Earth in Space
Understand Earth's rotation and revolution, why we have seasons, and how the Moon creates different phases and eclipses.
Earth's Rotation: Day and Night
The Earth rotates (spins) on its axis once every 24 hours. This is what gives us day and night. The side of Earth facing the Sun has daytime; the side facing away has nighttime.
The Earth spins on a tilted axis (23.5 degrees). One full spin = 24 hours = one day.
Key point: The Sun does not move around the Earth. It appears to move across our sky because the Earth is spinning. This was a major scientific discovery!
Earth's Revolution: Seasons
The Earth also revolves (orbits) around the Sun, taking 365.25 days (one year). Because Earth's axis is tilted at 23.5 degrees, different parts of Earth receive different amounts of sunlight throughout the year. This causes the seasons.
Why Do We Have Seasons?
Summer (in your hemisphere)
Your hemisphere is tilted toward the Sun. Sunlight hits more directly, days are longer, and it is warmer.
Winter (in your hemisphere)
Your hemisphere is tilted away from the Sun. Sunlight hits at an angle, days are shorter, and it is cooler.
Spring
Transitioning from winter to summer. Days get longer, temperatures rise gradually.
Autumn
Transitioning from summer to winter. Days get shorter, temperatures cool down.
Important: When it is summer in Australia (Southern Hemisphere), it is winter in Europe (Northern Hemisphere), and vice versa.
Moon Phases
The Moon orbits Earth about once every 29.5 days. As it orbits, we see different amounts of its sunlit side, creating the phases of the Moon. The Moon does not produce its own light -- it reflects sunlight.
The 8 Moon Phases
New Moon
Waxing Crescent
First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent
Waxing = getting bigger | Waning = getting smaller
Eclipses
An eclipse occurs when one object in space moves into the shadow of another. There are two types:
Solar Eclipse
The Moon passes between the Sun and Earth, blocking the Sun's light. The Moon's shadow falls on Earth.
Sun - Moon - Earth
Warning: Never look directly at the Sun during a solar eclipse!
Lunar Eclipse
The Earth passes between the Sun and Moon, casting Earth's shadow on the Moon. The Moon may appear reddish.
Sun - Earth - Moon
Safe to watch! The Moon can turn a beautiful red-orange colour.
Key Vocabulary
Rotation
Earth spinning on its axis. One rotation = 24 hours (one day).
Revolution
Earth orbiting around the Sun. One revolution = 365.25 days (one year).
Axis
An imaginary line through the centre of the Earth from pole to pole. Earth's axis is tilted at 23.5 degrees.
Eclipse
When one object in space casts a shadow on another (solar or lunar).
Waxing
The lit part of the Moon is getting larger (growing) each night.
Waning
The lit part of the Moon is getting smaller (shrinking) each night.
Worked Examples
When it is summer in Australia, what season is it in England? Why?
Step 1: Australia is in the Southern Hemisphere; England is in the Northern Hemisphere.
Step 2: When the Southern Hemisphere is tilted toward the Sun (summer), the Northern Hemisphere is tilted away (winter).
Answer: It is winter in England. The tilt of Earth's axis means the two hemispheres have opposite seasons.
What causes day and night?
Step 1: The Earth rotates on its axis once every 24 hours.
Step 2: The side facing the Sun receives light (day); the side facing away is in shadow (night).
Answer: Day and night are caused by Earth's rotation. As Earth spins, different parts face toward or away from the Sun.
During which phase can we see the entire lit face of the Moon?
Step 1: We see the Moon because it reflects sunlight. How much we see depends on the Moon's position relative to Earth and the Sun.
Answer: During a full Moon, the entire face of the Moon that faces us is lit by the Sun. This happens when the Earth is between the Sun and Moon.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What causes the seasons on Earth?
Question 2
How long does it take for Earth to complete one rotation?
Question 3
During a solar eclipse, what passes between the Sun and Earth?
Question 4
When the Moon appears to be getting larger each night, we say it is:
Question 5
When it is summer in the Southern Hemisphere, what season is it in the Northern Hemisphere?
Key Concepts Summary
- ●Rotation (spinning on axis) causes day and night (24 hours).
- ●Revolution (orbiting the Sun) combined with Earth's 23.5-degree tilt causes seasons (365.25 days).
- ●The hemispheres have opposite seasons because of the tilt.
- ●Moon phases (new, crescent, quarter, gibbous, full) are caused by the Moon's position as it orbits Earth.
- ●A solar eclipse = Moon blocks Sun; a lunar eclipse = Earth's shadow on the Moon.