Astronomy Chapter 3 Terms

aphelion

the point in its orbit where a planet (or other orbiting object) is farthest from the Sun

apogee

the point in its orbit where an Earth satellite is farthest from Earth

ellipse

a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same

gravity

the mutual attraction of material bodies or particles

Kepler’s first law

each planet moves around the Sun in an orbit that is an ellipse

Kepler’s second law

the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time

Kepler’s third law

the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit. p squared = a cubed

major axis

the maximum diameter of an ellipse

mass

a measure of the amount of material within an object

momentum

the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force

Newton’s first law

every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force

Newton’s second law

the change of motion of a body is proportional to and in the direction of the force acting on it

Newton’s third law

for every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions)

perihelion

the point in its orbit where a planet (or other orbiting object) is nearest to the Sun

perturbation

a small disturbing effect on the motion or orbit of a body produced by a third body

velocity

the speed and direction a body is moving—for example

waning 

after full moon to new moon, each night a smaller percentage of the moon is illuminated

waxing

after new moon to full moon, each night a larger percentage of the moon is illuminated

synodic

A complete cycle of lunar phases: 29.53 days

siderial

the time the moon takes to circle the sky once and return to the same position among the stars: 27.32 days

Meridian

a circle that passes through four points on the celestial sphere. Goes through due south, the zenith, due north, and the nadir.

asterism

not official constellation (group of stars or shape of stars)

Constellation

a group of stars in an area in the sky with clearly defined boundaries: 88 sectors

earths axis

line going through earth connecting the north and south poles

circumpolar zone

stars within 38 degrees of the north pole can never set

Polaris

directly above the north celestial pole, stays stationary

ecliptic

path the sun takes around the celestial sphere

circumpolar constellation

one that appears to rotate around the North or South Celestial Pole and, due to its proximity to that pole, never dips below the horizon as seen from a specific latitude

celestial equater

earth’s equator projected on the night sky.

Precession

the conical motion of an object
spinning rapidly on an axis

Eratosthenes

calculation for circumference of the earth

vernal equinox

The point when the sun crosses the equator and is “traveling northward”: first day of spring

autumnal equinox

The point when the sun crosses the celestial
equator and is traveling “southward”: first day of autumn

summer solstice

the point where the sun is farthest north in the sky: first day of summer

winter solstice

sun is farthest south in the sky: first day of winter

apparent magnitude

brightness of star (numeric scale) 1- brightest

6 and above not visible

aphelion

furthest point from the sun

Earths tilt

23.5 degrees

90- Altitude

altitude on equinox

Tropic of Cancer

an imaginary line of latitude located at approximately 23.5 degrees north of the Equator

tropic of capricorn

an imaginary circle of latitude about 23.5° south of the Earth's equator, marking the southernmost point where the sun can be directly overhead

perigee

the point in the orbit of the moon or a satellite at which it is nearest to the earth

artic circle

the farthest southern latitude in the Northern Hemisphere where the center of the Sun can remain continuously visible for 24 hours or continuously hidden for 24 hours in a year

antartic circle

the imaginary line of latitude at approximately 66.5° South where the sun remains above the horizon for at least 24 hours during the Southern Hemisphere's summer solstice and below it for 24 hours during the winter solstice.

heliocentric

suns at center followed motion of planets

retrograde

backwards motion

circumference of the earth

44,000km

Tycho Brache

discovered a supernova, built advanced astronomical instruments, and developed his own model of the solar system,

placing earth at the center of the universe and planets orbiting it

Johannes Kepler

formulated the three laws of planetary motion, demonstrating that planets move in elliptical orbits around the Sun and establishing a mathematical relationship between a planet's orbital period and its distance from the Sun

semi major axis

the distance from the center of an ellipse to its farthest point, or half the length of the ellipse's longest diameter

galileo galilei

used the scientific method to make astronomical observations using a telescope: discovered new planets, rings of saturn discovered

Issac Newton

laws of motion:

universal law of gravitation

Inertia

an objects tendency to resist

Copernicus 

heliocentric universe

heliocentric

Sun is at the center of the solar system, and the Earth and other planets orbit around it

synchronous rotation

when a body (for example, the Moon) rotates at the same rate that it revolves around another body

geocentric 

earth centered universe

tychonic

geocentric