Our place in space
Earth is one of eight planets in our solar system. The solar system is the Sun and everything that orbits it: planets, moons, dwarf planets, asteroids and comets.
The Sun
The Sun is a star, a giant ball of hot, glowing gas. It’s about 109 times wider than Earth and holds more than 99.8% of all the mass in the solar system. Its gravity holds everything in orbit around it. Sunlight takes about 8 minutes to reach Earth.
Earth’s movements
Rotation: day and night
Earth spins on its axis, an imaginary line from the North Pole to the South Pole. One full spin, a rotation, takes about 24 hours. The side facing the Sun has day; the side facing away has night.
Revolution: one year
Earth travels around the Sun in a path called an orbit. One trip, a revolution, takes about 365¼ days, or one year. The extra quarter day is why we add a leap day (February 29) every four years.
Why we have seasons
Earth’s axis is tilted about 23.5°. As Earth orbits the Sun, the tilt stays pointed the same way.
| Season (Northern Hemisphere) | What’s happening |
|---|---|
| Summer | The Northern Hemisphere tilts toward the Sun: more direct sunlight and longer days |
| Winter | The Northern Hemisphere tilts away: less direct sunlight and shorter days |
| Spring and fall | Neither hemisphere tilts strongly toward the Sun |
Seasons are not caused by Earth being closer to or farther from the Sun. In fact, Earth is closest to the Sun in early January!
The Moon
The Moon is Earth’s natural satellite. It orbits Earth about once every 27 days and has no light of its own; it reflects sunlight.
Phases of the Moon
As the Moon orbits, we see different amounts of its sunlit half. These are the phases of the Moon. A full cycle takes about 29.5 days.
| Phase | What we see |
|---|---|
| New moon | Dark (the lit side faces away from us) |
| Waxing crescent | A thin sliver on the right (growing) |
| First quarter | Right half lit |
| Waxing gibbous | More than half lit |
| Full moon | The whole face lit |
| Waning gibbous | Shrinking, more than half |
| Third (last) quarter | Left half lit |
| Waning crescent | A thin sliver on the left |
(This is how it looks from the Northern Hemisphere.)
The Moon always shows the same face to Earth because it rotates once for every trip around Earth.
Tides
The Moon’s gravity pulls on Earth’s oceans, causing tides. Most coasts get two high tides and two low tides each day. Nova Scotia’s Bay of Fundy has the highest tides in the world, up to about 16 metres!
The planets
| Planet | Type | Fun fact |
|---|---|---|
| Mercury | Rocky | Closest to the Sun; a year is just 88 days |
| Venus | Rocky | Hottest planet (about 465 °C) because of thick clouds trapping heat |
| Earth | Rocky | The only planet known to have life |
| Mars | Rocky | The "Red Planet"; has the tallest volcano, Olympus Mons |
| Jupiter | Gas giant | Biggest planet; its Great Red Spot is a giant storm |
| Saturn | Gas giant | Has spectacular rings of ice and rock |
| Uranus | Ice giant | Spins on its side |
| Neptune | Ice giant | Windiest planet; farthest from the Sun |
A memory trick: My Very Eager Mother Just Served Us Nachos.
The four inner planets are small and rocky. The four outer planets are huge and made mostly of gas or ice.
Pluto was called a planet until 2006. Now it’s classified as a dwarf planet, because it hasn’t cleared other objects from its orbit.
Other objects
- Asteroids: rocky chunks, mostly in the asteroid belt between Mars and Jupiter.
- Comets: balls of ice and dust that grow glowing tails when they come near the Sun.
- Meteors: "shooting stars," bits of space rock burning up in our atmosphere.
Common mistakes
- Thinking seasons happen because Earth gets closer to the Sun.
- Thinking the Moon’s phases are caused by Earth’s shadow. They come from our changing view of the lit half.
- Thinking the Sun moves across the sky. Earth’s rotation makes it look that way.
Try it yourself
- In a dark room, use a lamp as the Sun and a ball on a stick as the Moon. Turn slowly in a circle holding the ball. Watch its phases appear.
- Tilt a globe and walk it around a lamp. Notice when your country gets the most direct light.
- Make a scale model: if the Sun were a beach ball, Earth would be the size of a peppercorn about 25 m away!
Quick recap
- The solar system is the Sun and everything that orbits it.
- Earth’s rotation causes day and night; its revolution makes a year.
- Seasons are caused by Earth’s tilted axis.
- The Moon’s phases come from seeing different amounts of its lit side.
- Eight planets orbit the Sun: four rocky and four giants.