An Introduction to Astronomy Part II: Historical Development of Astronomy Lambert E. Murray, Ph.D. Professor of Physics.

An Introduction to Astronomy

Part II: Historical Development of Astronomy

Lambert E. Murray, Ph.D.

Professor of Physics

The Gift of the Greeks

The Greek philosophers were the first to realize that the universe was “comprehensible”– By careful observation of the motions of the stars

and planets they developed a “model” for the universe that satisfactorily explained the known universe for nearly 1500 years.

What facts can we learn about our “universe” by careful observation of the different objects in the day and night sky?

What do you know about the Sun’s Motion?

Where and when does the Sun rise and set? Are the days always the same length? Why? Why is it hotter in the summer and colder in the

winter? How high does the Sun get in the daytime sky?

Does this change during the year? Why don’t you see the stars during the day? What causes solar eclipses?

Facts about the Sun It rises in the East and sets in the West. It reaches different maximum heights in the

summer and winter. It rises north of East in the summer and

South of East in the winter. The length of day and night changes with

the seasons. Sometimes the sun is blotted out – a solar

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What do you know about the Moon’s Motion?

Where and when does the Moon rise and set? Does it rise and set at different times each night?

What direction is the Moon moving relative to the stars?

What causes the phases of the Moon? Is it the Earth’s shadow? Where would you expect to see a full moon?

Can you see the Moon during the day? What causes lunar eclipses?

Facts About the Moon I It has the same basic daily pattern as the Sun –

moving from East to West during the day/night. The moon changes its position relative to the stars

(and Sun) each night – moving slowly in an Eastward direction relative to the constellations.

The moon passes through phases, completing one cycle about every 28 days.

The moon can sometimes be observed during the daytime.

The Full Moon is seen when it is opposite the Sun in the sky, while a New Moon is seen near the Sun.

Sometimes the moon is blotted out – a lunar eclipse.

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Phases of the Moon

What do you know about the motion of the Stars?

What is a constellation? Do you always see the same constellations at night? How

do they change during the year? How do the Sun and Moon move relative to the stars? How do the stars appear to move during the night? Why can’t you see the stars during the day? What if we

had no atmosphere? What happens to constellations as you move north? Is everything that looks like a star a star? How can you

tell which are stars?

Facts About the Stars I There are a very large number of stars –

many are invisible to the naked eye. Most stars appear to move in fixed groups

(called constellations) with the same basic daily motion as the Sun and Moon, moving from East to West.

Stars are seen only at night (although the brightest ones are seen just before sunset and are still visible just after sunrise).

The North Star is approximately fixed in the night sky.

Facts About the Stars II Different constellations are visible at

different times of the year, and these constellations appear to move Westward during the year.

As one moves northward, the North Star appears to move upward in the night sky, while the stars in the south drop below the horizon.

Some stars (the wandering stars) appear to move among the other stars. These stars sometimes move in a bizarre manner.

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Star Tracks

Constellations

Constellations and Asterisms

Usually we think of a constellation as a particular grouping of stars that may “look” like some stick figure man, lion, etc. Many of these grouping of stars have been identified by various names in various nations over past history. To make things more uniform, the International Astronomical Union in 1928 divided the night sky into 88 well-defined regions (named constellations) associated with these well know star groupings.

An asterism is a group of easily identifiable stars which may be a part of one or more constellations.

The Winter

Triangle – An

Asterism

Early Models of the Solar System

A simple model to describe the motion of the stars in a 24 hour period might be to picture the stars on a spherical shell which rotate around the earth.

An alternate model, which works just as well, is for the earth to rotate inside the shell of stars.

In order to explain the motion of all the other celestial bodies, more spherical shells must be added to this model.

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Aristotle’s Model of the Universe Aristotle’s Model of the Solar System was based

upon celestial observations and upon terrestrial observations (fire and air always rise).

This diagram of his model indicated very little detail in the actual way the planets moved, but the position of the sun and various planets could be modeled by having the different shells move at different rates and at slightly different angles to one another.

A More Complex Model

The Celestial Sphere The celestial sphere is a model of the night sky

where we assume that all the stars in the heavens are attached to a sphere surrounding the Earth.

Positions on the celestial sphere are designated in one of two ways:– Local Altitude and Azimuth angles

– Declination and Right Ascension angles Declination is like the latitude angle on the Earth, but

measured from the Celestial Equator. This angle is measure in degrees.

Right Ascension is like the longitude angle on Earth, but measured from the Vernal Equinox. This angle is measured in hours, minutes, and seconds.

The Celestial Sphere

The Celestial Sphere with

Constellations

Additional Facts Known to Early GreekAstronomers

Aristotle had argued that the earth, moon, and sun were spherical objects based partly upon observations of eclipses (about 350 B.C.).

