

Year: Month: Day:

State or Territory:

City or Town Name:

The city or town name you enter above must be a city or town in the U.S. (if you would like to obtain information on rising and setting times of the Moon and Sun, but are outside of the United States, click here.) In the United States, the city or town name will be compared to a file with over 22,000 places listed. Letter case is not important as either upper- or lower-case letters or a combination can be used. Spelling out place name prefixes is important, however. For example, type East Orange, Fort Lauderdale, or Mount Vernon. The only exception is St., which is entered as an abbreviation with a period, as in St. Louis. Also, you need only enter as many characters as will unambiguously identify the place.

Note: The preceding moon phase diagram, computation table, and links are all courtesy of the United States Naval Observatory in Washington, DC. If you find this information useful, please visit the USNO's web site and let them know you appreciate their services and the information they provide. Alternately, you may wish to visit the USNO's Complete Sun and Moon Data For One Day page directly.

If you would like to view the phase of the Moon for any date and time from A.D. 1800 – 2199, click here.

To determine if the weather will cooperate with your observing plans, the latest forecast can be found at The Weather Underground:

Click for Iowa City, Iowa Forecast

Calibrating Your String

To calibrate your string, you will need the following equipment:

If you attempt to calibrate your twine at home, you may find masking tape useful because you don't want to mar or damage any surfaces. If you go to your local Kirkwood center, you may want to use an empty classroom that has a dry erase board or chalk board.

Once you've collected all of the equipment, calibrate your twine by following the instructions below.

  1. Make a mark such as the one shown below on a chalkboard or dry erase board (if necessary, simply use masking tape on a wall, window, or door). The distance between the vertical marks must be exactly 25 inches and the figure should be made at eye level.

    2. Wall marking

  2. Use masking tape to mark a line on the floor directly in front of the 8° figure on the wall. The tape line must be exactly 15 feet from the wall. When finished, your setup should look like this if seen from overhead:

    3. Wall mark--top view

  3. Make a knot at each end of the twine.
  4. Stand with your toes at the tape line on the floor and face the 25 inch (or 8°) mark on the wall.
  5. Taking your twine, extend your arms straight in front of you with elbows locked and grasp the twine so a finger and thumb of your left hand is at the point where the 8° mark and left vertical mark intersect. Be sure to work from left to right and start at one of the knots you tied.
  6. With one eye shut, stretch the twine so that a finger and thumb of your right hand is now at the point where the 8° mark and the right vertical mark intersect. At this point the twine should be pulled tight between the two vertical marks and blocks your view of the horizonal 8° line.
  7. Use a pen to mark the point where you held the string with your right thumb and finger.
  8. Take ahold of the mark you just made with your left thumb and finger then repeat steps 5 through 7. Repeat these steps to create five 8° segments connected end to end. You should now have a total of five 8° segments marked on your twine that span a total of 40°.
  9. Fold the twine between two consecutive pen marks. This folded segment should now be divided into two equal sections. Mark the point of the loop to divide the 8° segment into two 4° segments.
  10. Repeat for each of the other four 8° segments. When finished the string will now be divided into a total of 10 segments of 4° each.

Supplemental Questions

These questions are to be answered and submitted along with your individual observations and star chart:

  1. What is the direction of the Moon’s diurnal (daily) motion?
  2. Based on your observations, what is the right ascension and declination of the Moon’s ascending node? In which constellation is it found?
  3. What is the approximate size of the Moon’s diurnal motion?
  4. When is the best time to observe the Last-quarter Moon?
  5. How long is the Moon’s sidereal period? Its synodic period?