Earth / Environmental Science

Ch. 22: Origin of Modern Astronomy

Introduction

There are 100 billion stars in the Milky Way Galaxy There are billions of galaxies in the universe

Ancient Greeks

Astronomy – the science of the study of the universe Deals with the properties of properties of objects in space and

the laws under which the universe operates Golden Age of Astronomy – 600 B.C. – A.D. 150; centered in

Greece. Greeks used philosophical arguments and observations to

explain natural events. Greeks

Greeks developed the basics of geometry and trigonometry Measured the sizes and distances of the sun and the moon.

Aristotle

Concluded that the Earth is round because it always casts a curved shadow when it passes between the sun and the moon

Erastosthenes (276-194 B.C.)

Measured the size of the Earth. Observed the angles of the Sun at noon in two Egyptian cities:

Syrene and Alexandria. Observed that the angles differed by 7 degrees (1/50 of a

circle). Concluded that the circumference must be 50 times the

distance between the two cities. Syrene and Alexandria were 5000 stadia apart (1 stadia = 157.6 meters)

Thus, Earth’s circumference = 39,400 km Modern circumference = 40,075 km

Hipparchus (2nd century B.C.)

Cataloged the stars Determined the location of 850 stars and divided them into 6

groups based on their brightness. Measured the length of the year Developed a method for predicting the times of lunar eclipses

to within a few hours. Geocentric Model

The Earth was a sphere that stayed motionless at the center of the universe

The moon, sun and planets orbit the Earth. Beyond the planets was a transparent, hollow sphere on which

the stars traveled daily around the Earth – the Celestial Sphere. All heavenly bodies appeared to remain in the same relative

position to one another. The seven wanderers: Sun, moon, Mercury, Venus, Mars,

Jupiter and Saturn were each thought to have a circular orbit around Earth.

Heliocentric Model

Aristarchus (312-230 B.C.) the first Greek to believe in a sun-centered universe

In this model, Earth and the other planets orbit the Sun Used geometry to calculate the relative distances from Earth to

Sun, and from Earth to the moon. Calculated the size of the Sun and the moon using these

distances. Earth-centered view dominated Western thinking for nearly

2000 years. Ptolemaic System (Claudius Ptolemy)

Accounted for the movements of the planets Had the planets move in circular orbits around a motionless

Earth. Each planet moves slightly eastward among the stars;

periodically stops and reverses direction; then resumes an eastward motion.

The westward drift is retrograde motion.

Birth of Modern Astronomy

Scientists discovered a universe governed by natural laws Nicolaus Copernicus Tycho Brahe Johannes Kepler Galileo Galilei Sir Isaac Newton

Nicolaus Copernicus (1473-1543; Poland)

Believed that the motions of the heavens could best be explained by a rotating Earth.

Concluded that Earth is a planet Believed in a Heliocentric solar system Represented the planets’ orbits using circles – the perfect

geometric shape Circular orbits didn’t quite match what people saw.

Tycho Brahe (1546-1601, Denmark)

Built an observatory near Copenhagen Without telescopes, he designed and used pointers to measure

the locations of heavenly bodies His assistant, Johannes Kepler, kept Brahe’s observations and

used them Johannes Kepler (1571-1630)

Discovered that Mars’ orbit was elliptical Discovered three laws of planetary motion:

1. Path of each planet is an ellipse with the Sun at one focus 2. Each planet revolves so that it sweeps out equal areas in

equal time periods 3. The orbital period (P) is proportional to planet’s distance

from the Sun (a).(P2=a3); Thus, solar distances (a) can be calculated when periods of revolution (P) are known.

Galileo Galilei (1564-1642, Italy)

Described the behavior of moving objects. Built the first telescope and made discoveries:

1. Four satellites (moons) orbiting Jupiter 2. Planets are circular disks - not just points of light 3. Venus had phases (like our moon) 4. Surface of moon was not smooth 5. Sun had sunspots (dark regions)

Sir Isaac Newton (1642-1727, England)

Proposed Inertia – an object in motion will continue to move at a constant speed and in a straight line.

Formulated and tested the Law of Universal Gravitation. Universal Gravitation

Every body in the universe attracts every other body with a force directly proportional to their masses and inversely proportional to the square of the distance between them.