Using geometric techniques, the early Greek astronomers had determined approximate values for:– the diameter of the earth– the relative distances from the earth to the moon and to

the sun.– the relative diameter of the moon and the sun.

They could accurately predict the occurrence of eclipses.

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Aristarchus’ Method for Determining the

Relative Distance to the Sun and Moon

Note: This is a schematic.It is not totally accurate forthe Sun and Moon.

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Eratosthenes’ Method for

Determining the Earth’s

Radius

Light from the Sun

Aristarchus’ Proposal to Determine the Moon’s Distance

If we assume the Earth’s shadow is approximately the same diameter as the Earth, we can approximate the diameter of the Moon (by seeing how far the Moon moves through the Earth’s shadow). Thus:

Distance to Moon = Diameter of Moon/Angular size of Moon

Lunar Eclipse

This sequence of photographs shows the shadow of the Earth projected across the path of the Moons orbit.

Scientific Evidence for the Geocentric Model in 200 B.C.

All things fall to the earth - even objects from “space”.

The motion of the sun, moon, stars, and planets could be well explained using Ptolemy’s geocentric model.– The model was based upon “perfect circles”.– This model worked well for over a thousand

years. We can’t “feel” the earth move.

Arguments for a Heliocentric Model in 200 B.C.

Aristarchus proposed an alternate, heliocentric (sun-centered) model which could also explain the observed motions of the celestial bodies.

His major reason for proposing this model was the enormous size of the sun.

However, one observation decided against this model – there was no observed parallax of the stars.

The Failure of Parallax

Ptolemy Refines the Model

Ptolemy’s principle contribution to astronomy was his efforts in fine-tuning the geocentric model so that this model could accurately describe and predict the motions of the celestial bodies.

His model was based upon the concept of “perfect circles”.

Earth

Planet

Epicycle

Deferent

Ptolemy’s Simple Model for Planetary

Motion

Ptolemy’s Model for Retrograde Motion

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Ptolemy’s Model for Mercury and Venus

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Ptolemy’s Complete Geocentric Model

Ptolemy’s More Exact Model

Timeline of Ancient Astronomy

The Marriage of Aristotle and Christianity

• In the 13th century St. Thomas Aquinas blended the natural philosophy of Aristotle, which included the Ptolemaic model, with Christian beliefs.

• A central, unmoving Earth fit perfectly with prevalent Christian thinking, and various scriptures where found, whose literal interpretation, seemed to agree with this model.

o 1 Chronicles 16:30: “He has fixed the earth firm, immovable.”

o Psalm 96:10: “He has fixed the earth firm, immovable ...”

o Psalm 104:5: “Thou didst fix the earth on its foundation so that it never can be shaken.”

o Isaiah 45:18: “...who made the earth and fashioned it, and himself fixed it fast...”

Timeline of Renaissance Astronomy

Copernicus Proposes a “New” Model

A rebirth of astronomy occurred in the 14th century. As observations improved, continuous refinements to Ptolemy’s model were required.

Finally, by the 16th century the “corrected” Ptolemaic model had become very complex. Copernicus suggested the heliocentric model as a “simpler” geometrical model which would produce the same observed results, but fewer circles were required.

Copernicus’Model

Requirements of the Model

To be a correct model of the Solar system, Copernicus’ model had to agree with observations.

His model could explain retrograde motion as long as the inner planets had shorter periods than the outer planets (see next slide).

However, there was still the problem with the lack of observable parallax.

Copernicus’ Model for Retrograde Motion

Galileo: Father of Modern Astronomy

Galileo’s Careful Observations Put an End to the Geocentric Model

Galileo was the first person to direct a telescope toward the heavens. His observations had a profound impact on astronomy (and religion).– He observed the Moons of Jupiter– He observed the phases of Venus– He observed Sunspots on the Suns surface (and

later went blind).

Galileo’s Observations of Jupiter’s Moons

This observation verified that not everything orbited the Earth.

Galileo’s Observations of Venus

Like Ptolemy’s model Venus appears larger (thus closer) when we view its dark side. However, notice how much of Venus’ surface is illuminated when it is far from us!

Venus’ Phases in Ptolemy’s Model

Venus’ Phases in Copernicus’ Model

Galileo’s observations of the phases of Venus indicated the Venus must orbit the Sun – a major modification of Ptolemy’s

model – and the end of the geocentric model of the solar system.

Tyco Brahe Faults Copernicus Model

Copernicus originally utilized circular motion for the planets. But he found he could not reproduce the more accurate observations with such a model.

Tycho Brahe, rejected Copernicus’ model because of the lack of parallax. He proposed a slightly different geocentric model in which the Sun and Moon orbit the Earth, but all the other planets orbit the Sun.

