ASTR 1210 Exam 1

star

a large, glowing ball of gas that generates energy through nuclear fusion in its core; the Sun is the closest star (alive for millions of years)

planet

an object that 1. orbits a star, 2. is massive enough for its own gravity to give it a round shape, and 3. has cleared the neighborhood around its orbit

dwarf planet

an object that 1. orbits a star and 2. is massive enough for its own gravity to give it a round shape but 3. has NOT cleared the neighborhood around its orbit (ex. Pluto)

Solar System

the Sun and all of the objects which are gravitationally bound to it

galaxy

a gravitationally bound collection of a few hundred million to a trillion or more stars (+ dust, gas, and dark matter)

How are planets round-shaped?

Gravity crushes the mass into itself, creating a spherical shape.

How far is our Solar System from the center of the galaxy?

About halfway (28,000 light years)

How many galaxies does the observable Universe have?

About 100 billion galaxies

Speed of Light

3 × 10⁵ km/s (or 300,000 km/s)

light year

the distance light travels in one year (~10 trillion km = 6 trillion miles); the farther away we look in distance, the further back we look in time

Astronomical Unit (AU)

the average distance from the Earth to the Sun

1 AU = 150 million km

Stephen’s Quintet

a visual grouping of five galaxies

Light Distances

Earth to Moon: 1.3 light seconds

Sun to Pluto: 5.5 light hours

Sun to Proxima Centauri (closest neighbor to Sun): 4.2 light years

Size of Milky Way: ~3 million light years

What discovery did Edwin Hubble make?

Discovered that galaxies appear to be receding from us (1920s); more distant galaxies are receding faster from us than closer ones

Hubble Law

galaxies are moving away from Earth at speeds proportional to their distance; the farther a galaxy is from us, the faster it is receding.

Big Bang

the event through to mark the birth of the Universe

Raisin Bread Analogy

an analogy meant to illustrate how galaxies move farther away from each other as the fabric of space expands

parsec

a unit of length used to measure the large distances to astronomical objects (1 pc = 3.26 light years)

Andromeda

a galaxy that is moving toward our galaxy due to a mutual gravitational pull; there is enough material that gravity is pulling the galaxy toward the Milky Way despite the expansion of the universe

Basic Facts about the Universe

1. The Earth and other planets orbit the Sun.

2. The moon orbits the Earth.

3. The Earth’s rotational axis is titled with respect to the imaginary plane in which the Earth orbits the Sun.

constellation

a configuration of stars named after an object, person, or animal; there are 88 constellations; used to map the sky but have no scientific meaning

How many degrees is the Earth tilted on its rotational axis?

23.5 degrees; it is perpendicular to its celestial plane (north toward Polaris and south toward the pole)

ecliptic

the apparent path the Sun takes through the constellations in the course of a year

Zodiac

set of 12 constellations that the Sun and Moon move through (due to motion of Earth around the Sun); horoscope sign is determined by the zodiac constellation the Sun is in when you are born

seasons

due to the inclination of the Earth’s rotational axis relative to the orbital plane of the Earth around the Sun; the hemisphere experience longer/warmer days depends on where it is in its rotation around the Sun and how much direct radation they receive

equinox

day and night are equal length everywhere (spring and autumn)

solstice

day is either the longest or shortest of the year (summer and winter)

summer solstice

time of year when the sun is highest in the sky (23.5 degree latitude)

winter solstice

time of year when the sun is lowest in the sky (-23.5 degree latitude)

How long is the lag time between summer solstice and peak annual temperature?

3 week lag time (due to heat stored in the atmosphere from radiation that takes time to reach the surface)

Moon Phases

new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent; period is about 27 days

observed phases of the Moon are due to the relative positions of the Moon and Sun with respect to the Earth

solar eclipse

the blocking of all or part of the light from the Sun by the moon; eclipses are the best time to study the energetics of the Sun’s atmosphere, which gives rise to the solar wind (directly affects the Earth)

solar wind

a continuous, outward stream of electrically charged particles—mostly protons and electrons—boiling off the Sun's million-degree outer atmosphere (the corona) into space

total solar eclipse

the Moon fully covers the Sun (totality only lasts a few minutes)

partial solar eclipse

the Moon partially covers the Sun

annular eclipse

the outer ring of the Sun is visible

lunar eclipse

the passage of the Moon into the shadow of the Earth

total lunar eclipse

the Moon passes entirely through umbra

partial lunar eclipse

part of the Moon passes through umbra

penumbral lunar eclipse

the Moon passes through the penumbra

umbra

the dark, inner part of a shadow where an object completely blocks a light source

penumbra

the lighter, outer part of a shadow where a light source is only partially blocked

nodes

the points where the Moon’s orbit crosses the ecliptic plane of Earth

Why don’t eclipses happen every month?