F = G · m1m2/r2

Mass and Weight

Mass – the amount of matter an object possesses Weight – the force of attraction a heavenly body has on an

object Newton - orbits

Proved that the force of gravity combined with the tendency of a planet to remain in straight-line motion results in an elliptical orbit.

Redefined Kepler’s third law: it now takes into account the masses of the bodies involved and provides a method for determining the mass of a body when the orbit of one of the satellites in known.

Earth-Moon-Sun System

Stonehenge – probably an attempt at better solar predictions Motions of Earth

Rotation – the turning or spinning of a body on its axis Revolution – the motion of a body along a path around some

point in space. Precession – the slight movement of Earth’s axis

Rotation

Produces Day and Night 1 rotation = 24 hours Two kinds of days: Solar Day (the time interval from one noon

to the next) and Sidereal Day (the time it takes for Earth to make one complete rotation with respect to a star other than our Sun).

Revolution

Earth revolves around the Sun in an elliptical orbit (at 107,000 km/hr)

Average distance from the Sun is 150 million km (1.5 x 108 km).

Perihelion – Earth is closest to Sun – 1.47 x 108 km; ~Jan 3. Aphelion – Earth is farthest from Sun – 1.5 x 108 km; ~July 4.

The Elliptic

The apparent annual path of the sun against the backdrop of the celestial sphere is called the elliptic.

Generally, the planets and the moon travel in nearly the same plane as Earth.

Earth’s Axis and Seasons

Earth’s axis is tilted 23.5 degrees Because of this tilt, Earth has its yearly seasons!!! Spring Equinox (Mar 20 or 21) Autumn Equinox (Sep 22 or 23) Summer Solstice (Jun 21 or 22) Winter Solstice (Dec 21 or 22)

Precession

The direction of Earth’s tilt continually changes – every 26,000 years

Currently – axis points toward Polaris Axis will point toward Vega in year 14,000 In year 28,000 Earth will again point toward Polaris (North

Star) Motions of Earth-Moon System

Distance from Earth to moon is 384,401 km Perigee – moon is closest to Earth Apogee – moon is farthest from Earth Relative positions of Earth, moon and Sun are constantly

changing resulting in the changes in the appearance of the moon (phases)

Phases of the Moon

Lunar phases are a result of the motion of the moon and the sunlight that is reflected from its surface

Lunar Motions

Synodic Month – the cycle of the moon through its phases requires 29-1/2 days

The true period is 27-1/3 days – the sidereal month. The reason for this difference is shown in the next slide Moon’s period of rotation = its revolution around Earth = 27-

1/3 days

Moon

Two weeks of daylight followed by two weeks of darkness Temp on day side = 127o C. Temp on dark side = -173o C.

Eclipses

Solar Eclipse – occurs when the moon move in direct line between Earth and the Sun

Occurs during New Moon Phases During a new-moon or full-moon phase, the moon’s orbit must

cross the plan of the ecliptic for an eclipse to take place. Have 4 eclipses per year: 2 solar, 2 lunar.

Earth’s Moon

Diam. of moon = 3,475 km; unusually large compared to size of Earth (12,756 km)

Density = 3.3 times that of water (Earth’s mantle); Earth’s avg. density = 5.5 x water.

Gravitational attraction = 1/6 Earth Lunar Surface

Dark regions on moon resembled seas on Earth so they were named maria (Latin for sea).

Moon has no water or atmosphere - therefore, no weathering or erosion

No tectonic forces - no volcanoes Lunar Craters

Round depressions on the moon’s surface Larger craters are about 250 km in diam. Most were formed by the impact of rapidly moving debris A meteoroid 3 meters in diam. can create a 150-m wide crater

Highlands

Densely-pitted, light-colored areas There are mountain ranges within the highlands Maria – Dark, relatively smooth area on the moon’s surface;

Ancient beds of basaltic lava, originated when asteroids punctured the lunar surface, letting magma bleed out.

Rilles – long channels associated with maria; look similar to trenches

Regolith – soil-like layer of gray debris – composed of igneous rock, glass and fine lunar dust.

Lunar History

Origin of moon: a body the size of Mars impacted Earth. The impact would have liquefied Earth’s surface and ejected a huge quantity of crustal and mantle rock. A portion of this debris would have entered orbit around Earth where it combined to form the moon.