Astronomy probably began long before recorded history
(more than 5,000 years ago) when human began to track
the motion of celestial objects so they knew when to plant
their crops or prepare to hunt migrating herd.
The ancient Chinese, Egyptians, and Babylonians are well
known for their record keeping.
These cultures recorded the location of the Sun, Moon,
and the five visible planets as these objects moved slowly
against the background of “fixed” stars.
It was not enough to track the motions of celestial objects;
predicting their future positions (to avoid getting married
at an unfavorable time, for example) become important.
A study of Chinese archives shows that the Chinese
recorded every appearance of the famous Halley’s Comet
for at least 10 centuries.
Like most ancients, the Chinese considered comets to be
mystical, generally comet were seen as bad omen and
were blamed for a variety of disaster, from wars to plague.
The Chinese has an accurate records of “guest stars”.
Today we know that a “guest star” is a normal star, usually
too faint to be visible, which increases its brightness as it
explosively ejects gases from its surface, a phenomena we
call a Nova or Supernova.
The Golden Age of Astronomy
The Golden Age of early astronomy (600 B.C. –
AD 150) was centered in Greece.
The basics of Geometry and Trigonometry, which
they developed, were used to measure the size of
and distances to the largest-appearing bodies in the
heavens –Sun and the Moon.
The Golden Age of
early astronomy (600
B.C. –AD 150) was
centered in Greece.
The early Greeks held the incorrect Geocentric (geo =
Earth, centric = centered) view of the universe.
Orbiting the Earth
moon, Sun, and
known planets –
Mercury, Venus, Mar
s, Jupiter and Saturn
Beyond the planets was a
transparent, hallow celestial sphere
on which stars were attached and
travelled around the Earth.
The sun and the
moon were thought
to be perfect crystal
The famous Greek Philosopher
Aristotle (384-322BC) concluded
that Earth is spherical because it
always casts a curved shadow
when it eclipses the moon.
Although most of Aristotle’s teaching were
considered infallible by may for centuries after his
death, his belief in a spherical Earth was lost during
the Middle Ages.
Measuring the Earth’s Circumference
The first successful attempt to
establish the size of the Earth was
credited to Eratosthenes (276-194
Eratosthenes observed the angles of the noonday
Sun in two Egyptian cities that were roughly north
and south of each other- Syene (now Aswan) and
Finding the angles of the noonday
sun differed by 7 degrees, or 1/50
of a complete circle, he concluded
that the circumference of Earth
must be 50 times the distance
between these two cities.
The cities were 5,000 stadia
apart, giving him a measurement of
Many historians believe the stadia
was 157.6 meters, which would
make Eratosthenes’s calculation of
Earth’s circumference – 39,400
km.- a measurement very close to
the modern value of 40. 0705 km.
The Sun-Centered Universe
The first Greek to profess a
Sun-Centered, or Heliocentric
, (helios=sun, centric=centered)
universe was Aristarchus (312-
Because of the strong influence of Aristotle’s
writing, the Earth-centered view dominated Western
thought for nearly 2,000 years.
Mapping the Stars
The greatest among Greek
astronomers was Hipparchus, best
known for his star catalogue.
Hipparchus determined the
location of almost 850 stars, which
he divided into six groups
according to their brightness (the
system is still used today)
He measured the length of the year to within minutes of
the modern value and developed a method for predicting
the times of lunar eclipse to within a few hours.
Many of the Greek discoveries were lost
during the Middle Ages, the Earth-centered
view that the Greek proposed became
entrenched in Europe.
Much of our knowledge Of Greek astronomy
comes from a 13-volume treatise, Almagest (the
great work) which was compiled by Ptolemy in
With the decline of the Roman Empire around
fourth century, much of the accumulated
knowledge disappeared as libraries were
After the decline of Greek and Roman
civilization, the center of astronomical study
move east to Baghdad, where
fortunately, Ptolemy’s work was translated into
Arabic astronomers expanded Hipparchus’s star model
catalog and divided the sky into 48 constellations – the
foundation of our present-day constellation system.
