Lesson 6: Kepler's Laws

What two things do you notice when watching satellites?

1. Orbits are ellipses (slightly "squashed" circles). 2. The farther out a satellite is, the longer it takes to complete one orbit (orbital period).

Who figured out the laws of planetary motion?

Johannes Kepler, over 400 years ago.

What are Kepler's laws about?

They describe how planets move around the Sun. We now know they apply to anything orbiting anything else—including satellites orbiting Earth.

What is Kepler's 1st Law?

Orbits are ellipses, not perfect circles.

Why was Kepler's 1st Law a big deal at the time?

People assumed the heavens were "perfect," so orbits had to be perfect circles. But circles didn't match observations. Once Kepler allowed for ellipses, he could predict planet positions accurately.

What does Kepler's 2nd Law describe?

It describes how an object's speed changes during its orbit. In a circular orbit, speed stays constant. In an elliptical orbit, the object moves faster when closer in and slower when farther out.

What is Kepler's 3rd Law?

It connects how far away an object orbits to how long its orbit takes—the farther out, the longer the orbital period.

What three values are needed to use Kepler's 3rd Law?

• m = mass of the object being orbited (like Earth)

• a = semi-major axis (orbital distance)

• P = orbital period (how long one full orbit takes)

What is the simplified formula for Kepler's 3rd Law?

a³ = P² × m

What can you calculate using Kepler's 3rd Law?

If you know how far away something orbits, you can calculate how long its orbit takes—or the other way around.

Who explained why Kepler's laws work?

Isaac Newton, about 150 years later, explained that Kepler's laws are just a natural result of gravity.

What was Sputnik and when was it launched?

Sputnik was the first artificial satellite, launched by the Soviet Union in 1957. It carried only a small radio transmitter.

How did scientists figure out Sputnik's altitude?

Ham radio operators measured how often the signal repeated, which told them Sputnik's orbital period (96.2 minutes). Using Kepler's 3rd Law and Earth's known mass, they calculated its altitude—without ever directly measuring it.

What was Sputnik's altitude and orbital region?

About 600 km above Earth's surface, which places it in Low Earth Orbit (LEO).

What happened to Sputnik after launch?

Even at 600 km, there's still a trace amount of atmosphere that slowly dragged on Sputnik, causing it to lose energy. Combined with its slightly elliptical orbit, Sputnik eventually fell back into the atmosphere and burned up—about 3 months after launch.

What is the key takeaway about Kepler's laws?

Kepler's laws let you connect distance and orbital period using simple math. If you know one, you can calculate the other—which is how scientists figured out Sputnik's altitude just by timing its radio signal.