astr ch2-3

Geocentric theory

The model of the universe where Earth is at the center and all other celestial bodies orbit around it.

Retrograde

The apparent backward motion of a planet as observed from another planet.

Prograde

The normal direction of a planet's motion in its orbit, in the same direction as most other celestial bodies.

Epicycles

Small circular orbits that planets move in, according to the geocentric model, to explain retrograde motion.

Heliocentric theory

The model of the universe where the Sun is at the center and the planets, including Earth, orbit around it.

Conjunction

An alignment of two celestial bodies in the sky, typically when they are close to each other as viewed from Earth.

Opposition

The position of a planet when it is directly opposite the Sun in the sky, making it visible all night.

Kepler’s 3 laws

Laws describing planetary motion: 1. Orbits are ellipses, 2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time, 3. The square of the orbital period is proportional to the cube of the semimajor axis of its orbit.

Semimajor axis

The longest radius of an ellipse, half of the major axis, used in orbital mechanics.

Eccentricity

A measure of how much an orbit deviates from being circular, ranging from 0 (perfect circle) to 1 (parabolic).

Perihelion

The point in the orbit of a planet where it is closest to the Sun.

Aphelion

The point in the orbit of a planet where it is farthest from the Sun.

Astronomical unit (AU)

The average distance between the Earth and the Sun, approximately 149.6 million kilometers.

Newton’s 3 laws

Fundamental principles of motion: 1. An object at rest stays at rest, 2. F=ma (force equals mass times acceleration), 3. For every action, there is an equal and opposite reaction.

Inertia

The tendency of an object to resist changes in its state of motion.

Momentum

The product of an object's mass and its velocity.

Conservation of momentum

In a closed system, the total momentum before an event is equal to the total momentum after the event.

Angular momentum

The rotational analog of linear momentum, dependent on mass, velocity, and the distance from the rotation axis.

Conservation of angular momentum

In the absence of an external torque, the total angular momentum of a system remains constant.

Weight

The force exerted on an object due to gravity.

Surface gravity

The gravity experienced at the surface of a celestial body.

Orbital velocity

The speed required for an object to remain in a stable orbit around a celestial body.

Escape velocity

The minimum speed needed for an object to break free from the gravitational attraction of a body, without any further propulsion.