It wasn’t until some time after the tenth century that the
ancient Greeks’ contributions o astronomy were
reintroduced to Europe through the Arabic community.
The Ptolemaic model soon dominated European
thought as the correct representation of the
heavens, which created problems for anyone who found
errors in it.
The Birth of Modern Astronomy
Ptolemy’s Earth-centered universe was not discarded
Modern astronomy’s development was more than
scientific endeavor, it require a break from deeply
entrenched philosophical and religious views that had
been a basic part of Western society for thousands of
The work of five noted scientists undergo a transition
from an astronomy that merely describe what is
observed, to an astronomy that tries to explain what is
observed and more importantly why the universe
behaves the way it does.
For almost 13 centuries after the time of
Ptolemy, very few astronomical advances
were made in Europe, some were even
lost, including the notion of a spherical
The first great astronomer to emerge after the Middle
Ages was Nicolaus Copernicus (1473-1543) from Poland
After discovering Aristarchus’s writing, Copernicus
became convinced that Earth is a planet, just the other
five then-known planet.
The daily motions of the heavens, he reasoned could be
more simply explained by rotating Earth.
Having concluded that the Earth is a
planet, Copernicus constructed the
heliocentric model for the solar system
with the Sun at the center and the
Mercury, Venus, Earth, Mars, Jupiter,
and Saturn orbiting it.
This was a major break from the ancient and prevailing
idea that a motionless Earth lies at the center of all
movement in the universe.
At that this time it was considered heretical b many
Europeans. Anyone who refuse to denounce the
Copernican theory were burned at the stake.
Tycho Brahe was born of Danish
nobility three years after the death of
He persuaded King Frederick II to
establish an observatory near
Copenhagen, which headed.
There he designed and built pointers (telescope would not
be invented for a few more decades), which he used for 20
years to systematically measure the location of the
heavenly bodies in an effort to disprove the Copernican
His observations, particularly of
Mars, were far more precise than any
made previously and are his legacy to
With the death of his patron, the King
of Denmark, Tycho was forced to leave
Tycho moved to Prague in the present day Czech Republic, where, in
the last year of his life, he acquired an able assistant, Johannes Kepler.
Kepler retained most of the observations made by Tycho and put
them to exceptional use.
Ironically, the data Tycho collected to refute the Copernican view of
the solar system would later be used by Kepler to support it.
Armed with Tycho’s data, a good mathematical
mind, and, of greater importance, a strong
belief in the accuracy of Tycho’s work, Kepler
derived three basic laws of planetary motion
1. The path of each planet around the Sun,
while almost circular, is actually an ellipse, with
the sun at one focus.
This law allow us to calculate
astronomical events like
eclipses, comets, spacecraft
rendezvous, and satellite
2. Each planet revolves so that an imaginary line
connecting it to the Sun sweeps over equal areas in
equal intervals of time.
In order for a planet
to sweep equal areas
in the same amount
of time, it must
travel more rapidly
when it is nearer the
Sun (perihelion) and
more slowly when it
is farther from the
Kepler was devout and believed that the
Creator made an orderly universe and that this
order would be reflected in the positions and
motions of the planets.
The uniformity he tried to find eluded him for
nearly a decade.
Then in 1619, Kepler published his third law in
the Harmony of the Worlds
3. The planet orbital period squared is equal to its
mean solar distance cubed.
The solar distances of the planets can be
calculated when their periods of revolutions are
For example: Mars has an orbital period of 1.88
years, which squared equals 3.54. The cube root
of 3.54 is 1.52, and that is the average distance
from Mars to the Sun in astronomical unit.
Kepler’s law assert that the planets
revolve around the Sun, and
therefore support the Copernican
He was the greatest Italian
scientists of the Renaissance.
He was a contemporary of
Kepler, and like Kepler, strongly
supported the Copernican theory
of a Sun-centered solar system.
Galileo’s greatest contributions to science were his
descriptions of the behavior of moving objects, which
he derived from experimentation.
All astronomical discoveries before Galileo’s time
were made without the aid of a telescope.
