1. If the Moon is in the first-quarter phase on September 14, what date in October will the Moon be in full phase?

The Moon revolves around Earth every 29.5 solar days, and the full phase of the Moon occurs 7 3/8 days after first-quarter phase. This means the next full moon will still be in the month of September and we must wait one more cycle for a full moon in the month of October. Thus, the Moon must progress through 1.25 cycles and 29.5 plus 7 3/8, or 37 days, must pass. There are 30 days in September, so 16 days remain of this month, leaving 21 more days needed for a total of 37 days. The first full moon in October will, therefore, occur on October 21st.

2. A high tide is occurring at Charleston, S.C., which is at a longitude of 80°W. What other longitude is having high tide at this time?

The two high tides occur on opposite sides of Earth and so are separated by 180° Traveling 180° around Earth from 80°W in either direction gets us to 100°E.

Return to Previous Page