Astronomy Mid-Term

Astronomy

Scientific study of objects and phenomena beyond Earth.

Solar System

The Sun and all objects gravitationally bound to it.

Galaxy

A gravitationally bound collection of stars, gas, dust, and dark matter.

Milky Way

The galaxy containing our Solar System.

Light-year

Distance light travels in one year; a unit of distance, not time.

Astronomical unit (AU)

Average Earth–Sun distance, about 150 million km.

Speed of light

About 300,000 km/s in a vacuum.

Light-travel time

Looking far away means seeing an object as it was in the past.

Sunlight travel time

Sunlight takes about 8 minutes to reach Earth.

Moonlight travel time

Light takes about 1.3 seconds from Moon to Earth.

Nearest star beyond Sun

Proxima Centauri, about 4.2 light-years away.

Galactic center distance

About 25,000 light-years from the Sun in the lecture.

Andromeda distance

About 2.5 million light-years away.

Magellanic Clouds distance

About 160,000 light-years in the lecture's example.

Cosmic microwave background

Ancient radiation from the early universe.

Scientific notation

Uses powers of ten to express very large or small quantities.

Order of magnitude

Approximate size expressed as a power of ten.

Scale comparison

Compare astronomical objects using consistent size or distance units.

Why do distant objects show the past?

Their light takes time to reach us.

What does a telescope observe in a distant galaxy?

Light emitted long before the present.

Celestial sphere

Imaginary sphere used to map positions of objects in the sky.

Constellation

Recognizable pattern or defined region of stars in the sky.

Why are constellations useful?

They provide a reference background for tracking sky motions.

Daily apparent sky motion

Sun, Moon, and stars generally rise east and set west because Earth rotates eastward.

Celestial poles

Points in the sky aligned with Earth's rotation axis.

Circumpolar stars

Stars that never set at a given observing latitude.

Ecliptic

Apparent yearly path of the Sun against background stars.

Why does the Sun drift eastward against stars?

Earth revolves around the Sun.

Moon's motion against stars

Moon gradually moves eastward relative to background stars.

Moon phases

Changing appearance of Moon's sunlit half as seen from Earth.

Cause of Moon phases

Changing Sun–Earth–Moon viewing geometry, not Earth's shadow.

New Moon

Moon lies roughly toward the Sun; illuminated half faces away from Earth.

Full Moon

Earth-facing lunar hemisphere appears illuminated.

First-quarter Moon

Half of the visible Moon is lit while waxing.

Third-quarter Moon

Half of the visible Moon is lit while waning.

Waxing

Illuminated fraction of the Moon appears to increase.

Waning

Illuminated fraction of the Moon appears to decrease.

Solar eclipse

Moon passes between Earth and Sun and casts a shadow on Earth.

Lunar eclipse

Earth passes between Sun and Moon, shadowing the Moon.

Umbra

Region of complete shadow.

Penumbra

Region of partial shadow.

Why not an eclipse every month?

Moon's orbital plane is tilted relative to Earth's orbit.

Why can the Moon cover the Sun?

Their apparent angular sizes are nearly equal.

Annular solar eclipse

Moon appears too small to cover Sun fully, leaving a bright ring.

Cause of seasons

Earth's tilted axis changes daylight duration and Sun angle.

Why is summer warmer?

Longer days and more direct sunlight deliver more energy.

Are seasons caused by Earth–Sun distance?

No; axial tilt is the main cause.

June solstice in Northern Hemisphere

Sun follows its highest yearly path; longest daylight.

December solstice in Northern Hemisphere

Sun follows its lowest yearly path; shortest daylight.

Equinox

Sun rises due east and sets due west; day and night nearly equal.

Why do seasonal changes grow at high latitudes?

Tilt produces larger differences in daylight and solar angle.

At midnight where are meridian stars relative to Sun?

On the opposite side of the sky from the Sun.

Why are some constellations not visible at night in a season?

They lie near the Sun in the daytime sky.

Scientific method

Use observations, testable predictions, and revision of models.

Hypothesis

A proposed explanation that can be tested.

Scientific theory

Well-supported explanatory framework that makes testable predictions.

Can observations prove a theory absolutely true?

No; they can support it or rule out predictions.

Inductive reasoning

Build general ideas from observations.

Deductive reasoning

Use a model to predict particular outcomes.

Babylonians

Recorded eclipses and planetary positions and developed predictive arithmetic.

Greek astronomy

Used mathematical and geometrical models of the sky.

Geocentric model

Earth-centered model of planetary motion.

Heliocentric model

Sun-centered model of planetary motion.

Plato

Emphasized circular motion and mathematical reasoning.

Eudoxus

Developed an Earth-centered model using nested spheres.

Aristotle

Promoted an Earth-centered cosmos and different physics for heavens and Earth.

Ptolemy

Created a detailed geocentric predictive planetary model.

Epicycle

Small circular motion added to a planet's larger circular motion in geocentric models.

Retrograde motion

Temporary apparent backward motion of a planet against stars.

Heliocentric explanation of retrograde motion

Relative orbital motions make a planet appear to reverse direction.

Copernicus

Proposed a Sun-centered planetary model in the 1500s.

Paradigm change

Major change in a scientific explanatory framework.

Tycho Brahe

Collected exceptionally precise pre-telescope planetary observations.

Johannes Kepler

Used planetary data to find three empirical laws of orbits.

Galileo Galilei

Used telescopic observations to challenge traditional geocentrism.

Galileo's Jupiter moons

Showed not everything orbits Earth.

Galileo's Venus phases

Full range of phases ruled out the standard Ptolemaic model.

Did Venus's phases prove heliocentrism?

No; they ruled out the Ptolemaic prediction and supported compatible alternatives.

Galileo's Milky Way observation

Resolved it into numerous faint stars.

Galileo's sunspots

Showed the Sun was not a perfect unchanging sphere.

Galilean relativity

Uniform shared motion cannot be detected by simple internal motion alone.

Inertia

Motion continues unchanged unless acted on by a net force.

When did Galileo build his telescope?

1609, with published observations in 1610.

When did Copernicus propose heliocentrism?

Sixteenth century.

When did Kepler formulate his laws?

Early 1600s.

Kepler's first law

Planets orbit in ellipses with Sun at one focus.

Kepler's second law

Equal areas are swept in equal times; planets move faster near Sun.

Kepler's third law

For solar orbits, period squared equals semimajor axis cubed (years, AU).

Semimajor axis

Half the long diameter of an ellipse; describes orbit size.

Perihelion

Point in an orbit closest to the Sun.

Aphelion

Point in an orbit farthest from the Sun.

Where is orbital speed fastest?

At perihelion, closest to the Sun.

Where is orbital speed slowest?

At aphelion, farthest from the Sun.

Newton's first law

An object keeps constant velocity unless acted on by net force.

Newton's second law

Net force equals mass times acceleration (F = ma).

Newton's third law

Forces between two objects are equal and opposite.

Mass

Amount of matter; does not depend on location.

Weight

Force of gravity acting on an object.

Why does constant-speed circular motion require force?

Velocity direction changes, so there is acceleration.

Universal gravitation

All masses attract; force scales with mass product and inverse distance squared.