Phases of the Moon

Why This Matters

The Moon's phases aren't random. They're a direct, predictable result of orbital geometry. Understanding why the Moon looks different each night connects to fundamental concepts you'll see throughout astronomy: how light interacts with spherical bodies, relative positions in space, and the difference between rotation and revolution. When you grasp lunar phases, you're building the foundation for understanding eclipses, tidal forces, and even how we observe planets from Earth.

You'll be tested on your ability to explain why each phase occurs based on the Moon's position relative to Earth and the Sun, not just what each phase looks like. Don't just memorize the sequence; know what geometric relationship each phase illustrates and be ready to predict what an observer would see from different locations.

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The Geometry Behind Phases

The Moon doesn't produce its own light. It reflects sunlight. What we call "phases" are simply the changing angle between the Sun, Moon, and Earth as the Moon orbits us. Half of the Moon is always illuminated by the Sun; phases describe how much of that illuminated half we can see from Earth.

New Moon

• The Moon sits between Earth and the Sun, so the illuminated half faces entirely away from us, making the Moon invisible in our sky
• Orbital position: Earth, Moon, and Sun are roughly aligned. This alignment is called syzygy, with the Moon in the middle.
• Marks day 0 of the 29.5-day synodic month, the time it takes for the Moon to return to the same Sun-Earth-Moon alignment

Full Moon

• The Moon is opposite the Sun from Earth's perspective, so we see the entire illuminated hemisphere
• Orbital position: Earth sits between the Sun and Moon, another syzygy alignment
• Occurs roughly 14.75 days after New Moon. This halfway point represents maximum illumination and is when lunar eclipses can occur.

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Waxing Phases: Growing Illumination

During the first half of the lunar cycle, the illuminated portion visible from Earth increases each night. "Waxing" means growing. The lit area expands as the Moon moves from between us and the Sun to the opposite side of Earth.

Waxing Crescent

• A thin sliver appears on the Moon's right side (in the Northern Hemisphere) as sunlight begins reaching the portion we can see
• Orbital angle: The Moon has moved roughly $45°$ from the Sun-Earth line
• Best visible in the western sky just after sunset. The Moon sets a few hours after the Sun during this phase.

First Quarter

• Exactly half the visible face is illuminated. Despite the name, you're one-quarter through the full cycle, not seeing one-quarter of the Moon.
• Orbital angle: The Moon forms a $90°$ angle with the Sun-Earth line. This position is called quadrature.
• Rises around noon, sets around midnight, making it visible in the evening sky

Waxing Gibbous

• More than half but less than fully illuminated. "Gibbous" means swollen or humped.
• Orbital angle: The Moon has passed $90°$ and approaches $180°$ from the Sun
• The Moon appears to "fill in" from right to left, with each night showing slightly more illumination until Full Moon

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Waning Phases: Decreasing Illumination

After Full Moon, the process reverses. The illuminated portion shrinks each night as the Moon continues its orbit back toward alignment with the Sun. "Waning" means shrinking. The lit area decreases as we approach the next New Moon.

Waning Gibbous

• More than half remains illuminated, but decreasing nightly. The "shadow" creeps in from the right side.
• Orbital angle: The Moon has passed $180°$ and moves back toward $90°$ separation
• Rises after sunset, visible in the late night and morning sky. You'll often see this phase still up at dawn.

Last Quarter (Third Quarter)

• Half illuminated, but the opposite half compared to First Quarter. The left side is now lit (Northern Hemisphere).
• Orbital angle: The Moon again forms $90°$ with the Sun-Earth line, but on the opposite side from First Quarter
• Rises around midnight, sets around noon, making this phase dominant in the pre-dawn sky

Waning Crescent

• A thin sliver remains on the Moon's left side. This is the final visible light before the cycle resets.
• Orbital angle: The Moon approaches realignment with the Sun, less than $45°$ separation
• Best visible in the eastern sky just before sunrise. Sometimes called the "old Moon."

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Quick Reference Table

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Why Eclipses Don't Happen Every Month

Since New Moon and Full Moon both involve syzygy, you might wonder why we don't get eclipses every single month. The reason is that the Moon's orbit is tilted about $5°$ relative to Earth's orbital plane (the ecliptic). Most months, the Moon passes slightly above or below the exact alignment needed for a shadow to fall on Earth or the Moon. Eclipses only occur when the Moon crosses the ecliptic at the same time it's in syzygy. These crossing points are called nodes, and they line up with the Sun only about twice a year during eclipse seasons.

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Self-Check Questions

1. What geometric relationship do New Moon and Full Moon share, and why don't eclipses happen every month despite this alignment?

2. A student sees a half-illuminated Moon high in the sky at sunset. Is this First Quarter or Last Quarter? How do you know?

3. Compare Waxing Gibbous and Waning Gibbous. What do they share in appearance, and what differs about their position in the cycle and timing of visibility?

4. If the Moon is at quadrature, what two phases could it be in, and what additional observation would tell you which one?

5. Explain why "First Quarter" doesn't mean we see one-quarter of the Moon's surface illuminated. What does the name actually refer to?