Transcript 10Sept_2014

Homework 1
Unit 2. Problems 13, 16, 18,
Unit 3. Problems 9. 18, 19, 20
Unit 8. Problem 20
Unit 10. Problem 17, 18
For Honors: special assignment
(talk with me after the lecture if
you have not done this)
Reading: Units 1-11
For Honors:
One should write 15 page research proposal that will
discuss either a mission, or an instrument that will
make an advancement in our knowledge of
Astrophysics possible beyond the present day
horizont.The intended project should include an
extensive (1) introduction that shows the present day
ways to solve a particular problem, (2) a discussion of
the proposed solution, (3) the benefits that this solution
will have in terms of advancing our knowledge, (4)
bibliography with references to web resources and
original research papers. The proposal is due two
weeks before the end of the classes.
The Celestial Sphere I
• Since earliest times, humans have sought to
understand the night sky
• A useful model of the sky is called the
Celestial Sphere
• It is not real – it is simply a tool for
understanding and prediction
• Stars in the universe are located at
various distances from Earth, but
can be imagined as lying on a
sphere, with the Earth at its center.
• This sphere appears to rotate around
the Earth, giving the impression that
stars rise and set.
The Celestial Sphere II
•
Important Terms
– Zenith: The point directly overhead on the celestial
sphere (CS)
– Nadir: The point opposite the zenith on the CS
– North or south celestial pole: The point around
which the stars appear to rotate
– Celestial Equator: An extension of the Earth’s
equator expanded out to the surface of the CS.
– Horizon: The lower edge of the visible CS
Constellations and Asterisms
•
•
•
•
The human mind is very good at
recognizing patterns –
consequently we have found
and named patterns of stars on
the celestial sphere
The names of these patterns
have their origins in mythology
from all over the globe
Sometimes very hard to see!
These patterns are called constellations
–
–
•
88 internationally recognized
constellations, covering the entire sky
Star names frequently include the name
of the constellation in which they are
located
Some popular patterns are not
constellations – these are called
asterisms
–
–
Big Dipper
The Teapot
The Ecliptic
•
•
•
•
The ecliptic ‘belt’
on the celestial
sphere is tipped
relative to the
celestial equator due
to the 23.5°
inclination of the
Earth’s rotational
axis
In June, the Sun
appears north of the
celestial equator
In December, the
Sun appears south of
the celestial equator
Twice a year, the sun
appears on the
celestial equator –
these times are
called the equinoxes
The Seasons I
• The Earth’s
inclination is
ultimately responsible
for the change in
seasons.
– In June, the Northern
Hemisphere is tilted
towards the Sun
– In December, the
Northern Hemisphere
is tilted away from
the Sun
•
Common Myths:
– Summers are warmer because the Earth is
closer to the Sun than in Winter
• Actually, the opposite is true!
– The tilt of the Earth’s axis brings the
Northern Hemisphere closer to the Sun in
Summer, and farther from the Sun in Winter
• True, but this accounts for only a minute
fraction of the extra heating
The Seasons II
•
This tilt has two important
effects
– In Summer, the Sun
spends more time above
the horizon – days are
longer, resulting in more
heating
– In Summer, light from
the Sun strikes the
ground more directly,
concentrating the Sun’s
energy.
•
Summers are therefore
warmer than winters!
Precession I
•
•
The Earth spins about its axis like a top, but the Sun’s gravity
adds a little tug
This tug results in the axis of the Earth rotating, or precessing,
with a 26,000 year period
Precession II
• Thanks to precession,
Polaris (the North Star)
will not always be “The
North Star!”
• 6000 years ago, the
North Star was Thuban, a
star in the constellation
Draco
• In 12,000 years, the
Earth’s axis will point
toward Vega, a bright star
in Lyra
What Time Is It?
• There are many ways
to measure time on
Earth
– Sunrise to sunrise
• Problem – seasons
change sunrise times!
– The time between
successive crossings of
the meridian by the
Sun (Solar Day)
• Problem – inaccuracies
due to clouds
Length of Daylight Hours
•
The number of daylight
hours a place has depends
on that place’s latitude on
the Earth
– Regions close to the
northern pole get more
daylight hours during the
summer, and less in
winter
– Within the Arctic Circle
(higher than 66.5 degrees
latitude), there are some
summer days where the
Sun never sets!
– Regions close to the
equator get close to 12
hours of sunlight all year.
