Astronomy unit 5

Proto-planetary disc

A spinning disk of gas and dust around a new star — where planets start to form.

Accretion

When small pieces of dust and rock stick together because of gravity, forming bigger bodies → planetesimals → planets.

Formation of Earth’s layers

As Earth formed and heated, heavy materials sank (core) and lighter ones rose (crust and mantle); this is differentiation.

Giant Impact Theory

A Mars-sized object (Theia) hit young Earth, and the debris formed the Moon.

Lunar mare (maria)

Dark, flat lava plains formed when lava filled old craters — younger and smoother.

Lunar highlands

Light, rough, cratered areas — older and higher than maria.

Rotation

Spinning on an axis.

Revolution

Orbiting around another object.

Lunation period

The time it takes the Moon to go through all its phases — about 29.5 days.

Lit portion of the Moon

Half (50%) is always lit by the Sun.

Cause of lunar phases

We see different parts of the lit half as the Moon moves around Earth.

Light sides and dark sides grow (Right/Left to Right/Left)

Right to Left in the Northern Hemisphere; waxing = light on right, waning = light on left.

Waxing Crescent

Light on the right.

Waning Crescent

Light on the left.

Waxing Gibbous

Light on the right.

Waning Gibbous

Light on the left.

First Quarter

Right half lit.

Third (Last) Quarter

Left half lit.

Full Moon

Fully lit.

New Moon

Dark (between Earth and Sun).

8 Phases of the Moon in order

New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent.

Why we see only one side of the Moon

The Moon’s rotation and revolution take the same time (synchronous rotation), so one side always faces Earth.

Umbra

Darkest part of the shadow — total eclipse.

Penumbra

Lighter outer shadow — partial eclipse.

How a lunar eclipse happens

Earth moves between the Sun and Moon, covering the Moon with Earth’s shadow.

Why the Moon looks red in a lunar eclipse

Earth’s atmosphere bends sunlight; blue scatters, red/orange light passes through and lights the Moon.

Partial vs. total lunar eclipse

Partial: part of Moon in umbra; Total: entire Moon in umbra (red/orange).

Who can see a lunar eclipse

Anyone on the night side of Earth — about half the planet.

Why eclipses don’t happen monthly

The Moon’s orbit is tilted 5°, so the Sun, Earth, and Moon don’t line up perfectly each time.

How a solar eclipse happens

The Moon moves between the Sun and Earth, blocking sunlight.

Who can see a solar eclipse

Only on the daytime side; total eclipse in umbra, partial in penumbra.

Umbra, penumbra, antumbra meaning

Umbra = total eclipse, Penumbra = partial eclipse, Antumbra = annular eclipse (“ring of fire”).

Annular solar eclipse explanation

The Moon is farther from Earth, so it looks smaller and doesn’t fully cover the Sun.

Lunar phase for solar eclipse

New Moon 🌑

Lunar phase for lunar eclipse

Full Moon 🌕