Why we have seasons

Curriculum links

Quoted word for word from the official documents.

Step by step

The caption is what the class reads on screen; the talking points are in your notes drawer (N) when you present. Steps marked Pupils only appear only in the pupil lesson.

1. 01

   A tilted planet

   Presented and for pupils

   Caption

   Earth's spin axis is tilted. It leans about 23.4 degrees from straight up.

   Talking points

   • NASA gives Earth's obliquity, the tilt of its axis from a line at right angles to its orbit, as 23.44 degrees.
   • USNO: the axis stays nearly fixed in direction in space as Earth goes round the Sun once a year.

   Sources 1 2 5Present from this step →

2. 02

   June: summer in the north

   Presented and for pupils

   Caption

   In June the North Pole leans towards the Sun. It is summer in the north and winter in the south.

   Talking points

   • USNO gives the June solstice of 2027 as 21 June at 14:11 UTC. The scene opens at the solstice as the viewer's own ephemeris finds it, a few minutes before that time.
   • USNO: at a solstice the axis is most tilted towards or away from the Sun. The hemisphere tilted towards the Sun has its longest day and the other its shortest.
   • NASA Space Place: it is summer in June in the Northern Hemisphere because the Sun's rays hit that part of Earth more directly than at any other time of the year.

   Ask the class

   It is June. Which hemisphere is having summer?

   1. AThe Northern HemisphereRight answer
   2. BThe Southern Hemisphere
   3. CBoth of them

   WhyIn June the North Pole leans towards the Sun, so the Northern Hemisphere gets the most direct sunlight.

   Sources 3 4 5 8 9 10 11Present from this step →

3. 03

   December: summer in the south

   Presented and for pupils

   Caption

   Six months later the South Pole leans towards the Sun. Now it is summer in Sydney and winter in London.

   Talking points

   • USNO gives the December solstice of 2027 as 22 December at 02:42 UTC.
   • USNO: six months after the June solstice Earth is on the opposite side of the Sun, so the Northern Hemisphere is tilted away from the Sun and has winter. The axis itself still points the same way.

   Sources 4 5 8 9 10 11Present from this step →

4. 04

   Long days and short days

   Presented and for pupils

   Caption

   In June, London spends most of each turn in sunlight. Sydney spends most of it on the night side.

   Talking points

   • The clock runs at an hour a second. Watch how long each city stays on the lit side.
   • USNO, sunrise to sunset on 21 June 2027: Greenwich (London) 16 hours 38 minutes, Sydney 9 hours 54 minutes.
   • USNO, sunrise to sunset on 22 December 2027: Greenwich 7 hours 50 minutes, Sydney 14 hours 25 minutes.

   Ask the class

   On 21 June 2027 London gets 16 hours 38 minutes of daylight. About how much does Sydney get on the same day?

   1. AAbout 16 and a half hours
   2. BAbout 10 hoursRight answer
   3. CExactly 12 hours

   WhySydney is in the Southern Hemisphere, which leans away from the Sun in June. The US Naval Observatory gives it 9 hours 54 minutes of daylight that day.

   Sources 5 7 8 9 10 11Present from this step →

5. 05

   An equinox

   Presented and for pupils

   Caption

   At an equinox the Sun is overhead at the equator. London and Sydney both get about 12 hours of daylight.

   Talking points

   • USNO gives the March equinox of 2027 as 20 March at 20:25 UTC.
   • USNO glossary: an equinox is the time at which the centre of the Sun crosses the Earth's equator.
   • USNO, sunrise to sunset on 20 March 2027: Greenwich 12 hours 10 minutes, Sydney 12 hours 9 minutes.
   • This view is from the Sun, so the point with the Sun overhead is in the middle of the disc.

   Sources 4 6 7 8 9 10 11Present from this step →

6. 06

   Closest in January

   Presented and for pupils

   Caption

   Earth is closest to the Sun in early January, during the northern winter and the southern summer. So distance from the Sun does not cause the seasons.

   Talking points

   • USNO, 2027: perihelion (closest to the Sun) on 3 January at 02:33 UTC, aphelion (farthest) on 5 July at 05:06 UTC. The scene opens at perihelion.
   • NASA gives perihelion as 147.1 million km and aphelion as 152.1 million km, a difference of about 5 million km.
   • USNO: Earth's orbit is very close to a perfect circle, and the change in distance is too weak to cause the seasons.
   • Sizes and distances in this view are not to scale.

   Ask the class

   Why is it warmer in summer?

   1. AEarth is closer to the Sun in summer
   2. BOur half of Earth leans towards the Sun, so its rays hit us more directlyRight answer
   3. CThe Sun gives out more heat in summer

   WhyEarth is closest to the Sun in early January, which is winter in the north and summer in the south, so distance cannot be the cause. Summer comes when your hemisphere leans towards the Sun and gets its rays more directly.

   Sources 1 3 4 5Present from this step →

7. 07

   Find the June solstice

   Pupils only

   Caption

   Your turn. Run the clock forward to the June solstice and pause it there.

   Talking points

   • The check passes when the Sun's apparent longitude is within 2 degrees of 90 degrees. The USNO glossary defines a solstice as the moment the Sun's apparent longitude is 90 or 270 degrees, and 90 degrees is the June one. The Sun moves about 1 degree a day along its yearly path (360 degrees in a 365.24-day year), so that is about two days either side.

   Pupil task

   Play the clock and speed it up. Pause it at the June solstice, when the North Pole leans most towards the Sun.

   HintThe June solstice is around 21 June. Watch the date and slow the clock down as you get close.

   The viewer checks the task as the pupil works and says when it is done.

   Sources 1 4 6Open this step as a pupil →

8. 08

   Explain it

   Presented and for pupils

   Caption

   The tilt stays the same all year. As Earth goes round the Sun, first one hemisphere and then the other leans towards it.

   Talking points

   • NASA Space Place: as Earth orbits the Sun its tilted axis always points in the same direction, so different parts of Earth get the Sun's direct rays through the year.
   • A good answer says that in December the Southern Hemisphere leans towards the Sun, so Sydney gets more direct sunlight and longer days, while London leans away.

   Ask the class

   Explain to a friend why Sydney has summer when London has winter.

   AnswerWritten in the pupil’s own words, up to 280 characters. It is not marked.

   Sources 3 5Present from this step →

Sources

Every fact in this lesson comes from these sources, and each step lists the ones it uses.

1. Earth Fact Sheet, NASA NSSDCA
2. Notes on the Planetary Fact Sheets, NASA NSSDCA
3. What Causes the Seasons?, NASA Space Place
4. Earth's Seasons and Apsides, 2027, US Naval Observatory
5. The Seasons and the Earth's Orbit, US Naval Observatory
6. Astronomical Almanac Glossary, US Naval Observatory
7. Sun and Moon rise and set for one day (Greenwich and Sydney, 20 March, 21 June and 22 December 2027), US Naval Observatory
8. What is the Prime Meridian, and why is it in Greenwich?, Royal Museums Greenwich
9. Stand on the Prime Meridian, Royal Observatory Greenwich, Royal Museums Greenwich
10. Airy transit circle, An Etymological Dictionary of Astronomy and Astrophysics, Paris Observatory
11. Sydney (Observatory Hill) site information, Australian Bureau of Meteorology