Earth Motions, Celestial Concepts, Moon Phases, Gravity, Kepler, Newton, and Light (Lecture Notes Review)

A comprehensive set of QA flashcards covering Earth’s motions, celestial models, Moon phases, seasonal climates, planetary orbits, gravity, Kepler’s laws, Newton’s laws, and basic light theory, as derived from the lecture notes.

What are the two main motions of the Earth?

Rotation on its axis and orbital motion around the Sun.

What is the period of Earth's rotation?

Approximately 24 hours.

What is the period of Earth's orbit around the Sun?

About 1 year.

What is precession in the context of Earth's motion?

A slow conical motion of Earth's axis with a cycle of about 26,000 years, changing which star is near the north celestial pole.

What motion causes the Sun to rise and set?

The rotation of the Earth on its axis.

What are circumpolar stars?

Stars that never set for a given latitude; always visible near the celestial pole.

What condition makes a star circumpolar for you?

Its angular distance from the celestial pole is less than your latitude (closer to the pole than your latitude).

Why do stars appear to move across the sky?

Because of the Earth's rotation, not real stellar motion on short timescales.

What is the celestial sphere model used for?

A conceptual model to understand the positions and motions of celestial objects; not a physically accurate description of the universe.

What is the ecliptic?

The Sun’s apparent path on the celestial sphere, tilted about 23.5 degrees to the celestial equator.

What is the celestial equator?

The projection of Earth’s equator onto the celestial sphere.

What is declination?

The angular distance of an object north or south of the celestial equator; like latitude on the sky.

What is right ascension?

The angular distance eastward along the celestial equator from the vernal equinox; like longitude on the sky.

What is the maximum solar declination at solstices?

Approximately +23.5 degrees at the summer solstice and -23.5 degrees at the winter solstice.

Why isn’t the celestial sphere a fully accurate model of the universe?

Earth isn’t at the universe’s center; stars are at different distances and have proper motions; the model oversimplifies motion.

Why does the Sun’s declination vary over the year?

Because of Earth’s tilt, the Sun’s position in declination changes from +23.5° to -23.5° over the year.

Are seasons caused mainly by the Earth’s distance from the Sun? Why or why not?

No. The tilt of the axis and the resulting variation in sunlight distribution cause seasons; the hemispheres experience opposite seasons.

What determines the Moon’s phase?

The fraction of the Sun-lit half of the Moon that is visible from Earth, depending on the Sun–Moon–Earth geometry.

What do waxing and waning describe?

Waxing = increasing illumination; waning = decreasing illumination of the Moon’s visible face.

How long is the average lunar cycle (synodic month)?

About 29.5 days.

In which direction does the Moon rise and set?

Rises in the east and sets in the west.

How much of the Moon’s near side is illuminated at any time?

About half, with the visible fraction changing during the cycle.

What is perihelion in planetary orbits?

The closest approach of a planet to the Sun.

What is aphelion?

The farthest distance of a planet from the Sun.

What is the semimajor axis?

The average distance from a planet to the Sun, measured in astronomical units (AU).

What is orbital eccentricity?

A measure of how non-circular an orbit is; e = 0 for a circle; Earth’s eccentricity is about 0.016.

State Kepler’s third law in its simple form.

p^2 = a^3, where p is the orbital period in years and a is the semi-major axis in AU.

What did Galileo contribute to astronomy?

First telescopic observations that supported heliocentric ideas and refined planetary observations.

What are Newton’s second and third laws?

Second law: F = m a (acceleration is caused by net force). Third law: For every action, there is an equal and opposite reaction.

What is Newton’s universal law of gravitation in its simplest form?

The gravitational force between two masses is proportional to the product of the masses and inversely proportional to the square of the distance: F ∝ m1 m2 / r^2.

What happens to angular velocity when angular momentum is conserved and a system’s radius increases?

The rotation slows (spin slows) as the radius increases with the same angular momentum.

What is the speed of light in vacuum?

Approximately 3 x 10^5 kilometers per second.

What is a photon?

A quantum of light; the particle associated with electromagnetic radiation.

What is a light-year?

The distance that light travels in one year; used to measure astronomical distances (e.g., Proxima Centauri is about 4.3 ly away).

What is wavelength?

The distance between successive crests (or troughs) of a wave.

What is frequency in a wave, and how does it relate to wavelength?

Frequency is the number of crests passing a point per unit time; it is inversely related to wavelength.

How is photon energy related to frequency?

Photon energy increases with frequency (E = h f, where h is Planck’s constant).