Seasons, day length and distances in space
Summer is not the time when Earth is closest to the Sun. The real explanation is a 23.5° tilt, and it also explains why daylight stretches and shrinks across the year.
Lesson overview
What you'll learn in this lesson
Understand the seasons and the Earth's tilt, day length at different times of year, and the light year as a unit of astronomical distance
Key learning points
- • The tilt, not the distance
- • Intensity of sunlight
- • Day length across the year
- • Measuring distance in light years
This lesson at a glance
- 30 minutes
- 25 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 25
Visual introductionPicture this
Seasons, day length and distances in space
Summer is not the time when Earth is closest to the Sun. The real explanation is a 23.5° tilt, and it also explains why daylight stretches and shrinks across the year.
In a nutshell
Understand the seasons and the Earth's tilt, day length at different times of year, and the light year as a unit of astronomical distance
What you already know
Part 2 of 25
Before we start
What you already know
You should already know that Earth orbits the Sun and rotates once a day.
Key words
Part 3 of 25
Key words
Words you'll need today
- Axial tilt
- The 23.5° lean of Earth's axis relative to its orbit.
- Solstice
- The longest or shortest day of the year.
- Equinox
- When day and night are close to equal length.
- Light year
- The distance light travels in one year.
Reset break
Part 4 of 25
Pause
That's sitting 1 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 5 of 25
Learning cycle 1 of 2
Why seasons happen
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 6 of 25
Learn
The tilt, not the distance
Earth's axis leans about 23.5° and keeps pointing the same way as Earth orbits. For half the year the northern hemisphere tilts towards the Sun and for half it tilts away. Earth is actually slightly closer to the Sun in January, which is decisive evidence that distance is not the cause of the seasons.
Explore the idea
Part 7 of 25
Learn
Intensity of sunlight
When a hemisphere tilts towards the Sun, sunlight strikes it more directly, so the same energy is concentrated on a smaller area and heats it more. Tilted away, the same beam is spread over a larger area and passes through more atmosphere. Concentration of energy per square metre is the mechanism behind summer and winter.
Reset break
Part 8 of 25
Pause
That's sitting 2 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 9 of 25
Quick check
Part 10 of 25
Learning cycle
Part 11 of 25
Learning cycle 2 of 2
Day length and cosmic distances
A short piece of teaching, then a check to make sure it has landed.
Reset break
Part 12 of 25
Pause
That's sitting 3 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 13 of 25
Learn
Day length across the year
The tilt also changes how much of each rotation a place spends in sunlight. Near the June solstice the northern hemisphere has its longest day; near the December solstice its shortest. At the equinoxes day and night are close to equal everywhere. Inside the Arctic Circle the effect is extreme enough to give continuous daylight or darkness.
Explore the idea
Part 14 of 25
Learn
Measuring distance in light years
Space distances are too large for kilometres to be useful, so astronomers use the light year: the distance light travels in a year, about 9.5 trillion kilometres. It measures distance, not time. Because light takes time to arrive, looking at a star four light years away means seeing it as it was four years ago — telescopes look backwards in time.
Quick check
Part 15 of 25
Reset break
Part 16 of 25
Pause
That's sitting 4 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 17 of 25
Common mix-ups
Part 18 of 25
Common mix-ups
Things people often get wrong
- People often think summer happens because Earth is closer to the Sun. It is caused by the tilt.
- People often think a light year is a length of time. It is a distance.
Challenge round
Part 19 of 25
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Reset break
Part 20 of 25
Pause
That's sitting 5 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 21 of 25
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Earth is actually slightly closer to the Sun in January, which is decisive evidence that distance is not the cause of the ____.
Space distances are too large for kilometres to be useful, so astronomers use the light year: the ____ light travels in a year, about 9.5 trillion kilometres.
Challenge round
Part 22 of 25
Game · Recall cards
Seasons are caused by...
Card 1 of 4
Exit quiz
Part 23 of 25
Exit quiz
Show what you've learned
6 questions, marked together at the end. Nothing is timed.
1. Earth's axis is tilted at about...
2. The longest day in the northern hemisphere occurs near the...
3. Inside the Arctic Circle in midsummer there can be...
4. A star 10 light years away is seen as it was...
5. Earth is closest to the Sun in...
6. Light travels roughly...
Mastery quiz
Part 24 of 25
Marked quiz
End of lesson quiz: Seasons, day length and distances in space
4 questions, marked with the reasoning shown. No timer.
1. Seasons are caused by...
2. At an equinox...
3. A light year measures...
4. Sunlight in winter feels weaker because it is...
Lesson round-up
Part 25 of 25
Lesson round-up
Ready when you are
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Ask LunaPart 1 of 25 · Watch & discover