The moon’s orbit around the Earth is tilted 5° on the ecliptic

sidereal period

the time a celestial body takes to complete one full revolution or rotation relative to the distant, fixed stars: 27.3 days

synodic period

the time it takes for a celestial body to return to the same position or alignment relative to the Sun as viewed from Earth: 29.5 days

revolution

the motion of one body about another (ex. Earth revolves around the Sun in a year)

rotation

the turning of a body about its axis (ex. Earth rotates about its axis once per day)

inclination

the angle between the orbital plane of a revolving body and some fundamental plane (ex. the ecliptic); Pluto’s obrit is inclined 17° with respect to the ecliptic

Inner Planets

Mercury and Venus; seen in the morning/night skies; always lead or trail the Sun along the zodiac

Mercury orbital period = 88 days

Venus orbital period = 225 days

Outer Planets

Mars, Jupiter, and Saturn; visible to the naked eye; move through zodiac at a slower pace

Mars orbital period = 2 years

Jupiter orbital period = 12 years

Saturn orbital period = 30 years

retrograde motion

reverse of the normal direction; an apparent westward motion of a planet with respect to the stars, caused by the motion of the Earth

opposition

the position of a planet when it is opposite the Sun in the sky

elongation

the point at which Mercury or Venus appear farthest from the Sun as seen from the Earth

conjunction

an event in which a planet and Sun line up in the sky as seen from Earth

stellar parallax

slight back and forth shifting of star positions that occurs from viewing stars from different positions in Earth’s orbit

What was the benefit of astronomy to ancient cultures?

It was useful as a tool to predict weather/seasons and it was useful as a tool to tell time (sundials).

Calendar

our calendar is based on astronomical objects:

Weekdays - Sunday (Sun), Monday (Moon), Saturday (Saturn)

Length of day = time Sun takes to complete one circuit in the sky

modern science and the Greeks

• credited for developing the scientific method (western culture)

• first to use logic and geometry to explain nature—not superstition

• understood power of reasoning from observations

Thales

first astronomer; asked “what is the universe made of?”; believed the universe is mostly water, and the Earth is a flat disk floating on the infinite ocean

Pythagoras

created pythagorean theorem; claimed Earth is a sphere within a celestial sphere (the sphere is geometrically perfect)

Aristotle

Greek philosopher who claimed the Earth’s curved shadow during lunar eclipses proves the Earth is spherical

Aristarchus

claimed the Earth orbits the Sun/presented the first known heliocentric model of the universe; was right in principle but wrong in detail

Apollonius and Hipparchus

Greek philosophers/astronomers who laid the foundation for the Ptolemaic, geocentric model of the Universe

Ptolemy

100-170 AD Roman astronomer who proposed a geocentric model of the Universe; explained the occasional retrograde motion of Mars, Jupiter, and Saturn by claiming that the planets use epicycles along the planets’ orbits (loop-the-loop)—this model was believed for 1500 years in Western culture

Copernicus

1473-1543 AD astronomer; used the best data of planetary motions available and the earlier suggestion of a heliocentric system by Aristarchus; worked out the detailed geometry of the solar system; FLAW—assumed motions/orbits must be perfectly circular

Tycho Brahe

1546-1601 AD Danish astronomer who built an observatory for naked eye observations to make accurate measurements of the positions of stars and planets (within ~1 arcminute)—set Kepler up for success

produced accurate astrometry: measurements of astronomical objects’ positions were necessary to pave the way to the correct model

Johannes Kepler

1571-1630 AD German astronomer and apprentice of Tycho; used Tycho’s high quality astrometry to make accurate models of the solar system; Kepler gave up belief in circular orbit perfection because data contradicted the model

