Prehistoric Astronomy: The Night Sky

Observations and evidence

Science is based on observations and evidence; astronomy depends on light of all types

Model

A scientific explanation that changes with new evidence, such as geocentric to heliocentric or static to expanding universe

Peer review

Ensures claims (hypotheses) are tested by others

Hypothesis

A testable statement based on new observations

Scientific theory

A large collection of undisputed data and laws that support a complex idea

Scientific law

An observation in nature that recurs without fail

Faith/Philosophy

Beyond the limits of science

Prehistoric

The period of Western astronomy before 500 B.C., when cyclical motions of the Sun, Moon and stars were observed and time keeping and determining directions developed

Local Group

The level of structure containing the Milky Way, inside the Local Supercluster

Local Supercluster

The level of structure containing the Local Group, inside the universe

The Milky Way

Our own galaxy; a spiral galaxy with hundreds of billions of stars, 100,000 light years across and 10,000 light years thick, appearing as a whitish band easiest to see in the Northern Hemisphere in the summer months

Andromeda

Another galaxy; apart from galaxies like it, everything we see in the night sky resides in the Milky Way

Spiral galaxy

The type of galaxy the Milky Way is

Observable universe

Thought to be approximately 91 billion light years across and to contain 100 billion galaxies

Light year

The unit used to express astronomical distances, such as the 91 billion light year width of the observable universe

Scientific notation

Needed to efficiently express sizes ranging from the diameter of the observable universe (10^26) down to smaller than subatomic particles (10^-24)

Cyclic

Repeating on a day-to-day and year-to-year basis, which gave prehistoric people methods for time keeping and the ability to predict and plan future events

Monumental structures

Buildings such as Stonehenge that prehistoric people had incentive to build from astronomical observations

Asterisms

Recognizable patterns of bright stars that allow us to orient ourselves on earth and help us locate specific stars

Constellations

The word reserved by astronomers to describe an entire area of the sky and all the objects in it

Magnitude

A star's brightness

Absolute magnitude

A star's true brightness, or how bright it would appear to us if we could see all stars at the same distance away (33 ly)

Apparent magnitude

The brightness of a star as seen from Earth; the lower the number the brighter the star, and humans can see as low as 6 with the unaided eye

Celestial sphere

Sky map; an imaginary sphere surrounding Earth on which all objects in the night sky appear to be fixed in place at equal distances away

Celestial equator

Divides the sky into northern and southern hemispheres

North celestial pole

Along earth's axis of rotation in the northern hemisphere

South celestial pole

Along earth's axis of rotation in the southern hemisphere

Ecliptic

The path on the celestial sphere traced by the path of the sun

Declination

0-90 degrees north and south of the celestial equator

Right ascension

0-24 hours around the celestial equator

Zodiac

The very narrow band centered about the ecliptic in which the planets move relative to the stars

Prograde motion

Apparent motion of planets from west to east relative to the stars

Retrograde motion

When a planet moves from east to west relative to the stars

Geocentric model

The model of the solar system astronomers ultimately rejected, largely because of the need to explain retrograde motion

Equinox

Means "equal night"; the sun is directly over earth's equator so there are exactly 12 h of day and 12 h of night everywhere on earth

Vernal equinox

The point where the sun crosses the celestial equator moving south to north (March 20 — equal parts light and dark)

Autumnal equinox

The point where the sun crosses the celestial equator moving north to south (September 22 — equal time light and dark)

Summer solstice

The northernmost point on the ecliptic; the day in the northern hemisphere each year with the most daylight (June 21 — longest day of the year)

Winter solstice

The southernmost point on the ecliptic; the day in the northern hemisphere each year with the least daylight (Dec 21 — shortest day of the year)

Zenith

When a star is at its highest point in the sky; the sun's zenith is around noon ("high noon")