ASTR 390 – Moon Plotting
Goals: (1) Understand three planes in the sky: the celestial equator, the ecliptic, and the plane of the moon’s orbit. (2) Find the line of nodes. (3) Gain more familiarity with the constellations and the phases and motion of the moon.
With the exception of artificial satellites, the moon is the object that moves quickest through the sky. It goes 360 degrees every month; about 12 degrees per day. It has well known phases (new, crescent, quarter, gibbous, full) but most people have no idea where or when to expect the moon to appear despite its clockwork regularity.
This observational project follows the moon during one orbit = one month. The observation sounds simple: Each clear night, find the moon, and plot its position among the stars on the attached star map with a conspicuous dot. Note the time, day, and lunar phase. Use your best angle-measuring tricks to place the moon as accurately as possible: goal: within one moon diameter, about half a degree.
What’s so hard about that? (1) The moon follows a very predictable pattern that, alas, does not correspond very well to a human’s sleep cycle. A waxing moon, you will find, is well placed for plotting as soon as it gets dark. A waning moon, on the other hand, rises progressively later and later. Soon, the best time to mark its position will be predawn. That is, not a biorythmically propitious time of day for most students. (2) The moon will inevitably drift into parts of the sky that are unfamiliar, so the student will be learning new star patterns as the project progresses.
Keep good records, as usual: make a table like this one to turn in with your report.
Obs. # / Date / Time / Phase / Right Ascension / Declination / “Ecliptic declination” / Weather notes“Obs. #” is a cross-reference to the plotted moons on the sky map.
Right Ascension and Declination can be read off of the sky map axes.
“Ecliptic Declination”: how many degrees above or below the ecliptic is each moon?
Your report should include an estimate for the orientation of the line of nodes and how you estimated its direction (and a description of what the line of nodes is). Based on your answer, when would be a good time of year to look for lunar or solar eclipses? (Explain the connection between eclipses and the line of nodes.)
Also connect your observations to what you learned about rise times, setting times, and meridian crossing times as function of lunar phase.