Sunlight at the Planets (Earth & Space)

Lesson notes

Overview

Solar flux falls with the square of distance from the Sun. This topic is rated advanced and is number 97 of 98 in Earth & Space. Deeper models, formal results and harder reasoning.

What you are seeing

The panel above shows a bar chart that compares values on one scale, so differences in size are easy to see. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

The sunlight reaching a planet falls with the square of its distance from the Sun, W/m² ∝ 1/d². Earth receives about 1,361 W/m², Mercury about 9,100, Mars 590 and Jupiter only 50, while at Neptune the Sun is about 900 times fainter than at Earth. This flux, along with the planet’s reflectivity and atmosphere, determines its temperature. It is also important for deciding how big solar panels on a spacecraft must be.

Reading the relationship

The expression "W/m²" is a compact summary of the idea. It uses the symbols W, m; name what each one stands for before you start. Notice that it contains a squared term, so doubling that quantity changes the result by a factor of four, and a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “Greenhouse Gas Warming Potentials”, the topic before it in Earth & Space. Once you are comfortable, continue with “Moons of the Planets”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.

Where you might use it

Revising “Sunlight at the Planets” before a class, exam or certification
Forecasting weather and understanding climate records and their uncertainty
Planning for earthquakes, floods and other natural hazards in cities
Interpreting satellite images, space missions and the age of rocks and stars

Frequently asked questions

What is Sunlight at the Planets?

Solar flux falls with the square of distance from the Sun. It is an advanced topic in Earth & Space.

How do I use the Sunlight at the Planets simulation?

The panel at the top shows a bar chart that compares values on one scale, so differences in size are easy to see. Move the sliders one at a time, then press Space to pause the animation and look closer.

What does the formula mean?

The expression W/m² packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.

Who is this topic for?

Deeper models, formal results and harder reasoning. If the quiz feels hard, reread the lesson notes and try the previous topic first.

What should I learn before this?

Start with “Greenhouse Gas Warming Potentials”, the previous topic in Earth & Space. Each topic in a subject builds on the ones before it.

How do I track my progress?

Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.