Introduction

• Distance scale

• Mapping the Sky

• Earth’s orbit and inclination

• The Celestial Sphere

• Phases of the Moon

• Solar and lunar eclipses

• Motions of the planets

• Geocentric vs. Heliocentric Models

• Kepler’s Laws

• Newton’s laws of gravity and motion

• Bode’s Law

Basic Facts to Keep in Mind

• The Earth and the other planets orbit the

Sun

• The moon orbits the Earth

• The Earth’s rotational axis is tilted with

respect to the imaginary plane in which the

Earth orbits the Sun

Astronomical Unit

• Definition: The average distance from the

Earth to the Sun

• 1 AU = 150 million kilometers

= 150,000,000,000 meters

= 1.5x1011 meters

= 8.3 light minutes

(speed of light = 3x108 meters per second)

Other “light distances”

• Earth to Moon ~ 1.3 light seconds

• Sun to Pluto ~ 5.5 light hours

• Sun to Nearest Star ~ 4.2 light years

(i.e., Proxima Centauri)

Constellation – A configuration of stars

named after an object, person, or animal.

There are 88 constellations

We use constellations simply to

map the sky

They have no scientific meaning.

The mapping of the sky is similar to

the way we map the Earth

Mapping the Sky

• Right Ascension: 0 – 24 hours (East-West)

• Declination: -90 to 90 degrees (South-North)

Earth’s rotational axis points near

the star Polaris

Polaris

Celestial sphere: the apparent

sphere of the sky

Celestial Sphere, cont.

• Summer solstice: time of the year when thesun is highest in the sky

• Winter solstice: … lowest in the sky

• Spring (vernal) & Autumn equinox: days andnights are of equal length

Seasons are due to the inclination of the

Earth’s rotational axis relative to the orbital

plane of the Earth around the Sun

Ecliptic: the apparent path the Sun takes through

the constellations in the course of a year

Zodiac (Grk – zoo): Set of 12 constellations that

the Sun & Moon move through

Note: the constellations visible at

night change with the Seasons

The phases of the moon

• The moon orbits

the Earth

(Period ~ 27 days)

• The observed

phases of the

Moon are due to

the relative positions

of the Moon & Sun

with respect to the

Earth

Solar Eclipse: the blocking of all or part of

the light from the Sun by the Moon

Types:

1)Total

2)Partial

3)Annular

Lunar Eclipse: the passage of the

Moon into the shadow of the Earth

Types:

1)Penumbral

2)Partial

3)Total

Terrestrial Planets

Mercury Venus

Earth Mars

•Large Core-to-

Atmosphere

Ratio

•Few/No Moons

Jovian Planets

Jupiter Saturn

Uranus Neptune

•Small “Core”-to-

Atmosphere Ratio

•Rings

•Lots of Moons

Motion of the Planets in the Sky• The word “planet” is derived from the

Greek word for “wanderer”

• The inner planets Mercury & Venus

• The outer planets visible to the naked eye

• Seen in the morning/evening skies

• Always lead or trail the Sun along the zodiac

• Orbital periods for Mercury & Venus are 88 & 225 days,

respectively

• Mars, Jupiter & Saturn

• Move through the Zodiac at a slower pace

• Orbital periods of Mars, Jupiter, & Saturn are 2, 12 & 30

years, respectively

Motion of the outer planets

Motion of the outer planets: another view…

Motion of outer planets

• 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

A recap

• The planets orbit the Sun

• The orbits of the planets are coplanar

• The moon orbits the Earth

• The Earth’s rotational axis is not perpendicular

to the plane of the ecliptic

Plane of the

Ecliptic

A recap, cont.

• The seasons on the Earth are determined by the

amount of sunlight received per day, which is

related to the tilt of the Earth’s rotational axis

The manner in

which sunlight

strikes the Earth

Astronomical events as observed

from Earth

• The Sun, Moon, & planets all follow the same

path with respect to the stars

Reason: The orbits of the planets around the

Sun are coplanar

• Solar Eclipse

Reason: The moon occasionally blocks the light

from the Sun

Frequency of Solar Eclipses

Astronomical events as observed

from the Earth, cont.

• Lunar eclipse

Reason: The moon occasionally passes into the

shadow of the Earth

Why don’t eclipses occur every month?

Astronomical events as observed

from the Earth

•Phases of the Moon

Reason: The Moon

orbits the Earth, &

thus the Moon’s position

with respect to the Sun

& Earth changes with

time.

Periods for the Moon:

Sidereal = 27.3 days

Synodic = 29.5 days

Astronomical events as observed

from the Earth

• The appearance of Mercury & Venus only in the

morning/evening skies

Reason: Both planets orbit closer to the Sun

than the Earth, thus they always appear to be

near the Sun

• Occasionally retrograde motion of an outer

planet

Reason: This occurs when the Earth passes an

outer planet in its orbit

Some Definitions

• Revolution: The motion of one body about

another

• Examples:

1) The Earth revolves around the Sun.

2) The Moon revolves around the Earth

3) The Earth completes one revolution around the

Sun in 365 days

• Rotation: The turning of a body about its axis

• Example: The Earth rotates once about its axisevery 24 hours.

• Inclination: The angle between the orbital planeof a revolving body & some fundamental plane(e.g. the plane of the ecliptic)

• Example: The orbit of Pluto has an inclination of17o with respect to the plane of the ecliptic.

• Old belief – Geocentric: Earth-centered (Ptolemy, 2nd

century A.D.)

• New belief – Heliocentric: Sun-centered (Copernicus,1473-1543 A.D.)

Ptolemaic System

Modeling of observational data helped to

confirm the Heliocentric Model

• Tycho Brahe (1546-1601): Made accurate

measurements of the positions of stars & planets

• Johannes Kepler (1571-1630): interpreted

Tycho’s data

Phases of Venus

Old model.

Problem – Venus

would always be in

a crescent phase

Galileo (1564-1642):

Made use of a telescope

To discover:

• Phases of Venus

• 4 brightest moons of

Jupiter

Kepler’s Laws of Planetary Motion

• Each planet moves in an elliptical orbit about the

Sun, with the Sun at one focus of the ellipse.

• An imaginary line connecting the Sun with a

planet sweeps out equal areas in equal times as

the planet moves about the Sun.

• 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 (i.e., distance3 / period2 = constant ).

AU years 1

Newton’s Law of Gravity

• The gravitational force between 2 objects is proportionalto the product of their masses & inversely proportional tothe square of the distance between them

• Where,

G = constant

M1 = mass of object one

M2 = mass of object two

R = distance between mass one & mass two

• Gravity: the tendency of matter to attract matter. Gravityis very important for AST 105.

Force of Gravity

• Weak between two people because the masses

involved are small

• Strong between a person & Earth because the

Earth is massive & the distance is small

• Strong between Sun & planets because the Sun

is very massive

Newton’s Laws of Motion

• Every body continues in its state of rest or ofuniform motion in a straight line unless it iscompelled to change that state by the action ofsome outside force

• The change of motion (acceleration) isproportional to the force acting on the body &inversely proportional to the mass of the body(i.e., acceleration = Force/Mass)

• To every action there is an equal & oppositereaction

Titius-Bode Rule (Bode’s Law)

• Distance (AU) = 0.4+0.3(2n)

• “Rule” developed before asteroid belt was discovered

• Problems – 1) No underlying physics

2) Neptune & Pluto don’t fit