ast101h1 mid term

Daylight (Summer)

Longer days, shorter nights, Sun elevation ~70° above horizon.

Daylight (Winter)

Shorter days, longer nights, Sun elevation ~23° above horizon.

Seasons

Caused by Earth's axial tilt, not orbital distance.

Summer Solstice

Northern Hemisphere tilted toward Sun (June).

Winter Solstice

Southern Hemisphere tilted toward Sun (December).

Equinox

Day and night equal length; Sun crosses celestial equator.

Moon Spin

Tidally locked to Earth, same side always faces Earth.

Lunar Eclipse

Moon enters Earth's shadow, often appears red.

Solar Eclipse

Moon blocks Sun's light, rare, only during new moon.

Moon's Orbit Tilt

5° tipped relative to ecliptic; eclipses occur ~every 6 months.

New Moon

Moon aligned with Sun, dark; only time solar eclipses occur.

Waxing Crescent

3–4 days after new moon; right sliver visible.

First Quarter

1 week after new moon; half of Moon lit, rises midday.

Waxing Gibbous

10–11 days after new moon, rises 9 hrs after Sun.

Full Moon

Opposite Sun, ~2 weeks after new moon; lunar eclipses possible.

Waning Gibbous

After full moon, rises ~3 hrs after Sun.

Third Quarter

Half lit, rises midnight, ~1 week before new moon.

Waning Crescent

Few days before new moon, left sliver visible.

Retrograde Motion

Apparent backward motion of planets, explained by heliocentric model.

Epicycle

Ancient concept of circles within circles explaining retrograde.

Tycho Brahe

1546–1601, precise measurements of Mars, observed supernova.

Parallax

Method to measure distances to stars via shift in position.

Johannes Kepler

1571–1630, derived laws of planetary motion.

Kepler's First Law

Planets move in elliptical orbits.

Kepler's Second Law

Planets sweep equal areas in equal times; move faster closer to Sun.

Kepler's Third Law

Orbital period squared ∝ semi

Galileo

Observed Moon's craters, Jupiter's moons, Venus phases; challenged geocentrism.

Velocity

Speed with direction; rate of change of position.

Acceleration

Rate of change of velocity (speed or direction).

Newton’s First Law

Object in motion stays in motion unless acted upon.

Newton’s Second Law

F=ma; acceleration ∝ force, ∝ 1/mass.

Newton’s Third Law

For every action, equal and opposite reaction.

Momentum

p=mv; product of mass and velocity.

Angular Momentum

L=mvr; conserved in orbital motion.

Universal Gravitation

F=G(M1M2/d²); attraction between all masses.

Sun Color

White, not yellow; color shift from atmosphere scattering.

Constellation

Apparent grouping of stars into patterns.

Celestial Sphere

Imaginary sphere where stars appear fixed.

Celestial Pole

Projection of Earth's axis onto sky (NCP & SCP).

Celestial Equator

Projection of Earth's equator into space.

Ecliptic

Path of Sun across sky; plane of Earth's orbit.

Zodiac

Band of constellations along ecliptic.

Ellipse

Oval orbit shape, defined by eccentricity.

Orbital Period

Time for one revolution around Sun.

Tides

Caused by gravitational differences between Earth’s near/far sides.

Tidal Locking

Rotation period equals orbital period (Moon to Earth).

Sun Core

Central region; nuclear fusion occurs.

Photosphere

Visible surface of Sun.

Corona

Outer atmosphere; visible during eclipse.

Chromosphere

Layer between photosphere and corona, reddish glow.

Hydrostatic Equilibrium

Balance between gravity pulling in and pressure pushing out in Sun.

Fusion

Combining nuclei; Sun fuses hydrogen into helium.

Fission

Splitting heavy nuclei; not how Sun generates energy.

Sunspots

Cooler, darker regions on Sun caused by magnetic activity.

Solar Cycle

~11

Granulation

Pattern on Sun's surface from convection currents.

Solar Flare

Sudden energy release on Sun, causes bursts of radiation.

Prominence

Large, bright feature of plasma extending outward from Sun.

Aurora

Northern/southern lights; caused by solar wind interacting with atmosphere.

Light Year

Distance light travels in one year (~9.46 trillion km).

Astronomical Unit (AU)

Average Earth

Parsec

Distance at which 1 AU subtends an angle of 1 arcsecond (~3.26 ly).

Apparent Magnitude

How bright a star appears from Earth.

Absolute Magnitude

Intrinsic brightness at 10 parsecs.

Spectroscopy

Study of light spectra to determine composition of stars.

Electromagnetic Spectrum

Range of all light: radio, microwave, infrared, visible, UV, X

Photosynthesis Connection

Sunlight drives life on Earth through plant energy conversion.

Solar Wind

Stream of charged particles from Sun affecting Earth's magnetosphere.

Earth’s Magnetic Field

Protects Earth from solar wind and cosmic radiation.

Precession

Slow wobble of Earth's axis (~26,000 years).

Eratosthenes

Measured Earth's circumference using shadows (~200 BCE).

Copernicus

Proposed heliocentric model of solar system.

Ptolemy

Geocentric model with epicycles; lasted 1400 years.

Heliosphere

Bubble

Milky Way

Our galaxy, ~100–400 billion stars, Sun located in Orion Arm.