Tycho’s Model

But What about the Scriptural Evidence for the Geocentric Model?

As more and more evidence began to build which indicated the correctness of Copernicus’ model, faithful Christians had to ask some fundamental questions about their interpretation of scripture.

By the end of the 17th century, most Christians had come to accept the heliocentric model.

These Christians had to make adjustments to their interpretation of certain scriptures: the Earth being “fixed” must be interpreted differently.

Kepler’s Laws of Planetary Motion

Based upon 50 years of careful observations by Tycho Brahe, Kepler, a mathematician, derived three laws of planetary motion:

1. All bound objects orbit the sun in elliptical orbits.

2. As an object orbits the sun, it sweeps out equal areas in equal times.

3. The square of the orbital period is proportional to the cube of the semi-major axis.

Kepler’s Law of Equal Areas

A highly elliptical orbit such as this is characteristic of comets.

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Kepler’s 3rd Law – Orbital Periods

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Using Kepler’s 3rd Law, we can relate the orbital period of other planets to that of the Earth:

Bode’s Law

Bode’s Law is a simple relationship which can be used to remember the approximate distance of each planet from the Sun.

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Orbital Periods of Visible Planets

Planet Approx Dist

(Bode) [A.U.]

Actual Dist

[A.U.]

Approx Period

True Period

Mercury .4 .387 92 days 88

Venus .7 .723 214 days 225

Earth 1.0 1.0 365 days 365

Mars 1.6 1.52 739 days 687

Asteroids 2.8 (Ceres) 2.77 4.7 yrs Ceres 4.6 yrs

Jupiter 5.2 5.2 11.86 yrs 11.86 yrs

Saturn 10.0 9.54 31.62 yrs 29.46

Newton’s Laws of Motion

[Law of Inertia] All objects remain at rest, or move with constant speed along a straight line, unless acted upon by some outside force.

The acceleration of a body is proportional to the force applied and the mass of the body

For every action, there is an equal and opposite reaction.

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Newton’s Law of Gravity Any two objects in the universe experience a force

of mutual attraction. This force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.

Based upon this law and the basic laws of motion, Newton was able to derive all of Kepler’s laws of planetary motion!

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Demonstration of Orbital Motion in Gravitational Fields

Simple Orbital Motion (Kepler’s three laws)

– Elliptical motion

– Equal areas in Equal times

– Circularizing Orbits

– Unbound Motion Multiple Planets Orbiting a Single Sun and Orbital

Stability Gravitational Boosts Comets and Meteor Showers Multiple Sun Systems

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Eclipses and Eclipse Seasons

Lunar Eclipse

This sequence of photographs shows the shadow of the Earth projected across the path of the Moons orbit.

Lunar Eclipses Lunar eclipses occur when the moon passes through

the shadow of the Earth

The location of the moon relative to the Earth’s shadow determines the type of eclipse that occurs.

Recall that the size of the Earth’s Shadow is roughly three times the size of the Moon.

Eclipse Geometry

Types of Lunar Eclipses

Solar Eclipse

Solar Eclipses

A solar eclipse occurs when the moon passes between the Earth and the Sun

A movie of the motion of the Moons shadow across the Earth– The area of total shadow is relative small– The Earth rotates as the Moon passes by

producing a curved path for the shadow.

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Total and Partial Eclipses

Those located at X observe a total eclipse, whilethose located at Y observe only a partial eclipse.

Annular Eclipses

Those located a A observe an annular ecliplse, whilethose located at P only observe only a partial eclipse.

Length of Eclipses

The maximum duration of a total lunar eclipse is about 1 hour and 47 minutes, the time it takes for the Moon to pass through the Earth’s Umbra.

The length to time for a solar eclipse can be anywhere from a few seconds, up to a maximum of 7 and a half minutes, depending upon the size of the Moon’s Umbra at the Earth’s surface.

Solar Eclipse Paths through 2017

Eclipse Seasons

Why don’t a solar and a lunar eclipse occur every month?

The Moon’s orbit around the Earth is tilted relative to the orbit of the Earth around the Sun.

This means that there are “eclipse seasons” that occur about every 6 months. But even then eclipses do not always occur, because of the relative position of the Sun, Earth, and Moon.

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Eclipse Seasons

Seasons of the Year and Time

Seasons are Caused by the Earth’s Tilt

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Geocentric View

Seasonal Heating Effects

Time is Measured by the Earth’s Rotation and Revolution

The Solar Day and Time Zones The Sidereal Day (measured relative to the

stars) – 23 hrs 56 min Sidereal Month (measured relative to the

stars) – 27.5 days Synodic Month (Lunar Month) – 29.5 days Solar Year – 365.25 days

Sidereal Day vs. Solar Day

Synodic Month

vs.Sidereal Month

Time Zones

End of Part II