Moon Phases: Crash Course Astronomy #4

Summary

This script delves into the celestial mechanics behind the Moon's phases, explaining how its spherical shape and orbit around Earth create the ever-changing lunar spectacle we observe. From the New Moon to the Full Moon and back, the script details the progression of lunar phases, their dependence on the angle of sunlight, and the resultant changes in the Moon's visibility and surface features. It also touches on the poetic phenomenon of Earthshine and the comforting predictability of the Moon's monthly cycle, inviting viewers to embrace the cosmos as their celestial neighborhood.

Takeaways

• 🌕 The Moon is a giant rock sphere reflecting sunlight, making it appear bright to us.
• 🌗 The Moon's phases are caused by the geometry of its orbit around Earth and its spherical shape, which always leaves half of it illuminated.
• 📏 The 'terminator' is the line dividing the Moon's lit day side from the unlit night side, key to understanding its phases.
• 🌑 The New Moon occurs when the Moon is between Earth and the Sun, showing only its unilluminated side.
• 🌒 A 'waxing crescent' Moon is visible as a thin crescent with horns pointing away from the Sun, as the Moon moves eastward in its orbit.
• 🌓 The 'first quarter' Moon is when the Moon is 90° away from the Sun, and we see exactly half of its lit side.
• 🌔 The 'waxing gibbous' phase describes the Moon as it becomes more than half full, continuing to grow in illumination.
• 🌕 The 'full Moon' happens when the Moon is opposite the Sun in the sky, and we see its entire lit side.
• 🌖 The 'waning gibbous' phase follows the full Moon, as the illuminated portion begins to shrink.
• 🌗 The 'third quarter' Moon is similar to the first quarter but with the lit and dark sides reversed, occurring when the Moon is 270° from the Sun.
• 🌘 The 'waning crescent' is the thinning phase before the cycle returns to the New Moon, visible close to sunrise and sunset.
• 🌑 The Moon's cycle repeats every 29.5 days, which is the basis for the length of a month.
• 🌌 From the Moon's perspective, the Earth would show phases opposite to those of the Moon as seen from Earth, with 'full Earth' visible during a new Moon from Earth.
• 🌙 The 'Earthshine' effect is caused by the Earth reflecting sunlight onto the Moon's unlit side, making it visible even when not fully lit.

Q & A

Why does the Moon appear to change its shape every day?

The Moon's appearance changes daily due to its spherical shape and its orbit around the Earth. As it orbits, the angle at which sunlight illuminates the Moon and the view from Earth changes, resulting in different visible phases.

What causes the Moon to look bright to us?

The Moon looks bright because it reflects sunlight onto Earth. Despite its surface being quite dark, similar to a chalkboard or asphalt, the Sun illuminates it, and it reflects that light to us.

What is the 'terminator' in the context of the Moon's phases?

The 'terminator' is the line that divides the lit day side of the Moon from the unlit night side. It is the boundary between the areas of the Moon that are illuminated by the Sun and those that are in shadow.

Why is the Moon sometimes visible during the day and sometimes at night?

The Moon is visible during both day and night because its orbit around the Earth is not synchronized with the Earth's rotation. It can be seen at different times depending on its position relative to the Sun and Earth.

What is the significance of the term 'New Moon'?

A 'New Moon' occurs when the Moon is positioned between the Earth and the Sun, making the illuminated side of the Moon face away from Earth, and we see the unilluminated side. It marks the beginning of the lunar cycle.

How often does the Moon orbit the Earth?

The Moon orbits the Earth roughly once every month, which is the basis for the term 'month' and the length of time it takes the Moon to go through all its phases, approximately 29.5 days.

What is the term for the phase of the Moon when it appears half-lit from Earth?

When the Moon appears half-lit from Earth, it is in the 'first quarter' phase. This occurs when the Moon is 90° away from the Sun in the sky, and the terminator line cuts right down the middle of the visible face of the Moon.

What is the term 'gibbous' used to describe in the context of the Moon's phases?

The term 'gibbous' is used to describe a phase of the Moon that is more than half-lit but not yet full. It indicates a swollen or convex shape, and when the illuminated portion is increasing, it is referred to as the 'waxing gibbous' phase.

Why does the full Moon rise at sunset and set at sunrise?

The full Moon rises at sunset and sets at sunrise because it is positioned opposite the Sun in the sky when it is full. This means the Earth is between the Moon and the Sun, allowing us to see the fully illuminated half of the Moon.

What is 'Earthshine' and how does it affect the appearance of the crescent Moon?

Earthshine is the glow seen on the otherwise dark part of the Moon, caused by the Earth reflecting sunlight onto the Moon. This effect makes the thin crescent Moon appear to have a ghostly face, as the nearly full Earth illuminates the unlit portion of the Moon.

How does the perspective of the Moon's phases differ when viewed from the Moon?

When viewed from the Moon, the phases are exactly 180° reversed compared to when viewed from Earth. For example, during a new Moon on Earth, the Earth as seen from the Moon would be full, due to the Earth being opposite the Sun as viewed from the Moon.