History of Astronomy

Arny, 3rd Edition, Chapter 1

Prehistoric Astronomy

• Early astronomical observations

revealed the obvious:

Planets as a distinct class of objects

different from the stars

Rising of the Sun in the eastern sky

and its setting in the west

Changing appearance of the Moon

Eclipses

As the moon moves (revolves)

around the Earth, it looks like it has different shapes. The shape of the moon does not

really change. It just changes its location in space.

Motions of the Moon

• Moon reflects the light from the sun

• The moon rotates and revolves around the Earth

• The changing relative positions of the moon, Earth and sun cause the phases of the moon, eclipses and tides

• The moon rotates once on its axis in 29.5 days.

The Moon's phases are caused by our seeing different amounts of its illuminated surface.

Phases of the Moon

EclipsesOccurs when:

- moon’s shadow hits Earth or- Earth’s shadow hits moon When an object in space comes

between the sun and a third object it casts shadow on that object = eclipse

SOLAR ECLIPSE

•Moon moves in a line directly between Earth and the Sun

•Can only occur during the new-Moon phase

Umbra – darkest part of the shadow

- cone shaped

LUNAR ECLIPSE

•Moon moves within the shadow of Earth•Only occurs during the full-Moon phase

Because the Moon's orbit isinclined about 5 degrees to the plane of the ecliptic, during most of the times of new- and full-Moon the Moon is above or below the plane, and no eclipse can occur.

The usual number of eclipses is four per year.

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Prehistoric Astronomy

– Many astronomical phenomena are cyclic and gave prehistoric people:

Methods for time keeping

Ability to predict and plan future events

Incentive to build monumental structures such as Stonehenge

– Modern civilization no longer relies on direct astronomical observations for time keeping and planning.

– Studying the night sky provides link to past.

Stonehenge, a stone monument built by the ancientBritons on Salisbury Plain, England. Its orientationmarks the seasonal rising and setting points of theSun. (Courtesy Tony Stone/Rob Talbot.)

History of Astronomy

The direction of the rising and setting Sun changes throughout the year. At the equinoxes (S&F) the rising and setting points are due east and west. The sunrise direction shifts slowly northeast from March to the summer solstice, whereupon it shifts back, reaching due east at the autumn equinox. The sunrise direction continues moving southeast until the winter solstice. The sunset point similarly shifts north and south. Sunrise on the summer solstice at Stonehenge. (Courtesy English Heritage.)

History of Astronomy

Prehistoric Astronomy

• The Celestial Sphere

–Vast distances to stars prevents us from sensing their true 3-D arrangement

–Naked eye observations treat all stars at the same distance, on a giant celestial sphere with the Earth at its center

Astronomical coordinate system on the celestial sphere

History of Astronomy

The Sun's path across the background stars is called the ECLIPTIC. The Sun appears to lie in Taurus in June, in Cancer during August, in Virgo during October, and so forth. Note that the ecliptic is also

where the Earth's orbital plane cuts the celestial sphere.

History of Astronomy

Prehistoric Astronomy• Constellations

–Constellations are fixed arrangements of stars that resemble animals, objects, and mythological figures

–Stars in a constellation are not physically related

–Positions of stars change very slowly; constellations will look the same for thousands of years

–Origin of the ancient constellations is unknown, probably served for tracking the seasons and navigation

History of Astronomy

The two constellations Leo, (A), and Cygnus, (B), with figures sketched in to help you visualize the animals they

represent. (Photo (A) from Roger Ressmeyer, digitally enhanced by Jon Alpert. Photo (B) courtesy Eugene Lauria.)

Constellations

The modern constellations are officially designated and charted by the International Astronomical Union (IAU), and they define very specific regions of the sky.

Every bit of the sky is covered by the 88 “official” constellations, each including the pattern of stars associated with it in the ancient legends.

History of Astronomy

• The Seasons

–The Earth is closest to the Sun in January, which is winter in the northern hemisphere

What causes seasons?

