Observation Activity A

The Moon's Orbit

Discussion. One of the best ways to relate a number of astronomy textbook facts and concepts to "real" life is to observe the sky regularly.  In doing this and other observational activities, you will become more acquainted with the night sky and what transpires there from day to day, month to month, season to season.

In this observational activity, you will plot the orbit of the moon against the starry background of the sky by observing the moon over a one- to two-month period.  Use a start chart to plot your observations.

Procedure.  Each clear night that the moon is in the sky, locate the moon and label its location against the background of stars on your photocopied star chart. If the star chart has a black background, use a white felt tip pen to write on the chart. Label the location of the moon by date and indicate its phase (crescent, first quarter, gibbous, full, third quarter, etc.).

Over a period of a month or two, your observations will mark out the orbit of the moon in the sky.  Sketch a smooth line through your labeled positions to show the orbit.  (NOTE:  Although the moon's orbit gradually precesses with a period of 19 years, it will not move appreciably over one or two months.)

If you continue these observations over a longer period, you will notice that the moon reaches first-quarter, full, and third-quarter phase farther east each month.  This is because the sun is moving eastward along the ecliptic, and the phases of the moon are related to the location of the sun.