In 1609, Galileo heard that a Dutch lens maker had
devised a system of lenses that magnified objects.
Apparently without ever seeing the telescope, Galileo
constructed his own, which magnified distant object
three times the size seen by the unaided eye.
He immediately made
others, the best having
magnification of about
telescope, Galileo was
able to view the
universe in a new way.
Galileo made many discoveries that
supported the Copernican view of
the Universe including the following:
1. The discovery of Jupiter’s
four largest satellite or moons.
This find dispelled the old idea that Earth was the
sole center of motion in the universe; for
here, plainly visible, was another center of motion
2. The discovery that the planets are circular
disk rather than just points of light, as was
This indicated that the
planets must be Earth-like
revolved around the sun.
3. The discovery that Venus exhibits phases just as the
Moon does and that Venus appears smallest when it is
in full phase and thus is farthest from Earth
Venus orbits its
source of light –the
In the Ptolemaic system, the orbit of
Venus lies between Earth and the Sun,
which means that only the crescent
phases of Venus should ever be seen
4. The discovery that Moon’s surface is not a
smooth glass sphere, as the ancient had
Galileo saw mountains, craters, and plains
indicating that the Moon was Earth-like.
He thought that plains
might be bodies of
water, and this idea was
strongly promoted by
others, as we tell from the
names given to these
features (sea of
tranquility, sea of
5. The discovery that the sun (the viewing of
which may have caused the eye damage that
later blinded him) has sunspot – dark regions
caused by slightly lower temperatures.
Galileo tracked the movement of
these spots and estimated that
rotational period of the Sun as just
under a month. Hence, another
heavenly body was found to have
both “blemishes” and rotational
In 1616, the Church condemned the Copernican Theory as
contrary to Scripture because it did not put humans as their
rightful place at the center of Creation, and Galileo was told to
abandon this theory.
Undeterred, Galileo began writing his famous work, Dialogue of
the Great World Systems.
Despite poor health, he completed the project and in 1630 went
to Rome, seeking permission from Pope Urban VIII to publish.
Since the book was a dialogue that expounded both the
Ptolemaic and Copernican system, publication was allowed.
However, Galileo’s detractors were quick to realize that he was
promoting Copernican view at the expense of the Ptolemaic
The sale of the book was quickly halted, and Galileo was called
before the Inquisition.
Tried and convicted of proclaiming doctrines contrary to
religious teaching, he was sentenced to permanently house
arrest, under which he remained for the last 10 years of his life.
Despite this restriction, and his grief following the death of his
eldest daughter, Galileo continued to work.
In 1637 he became totally blind, yet during the next few years
he completed his finest scientific work, a book on the study of
motion in which he stated that the natural tendency of an object
in motion is to remain in motion.
Later, as more scientific evidence in support of the Copernican
system was discovered, the Church allowed Galileo’s works to
Sir Isaac Newton
Sir Isaac Newton (1642-
1727) was born in the year
of Galileo’s death.
His many accomplishments
in Mathematics and
Physics led a successor to
say that “Newton was the
greatest genius that ever
At the age of 23, he envisioned a force that
extends from Earth into space and holds the
Moon in orbit around Earth.
He was the first to formulate and test the Law of
Universal Gravitation. It states that:
“Every body in the universe attracts every
other body with a force that is directly
proportional to their masses and inversely
proportional to the square of the distance
The Law of gravitation
also states that the greater
the mass of an object, the
greater its gravitational
The division of the sky into areas.
Constellation (con = with, stella = star)
Usually named in honor of mythological
character or great heroes.
Sometimes it takes a bit of imaginations to make
out the intended subjects, as most constellation
were probably not thought of as likeness in the
Although the stars that make up constellation all appear
to be the same distance from Earth, this is not the case.
Some are many times farther away than others.
Thus, stars in a particular constellation are not
associated with each other in any important physical
Various cultural groups, including Native Americans
and the Chinese, attached their names, pictures, and
stories to the constellations.
For example, the constellation Orion the hunter was
known as the White Tiger to ancient Chinese observer.
