Transcript Motion in the Sky & Getting to know the Sky

Outline - Jan. 19, 2010
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Units of measurement for the course (pg. 6)
•
Our place in the universe (pgs. 2-5)
•
The universe as a time machine (pgs. 7 and 8)
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Motion in the universe (pgs. 15-20)
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What can we learn by looking at the sky without a telescope? (Ch. 2)
To get the notes, go to
http://firedrake.bu.edu/AS102/AS102.html
and click on “Class Notes”
Units of Measurement
•
Time: seconds, minutes, years, “millions of years” (106 years = 1 Myr), “billions
of years” (109 years = 1 Gyr)
•
SI (kilograms, kilometers) + some “astronomer’s favorite units”
•
“Solar masses” (Msun) = the mass of an object measured in units of the sun’s
mass
•
“Solar luminosities” (Lsun) = the intrinsic brightness of an object measured in
units of the sun’s intrinsic brightness
•
Astronomical Unit (AU) = distance between the earth and the sun = 1.5x108 km
•
Light year (ly) = the distance light can travel in 1 year = 9.46x1012 km
•
Parsec (pc) = 3.26 light years
Examples: Solar Masses and Soar Luminosities
Mass of the sun = 1.99x1030 kg = Msun
Luminosity (intrinsic brightness) of the sun = 3.84x1026 W = Lsun
Mass of Sirius = 4.02x1030 kg = Msirius
Luminosity of Sirius = 9.75x1027 W = Lsirius
In units of the sun’s mass, the mass of Sirius is
4.02x1030 / 1.99x1030 = 2.02
Msirus = 2.02 Msun
In units of the sun’s luminosity, the luminosity of Sirius is
9.75x1027 / 3.84x1026 = 25.4
Lsirius = 25.4 Lsun
Site: Alborz Moutains (Iran), morning of Jan. 25, 2007
Our place in the universe
Earth is a small, rocky planet orbiting a rather ordinary star (the Sun) at
a distance of 1.5x108 km = 1 AU = 1.58x10-5 ly = 8.3 “light minutes”
Artist’s conception of
the Milky Way galaxy
Sun (and all the planets)
orbit the center of the
Milky Way at a distance
of about 28,000 ly
(about halfway across
the disk of the galaxy)
One orbit of the sun
around the galaxy takes
about 230 million years
to go once around the
galaxy
Sun has made about 20
trips around the galaxy
since it was born
The “Local Group” of Galaxies
Andromeda / M31
Small
Magellanic
Cloud (LMC)
Large
Magellanic
Cloud (LMC)
About 30 galaxies in total, dominated by two large spiral galaxies (Milky Way and M31);
remaining galaxies are “dwarf” galaxies. Local Group is about 2.5x106 ly = 2.5 Mly in diameter.
Milky Way and M31 are approaching each other at a relative speed of about 300 km/s.
Galaxies are Gregarious
(prefer to live in small “groups” or big “clusters”)
Small portion of the Virgo Cluster, the closest large galaxy cluster (contains about
1,500 galaxies, distance about 60 Mly)
The Local Network of “Superclusters” and Voids
Galaxies cluster together to form
“groups” or “clusters” of galaxies
Clusters of galaxies cluster
together to form “superclusters”
of galaxies
We belong to the
Virgo Supercluster
Voids are regions of space
where, for some reason, galaxies
had a hard time growing
Each white dot in the picture represents a galaxy.
Pause for reflection…
Spiral galaxy M87; distance = 12 Mly = 12,000,000 ly
We keep quoting distances in units of “light years” (and
even millions of light years), what does this really mean
when we look through the telescope at these objects?
The Universe as a Time Machine
When you look up at the sky tonight, you will see light from the stars that
is arriving at Earth tonight.
The farther an object is from Earth, the longer it has taken the light to
arrive at your eyes (and the farther back in time you are seeing when
you look at the more distant object).
