HomeNAAP LabsLunar Phases › Instructor Resources

Lunar Phases Lab — Instructor Resources


The NAAP Lunar Phases Lab pedagogical objectives want students not only able to identify the phases of the moon but also to understand the geometry giving rise to them from various perspectives and to be able to correlate those perspectives.


Course Integration

The module should generally used after the horizon coordinate system has been introduced. Though, if the horizon segment of the module isn't emphasized that prerequisite is optional.


  • One Day Only

    The same day is looped over and over again. As indicated by the sun rising directly east and setting directly west, the sun's declination = 0°. Likewise, the declination of the moon is zero.

  • Implied Observer Location

    Graphically, the stick figure on the small globe looks to be situated on the equator. The phase panel also generates shading consistent with an observer at the equator given that the sun and moon have declination = 0°. However, the horizon plane indicates the observer is at a mid-northern latitude (about 40°N).

  • No Libration

    Even when a single day is looped over and over, diurnal libration changes the observable lunar face by about 1°. This effect is not shown.

  • No Eccentricity

    Eccentricity effects of both the moon's orbit around the earth and the earth's orbit around the sun are ignored.

  • Scale

    The physical size of the moon and earth are approximately proportional (moon radius / earth radius ≈ 1/4). But the size of the moon's orbit is not in correct proportions to the displayed moon and earth sizes (moon radius / moon orbital radius ≈ 60).

  • Phases

    “Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively.” (U.S. Naval Observatory Lunar FAQ). When the exact phase occurs is independent of the observer.

    An attempt was made to balance when the moon “looks” like that phase and the technical definition. For example, a full moon is technically 100% illuminated, but ±1% illumination tolerance is used. Similarly, the first and third quarters are 50% illuminated but a tolerance corresponding to about ± 4.4% tolerance is used.