The scale of space and light

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

   The Solar System at true scale

   Presented and for pupils

   Caption

   This is the Solar System at its true scale, where the planets are far too small to see.

   Talking points

   • NASA average distances from the Sun: Earth 149.6 million km, Jupiter 778.5 million km, Neptune 4,515.0 million km.
   • Sizes and distances are both true here, so the labels mark where the bodies are.

   Sources 1 2Present from this step →

2. 02

   Sunlight on its way out

   Presented and for pupils

   Caption

   Light travels about 300,000 km every second, and still it takes more than four hours to reach Neptune.

   Talking points

   • The speed of light is exactly 299,792,458 m/s (NIST). Light times from NASA's average distances: Mercury 3.2 minutes, Venus 6.0, Earth 8.3 (8 minutes 19 seconds), Mars 12.7, Jupiter 43.3, Saturn 1 hour 20 minutes, Uranus 2 hours 39 minutes, Neptune 4 hours 11 minutes.
   • AQA's equation, distance travelled = speed × time, rearranges to time = distance ÷ speed.
   • Edexcel's data sheet gives the speed of light as 3.0 × 10⁸ m/s, the value used in the question.

   Ask the class

   Jupiter is 778.5 million km from the Sun. Light travels at 3.0 × 10⁸ m/s, which is 300,000 km/s. How many minutes does sunlight take to reach Jupiter?

   Answer43 minutes. Answers from 42 to 44 are marked right.

   Whytime = distance ÷ speed = 778,500,000 km ÷ 300,000 km/s = 2,595 s, and 2,595 ÷ 60 = 43 minutes.

   Sources 1 5 11 12Present from this step →

3. 03

   Calling Mars

   Presented and for pupils

   Caption

   On 19 February 2027 Mars is opposite the Sun in our sky and 101.4 million km from Earth, and a radio message still takes minutes to get there.

   Talking points

   • AQA: all types of electromagnetic wave, radio waves among them, travel at the same velocity through a vacuum (space), the speed of light.
   • JPL Horizons gives Earth to Mars at that moment as 0.6779 au, 101.4 million km, with a one-way light time of 5.64 minutes.
   • NASA: Mars comes as close as 54.6 million km and goes as far as 401.4 million km from Earth, so a message takes from about 3.0 to 22.3 minutes each way.
   • True scale: the labels mark the three bodies.

   Ask the class

   A radio message travels at the speed of light, 300,000 km/s. How many minutes does it take to cross the 101.4 million km to Mars?

   Answer5.6 minutes. Answers from 5.4 to 5.8 are marked right.

   Why101,400,000 km ÷ 300,000 km/s = 338 s, and 338 ÷ 60 = 5.6 minutes. JPL's Horizons system gives 5.64 minutes for that moment.

   Sources 3 5 7 11Present from this step →

4. 04

   The light year

   Presented and for pupils

   Caption

   A light year is the distance light travels in a year, about 9.5 trillion km.

   Talking points

   • IAU: the light year is the distance light travels in one Julian year (365.25 days) in a vacuum, 9,460,730,472,580.8 km. NASA rounds it to about 9.5 trillion km, and Edexcel's data sheet to 9.5 × 10¹² km.
   • NASA: the nearest star to the Sun, the red dwarf Proxima Centauri, is 4.24 light years away, so the light we see from it set off 4.24 years ago.
   • The IAU notes that for studying the Milky Way astronomers usually choose another unit, the parsec. Edexcel's data sheet gives one parsec as 3.26 light years.

   Ask the class

   Proxima Centauri is 4.24 light years away. A light year is 9.46 trillion km. How far away is Proxima Centauri, in trillion km?

   Answer40.1 trillion km. Answers from 39.6 to 40.6 are marked right.

   Why4.24 × 9.46 = 40.1 trillion km, which is 4.0 × 10¹³ km.

   Sources 6 8 12Present from this step →

5. 05

   Our galaxy from outside

   Presented and for pupils

   Caption

   This is a model of our galaxy, the Milky Way, seen from outside, and the Sun is one of its stars.

   Talking points

   • NASA: the Milky Way is a spiral galaxy about 100,000 light years in diameter, and our solar system is 26,000 light years from its centre. Seen from above, it would look like a large pinwheel.
   • Chandra (NASA's X-ray observatory): our solar system is inside the galaxy, so a view of it from outside can only be an artist's picture, pieced together from many kinds of observation. This one is a model too, and its distance from the planets is not to scale.
   • AQA: our solar system is a small part of the Milky Way galaxy.

   Ask the class

   The Milky Way is about 100,000 light years across. How long does light take to cross it?

   1. AAbout 100,000 yearsRight answer
   2. BAbout 100,000 days
   3. CAbout 8 minutes

   WhyA light year is the distance light travels in one year, so 100,000 light years takes light about 100,000 years.

   Sources 9 10 11Present from this step →

6. 06

   Minutes, hours, years

   Presented and for pupils

   Caption

   Light takes minutes to reach us from the Sun, hours from Neptune, years from the nearest star and tens of thousands of years from the middle of the galaxy.

   Talking points

   • From Earth: the Sun 8 minutes 19 seconds of light travel on average; Neptune 4,319 to 4,711 million km (NASA), 4.0 to 4.4 hours; Proxima Centauri 4.24 years; the centre of the Milky Way 26,000 years.
   • The galaxy here is the same model as in the last step, and its distance from the planets is not to scale.

   Ask the class

   Put these in order of distance from Earth, nearest first.

   They start in this order: Proxima Centauri, Neptune, The centre of the Milky Way, The Sun.

   Answer

   1. The Sun
   2. Neptune
   3. Proxima Centauri
   4. The centre of the Milky Way

   WhyThe Sun is 8 light minutes away, Neptune about 4 light hours, Proxima Centauri 4.24 light years and the centre of the Milky Way about 26,000 light years.

   Sources 4 5 6 8 9Present from this step →

7. 07

   Four hours of light

   Pupils only

   Caption

   Your turn: find the planet that sunlight takes about four hours to reach, and click on it or step to it with the arrows.

   Talking points

   • NASA: Neptune is on average 4,515.0 million km from the Sun. At 299,792 km/s light covers that in 15,060 s, 4 hours 11 minutes.

   Pupil task

   Focus on the planet that sunlight takes about four hours to reach.

   HintIt is the farthest planet from the Sun.

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

   Sources 1 5Open this step as a pupil →

Sources

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

1. Planetary Fact Sheet (metric), NASA NSSDCA
2. Notes on the Planetary Fact Sheets, NASA NSSDCA
3. Mars Fact Sheet, NASA NSSDCA
4. Neptune Fact Sheet, NASA NSSDCA
5. Speed of light in vacuum, CODATA value, NIST
6. Measuring the Universe: the IAU and astronomical units, International Astronomical Union
7. Mars from Earth at 2027-02-19 15:44 UT (distance and one-way light time), JPL Horizons
8. Sun: Facts, NASA Science
9. The Milky Way Galaxy, Imagine the Universe, NASA Goddard
10. How can you take a picture of our Galaxy if we are in it?, Chandra X-ray Observatory (NASA)
11. AQA GCSE Physics (8463) specification, version 1.1
12. Pearson Edexcel GCSE (9-1) Astronomy (1AS0) specification, issue 3