The Big Dipper is visible
in the Northern Sky
every night, all night long
Alkaid
Mizar
Alioth
The light that you see tonight from Dubhe
left the star before your Grandmother
was born! It also left Dubhe 45 years
before the light that you see tonight from
Merak left Merak.
Megrez
Phecda
Dubhe
Merak
Note: the star that is the farthest away is
also seen to be the brightest in the sky.
What does that tell you?
How can we study the evolution of the universe?
(the universe is REALLY old - 13.8 billion years)
Problem: Humans live for a short time (100 years) compared to
the age of the universe
We don’t have the luxury of watching the universe undergo
changes over our lifetimes (it actually changes very little on
time scales less than a few million years)
Tactic: Study vast collections of objects in the universe (i.e.,
galaxies) that are located at different distances (= different look
back times) and compare them to each other
Are the galaxies that are located 5 to 10 billion light years from
us significantly different from the galaxies that are only a few
million light years away?
The universe is full of galaxies!
There are over 2000
galaxies in the image, and
in the observable universe
there are at least 100
billion galaxies.
The look back times to the
galaxies in this image
range from 0.5 billion to 9
billion years.
In astronomy, we cannot
separate distance from
time. This means the
“observable universe”
corresponds to how far
back in time we can see.
Motion in the Universe
Is it ever possible to be truly “still” on Earth?
Observed Motions
•
Rotation of the Earth on its axis
•
Revolution of the Earth about the Sun
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Revolution of the Sun about the center of the Milky Way
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Motions of galaxies in the Local Group
•
“Recessional” velocities of galaxies outside the local group due to
expansion of the universe
The first 4 motions above are related to conservation laws in
physics (angular momentum) and law of gravity. The last is
related to the birth of the universe in the Big Bang.
How to Get a Law Named After You
(discover something really fundamental)
Edwin Hubble
Hubble’s Law: the farther a galaxy is from
us, the faster it is receding from us
Hubble’s Law is proof that the universe as a whole is expanding (getting larger)!
Raisin Bread Analogy to the Expanding Universe
http://spiff.rit.edu/classes/phys301/lectures/lambda/RaisinBread.swf
The program plots the distance of each of the raisins, as viewed from one particular
raisin, and the rate at which the distances to the raisins is increasing (i.e., the velocity)
Getting to know the sky
What things (not related to weather) can you see in
the sky without the aid of a telescope?
A few things you can see in the sky without a telescope
With the exception of stars that are near the north and
south poles, all astronomical objects “rise” & “set”
Constellations - 88 in total
Some constellations are easy to recognize from the patterns of stars. Very
few are easy to imagine as their corresponding image (mythological
character or animal). Orion (the hunter) is one of the exceptions!
Cassiopeia - The Queen (mother to Andromeda); see
Clash of the Titans (they’re all in the sky)
Constellations have well-defined borders on
the sky (set by the International
Astronomical Union in 1928)
Cassiopeia is one of the northern
“circumpolar” constellations, which means
she never rises or sets
Constellations of the Zodiac
(the “sun sign” constellations of astrology)
Called the sun
signs because
they are all
located along
“the ecliptic”, the
path of the sun
through the sky
(= the earth’s
orbit around the
sun).
Over the course
of a year, the sun
(as viewed from
the earth),
travels through
each of these
constellations.
Zodiacal Constellations and the Sun
Which constellations could you SEE (i.e., on a given night)?
What determines which constellations you can see on a
given night?
• Time of year (summer vs. winter constellations)
• Latitude (can’t see below your own horizon; e.g., if you live in
Lima, Peru you will never see Cassiopeia)
Since the constellations you see repeat over and over with the
same seasons, it’s easy to notice a 1-year pattern to the seasons.
Cause of the Seasons
Over the course of the year in Boston, what will you notice about:
1. The height of the sun at noon
2. The rising and setting points of the sun (i.e., is it always due
east and due north, respectively?)
