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Difficulty: MediumEarth, Moon, and Sun Interactions

Beginning immediately after a New Moon, in what chronological order do the following lunar phases appear to an observer on Earth during a single monthly orbital cycle?

  1. 1Waxing Crescent
  2. 2First Quarter
  3. 3Full Moon
  4. 4Third Quarter

Answer

Waxing Crescent → First Quarter → Full Moon → Third Quarter
Directly after a New Moon, the Moon moves along its orbit so that sunlight begins striking the side facing Earth, creating a Waxing Crescent. About one week into the roughly 29.5-day cycle, one-quarter of the orbit is complete, displaying the First Quarter (half lit). At approximately two weeks, the Moon reaches the opposite side of Earth from the Sun, producing a Full Moon. By the third week, as the illuminated fraction decreases, the Moon reaches the Third Quarter phase before finishing its cycle.

Step-by-Step Solution

1
Establish the starting point and direction of the lunar phase cycle
The cycle starts at New Moon (day 0), when the Moon lies between Earth and the Sun and is not illuminated from Earth's viewpoint.
Tracking phase order requires advancing sequentially from the designated starting condition.
2
Sequence the waxing phases leading up to maximum illumination
As the Moon travels counterclockwise in its orbit, visible illumination expands from a Waxing Crescent (days 1–6) to First Quarter (day 7, half illuminated), then continues expanding toward Full Moon (day 14, fully illuminated).
The term 'waxing' denotes growing illumination during the first half of the synodic month.
3
Sequence the waning phases following maximum illumination
After the Full Moon, the illuminated portion diminishes through Waning Gibbous to Third Quarter (day 21, half illuminated) and Waning Crescent before returning to New Moon.
The term 'waning' denotes shrinking illumination during the second half of the synodic month.

Key Concept

Lunar phase cycle and relative orbital geometry of the Earth, Moon, and Sun
Estimated Time:1m 0s
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Topics

Earth, Moon, and Sun Interactions

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