1 © Sierra College Astronomy Department The Earth in the Universe Observations and Models Presented...

11© Sierra College Astronomy Department© Sierra College Astronomy Department

The Earth in the UniverseThe Earth in the UniverseObservations and ModelsObservations and Models

Presented ByPresented By

Professor Harry L. F. HoupisProfessor Harry L. F. Houpis

© Sierra College Astronomy Department© Sierra College Astronomy Department 22

The Celestial SphereThe Celestial Sphere

Celestial sphere is the sphere of heavenly objects that seems to center on the observer.

Celestial pole is the point on the celestial sphere directly above a pole of the Earth. In the Northern Hemisphere the north celestial pole

is very near to the star Polaris. In the Southern Hemisphere, there is no obvious

star near the south celestial pole. Height of the celestial pole above the horizon is equal

to your latitudeAll celestial objects move from east to west on a

minute-by-minute basis.


A constellation (from the Latin, meaning “stars together”) is an area of the sky containing a pattern of stars named for a particular object, animal or person.

The earliest constellations were defined by the Sumerians as early as 2000 B.C.

The 88 constellations used today were established by international agreement.

An asterism is an unofficial arrangement of stars.

The Celestial SphereThe Celestial Sphere


The Sun’s Motion: A YearThe Sun’s Motion: A YearTiming

The Sun appears to move constantly eastward among the stars (on a day-to-day basis).

The time the Sun takes to return to the same place among the stars is about 365.24 days.

Consequently, the stars rise about 4 minutes earlier each day.

Paths in the Sky The celestial equator is a line on the celestial sphere

directly above the Earth’s equator The ecliptic is the apparent path of the Sun on the celestial

sphere The zodiac is the band that lies 9° on either side of the

ecliptic on the celestial sphere and contains the constellations through which the Sun passes


Observation: The PlanetsObservation: The Planets

Five planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter, Saturn.

Planets lack the simple, uniform motion of the Sun and Moon.

These planets always stay near the ecliptic.Mercury and Venus never appear very far from

the position of the Sun in the sky.Planets sometimes stop their eastward motion

and move westward against the background of stars. This is called retrograde motion.


Pre-Copernican ModelsPre-Copernican Models

Models of Claudius Ptolemy (circa A.D. 150)To account for retrograde motion, the epicycle is

introduced – the epicycle is the circular orbit of a planet, the center of which revolves around the Earth in another circle.



Models of Claudius Ptolemy (continued)Needed to “tie” Mercury and Venus to the

Sun.To account for other non-uniform variations

in the planets’ motions while maintaining the principle that the Universe was constructed of circles and spheres, the following constructs were introduced: Deferent and equant (note that the Earth is no

longer at the center of the Universe) “Embedded” spheres



Models of Tycho Brahe (born 3 years after Copernicus died)

Placed all the planets around the Sun except the Earth.

The Earth-less Solar System then orbited around the stationary Earth

Other Variations and ModelsSee http://www.csit.fsu.edu/~dduke/models


Criteria for Scientific ModelsCriteria for Scientific Models

Three criteria for scientific models: Model must fit the data Model must make predictions that can be tested and be of such

a nature that it would be possible to disprove it Model should be aesthetically pleasing - simple, neat, and

elegant (Occam’s razor) Ptolemy’s and Tycho’s models meet the first two criteria

for a good scientific model fairly well but it is much less successful with the third (aesthetically pleasing).

400 years before Ptolemy, the Greek philosopher Aristarchus proposed a moving-Earth solution to explain celestial motions.

Ptolemy and others discredited Aristarchus’s model, but used wrong assumptions to do so.


Nicholas CopernicusNicholas Copernicus

Copernicus, a contemporary of Columbus, worked 40 years on a heliocentric (sun-centered) model for two reasons: Ptolemy’s predicted positions for

celestial objects had become less accurate over time.

The Ptolemaic model was not aesthetically pleasing enough.

His system revived many of the ideas of the ancient Greek Aristarchus.