Earth’s distance from the sun varies throughout the year

Tilt! Because of the tilt of Earth’s axis, the amount of radiation received by Northern and Southern Hemispheres varies through the year

The Earth’s rotation axis is tilted 23.5º

The rotation axis of the Earth maintains nearly exactly the same tilt and direction from year to year

The northern and southern hemispheres alternately receiving (on a yearly cycle) the majority of direct light from the Sun

This leads to the seasons

Tilt of the Earth’s axis towards or away from the sun creates the seasons

Earth’s Seasons

When the north pole tilts toward thesun, it gets more radiation – more warmth

during the summer

SUMMER (Northern Hemisphere)

WINTER (Southern Hemisphere)

When the north pole tilts toward thesun, the south pole tilts away

So when it’s summer in the north, it’s winter in the south

Tilt of the Earth’s axis towards or away from the sun creates the seasons

Earth’s Seasons

When the north pole tilts away from the sun, it gets less radiation –

So it’s colder during the winter

WINTER (Northern Hemisphere)

SUMMER (Southern Hemisphere)

When the north pole tilts away from thesun, the south pole tilts toward it…

When it’s winter in the north, it’s summer in the south

History of Astronomy

Prehistoric Astronomy

Solstices – WINTER and SUMMER

Equinoxes – SRING and FALL

The SOLSTICES (about June 21 and December 21) are when the Sun rises at the most extreme north and south points

The EQUINOXES(equal day and night and about March 21 and September 23) are when the Sun rises directly east

History of Astronomy

Early Ideas of the Heavens

• CLASSICAL 500 B.C. – A.D. 1400

• Ancient Greek Astronomers

–Through the use of models and observations, they were the first to use a careful and systematic manner to explain the workings of the heavens

–Limited to naked-eye observations

History of Astronomy

Early Ideas of the Heavens

- By 300 B.C., Aristotle presented naked-eye observations for the Earth’s spherical shape:

–Shape of Earth’s shadow on the Moon during an eclipse

• A traveler moving south will see stars previously hidden by the southern horizon

The Shape of the Earth

- Pythagoras taught as early as 500 B.C. that the Earth was round, based on the belief that the sphere is the perfect shape used by the gods

History of Astronomy

During a lunar eclipse, we see that the Earth's shadow on the Moon is curved. Thus the Earth must be round.

As a traveler moves from north to south on the Earth, the stars that are visible change. Some disappear below the northern horizon, whereas others, previously hidden, become visible above the southern horizon. This variation would not occur on a flat Earth.

History of Astronomy

Early Ideas of the Heavens

The Size of the Earth

–Eratosthenes (276-195 B.C.) made the first measurement of the Earth’s size

–He obtained a value of 25,000 miles for the circumference, a value very close to today’s value

History of Astronomy

Eratosthenes's calculation of the circumference of the Earth. The Sun is directly overhead on the summer solstice at Syene, in

southern Egypt. On that same day, Eratosthenes found the Sun

to be 7° from the vertical in Alexandria, in northern Egypt. Eratosthenes deduced that the

angle between two verticals placed in northern and

southern Egypt must be 7°.

History of Astronomy

Early Ideas of the HeavensDistance and Size of the Sun and Moon

–The sizes and distances of the Sun and Moon relative to Earth were determined by Aristarchus about 75 years before Eratosthenes measured the Earth’s size

–These relative sizes were based on the angular size of objects

–Aristarchus realizing the Sun was very large proposed the Sun as center of the Solar System, but the lack of parallaxargued against such a model

History of Astronomy

Aristarchus used the size of the Earth's

shadow on the Moon

during a lunar eclipse to

estimate the relative size of the Earth and

Moon.

History of Astronomy

How to determine linear size

from angular

size.

History of Astronomy

Aristarchus estimated the relative distance of the Sun and Moon by observing the

angle A between the Sun and the Moon when the Moon is exactly half lit. Angle B must

be 90° for the Moon to be half lit. Knowing the Angle A, he could then set the scale of

the triangle and thus the relative lengths of the sides.

