Argument of periapsis

Okay, imagine you are driving a toy car around a circular track. The track is like the path that planets take around the sun. Now, if you look closely, you'll notice that the toy car does not always go at the same speed. Sometimes it goes faster and sometimes slower. This is because of gravity, which is like a big invisible hand pulling the car towards the center of the circle.

Now, let's imagine that the toy car is a planet and the circular track is its orbit around the sun. When the planet is closest to the sun (at the closest point of its orbit), it is moving faster because the sun's gravity is pulling it harder. This point is called the periapsis (pronounced "pear-ee-AP-sis").

But here's where it gets a little bit tricky. Sometimes, the planet's path is not perfectly circular – it's a little bit elongated or squished. When this happens, the periapsis is not always at the same point. Instead, it can move around the orbit, kind of like playing a game of duck-duck-goose with the other points on the path.

The angle at which the periapsis is located on the orbit is called the argument of periapsis. Think of it like a clock – the angle is measured in degrees, just like the hours on a clock. So if the periapsis is located at the 3 o'clock position on the orbit, the argument of periapsis would be 90 degrees.

Scientists use the argument of periapsis to calculate how a planet's orbit is changing over time. By tracking the movement of the periapsis, they can learn a lot about the planet's gravitational interactions with other objects in the solar system.