Download ch 2 the sky

THE STARS
Chapter 2 Section 1
Why do we see what we see?
When looking out into the night sky we
need to remember that we live on a
planet that rotates
 The apparent movement of the stars in
the sky is because of the Earth’s rotation

Constellations

Constellations – patterns of the night sky
that resemble something
◦ Ancient cultures named groups of stars after
heroes, gods, and/or mythical beasts

International Astronomical Union
established 88 official constellations
◦ Now a constellation represents a region of
the sky, not a group of stars
Other classifications

In addition to the
88 constellations
the sky contains
a number of less
formally defined
groupings called
asterisms.
◦ Probably the best
known is the Big
Dipper
http://en.wikipedia.org/wiki/File:Big_dipper_from_the_kalalau_lookout_at_the_kokee_state_park_in_hawaii.jpg
The Brightness of Stars
Magnitude Scale is a system used by
astronomers to be able to classify
brightness of stars
 When we look into the sky which star is
the brightest? Do you think that is the
brightest star in the universe? Largest?

The Brightness of Stars
Brightness for us depends on distance to the
star
 Apparent visual magnitude describes how the
stars look to human eyes seen from Earth

◦ This doesn’t really mean that the sun is the largest or
brightest star in the universe, but to us it seems that way
Smaller apparent magnitude = brighter
Magnitude and Intensity

Apparent Magnitude vs. Absolute Magnitude
◦ Apparent has to do with our distance to the
object, closer it is larger/brighter it seems
◦ Absolute deals with the idea of intensity
 This is the measure of the light energy from a star
that hits one square meter in one second
 This will give us a number so we have a solid number
for its intensity
The Celestial Sphere
Chapter 2 Section 2
What is the Celestial Sphere?

Celestial sphere is an imaginary sphere of
very large radius surrounding Earth and
to which the planets, stars, sun, and moon
seem to be attached
http://csep10.phys.utk.edu/astr161/lect/celestial/celestial.html
Ancient astronomers believed
that the sky was a great sphere
with stars stuck on the
inside…they didn’t have the
knowledge or advances in
technology to be able to
understand the sky completely,
but we still use the celestial
sphere as a model to help
understanding of the sky
A Model of the Sky

Horizon – circular boundary between
Earth and the sky
 Part of the sky that we can see
http://en.wikipedia.org/wiki/Zenith

http://en.wikipedia.org/wiki/Horizon
Zenith is the point on the sky directly
overhead
A Model of the Sky

Stars in the northern sky appear to
revolve around a point called the north
celestial pole which is the point on the
sky directly above Earth’s north pole
(same thing in the southern sky)
◦ The star Polaris (North Star) happens to lie
very near the north celestial pole and thus
hardly moves as Earth rotates
◦ There isn’t a star in the southern sky like
Polaris
A Model of the Sky

Celestial
equator is an
imaginary line
directly above
Earth’s equator
http://en.wikipedia.org/wiki/File:AxialTiltObliquity.png
Latitude and the Sky
What we see in the sky depends on our
latitude
 What we see here in Michigan is a lot
different than what someone sees in
Australia

Latitude and the Sky
Constellations near the north celestial
pole never set below the horizon
 These are called north circumpolar
constellations

◦ Ex. Ursa Major and Cassiopeia

Constellations near the south celestial
pole will never rise above the horizon for
an observer in the northern hemisphere
and vice versa
Precession

Precession is the slow change in the
direction of Earth’s axis of rotation
◦ One cycle takes nearly 26,000 years
As you watch the rotation you
can start to see the wobbling on
its axis
http://en.wikipedia.org/wiki/File:Gyroscope_precession.gif
Angular distance – a measure of the separation
between two objects in the sky
 Angular diameter – a measure of the size of an
object in the sky
 Scientific model – an intellectual concept
designed to help us think about a natural process
without necessarily being a conjecture of truth
 South circumpolar constellation – one of the
constellation near the south celestial pole that is
to seen to rise or set
 Horizon – circular boundary between Earth and
the sky

