AS101 Lesson 1

Astronomy (global effort)

Study of the universe done by scientists worldwide through collaboration and shared data

Scientific method

Process of observing, forming hypotheses, testing, and revising based on evidence

Scientific notation

A way to write very large or small numbers using powers of 10

Astronomical Unit (AU)

Average distance between Earth and Sun (~150 million km) used for solar system distances

Light-year (ly)

Distance light travels in one year (~9.46 trillion km), used for stars and galaxies

Universe

Everything that exists including all matter, energy, space, and time

Galaxy

A large system of stars, gas, and dust held together by gravity

Milky Way Galaxy

Our home galaxy containing about 100 billion stars

Local Group

A cluster of galaxies that includes the Milky Way

Supercluster

A group of galaxy clusters forming the largest structures in the universe

Solar system

The Sun and all objects that orbit it, including planets, moons, asteroids, and comets

Star

A massive, glowing ball of gas that produces its own energy through nuclear fusion

Planet

A non-luminous object that orbits a star and reflects its light

Scale of the universe

The idea that objects increase in size dramatically from Earth to the universe

Powers of 10

A way of comparing sizes and distances by factors of 10 in astronomy

Cosmic calendar

A model that compresses the 13.8 billion year history of the universe into one year

Big Bang

The event that marked the beginning of the universe about 13.8 billion years ago

Temporal perspective

Understanding the universe in terms of time and its long history

Human existence (cosmic calendar)

Humans appear only in the last few seconds of December 31

Constellation

A pattern of stars seen from Earth that appear close together

Reality of constellations

Stars in constellations are actually at different distances and not physically grouped

Celestial sphere

An imaginary sphere surrounding Earth onto which stars appear to be fixed

Zenith

The point in the sky directly above an observer

Meridian

An imaginary line running from north horizon through zenith to south horizon

Celestial equator

Projection of Earth’s equator onto the celestial sphere

Star motion (apparent)

Stars appear to move east to west across the sky

Star motion (actual cause)

Earth rotates west to east, creating the illusion of star movement

Circumpolar stars

Stars that never rise or set and circle around Polaris

Angular size

The apparent size of an object in the sky measured in degrees

Moon angular size

About 0.5 degrees in the sky

Fist measurement

About 10 degrees when held at arm’s length

Hand span measurement

About 20 degrees when held at arm’s length

Precession

Slow wobble of Earth’s rotational axis over ~26,000 years

North Star (Polaris)

The current star near Earth’s rotational axis in the northern sky