Astronomers use constellations when they want to
roughly identify the area of the heaven they are
Some of the brightness stars in the heavens were given
proper names such as Sirius, Arcturus, and Betelgeuse.
The brightness stars in a constellation are generally
names in order of their brightness by the letter of the
Greek alphabet –alpha (α), beta (β), and so on –
followed by the name of the parent constellation.
For example, Sirius, the brightness star in the
constellation Canis Major (larger Dog) is also called
Alpha (α) Canis Majoris.
Motion of Earth
The main consequences of Earth’s rotation
are day and night.
Earth’s rotation has become a standard
method of measuring time because it is so
dependable and easy to use.
Earth’s rotation is measured in two ways,
making two kinds of days.
1) Mean Solar Day
The time interval from one noon to the
next, which averages about 24 hours.
Noon is when the Sun has reached its
zenith (highest point in the sky).
2) Sidereal (sider=star, at=pertaining to) day
Is the time it takes for Earth to make one complete
rotation (3600) with respect to a star other than our sun.
The sidereal day is measured by the time required for a
star to reappear at the identical position in the sky.
The sidereal day has a period of 23 hours, 56
minutes, and four seconds, which is almost four minutes
shorter than the mean solar day.
This difference results because the directions to distant
stars changes only infinitesimally, whereas the direction
to the Sun changes by almost 1 degree each day.
around the Sun in
an elliptical orbit at
an average speed of
107, 000 km per
Its average distance from the sun is 150
Because of its elliptical orbit. Earth’s
distance from the sun varies.
Earth’s axis is
This angle is very important to Earth’s
inhabitants because the inclination of Earth’s
axis causes the yearly cycle of seasons.
A very slow movement of the Earth is called
Although Earth’s axis
same angle of tilt, the
direction in which the
axis point continually
As a result, the axis
traces a circle in the
At present time, the axis points toward the bright
In AD 14,000, it will point toward the bright star
Vega, which will then be the North Star for about
a thousand years or so.
The period of precession is 26,000 years.
By the year 28,000, Polaris will once again be the
Precession has only a minor effect on the season
because Earth’s angle of tilt changes slightly.
Motion of the Earth-Moon System
Earth has one natural satellite, the Moon.
In addition to accompanying Earth in its annual
trek around the Sun, our Moons orbits Earth
about once each month.
When viewed from a Northern Hemisphere
perspective, the Moon moves counterclockwise
(eastward) around the earth.
The Moon’s orbit is
elliptical, causing the
to vary by about 6
The cycle of the Moon through its phases
requires 29 ½ days – a time span called the
was the basis
for the first
Sidereal Month is the apparent period of the
Moon’s revolution around the Earth and not the
true period, which takes only 27 1/3 days.
As the moon orbits Earth, the Earth-Moon system
also moves in an orbit around the Sun.
after the Moon has
made a complete
Earth, it has not yet
reached its starting
position with respect to
the Sun, which is
directly between the
Sun and the Earth.
An interesting fact concerning the motions of the
Moon is that its period of rotation around its axis
and its revolution around the Earth are the same -
27 1/3 days.
Because of this, the same lunar hemisphere
always faces Earth.
All of the landings of the manned Apollo missions
were confined to the Earth-facing side.
Only orbiting satellite and astronaut have seen the
back side of the Moon.
Because of the Moon rotates on its axis only once
every 27 1/3 days, any location on its surface
experiences periods of daylight and darkness
lasting about two weeks.
Along with the absence of
atmosphere, accounts for the
high surface temperature of
1270C (2610F) on the day side
of the moon and the low
surface temperature of -1730C
(-2800F) on its night side.
Phases of the Moon
The first astronomical phenomenon to be
understood was the regular cycle of the Phases of
On a monthly basis, we observe the phases as a
systematic change in the amount of the Moon that
Phases of the Moon
We will choose the “new-Moon” position in the
cycle as the starting point.
About 2 days after the new Moon, a then silver
(crescent phase) can be seen with the naked eye
low in the western sky just after sunset.
