## Essential question How do moons form and behave, and why does Earth's Moon matter for life on Earth?

## Quick warm-up (1 minute) Look at a picture of the night sky (or imagine it). What is one thing you already know about the Moon or other moons?

### Vocabulary you will use - Moon (natural satellite): a natural object that orbits a planet or dwarf planet. - Gravity: the attractive force between masses. - Orbit: the path an object follows as it goes around another object. - Tide: the regular rise and fall of ocean water level. - Tidal locking: when an orbiting object rotates in the same amount of time it takes to orbit, so it keeps the same face pointed toward its partner.

## How do moons get there? Scientists think moons form in a few common ways:

1. Made with the planet: material around a young planet clumps together into moons (many large, regular moons). 2. Captured: a planet's gravity grabs a passing asteroid or dwarf planet (often small, irregular moons). 3. Big impact: a huge collision throws material into orbit that later clumps into a moon (a leading idea for Earth's Moon).

Not every planet has moons. Mercury and Venus have zero known moons.

## Known moons in our Solar System (snapshot) Counts can change as astronomers discover tiny moons and as objects get confirmed by international naming groups.

| Planet | Known moons | |---|---:| | Mercury | 0 | | Venus | 0 | | Earth | 1 | | Mars | 2 | | Jupiter | 97 | | Saturn | 274 | | Uranus | 29 | | Neptune | 16 |

Why so many for the giant planets? They are massive and have strong gravity, so they can collect and keep many moons.

## Remarkable moons worth knowing (quick tour) Pick a few of these to highlight based on time.

- Io (Jupiter): extremely active volcanoes. - Europa (Jupiter): icy crust with strong evidence for a deep ocean. - Ganymede (Jupiter): largest moon in the Solar System. - Titan (Saturn): thick atmosphere and lakes of liquid methane and ethane. - Enceladus (Saturn): ice geysers that spray material into space. - Triton (Neptune): orbits backward compared with Neptune's spin, suggesting it may have been captured. - Phobos and Deimos (Mars): small, irregular moons that look like captured asteroids.

Teacher note: If students ask "Could there be life?", focus on what scientists look for: liquid water, energy, and the right chemistry.

# Earth's Moon (deep dive) In the rest of this lesson, you will focus on Earth's Moon and why it matters.

## Moon basics - Average distance from Earth: about 384,400 km. - The Moon takes about 27.3 days to orbit Earth once (sidereal month). - New Moon to New Moon takes about 29.5 days (synodic month). - The Moon is slowly moving away from Earth over time.

Even though the Moon looks like it "follows" Earth, Earth and the Moon actually move together around a shared center of mass.

## The Moon's 'weird' orbit (in a good way) The Moon's orbit is not a perfect circle. Sometimes it is a little closer (perigee) and sometimes a little farther (apogee).

The Moon's orbital path is also tilted about 5 degrees compared with Earth's orbit around the Sun. This is one reason eclipses do not happen every month.

If you want a challenge question: - What would change if the Moon's orbit were not tilted at all?

## Tidal locking (why we see one face) A common mistake is to say "the Moon does not spin." It does spin.

- The Moon spins once on its axis in the same time it orbits Earth once. - That is why the same face stays pointed toward Earth. This is called tidal locking (or synchronous rotation).

### Near side, far side, and "dark side" - The near side is the half that faces Earth most of the time. - The far side is the half that faces away most of the time. - The far side is not always dark. It gets sunlight during its own day, just like the near side.

### Extra interesting detail (optional) Because the Moon's orbit is a little tilted and a little oval, the Moon appears to wobble (called libration). Over time, we can see a little more than half of the Moon's surface from Earth.

## Tides and the Moon The Moon's gravity pulls on Earth. It pulls a little more strongly on the side of Earth that is closer to the Moon than on the side that is farther away. This difference helps create two bulges in Earth's oceans:

- one bulge on the side facing the Moon - one bulge on the opposite side

As Earth rotates, a coastline can pass through: 1. a bulge (high tide), 2. a low point (low tide), 3. the other bulge (high tide), 4. the other low point (low tide).

That is why many coasts have two high tides and two low tides each day.

### Why do tides happen later each day? Earth is spinning, but the Moon is also moving along its orbit. So Earth needs a little extra time to "catch up" to the Moon's position. Tide timing shifts about 50 minutes later each day.

### Spring tides and neap tides (patterns) - Spring tides: when the Sun, Earth, and Moon line up (New Moon or Full Moon). Tides are extra strong. - Neap tides: when the Sun and Moon pull at right angles (First Quarter or Last Quarter). Tides are less strong.

Reality check: real tide heights also depend on local coastline shape, ocean depth, and weather.