Loading module…

What you will learn in The Solar System in Motion

1. A tour of the solar system
Eight planets and one star, drawn where they are today from the orbital elements rather than arranged on a page. The first thing the true positions show is the thing every diagram hides: the planets are nowhere near evenly spaced.
2. The scale problem
No picture of the Solar System can be true to scale and show anything. Every one enlarges the planets, squeezes the distances, or both — and almost none of them says so. Here the learner rebuilds the textbook diagram from two sliders and reads off how wrong it is.
3. Orbits are ellipses
An orbit is an ellipse with the Sun at one focus — but almost every planetary orbit is so close to a circle that the difference cannot be seen, and the diagrams that make it visible are exaggerating.
4. Faster when closer
A planet does not travel at a steady speed. It hurries through the part of its orbit nearest the Sun and dawdles through the far part — and the rule behind it is that the line to the Sun sweeps equal areas in equal times.
5. Why the outer planets crawl
Mercury goes round the Sun in 88 days and Neptune in 165 years. Drag ten years past in the Solar System view and watch one planet race and the other barely move — then see the two reasons the outer orbits take so long.
6. Orbital resonance
When two orbital periods form a simple ratio, the bodies meet in the same few places every time. That single fact keeps moons locked together, carves gaps in the asteroid belt, and lets two crossing orbits stay clear of each other forever.
7. Axial tilt and the seasons
The most common misconception in the whole subject is that summer happens because Earth is nearer the Sun. Three experiments on one globe settle it: closest approach falls in northern winter, a circular orbit leaves the seasons as they were, and an upright Earth has no seasons at all.
8. The backward planets
Every two years or so Mars seems to stop among the stars, drift backwards for about eleven weeks, and then carry on. It never actually turns round. The loop is what a direction does when the observer, on the faster inside track, overtakes.
9. What counts as a planet
Pluto stopped being a planet in 2006 by a vote, and that makes the decision feel arbitrary. The definition is three questions, and the third — has it cleared its orbit? — separates planets from dwarf planets by a factor of more than a thousand, with nothing in between.

What you will learn in The Solar System in Motion

1. A tour of the solar system

Eight planets and one star, drawn where they are today from the orbital elements rather than arranged on a page. The first thing the true positions show is the thing every diagram hides: the planets are nowhere near evenly spaced.

2. The scale problem

No picture of the Solar System can be true to scale and show anything. Every one enlarges the planets, squeezes the distances, or both — and almost none of them says so. Here the learner rebuilds the textbook diagram from two sliders and reads off how wrong it is.

3. Orbits are ellipses

An orbit is an ellipse with the Sun at one focus — but almost every planetary orbit is so close to a circle that the difference cannot be seen, and the diagrams that make it visible are exaggerating.

4. Faster when closer

A planet does not travel at a steady speed. It hurries through the part of its orbit nearest the Sun and dawdles through the far part — and the rule behind it is that the line to the Sun sweeps equal areas in equal times.

5. Why the outer planets crawl

Mercury goes round the Sun in 88 days and Neptune in 165 years. Drag ten years past in the Solar System view and watch one planet race and the other barely move — then see the two reasons the outer orbits take so long.

6. Orbital resonance

When two orbital periods form a simple ratio, the bodies meet in the same few places every time. That single fact keeps moons locked together, carves gaps in the asteroid belt, and lets two crossing orbits stay clear of each other forever.

7. Axial tilt and the seasons

The most common misconception in the whole subject is that summer happens because Earth is nearer the Sun. Three experiments on one globe settle it: closest approach falls in northern winter, a circular orbit leaves the seasons as they were, and an upright Earth has no seasons at all.

8. The backward planets

Every two years or so Mars seems to stop among the stars, drift backwards for about eleven weeks, and then carry on. It never actually turns round. The loop is what a direction does when the observer, on the faster inside track, overtakes.

9. What counts as a planet

Pluto stopped being a planet in 2006 by a vote, and that makes the decision feel arbitrary. The definition is three questions, and the third — has it cleared its orbit? — separates planets from dwarf planets by a factor of more than a thousand, with nothing in between.