2.2 Time of Month Tutorial



We continue our discussion of the lunar phases diagram with the positions of the Moon as it orbits the Earth each month, again viewed from above the North pole. We number 8 positions in a circle around the Earth. Since all of the Moons are illuminated from the right, the left side of each one is shaded. As seen from Earth, however, the illuminated portion of the near side of the Moon varies around the orbit. Depictions of the Moon's phases as seen from Earth are shown along the left.

The names given to each phase have also been labeled. The four cardinal points in the orbit have special names. Orbital phase 1, placed directly between the Earth and Sun, is called New as it marks the traditional beginning of a new month. Orbital phase 5, placed directly opposite the Sun, is called Full as the Moon is observed to be fully illuminated here. Orbital phases 3 and 7 are called first and third quarter, respectively, as they mark the points one-quarter and three-quarters through the month. (Note, however, the word "quarter" does not refer to the illumination: the quarter Moon is actually precisely one-half illuminated.)

At any point between these four principle phases, the nomenclature is fixed by the trend in the illumination and the shape of the Moon. Between New and Full Moon, the illumination is increasing from night to night, so the Moon is said to be waxing. Similarly, between Full and New Moon, the illumination is decreasing or waning. Between third quarter and first quarter, less than half of the Moon is illuminated, so the Moon is crescent-shaped. Between first quarter and third quarter, more than half of the Moon is illuminated, so the Moon has a gibbous shape.

As a check that you understand the connection between time of month (lunar phase) and the appearance of the Moon, select a time of month from the menu on the right. A box will appear around the corresponding Moon representations.

Lunar phases web tool site map

  1. Introduction
  2. Tutorial
    1. Time of Day
    2. Time of Month
    3. Apparent Direction of Moon
  3. Lunar Phases Tool and Quiz
  4. Going Beyond the Basics
  5. Credits and Feedback
  6. Note for Teachers