Cosmos Notebook

Learning paths · Path

Measuring the Solar System

From a stick's shadow to the mass of Jupiter — measure the Earth, the Sun and a giant planet yourself, following the seasons.

AstrophysicsBeginner3–6 months · follows the seasons5 stages · 7 steps + 1 optional

Each stage builds on the last: first distances on Earth, then the size of the Earth itself, then the Sun, then the mass of Jupiter, and finally a computer model that puts it all together. Some steps have to wait for the right sky — an equinox, a sunny day, Jupiter in the evening — so do whatever the sky allows and come back to the rest.

Safety: never look at the Sun, directly or through any optics. The Sun steps here only ever use a projected image.

  1. Any timeWeeks 1–2

    Find your way around

    Get to know the sky and learn the trick astronomers use to measure distances.

    1. UseStellarium — learn four constellations you can see from home
    2. AlsoSketch the Moon on three evenings a few days apart and note how its shape and position change
    3. DoMeasure distance by parallax

    Checkpoint Explain parallax using your thumb, and why it gets harder to measure for things that are further away.

  2. EquinoxMar or Sep

    Measure the Earth

    At the spring or autumn equinox, a stick's shadow is enough to measure the whole planet.

    1. DoMeasure the Earth like Eratosthenes

    Checkpoint Eratosthenes' method only works because the Sun's rays arrive almost parallel. Explain why that is, and what would go wrong if the Sun were much closer.

  3. Sunny daysOver 2 weeks

    Measure the Sun

    Project the Sun safely to measure how big it is, then follow its spots to time its spin.

    1. DoMeasure the Sun's diameter with a pinhole
    2. DoTrack sunspots to time the Sun's spin

    Checkpoint Your sunspots took about two weeks to cross the Sun's face. What does that tell you about how long the Sun takes to spin once?

  4. Winter evenings2–4 weeks

    Weigh a planet

    Jupiter is bright in the evening sky every winter. Time its moons through binoculars and Kepler's third law gives you its mass.

    1. DoWeigh Jupiter
    2. ReadAstrophysics for People in a Hurry

    Checkpoint In one paragraph, explain how timing Jupiter's moons tells you how heavy Jupiter is.

  5. Any timeWeeks after

    Model it

    Build the Solar System in code and check your measurements against it.

    1. DoSimulate an orbit in code
    2. DoSimulate an exoplanet transit — find planets around other stars the same way Optional
    3. UsePhET Interactive Simulations — the "Gravity and Orbits" simulation Optional

    Checkpoint Put your measured mass of Jupiter into the orbit simulator. Do its moons go round in about the times you observed?

Method

How every project runs

  1. Learn just enough. One short video or chapter on the idea.
  2. Predict. Write down what you expect before measuring.
  3. Do it. Record everything, including the messy bits.
  4. Explain it. A one-page write-up for a friend — share it with the community if you like.
  5. Follow the questions. They become the next project.