Time Zones
•
•
The globe is divided into
24 time zones, designed
such that local noon
roughly corresponds to
the time when the sun is
highest in the sky
If it is noon on the
Prime Meridian in
Greenwich, UK, it is
midnight on the opposite
side of the world. This
midnight line is called
the International Date
Line
The Phases of the Moon
• As the Moon moves around the
Earth over its 29.5 day cycle,
one half of its surface is always
lit by the sun
• From Earth, we see only
portions of the illuminated
surface, giving the appearance
of phases of the Moon
– Full Moon: The Earth is between
the Moon and the Sun, so we see
all of the illuminated surface
– New Moon: The Moon is
between the Earth and the Sun, so
we see none of the illuminated
surface
During the winter the temperature is lower because
the Sun
•
•
•
•
A. Stops moving
B. has lower temperature
C. is farther away from the Earth
D. does not rise as high in the sky
Size of the Earth
• Eratosthenes (296-195 b.c.e.)
wanted to know the size of the
Earth
• He noted that the sun could be
seen from the bottom of a well
in Syene, so the Sun must be
directly overhead
• Then he measured the angle
the Sun made with the horizon
in Alexandria (7 degrees)
• Calculated a diameter of
13,000 km, almost exactly
correct!
Measuring Angular Diameter
• In Astronomy, we will frequently
estimate the sizes of planets, etc.
• To do this, we measure the angle that
the object makes in the sky.
• We say that an object subtends an
angle (A) in the sky
•
•
For example, the moon subtends
0.5 degrees.
The Sun also subtends 0.5
degrees, which is why solar
eclipses are so beautiful!
Measuring Linear Diameter
• If we measure the
angle subtended by
an object in the
sky (A), and we
know the distance
to it (d), we can
calculate its actual,
linear diameter
(L)!
The Motion of the Planets
• Because the planets’
orbits all lie in more or
less the same plane,
the paths of the planets
through the sky all lie
close to the ecliptic,
appearing to move
through the
constellations of the
zodiac
• Only Pluto seems to
move far from the
ecliptic
Retrograde Motion
•
•
•
As the Earth catches up to
the orbital position of
another planet, that planet
seems to move backwards
through the sky.
This is called retrograde
motion
Posed a frustrating problem
to the ancients – if all
planets moved in perfect
circles, how could they
move backwards, and why
only occasionally?
Geocentric Models
•
•
•
•
Models in which
everything revolves around
the Earth are called
Geocentric models.
From earliest Greek times,
this kind of model was
used to describe the
heavens
Planets and stars resided on
their own spheres, each
tipped slightly relative to
each other. This
reproduced the motion of
the planets and Sun
through the sky.
Did not explain retrograde
motion!
Epicycles
• Ptolemy (100-170 C.E.) improved the
geocentric models by including
epicycles
– Planets were attached to small circles
(epicycles) that rotated.
– These epicycles were attached to a larger
circle, centered on Earth
•
•
This can be visualized as a
planet attached to a Frisbee,
attached to a bicycle wheel
with the Earth at the center.
Did a fair job of reproducing
retrograde motion.
Heliocentric Models
• Nicolas Copernicus devised a
heliocentric (Sun-centered)
model in which everything,
including the Earth, revolves
around the Sun
• Retrograde motion is a natural
result of these models!
• Copernicus was also able to
measure the relative distances
between the Sun and the
planets
Mercury and Venus
•
•
It was found that Mercury and
Venus were closer to the Sun than
the Earth, as they were never
found very far from the Sun in the
sky
•
•
Mercury’s greatest elongation, or
angular separation from the Sun, is
never more than 28 degrees
Venus’s greatest elongation is never
more than 47 degrees
Mercury is therefore closer to the
Sun than Venus
Tycho Brahe (1546-1601 C.E.)
• Built instruments to
measure the positions
of planets very
accurately (~1 arc
minute)
• Found that comets moved outside
of the Earth’s atmosphere
• Witnessed a supernova and
concluded that it was much farther
away than any celestial sphere
• As he could detect no parallax
motion in the stars, he held that the
planets go around the Sun, but the
Sun, in turn, orbits around the Earth
Johannes Kepler (1571-1630
• Using Tycho Brahe’s data, discovered
that planets do not move in circles
around the Sun, rather, they follow
ellipses with the Sun located at one of
the two foci!