Day after day, the sun appears to take a slightly different
track in the sky, and the track correlates with the seasons.
Path of the Sun at Different Times of Year
From March 21-22 (spring equinox) to
September 22-23 (autumnal equinox), the
sun rises and sets north of due east/due
west, so the days are long.
From September 22-23 to March 21-22,
the sun rises and sets south of due
east/due west, so the days are short.
Equniox = equal night
The height of the sun at noon and its
rising/setting point depend upon your
latitude on earth. The poles are truly
extreme (constant day/night for months).
Midnight Sun
Near the earth’s poles, the sun never sets during mid-summer.
You pay for all this summer sun by never seeing the sun in mid-winter!
Tilt of the Earth Relative to the Sun’s Rays
The earth’s equator is
inclined at 23.5o to the
plane of its orbit around
the sun. This is the true
cause of the seasons.
What season is it in
South America?
What season is it in North
America?
Solar “Irradiance”
equator
mid-latitude
(similar to Boston)
polar climates
The greater is the angle between the surface of the earth and the sun’s rays, the
more power per unit area the surface of the earth receives.
Seasons and the Earth’s Orbit
You experience winter when your hemisphere is pointed away from the direction of the
sun, and summer when your hemisphere is pointed toward the direction of the sun.
Precession of the Equinoxes
Like a spinning top, the earth’s rotation axis
“precesses”, constantly changing the direction
of the North pole with respect to the sky.
This “minor motion” is very slow (takes 26,000
years to complete), but is important to
navigation by the stars!
Right now, the North Star is “Polaris” (the tail
star of the Little Dipper). Five thousand years
ago the North Star was Thuban, and in 14,000 it
will be Vega.
Phases of the Moon
Names of phases, left to right:
Waning Crescent, Third Quarter, Waning Gibbous, Full, Waxing Gibbous, First
Quarter, Waxing Crescent
The amount of the moon’s face that is “lit up” increases daily from New Moon
to Waxing Crescent to Full, then decreases daily from Full Moon to Waning
Crescent to New Moon.
“Shadow of the Earth” does not cause the phases of the moon
Earth and moon, both showing the “First Quarter” phase, seen from space.
Cause of Phases of the Moon:
Changing angle between the earth and sun
At new moon, you look in
the same direction to see
both the sun and moon.
At new moon the far side
of the moon is in total
daylight!
At full moon, you look on
opposite sides of the
sky to see the sun and the
moon. At full moon, the
far side of the moon is in
total darkness!
At the quarter moons, the
earth, moon, and sun
make a right-angle
triangle.
When does the moon rise and set?
(make it simple - assume 12 hours from rise to set)
On the diagram, you can see anything that is
180o from you (the extent of your horizon)
Sunrise = 6am, Sunset = 6pm
The moon rises about 50 minutes
later from one day to the next.
New moon must rise and set with the
sun (6am and 6pm), respectively.
Full moon must rise when the sun is
setting (6pm), and must set at sunrise
the following day (6am).
First quarter is mid-way between new
and full, so it must rise at noon (i.e. 6
hours later than the new moon rises)
and set at midnight (i.e., 12 hours after
it rose).
Choose a spot on the earth to stand, then ride along
with the earth as it rotates counter-clockwise
Third quarter is mid-way between full
and new, so it must rise at midnight
(i.e., 3 hours later than the full moon
rises), and set at noon (i.e., 12 hours
after it rose).
Solar and Lunar Eclipses
Total lunar eclipse
time sequence
Total solar
eclipse time
sequence
Why don’t we get a solar eclipse with every new moon
and a lunar eclipse with every full moon?
Moon’s orbit is inclined at 5o to Earth’s orbit. It is only when the moon is
precisely in the same plane as the earth’s orbit that eclipses can occur.
Minute Paper
A few sentences on one of the following:
* something you found particularly interesting today
* something you found particularly confusing today
* questions on things from today that you would like to know more about
Be sure to PRINT your name legibly