Astronomy

Start here: geocentric views versus heliocentric reality, the scale of the solar system, and gravity as the organizing force.

Moon

Why the Moon cycles through phases, why a month of phases is longer than one orbit, why we only ever see one face, and why eclipses are rare.

Solar System

Here

Kepler's three laws, the scale of planetary orbits, why inner planets move faster, and retrograde motion as a frame effect.

Solar System

The planets are not scattered at random. They orbit the Sun on nearly circular, nearly coplanar paths, all in the same direction, with timing set by a single force. Three laws found by Johannes Kepler — before Newton explained why — capture the pattern.

Kepler’s Laws

Three laws found by Johannes Kepler capture the pattern. Click any law to bring up its description and an interactive — the first is open to start.

Each planet moves on an ellipse with the Sun at one focus. Most planetary orbits are only slightly elongated, so they look almost circular, but the offset matters for precise predictions. Drag the slider to stretch the orbit and watch the Sun sit at a focus — never the centre.

Eccentricitye = 0.50

In units of years and astronomical units, the constant in the third law is exactly one for objects orbiting the Sun:

\left(\frac{T}{\text{yr}}\right)^2 = \left(\frac{a}{\text{AU}}\right)^3.

This is why larger orbits take dramatically longer: Mercury laps the Sun every $0.24$ years while Neptune needs about $165$ . Gravity is the reason — closer to the Sun the attraction is stronger, demanding a higher orbital speed, and the path around is shorter too. Inner planets are both faster and have less ground to cover, so they lap the outer ones again and again.

Heliocentric Motion

A heliocentric model places the Sun at the center. This is a convenient reference frame for viewing the planets, as their primary motion is simply around the Sun. But many planets have moons, which are not solely orbiting the Sun. The Moon orbits Earth, while the Earth-Moon system orbits the Sun. So in a heliocentric view, the Moon’s path is the combination of both motions — a small loop riding along a big one.

The same two-frame thinking explains a famous puzzle. Throughout most of the year, an outer planet like Mars drifts slowly eastward against the stars. For a few weeks it appears to stop, reverse into retrograde, and then resume. Since Earth is on a faster inner orbit, we overtake Mars on the inside. From our point of view, Mars appears to slide backward against the distant background.

Problem Solving

Kepler's Third Law for a New Planet

Problem

A planet orbits the Sun at an average distance of $a = 4\,\text{AU}$ . How long is its year?

Use

\left(\frac{T}{\text{yr}}\right)^2 = \left(\frac{a}{\text{AU}}\right)^3.

Solve

T^2 = 4^3 = 64 \quad\Rightarrow\quad T = \sqrt{64} = 8\,\text{years}.

Check

A larger orbit means a longer year, and $8$ years for $4\,\text{AU}$ is reasonable — it falls between Mars ( $1.5\,\text{AU}$ , $1.9$ yr) and Jupiter ( $5.2\,\text{AU}$ , $11.9$ yr).

Jupiter's Distance in Kilometers

Problem

Jupiter is about $5.2\,\text{AU}$ from the Sun. Estimate that distance in kilometers, using $1\,\text{AU} \approx 1.496 \times 10^8\,\text{km}$ .

Solve

5.2 \times (1.496 \times 10^8) \approx 7.78 \times 10^8\,\text{km}.

Check

That is several times Earth’s distance from the Sun, which matches Jupiter’s place well out in the solar system — and shows why AU is the friendlier unit.

Orbits Checkpoint

Question 1 of 4

Kepler's second law says a planet sweeps equal areas in equal times. What does this imply about its speed?

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