Nicholas CopernicusNicholas Copernicus Model Specifics:

The Earth rotates under a stationary sky (which gives the same observations as a rotating celestial sphere and a stationary Earth).

The Earth revolves around a stationary Sun, which appears to move among the background stars. (Projection is the key concept)

His model explains the generally west to east motion of the planets.

Observed retrograde motion of planets (such as Mars) is explained more simply and conclusively.

Copernicus had the Moon revolving around the Earth. All others circled the Sun.


Nicholas CopernicusNicholas Copernicus

Model Specifics (continued) The Sun’s apparent motion north and south

of the equator is explained by having the Earth’s equator tilted with respect to the planet’s orbit around the Sun.

Closeness of Mercury and Venus to Sun is easily explained.

However, the Copernican model turns out to be no more accurate (relative to the observations) than the geocentric models


Tycho BraheTycho Brahe

Tycho was born 3 years after Copernicus died.

Tycho built the largest and most accurate naked-eye instruments yet constructed.

He could measure angles to within 0.1º, close to the limit the human eye can observe.


Tycho Brahe and Johannes KeplerTycho Brahe and Johannes Kepler

In 1600, a year before Tycho died, Kepler accepted a position as Tycho’s assistant, working on models of planetary motion.

Tycho’s best data had been gathered for Mars.Based on circles and epicycles Kepler’s best

model for Mars matched Tycho’s data to an accuracy of 0.13º.

Yet, this error exceeded the error in Tycho’s measurements, which bothered Kepler.

Kepler’s persistence led him to abandon circles and try other shapes. The shape that worked for Mars and all other planets was the ………..


Johannes KelperJohannes Kelper

The Ellipse The ellipse is a geometrical shape every point of

which is the same total distance from two fixed points (the foci).

Eccentricity is the distance between the foci divided by the longest distance across (major axis).

Astronomers refer to the semi-major axis distance and eccentricity.

Consequences Kepler developed his three laws The heliocentric theory worked far better than the old

geocentric theory with regard to predictions Kepler’s empirical laws were later explained by the

use of Newton’s dynamical laws of motion and gravitation – the first unified theory of physics


Galileo GalileiGalileo Galilei

Galileo was born in 1564 and was a contemporary of Kepler.

Galileo built his first telescope in 1609, shortly after hearing about telescopes being constructed in the Netherlands.

Galileo was the first person to use a telescope to study the sky.


Galileo GalileiGalileo Galilei

Galileo made 5 important observations: Mountains and valleys on the Moon Sunspots More stars than can be observed with the naked eye Four moons of Jupiter Complete cycle of phases of Venus

Though Galileo’s first three observations do not disprove the geocentric theory, they cast doubt on the assumption of perfection in the heavens.


Galileo GalileiGalileo GalileiSatellites of Jupiter In 1610 Galileo discovered that Jupiter had four

satellites of its own, now known as the Galilean moons of Jupiter.

Jupiter and its orbiting moons contradicted the Ptolemaic notions that the Earth is the center of all things and if the Earth moved it would leave behind the Moon.

The Phases of Venus Galileo observed that Venus goes through a full set

of phases: full, gibbous, quarter, crescent. Venus’s full set of phases can be explained by the

heliocentric theory. The Ptolemaic theory predicts that Venus will

always appear in a crescent phase, which is not borne out by the observations.


SummarySummary The Pre-Copernican Era

Ancient Greek desire to explain the Universe with the “most perfect” of all geometries: circles and spheres

“Common sense” and human intuition were all that were needed to explain the observations

The Earth was the center of the Universe and the heavens followed a different physics

The “science” of the times was not held hostage to observations – models were only for calculation purposes and not necessarily a required reality

The Copernican Era What’s with the circles? – “around the corner” vs “direct

consequences” inquiries How common is common sense and how reliable is intuition? Unification is the holy grail of science Models are held hostage to observations and fundamental

symmetries (realities?) are sought

1 © Sierra College Astronomy Department The Earth in the Universe Observations and Models Presented...

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