Question

How is the Moon positioned with respect to Earth and the Sun when it is (a) full, (b) new, (c) gibbous, (d) crescent?

Short answer

(a) Opposite Earth from Sun; (b) Between Earth and Sun; (c) Not in line, partial illumination; (d) Small angle with Earth and Sun showing less than half illuminated.

Step-by-step solution

Understanding the Full Moon

When the Moon is full, it is positioned on the opposite side of the Earth from the Sun. In this alignment, the Earth is between the Sun and the Moon, allowing the side of the Moon facing Earth to be fully illuminated by the Sun's light. This means the Sun, Earth, and Moon form a straight line.

Understanding the New Moon

During the new moon phase, the Moon is positioned between the Earth and the Sun. Because of this alignment, the side of the Moon that is illuminated by the Sun is facing away from Earth, resulting in the Moon being invisible to us from the Earth's surface. Here too, the Sun, Moon, and Earth generally align in a straight line.

Understanding the Gibbous Moon

A gibbous moon occurs between the first quarter and full moon phases (waxing gibbous), or between full moon and last quarter phases (waning gibbous). In this position, more than half but not all of the lunar surface visible from Earth is illuminated by the Sun. The Sun-Earth-Moon alignment is such that the Moon is not in a straight line with them, leading to this partial illumination.

Understanding the Crescent Moon

During the crescent moon phase, less than half of the Moon's disc is illuminated. This occurs just after the new moon in the waxing crescent phase or just before the new moon in the waning crescent phase. The alignment has the Moon at a small angle with the Earth and Sun, where only a small slice of the Moon's surface is lit and visible from Earth.

Key concepts

Moon Phases

The Moon goes through a cycle of phases, repeating approximately every 29.5 days. These phases range from new moon to full moon, with several transitional phases in between. Each phase depends on the Moon's position relative to the Earth and Sun.
The four primary phases—new, first quarter, full, and last quarter—are distinct stages where specific parts of the Moon are illuminated by the Sun. In between these phases are the waxing and waning crescent and gibbous phases.

• New Moon: This is when the Moon is nearly invisible from Earth, as its illuminated side faces away from us.
• Crescent Moon: A slim portion of the Moon is visible, happening just after the new moon (waxing) or just before the new moon (waning).
• Gibbous Moon: More than half of the Moon is visible, occurring before a full moon (waxing) and after a full moon (waning).
• Full Moon: The whole face of the Moon is visible, fully illuminated by the Sun.

Understanding these phases helps in grasping the cyclical nature of the Moon's journey around Earth.

Sun-Earth-Moon Alignment

The alignment of the Sun, Earth, and Moon is crucial in determining the Moon's phase. This alignment dictates how much of the Moon's surface is illuminated and visible from Earth.
During a full moon, the Earth is positioned between the Sun and the Moon. This arrangement allows sunlight to reflect off the entire surface of the Moon that faces Earth.
In a new moon, the Moon is between the Earth and the Sun. Here, the Moon's illuminated side faces away from the Earth, making it invisible to us.

• Straight Line Alignment: Occurs during new and full moon phases, causing significant tidal effects due to gravitational forces.
• Non-linear Alignment: Ensures none of the primary celestial bodies are perfectly aligned in a straight line, usually seen in the crescent or gibbous phases.

Comprehending these alignments aids in predicting tidal phenomena and understanding lunar visibility.

Waxing and Waning

Waxing and waning describe the Moon's changes in illumination. These terms signify whether the visible illuminated part of the Moon is increasing or decreasing.
Waxing means growing. During this period, more of the Moon becomes visible. It starts after the new moon and progresses through waxing crescent, first quarter, and waxing gibbous, up to a full moon.
Waning means shrinking. Following a full moon, the illumination decreases through waning gibbous, last quarter, and waning crescent, leading back to a new moon.

• Waxing Phases: Crescent and gibbous phases as the moon heads towards full visibility.
• Waning Phases: Crescent and gibbous phases as the moon loses visibility.

These cycles allow us to predict the Moon's appearance on any given night.

Lunar Illumination

Lunar illumination refers to the portion of the Moon's surface that is lit by the Sun and visible from Earth. This changes throughout the lunar cycle due to the Moon's orbit around Earth.
The amount of illumination depends entirely on the Sun-Earth-Moon geometric relationship:

• Full Moon: Full illumination due to direct sunlight.
• New Moon: No visible illumination as the Moon's lit side faces away from Earth.
• Gibbous Moon: More than half illuminated as the Moon is either approaching or receding from full moon.
• Crescent Moon: Less than half illuminated when transitioning from new to full or full to new.

This concept is crucial for understanding lunar visibility and its effects on Earth, such as tides and night lighting.