The Origin of
Modern Astronomy
Nicolai Copernicus
(1473-1543)
Goals for Today
• Complete our study of the Moon:
lunar phases, the Sun-Earth-Moon
geometry, and the eclipses
• Tides and their origin
• Earth's place in the cosmos; a little
bit of history…
Assigned Reading
• Unit 10: Geometry of Sun, Earth, and
Moon
• Unit 19: Tides
•  Units 11 and 12: Understanding the
Motions of Planets, and the Birth of
Modern Astronomy
Announcements
Homework # 1 starts today; it is due on
Thursday, Sept 29th
Astronomy Helpdesk is now available:
Mondays through Thursdays, 7-9 pm, in
Hasbrouck 110
Lunar Phases
Survey Question
A third quarter moon is rising. What time is it?
1) sunset
2) sunrise
3) midnight
4) noon
Survey Question
A third quarter moon is rising. What time is it?
1) sunset
2) sunrise
3) midnight
4) noon
Earth Rising on the Moon
Apollo 11
Survey Question
In this picture of the Earth rising, what is the
predominant direction of the sun in relation to
the observer?
1) above the observer
2) below the observer
3) behind the observer
4) behind the Earth
Survey Question
In this picture of the Earth rising, what is the
predominant direction of the sun in relation to
the observer?
1) above the observer
2) below the observer
3) behind the observer
4) behind the Earth
Solar Eclipses
The Sun is physically 400 times larger than
the Moon.!
Why can it be obscured by the Moon?!
The Sun is 400 times further
away than the Moon
Angle diameter (in radians) = diameter/distance
Full circle = 2! (in radians)=360 degrees=360o
1o=60 arminutes = 60
1 = 60 arcseconds = 60
Angular Size
Some Examples:!
Horizon to zenith (point overhead)
!
! 90o !
Your fist at arm's length !
!
!
! 10o !
Sun or Moon seen from Earth !
!0.5o = 30' !
Smallest detail visible to naked eye ! 1' = 60'' !
!
Smallest detail visible by a single telescope from
!Earth's surface: !1'' !
!
Angular size of Pluto: 0.15”!
!
Betelgeuse (largest star) seen from Earth 0.004'' !
!
Geometry of Moon and Sun
•  Both the Moon and the Sun have angular diameter of about 0.5
degree, the exact value at a given time depending on the exact
distance.
•  Therefore, we can periodically have solar eclipses
Geometry of Solar Eclipses
!
!
Looking Back on an Eclipsed Earth; 1999 August 11 !
Credit: Mir Space Station Crew!
Annular Eclipse
October 3, 2005
Total and Annular Eclipse
It takes about an hour for the
moon to cover the sun. Why?
The moon moves at an angular speed of!
360 deg/(28x24 hr) = 0.5 deg/hr.!
!
The angular size of the moon is about 0.5 deg.!
!
How come there isn't an eclipse
every full moon and new moon?
The main reason is that the moon’s orbit is tilted from the
earth’s orbit.!
!
However, twice a year the nodes of the moon’s orbit will
align with the Earth-Sun direction, and total eclipses will
be possible.!
!
A total eclipse from a given point on the surface of the
Earth is not a common occurrence. For example, next
solar eclipse in the USA will occur on 21 August 2017;
the following one will be on 8 April 2024. !
Survey Question
The phase of the moon at a solar eclipse is?
a) new
b) 1st quarter
c) full
d) it depends on the year
Survey Question
The phase of the moon at a solar eclipse is?
a) new
b) 1st quarter
c) full
d) it depends on the year
Lunar Eclipse
(the Moon in the Earth’s Shadow)
!
!
Effects of Moon on Earth:
Tides
To sun
Spring Tides
Occur at every new and full moon
Neap tides!
Occur at every first- and third-quarter moon
Friction drags the tidal bulges eastward
out of the direct earth-moon line
Earth's rotation slows down by 0.0023 s/100 years.!
Only 900 million years ago, Earth' day was 18 hrs long.!
The moon's orbit is growing larger by about 4 cm/yr.!
We see only one side of the moon, because the tidal friction
has slowed the rotation of the moon so that its rotation and
revolution are synchronized
And Now, Onto the Discussion
of the Birth of Modern
Astronomy!
Archeoastronomy
•  There is evidence that Human kind has paid very closed
attention to the sky, its motions and periodicity.
•  Archeoastronomy is a blend of superstition, astrology,
religion, but also common sense
•  Greeks were the first to try to understand (logically) the
universe.
•  Pythagoras: universe ruled by geometrical, mathematical
relationship --- This notions still fundamental today
•  All ingredients there for the methods of modern science: have a
theory and test it against reality – the theory may or may not
survive…
•  Ultimately, Greek “science” still a blend of scientific method and
“philosophical” believes.
•  True scientific method had to wait for Galileo
Not to scale
Aristotle Universe:
Three Basic Assumptions
•  The Earth was at the center of the
Universe.
•  The only motion in the heavens was uniform
circular motion
•  The heavens were more perfect than the
Earth, and objects in the heavens were
eternal.
"Common Sense"
•  If the Earth actually spun
on an axis, why didn't
objects fly off the spinning
Earth?
•  If the Earth was revolving
around the sun, why didn't
it leave behind the birds
flying in the air?
•  If the Earth were actually
on an orbit around the sun,
why wasn't a parallax
effect observed?
Two problems for the
Geocentric model:
(1) Change of brightness
(2) Retrograde Motion
Ptolemy Solution:
add a special fix:
epicycle (circle on
circle)
Brightness changes
because of distance
change
Ptolemic Model
The Copernican Revolution:
The Heliocentric System
Nicolai Copernicus (1473-1543).
The Sun, not the Earth, was the
center of the Solar System.
The Earth is just another planet
(the third outward from the Sun),
and the Moon is in orbit around
the Earth, not the Sun.
The stars are distant objects that
do not revolve around the Sun.
"
"
"
Retrograde Motion and Varying
Brightness of the Planets
The planets in such a system naturally vary in brightness
because they are not always the same distance from the Earth.
The retrograde motion could be explained in terms of
geometry and a faster motion for planets with smaller orbits.
"
"
Galileo Galilei
Galileo discovered that:
!  our Moon has craters (is a `mound of
rocks’);
!  Jupiter has it's own moons;
!  the Sun has spots;
!  Venus has phases like our Moon;…
The `heavens’ are imperfect!
These discoveries confirmed the
Copernican hypothesis that the Earth
was just another planet.
(1564-1642), Italian
Imperfect and Changing
Universe
Sun had dark patches on Sun. The
motion of such sunspots indicated that
the Sun was rotating on an axis.
Jupiter had 4 moons; thus planets could
have moons circling it that would not
be left behind as the planet moved
around its orbit
The Moon was not smooth, but was
covered by mountains and craters.
The Phases of Venus
Venus goes through a complete set of phases, just like the
Moon. This was the first empirical evidence that allowed a
definitive test of the geocentric and heliocentric models.
The introduction of the `scientific
method’ (formulate an hypothesis, test the
hypothesis against reality, verify or discard the
hypothesis if data do not confirm it) marks the
birth of Modern Science, and Astronomy