Transcript Introduction to Astronomy - Western Oregon University

Introduction to Astronomy
Astronomy is the study
of the universe
Motions of the Earth-Moon System
Earth
Moon
Average Earth-Moon distance:
384,401 km (238,329 miles).
The Earth and Moon from a distance of 10 million km
You are here
12 humans have been here
• Phases: systematic change in the amount of the
Moon that appears illuminated.
Moon’s orbital period = 27.3 days (sidereal month)
Cycle of lunar phases = 29.5 days (synodic month)
Motions of the Earth-Moon System
• Solar Eclipse: when
the Earth moves into
the Moon’s shadow.
• Lunar Eclipse: when
the Moon moves into
Earth’s shadow.
Full Solar Eclipse on November 14th 2012 as seen from Australia.
An eclipse will only occur if a new- or full-Moon
phase occurs along the plane of Earth’s orbit.
Motions of the Earth-Moon System
• Revolution and
Rotation of Moon =
27.3 days:
– Same lunar
hemisphere always
faces the Earth.
Ancient Astronomy
Ancient Astronomy
• The “Golden Age” of ancient Greek astronomy
(600 B.C. – A.D. 150)
– Geocentric (Earth-centered) model for the universe
widely accepted – dominated western thought for
nearly 2,000 years.
– Believed the Earth to be a perfect sphere.
– Understood what causes the phases of the moon.
Ancient Astronomy
• Aristarchus (310-230 B.C.):
– The Sun is many times larger than the Earth
and is located at a great distance.
– Proposed a Heliocentric (Sun-centered)
model for the universe.
Ancient Astronomy
• Hipparchus (2nd century B.C.):
– Constructed a star catalogue that included locations
of almost 850 stars.
– Divided stars into six groups according to brightness
(similar system is used today).
– Measured the length of a year to within minutes of the
modern value.
Measuring Earth’s Circumference:
• Aristotle (384-322
B.C.): presented
evidence that the
Earth is spherical.
• Eratosthenes
(276-194 B.C.):
measured Earth’s
circumference as
39,400 km.
Ancient Astronomy
• The Ptolemaic System:
– Developed by the Greek astronomer, Ptolemy in the
2nd century A.D.
– The Moon, Sun, planets and stars all follow perfectly
circular orbits around a stationary Earth.
– Provided a model to explain the apparent retrograde
motion of planets.
East
West
October 2011- July 2012 apparent path of Mars
relative to background stars.
Origins of Modern Astronomy
• Nicolaus Copernicus
(1473-1543)
– Proposed that Earth is
a planet and that all
planets revolve around
the Sun.
– Used perfectly circular
orbits to explain the
motion of the planets.
Origins of Modern Astronomy
• Giordano Bruno (1548-1600)
– Dominican Monk that preached
the Copernican model of the
Universe.
– Proposed that stars were other
suns with their own worlds
orbiting them.
– Tried for heresy by the Roman
Inquisition and sentenced to
death.
Origins of Modern Astronomy
• Tycho Brahe (1546-1601)
– Measured the motions of
planets with more precision
than any made previously.
– Recognized the principle of
stellar parallax which is the
apparent shift of nearby stars
with respect to those farther
away from the Earth.
• Stellar Parallax: back and forth motion of
nearby stars relative to more distant stars due to
Earth’s orbital motion.
Tycho’s model for the Solar System
Origins of Modern Astronomy
• Kepler (1571-1630)
– Used Tycho’s
observations, but
supported the Copernican
model of the Solar
System.
– Derived 3 basic laws of
planetary motion.
Kepler’s Laws
1. The path of each
planet around the Sun
is an ellipse, with the
Sun at one focus.
2.
Each planet revolves so that an imaginary
line connecting it to the Sun sweeps over
equal areas in equal intervals of time.
Perihelion
Aphelion
Kepler’s Laws
3. The orbital periods of the planets and
their distances to the Sun are
proportional.
p2 = d3
p = orbital period of planet around the Sun
d = distance from planet to the Sun
1 Astronomical Unit (AU) = the mean distance
from the Earth to the Sun (93 million miles)
Origins of Modern Astronomy
• Galileo (1564-1642)
– Confirmed the
Copernican theory of a
Sun-centered solar
system.
– Constructed his own
telescope which
allowed for many
significant discoveries.
Understanding Motion
• Galileo demonstrated that moving objects, once moving
continued in motion without the application of forces.
– Assuming there are no opposing forces such as
friction or air resistance.
• Force: basically a push or a pull that alters the motion of
an object.
• Inertia: the property by which objects resist changes in
motion.
Galileo’s Discoveries
• Discovery of 4 moons
orbiting Jupiter –
indicated that Earth was
not the center of all
motion.
• Planets are not points of
light, but are spheres like
Earth.
• Venus has phases just
like the Moon.
Galileo’s Discoveries
• The Moon’s surface is
not a smooth glass
sphere – it has
craters, mountains,
and plains.
• Discovery of sunspots
on the surface of the
Sun – the motion of
these spots indicated
that the Sun rotates.
Origins of Modern Astronomy
• Sir Isaac Newton
(1642-1727)
– Formulated the laws
of motion and
gravitation that
govern all bodies in
the universe.
Newton’s Laws
• The Law of Inertia:
every object continues in a state of rest or of
uniform speed in a straight line unless acted on
by a force.
• The inertia of an object depends on its mass.
• Mass = the quantity of matter in an object.
Gravitation
• The Law of Universal Gravitation: every
body in the universe attracts every other
body with a force that is:
– directly proportional to their masses.
– inversely proportional to the square of the
distance between their centers of mass.
d = 6,400 km or 4,000 miles
Gravity is a mutual attraction between both masses.
G (universal gravitational constant) = 6.67 x 10-11 N·m2/kg2
Orbits result from a combination of
gravitational attraction and an object’s inertia.
Refining Kepler’s Models
• Orbit of a planet is not a perfect ellipse
due to gravitational influences from other
planets.
• Perturbation: any deviation in the orbit of
a body from its predicted path.
Refining Kepler’s Models
• Neptune was discovered because of its
gravitational effect on the orbit of Uranus.
• Newton combined Kepler’s 3rd law with his
law of gravitation - the mass of a body can
be calculated when the orbits of its
satellites are known.
Albert Einstein (1879-1955)
• Tiny discrepancies in the orbits of
planets could not be predicted by
Newton’s Laws alone.
• General Theory of Relativity predicted
these remaining perturbations.