APS 105: Our Place in the Universe

Vocabulary flashcards covering astronomical scale units, solar system distances, planetary classification, light travel time, and scale models based on lecture notes by Prof. William Moore.

Astronomical Unit (AU)

The average distance between the Earth and the Sun, defined as approximately $150\text{ million kilometers}$ ($149,597,870.691\,\text{km}$).

Light-Year (LY)

The distance light travels in one year, equal to $9 \times 10^{15}\,\text{m}$, $9 \times 10^{12}\,\text{km}$, or $5.9 \times 10^{12}\,\text{miles}$.

Gravitational Dominance of the Sun

The distance threshold up to which the Sun's gravity dominates space, extending approximately $1\,\text{LY}$.

Nearest Star Distance

The distance from our Solar System to the closest neighboring star, equal to $4.2\,\text{LY}$ (or space enough to fit $27\text{ million}$ Suns).

Milky Way Galaxy Dimensions

The span of our galaxy, measuring about $100,000\,\text{LY}$ across, with our Solar System positioned approximately $26,000\,\text{LY}$ from the center.

Light Travel Time Principle

The observational fact that distance equals time in the past, meaning light received from an object $1\,\text{LY}$ away is $1\,\text{year}$ old.

Earth-Moon Distance Scale

A physical spacing along which exactly $30$ Earths can fit between the Earth and the Moon, equal to approximately $250,000\,\text{miles}$.

Scale Model

A visual or physical representation where all sizes and distances are scaled by the exact same proportion, preserving shapes and angles.

Basketball Scale Model

A scale model where Earth is represented by a basketball ($1\,\text{inch} = 1000\,\text{miles}$), the Moon by a tennis ball $21\,\text{feet}$ away, Mars by a softball, and the Sun as an $8\text{-story}$ tall object $1.5\,\text{miles}$ away.

Terrestrial Planets

The inner solar system planets—Mercury, Venus, Earth, and Mars—located within $1.5\,\text{AU}$ of the Sun.

Jovian Planets

The outer gas giant planets—Jupiter, Saturn, Uranus, and Neptune—located beyond Mars' orbit between $5.2\,\text{AU}$ and $30.1\,\text{AU}$ from the Sun.

Mote of Dust

Carl Sagan's description of planet Earth as seen from distant space, suspended in a sunbeam.