1. Introduction


The appearance or shape of the Moon gradually changes through the course of a month. This cycle, the cycle of lunar phases, may be explained if we make the following assumptions:
  1. The Moon is spherical in shape.
  2. The Moon is not self-luminous, but rather reflects the light of the distant Sun.
  3. The Moon circles the Earth once per month.
You can demonstrate how the observed phenomena are a consequence of these assumptions by holding a ball at arm's length in a room illuminated by a single lamp in the corner. Watch the changing illumination of the ball as you gradually spin in place one full circle. The phase of the Moon (the ball) depends on where it is in its orbit with respect to the Sun (the lamp).

Similarly the day/night cycle may be explained by the following assumptions:

  1. The Earth is spherical in shape.
  2. The Earth is illuminated by the distant Sun.
  3. The Earth rotates on its axis once each day.
The time of day depends on how much the observer has rotated with respect to the Sun.

Now both the daily and monthly motions occur simultaneously. So for example, if we want to know in which direction to look for the Moon, we need to know both the time of day and the time of the month (the lunar phase). Answering questions that combine these two motions can be a bit confusing at first. The purpose of this web site is to introduce you to a lunar phases diagram which you can use to solve these kinds of problems.

The layout of the site is straightforward. First, there are three tutorial pages that explain the elements of the lunar phases diagram one-by-one. Second, there is the lunar phases tool and quiz. The tool is an interactive version of the diagram and can be used to solve the problems posed by the quiz. Once you can answer three questions correctly in a row, you have mastered the topic! Finally, for the curious student who wonders whether we have cut some corners in our basic explanation, there is a page with all the caveats and details we did not mention earlier.

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