FC - MoonIntro

40 question-and-answer flashcards covering fundamental facts, orbital mechanics, surface geology, interior structure, environment, origin, and exploration of the Moon.

Approximately how far is the Moon from Earth on average?

About 385,000 km

What is the approximate diameter of the Moon?

Roughly 3,500 km

What is the Earth-to-Moon mass ratio?

About 81 : 1 (Earth is 81 times more massive)

What is the Moon’s sidereal orbital period around Earth?

27.3 days

What is the synodic period of the Moon and what does it control?

29.5 days; it governs the repeating illumination geometry that produces the lunar phases

Why does the Moon keep nearly the same face toward Earth?

Because it is in synchronous rotation—its rotational and orbital periods are equal

What is the eccentricity of the Moon’s orbit?

e ≈ 0.0549, making the orbit slightly elliptical

What is the inclination of the Moon’s orbital plane to the ecliptic?

About 5.145°

What is the Moon’s obliquity (axial tilt to its orbital plane)?

Approximately 6.688°

What climatic consequence follows from the Moon’s tiny obliquity?

The Moon experiences virtually no seasons

What popular term describes the larger-than-average apparent size of the Moon at perigee?

“Super Moon”

What celestial body primarily causes tides on Earth?

The gravitational pull of the Moon

At what measured rate is the Moon receding from Earth?

About 3.8 cm per year, determined by laser ranging

About what percentage of the lunar surface can be seen from Earth due to librations?

Roughly 59 %

What is the amplitude of lunar libration in longitude?

Approximately 7.9°

What is the amplitude of lunar libration in latitude?

Approximately 6.7°

How often do the cyclic illumination conditions of Earth–Moon–Sun geometry repeat?

Every 29.5 days, producing the familiar lunar phases

Under what orbital condition do eclipses occur?

When Earth, Moon, and Sun align as the Moon crosses the ecliptic at its nodes

Why can the Moon completely cover the Sun during a total solar eclipse?

Because the apparent angular sizes of the Moon and Sun are nearly identical as viewed from Earth

What are lunar maria?

Dark, basaltic plains formed by ancient lava flows, often inside large impact basins

What are lunar highlands (terrae)?

Light-colored, elevated regions with rugged terrain and mountain ranges

Roughly how many impact craters larger than 1 km exist on the Moon?

About half a million

What is the name and size of the Moon’s largest known impact basin?

South-Pole–Aitken basin; ~2,200 km in diameter and up to 13 km deep

How is crater density used in lunar geology?

The number of craters per unit area serves as a relative dating tool for surface ages

What is the typical regolith thickness in lunar maria versus highlands?

About 3–5 m in maria and 10–20 m in highlands

What is the surface gravity on the Moon?

1.625 m/s², roughly 17 % of Earth’s gravity

What are “mascons” in the lunar gravity field?

Mass concentrations beneath large impact basins that create gravity anomalies

What early stage is inferred from lunar chemistry indicating crust formation?

A global magma ocean that differentiated into a lower-density crust

What is the mean density and moment-of-inertia coefficient of the Moon?

ρ ≈ 3.344 g/cm³ and MoI ≈ 0.392 (more uniform than Earth)

By how much is the lunar center of mass (CoM) offset from its center of figure (CoF)?

About 1.8 km toward Earth

What do seismic experiments suggest about the Moon’s interior?

It is dry, largely homogeneous, has a distinct crust, partially molten deep interior, and an uncertain small core

What radiometric ages are typical for returned lunar rocks?

Approximately 3.3 – 4.5 billion years old

How do lunar rocks differ from most Earth rocks in volatile content?

They are extremely depleted in volatiles and water-formed minerals

Where was the famous lunar meteorite Allan Hills 81005 discovered?

In Antarctica’s Allan Hills region in January 1982

What is the strength of the Moon’s crustal magnetic field today?

About 100 nT, ~1/100 of Earth’s field

What is the total mass of the Moon’s extremely thin atmosphere?

Roughly 10⁴ kg

Why can ice persist in some lunar polar craters?

Permanent shadow keeps temperatures very low, allowing long-term ice accumulation

What did NASA’s LCROSS impact experiment in 2009 detect in a polar crater ejecta plume?

Volatiles indicating about 5–6 % water ice by mass

What is the currently favored theory for the Moon’s origin?

A giant impact between early Earth and a Mars-sized body, ejecting mantle material that re-accreted into the Moon

During which years did the Apollo program land astronauts on the Moon, and how much rock did they return?

1969–1972; six landings returned about 381.7 kg of lunar samples