ASTRONOMY 195: OBSERVATIONAL ASTRONOMY

Homework #2: NAKED EYE OBSERVATIONS OF THE MOON

Due November 13, 2014

Part I: PLOTTING THE MOON'S ORBIT:

The goal of this homework is to observe the changing phases and positions of the moon on the sky over the next 9 weeks, and thereby find both the synodic, and sidereal period of the moon. You will need to use an angle measuring device such as your fist, and pinkie finger, which you

can assume subtend 11 and 2 respectively.

What you need to do:

o Observe the moon whenever it is clear, note the time and date in the observing table. If it were clear every night, an observation every two days would be ideal. It won’t be clear every night, so make observations whenever it is clear. I expect you will have at least 15 observations over the next 63 days.

o Note the moon’s phase, and make a quick sketch of the moon’s appearance in the observing table.

o Measure the angular distance between the moon and at least two, preferably three bright stars, and record this information. This will be difficult to do sometimes, particularly when the moon is full, but do your best. Use your star finders to help identify stars.

o Plot the moon’s position on the star charts (I’ve included copies at the end) using the information in the table. Use the star angles to triangulate the position of the moon on the charts. You may find it easiest just to ”eye-ball” the position of the moon on the charts.

o Look for "earthshine". When the moon is only a few days old it appears as a sliver. Despite the fact that the dark side of the moon is not in sunlight, it is still faintly visible. This is because the dark side is illuminated by solar light reflected off of the earth. This phenomenon is termed earthshine. Be sure to note what nights you can see earthshine.

o Make an observation of the full moon when it rises to determine if it is bigger on the horizon (see question 12 below).

MAKE THE OBSERVATIONS -- “observations” taken by looking it up on the web do not count, and will be penalized.

Date Time Phase, Drawing Star 1 Angle 1

Star 2 Angle 2

Star 3 Angle 3

Earth Shine?

Using your charts and table, answer the following questions:

1. Calculate the Moon’s “Sidereal Period”. Inspect your log to find two dates (separated by about a month) on which the moon appeared in the same place in the sky. How long did it take the moon to return to the same place with respect to the stars? This is called the sidereal period of the moon.

2. Calculate the Moon’s "Synodic Period". Find two drawings of the Moon in your logs that show the same lunar phase. What is the period for the lunar phases, i.e. what is a “lunar month”? This is termed the synodic period of the moon.

3. Briefly explain why the sidereal period differs from the synodic period.

4. What direction does the moon move with respect to the stars on the sky?

5. What is the approximate motion of the moon with respect to the stars (in degrees) in 24 hours?

6. At what right ascension does the moon’s path cross the celestial equator on its travel from the southern celestial hemisphere, to the northern hemisphere? This is called the ascending node of the moon’s orbit. I have marked the ascending node of the sun’s orbit on your star charts to get you thinking right.

7. What is the inclination of the moon’s orbit to the ecliptic? In other words, how far does the moon stray from the ecliptic plane? Use your data to estimate this.

8. What time does a full moon rise? Explain.

9. When is a 3rd quarter moon crossing the meridian? Explain.

10. What time does a 1st quarter moon set? Explain.

11. The “age” of the moon is the time in days from when the moon is new (which we call 0 days old). If the moon is 0 days old when it crosses the plane of the ecliptic (at an ascending or descending node) what celestial event is occurring?

12. Is the full moon larger on the horizon? A rising full moon appears to be larger than it does when it is high in the sky. Devise a method by which you can measure its size, and make the observations. Record your method and observations below. Is the moon truly larger?

Ascending node

of Sun’s orbit