Kepler’s First Law

The orbit of each planet about the Sun is an ellipse with the Sun at one focus (not geometric center)

eccentricity

the measurement of how much a geometric path/orbit deviates from a perfect circle

(distance from center to focus/semimajor axis) = c/a

perihelion

the point in a planet’s orbit when it is closest to the Sun

aphelion

the point in a planet’s orbit when it is farthest from the Sun

Kepler’s Second Law

An imaginary line connecting the Sun with a planet sweeps out equal areas in equal times as the planet moves about the Sun

Kepler’s Third Law

The cube of the distance from the Sun divided by the square of the time required to traverse the orbit is a constant and is the same for every planet

What did Kepler note about the motion of more distant planets?

More distant planets move slowly relative to closer planets; speculated that this might result from a “force” from the Sun

Galileo Galilei

1564-1642 AD Italian astronomer best known for using the telescope for astronomical purposes; his discoveries added support to the heliocentric model of the solar system; also known for testing gravitational acceleration by simultaneously dropping two different sized masses from the top of the Tower of Pisa

Objection to Heliocentric Model

Aristotle believed Earth could not be moving because if it were, objects such as birds, falling stones, and clouds would be left behind

Rebuttals to Objection of Heliocentric Model

Galileo: a rolling ball; a moving object remains in motion unless a force acts to stop it; objects sharing Earth’s motion through space with continue to move

Tycho: noncircular motion contradicted Aristotle’s claim that the heavens must be perfect and unchanging; Tycho had observed comet/supernova, meaning the heavens were not static

lack of parallax: stars are farther away and more numerous than Tycho and others believed

science

the intellectual and practical activity encompassing the systematic study of the structure and behavior of the physical and natural world through experimentation and observation

scientific method

principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation

steps: observation of phenomena, formation of hypothesis, experimentation to demonstrate truth/falseness, conclusion that validates or modifies hypothesis

Occam’s Razor

a problem-solving rule of thumb stating that when you have competing explanations for something, you should prefer the one that makes the fewest assumptions (simplest is likely correct)

theory

a model that makes predictions that survive repeated and varied testing

astrology

the apparent positions of the Sun, Moon, and planets among the stars in our sky influence human behavior

cumulative effect

south-moving air veers to the southwest; a phenomenon where small, repetitive, or continuous forces build up over vast spans of time or distance to create massive, observable cosmic changes

coriolis effect

the apparent curving or deflection of an object's path when it moves freely across a rotating reference frame; planet rotation causes air moving on a planet to deviate from a straight line trajectory

felt on a spinning carousel

the effect makes hurricanes spin counter-clockwise in the Northern hemisphere

leap year

a day is added to the calendar every 4 years to make up for the fact that a year is actually 365.25 days, not 365 days

speed

distnace traveled per unit of time

velocity

speed AND direction

acceleration

change in velocity with time (constant velocity is not felt, but acceleration is)

Acceleration Due to Gravity

g = 9.8 m/s²

momentum

mass times velocity (p = mv)

a force must be applied to change the momentum

angular momentum

the momentum an object has when spinning on its axis (ex. Earth, an ice skater, etc.)

changes from the application of a torque (twisting force)

Isaac Newton

1642-1727 AD; born just after Galileo’s death; developed calculus and used math to develop scientific ideas

fundamental contributions: three laws of motion, law of gravity, applying the laws of physics to the heavens

Newton’s First Law of Motion

An object moves a constant velocity if there is no net force acting upon it (objects at rest stay at rest—concept of inertia)

Newton’s Second Law of Motion

Force = Mass x Acceleration

Newton’s Third Law of Motion

For any force, there is always an equal and opposite reaction force.

What conservation principles do Newton’s laws arise from?

Conservation of momentum, conservation of angular momentum, and conservation of energy

Conservation of Momentum

Total momentum of interacting objects does not change unless an external force is applied (ex. billiard balls)

Conservation of Angular Momentum

In the absence of an external torque, the total angular momentum of a set of interacting objects does not change.

Conservation of Energy

Objects gain or lose energy only by exchanging it with other objects (ex. kinetic, radiative/light, potential)