History of Astronomy

Early Ideas of the HeavensThe Motion of the Planets

–Because of the general east to west motion of objects in the sky, geocentric theories were developed

– Eudoxus (400-347 B.C.) proposed a geocentric model in which each celestial object was mounted on its own revolving transparent sphere with its own separate tilt

– The faster an object moved in the sky, the smaller was its corresponding sphere

– could not explain retrograde motion

History of Astronomy

Motion of the Earth around the Sun causes stellar parallax. Because the stars are so remote, this is too small to be seen by the naked eye. Thus the ancient Greeks incorrectly deduced that the Sun could not

be the center of the Solar System.

History of Astronomy

Cutaway view of the geocentric model of the Solar System according to Eudoxus. (Some spheres omitted for clarity.)

Ptolemaic system

•Geocentric model

•To explain retrograde motion, Ptolemy used small circles, called epicycles

Early Ideas of the Heavens

The universe according to Ptolemy, second century A.D.

Retrograde motion as explained by Ptolemy

History of Astronomy

Astronomy in the Renaissance

Nicolaus Copernicus (1473-1543)

–reconsidered Aristarchus’s heliocentric model with the Sun at the center of the solar system

–Constructed a model of the solar system that put the Sun at the center, but he used circular orbits for the planets

History of Astronomy

How Copernicus calculated the distance to the planets. (A) When an inner planet appears in the sky at its

farthest point from the Sun, the planet's angle on the sky away from the Sun, A, can be measured. You can see from the figure that at the same time angle B is

90°. The planet's distance from the Sun can then be

calculated with geometry, if one knows the measured value of angle A and the fact that the Earth-Sun

distance is 1 AU.

(B) Finding the distance to an outer planet requires determining how long it takes the planet to move from being opposite the Sun in the sky ( the planet rises at sunset) to when the Sun-Earth-planet angle is 90° (the planet rises at noon or midnight). Knowing that time interval, one then calculates what fraction of their orbits the Earth and planet moved in that time. Multiplying those fractions by 360° gives the angles the planet and Earth moved. The difference between those angles gives angle B. Finally, using geometry and the value of angle B as just determined, the planet's distance from the Sun can be calculated.

History of Astronomy 52

Astronomy in the Renaissance

Tycho Brahe (1546-1601)

–Designed and built instruments of far greater accuracy

–Made meticulous measurements of the planets

–Proposed compromise geocentric model

History of Astronomy

Astronomy in the Renaissance

Johannes Kepler (1571-1630)

–Upon Tycho’s death, his data passed to Kepler, his young assistant

–Using the very precise Mars data, Kepler showed the orbit to be an ellipse

Kepler’s law of equal areas

1st law-Orbits of the planets are ELLIPTICAL.

Kepler's three laws of orbital motion:

2nd law – Planets revolve around the sun at varying speed.

History of Astronomy

3rd law – There is a proportional relation between a planet’s orbital period and its distance to the sun (measured in

AU).

History of Astronomy

Astronomy in the Renaissance• Galileo (1564-1642)

– First person to use the telescope to study the heavens and offer interpretations

• The Moon’s surface has features similar to that of the Earth The Moon is a ball of rock

• The Sun has spots The Sun is not perfect, changes its appearance, and rotates

• Jupiter has four objects orbiting it The objects are moons and they are not circling Earth

• Milky Way is populated by uncountable number of stars Earth-centered universe is too simple

• Venus undergoes full phase cycle Venus must circle Sun

History of Astronomy

Astronomy in the Renaissance

–Credited with originating the experimental method for studying scientific problems

–Deduced the first correct “laws of motion”

–Was brought before the Inquisition and put under house arrest for the remainder of his life

History of Astronomy

Isaac Newton & Birth of Astrophysics

• Isaac Newton (1642-1727) was born the year Galileo died

• He made major advances in mathematics, physics, and astronomy

• He pioneered the modern studies of motion, optics, and gravity and discovered the mathematical methods of calculus

• It was not until the 20th century that Newton’s laws of motion and gravity were modified by the theories of relativity

History of Astronomy

The Growth of Astrophysics• New Discoveries

– In 1781, Sir William Herschel discovered Uranus; he also discovered that stars can have companions

– Irregularities in Uranus’s orbit together with law of gravity leads to discovery of Neptune

• New Technologies

– Improved optics lead to bigger telescopes and the discovery of nebulas and galaxies

–Photography allowed the detection of very faint objects