During the following week, the illuminated
portion of the moon that is visible from the Earth
increases (waxing) to a half-circle (first-quarter
phase) that can be seen from about noon to
In another week, the complete disk (full-Moon
phase) can be seen rising in the east as the Sun
sinks in the west.
During the next two weeks, the percentage of the
Moon that can be seen steadily declines
(waning), until the Moon disappears altogether
The lunar phases are a consequence of the
motion of the Moon and the sunlight that is
reflected from its surface.
Half of the Moon that is illuminated at all times.
But on the Earthbound observer, the percentage
of the bright side that is visible depends on the
location of the Moon with respect to the Sun and
When the moon lies between the Sun and
Earth, none of its side faces Earth, so we see the
new-Moon (“no moon”) phase.
When the moon lies on the side of Earth
opposite the Sun, all of its lighted side faces
Earth, so we see the full Moon.
Eclipses of the Sun and Moon
When the Moon moves in a line directly between
Earth and the Sun, which can occur only during
the new-Moon phase, it casts a dark shadow on
Earth, producing a solar eclipse (eclipsis=failure to
The Moon eclipse (lunar eclipse) when it moves
within Earth’s shadow, a situation that is possible
only during the full-Moon phase.
The Moon’s orbit is inclined about 50 to the plane
of the ecliptic. Thus,
a) during the most new-
Moon phases, the
shadow of the Moon
passes either above or
b) During most full-
Moon phases, the
shadow of Earth misses
An eclipse can only take place when a new- or full
Moon phase occurs while the Moon’s orbit
crosses the place of the ecliptic.
Crossing the plane of ecliptic are normally met
twice a year, therefore the usual number of
eclipses is four.
These occur a set of one solar and one lunar
eclipse, followed six months later with another set.
Occasionally the alignment is such that three
eclipses can occur in a one month period –at the
beginning, middle, and the end.
These occur as a solar eclipse flanked by two
lunar eclipse, or vice versa.
It also occasionally happens that the first set
of eclipses for the year occurs at the very
beginning of a year, the second set in the
middle, and the third set occurs before the
calendar year ends, resulting in six eclipses
in the year.
More rarely, if one of these sets consists of
three eclipses, the total number of eclipse is
a year can reach seven, which is the
Total eclipses are visible only to people in the
dark part of the Moon’s shadow (umbra), while a
partial eclipse is seen by those in the light portion
1. Why do we use the mean solar day to
measure time rather than the sidereal
2. Why did the ancients believed that
celestial objects had some control over
3. What major change did Copernicus
make in the Ptolemaic system? Why
was this change philosophically
4. What was Tycho Brahe’s contribution to science?
5. Does Earth move faster in its orbit near perihelion
(January) or near aphelion (July)? Keeping your answer
to previous question in mind, is the solar day longest in
January or July?
6. Use Kepler’s third law (p2 = d3 ) to determine the
period of a planet whose solar distance is:
a) 10 AU
b) 1 AU
c) 0.2 AU
7. Use Kepler’s third law to determine the
distance from the sun of a planet whose
a) 5 years
b) 10 years
c) 10 days
8. Did Galileo invent the telescope?
9.Of what value are constellations to
10. Explain the difference between the
mean solar day and the sidereal day.
11. What is the different about the crescent
phase that precedes the new-Moon phase
and that which follows the new-Moon
12. What phases of the Moon occurs
approximately one week after the new-
Moon? Two weeks?
13. Currently, Earth is closest to the sun
(perihelion) in January (147 million km) and
farthest from the sun in July (152 million km).
As the result of the precession of Earth’s
axis, 12,000 years from now perihelion ( closest)
will occur in July and aphelion (farthest) will take
place in January. Assuming no other
changes, how might this change average summer
temperature for your location? What about
average winter temperature? What might the
impact on the biosphere and hydrosphere?
14. In what ways do the interactions
between Earth and its Moon influence the
Earth system? If Earth did not have a
Moon, how might the
atmosphere, hydrosphere, geosphere, and
biosphere be different?
15. Describe the locations of the
Sun, Moon, and Earth during a solar
eclipse and during a lunar eclipse.