The Cycles of the Sun
Chapter 2 Section 3
Rotation vs. Revolution

Rotation is the turning of a body on its
axis
 Earth rotates on its axis once every day (24 hours)

Revolution is the motion of a body
around a point located outside the body
 Earth revolves around the sun once a year (365 ¼
days)
The Annual Motion of the Sky
The apparent path of the sun around the
sky is called the ecliptic
 The eastward motion of the sun along the
ecliptic is a consequence of Earth’s
moving around its orbit

◦ Takes 365.25 days for the Earth to orbit the
sun
Equinox vs. Solstice




Vernal Equinox is the place where the sun
crosses the celestial equator moving northward
(about March 21st)
Autumnal Equinox is the place where the sun
crosses moving southward (about September
22nd)
Summer Solstice is the place where the sun
reaches its farthest point north (about June
22nd)
Winter Solstice is the place where the sun
reaches its farthest point south (about
December 22nd)
The Seasons
The main reason for the seasons is
because of the tilt (how it sits on its axis)
of the Earth
 Earth’s distance to the Sun has very little
influence on why we have seasons


Demonstration of rotation/revolution and
an explanation for the seasons
◦ http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html
The Seasons

Because the Earth’s orbit is slightly
elliptical our position in relation to the
sun changes
◦ Perihelion – closest point to the sun (Jan 4)
◦ Aphelion – farthest point to the sun (July 4)
http://www.museum.state.il.us/exhibits/ice_ages/images/aphelion.gif
The Seasons
What Earth looks like on
the first days of each
new season
Overhead look
how Earth travels
around the sun
The Motions of the Planets
Chapter 2 Section 4
The Moving Planets

Most of the planets of our solar system
are visible to the unaided eye, though they
produce no light of their own, but we see
them because sunlight reflects off of them
◦ Mercury, Venus, Mars, Jupiter, and Saturn can
all be seen with out the aid of a telescope
 Figure 2-21 shows the location of Mercury and Venus at
sunrise and sunset
The Moving Planets
Evening star is any planet visible in the
evening sky
 Morning star is any planet visible in the
morning sky

◦ Please remember that these are planets and
not really stars
 Venus is probably the best example because it is the
brightest
http://www.google.com/imgres?q=venus+morning+star&um=1&hl=en&tbm=isch&tbnid=ppwKAppLPNMLBM:&imgrefurl=http://earthsky.org/page/61&docid=4V00MNhp3xDTFM&w=300&h=212&ei=mZ1oTvWYAsdgQe1u4jiDA&zoom=1&iact=hc&vpx=436&vpy=287&dur=3750&hovh=169&hovw=240&tx=126&ty=114&page=9&tbnh=141&tbnw=185&start=133&ndsp=18&ved=1t:429,r:2,s:133&biw=1195&bih=628
Astrology

Ancient astrologers defined a zodiac as a
band 18˚ wide centered on the ecliptic
◦ This band was divided into12 segments named
for the constellations along the ecliptic – the
signs of the zodiac

Horoscope shows the location of the sun,
moon, and planets among the zodiacal
signs with respect to the horizon at the
moment of a person’s birth as seen from
that longitude and latitude
Astrology

As time as gone on precession has moved
the constellations so that they no longer
match the zodiacal signs
 If you were born on or between November 30th
and December 17th, the sun was passing through
the corner of the nonzodiacal constellation
Ophiuchus, and you have no official zodiacal sign

Astrology makes sense only when we
think of the world as the ancients did
 This is why modern science left astrology behind
centuries ago
Astronomical Influences on
Earth’s Climate
Chapter 2 Section 5
The Hypothesis

Milankovitch hypothesis states that
changes in the shape of Earth’s orbit, in
precession, and in inclination affect Earth’s
climate and trigger ice ages
The Hypothesis –
st
1
motion
Astronomers know that the elliptical shape
of our orbit varies slightly every 100,000
years
 Right now our orbit is 1.7% closer than
average to the sun during northernhemisphere winters and 1.7% farther away
during northern-hemisphere summers

 This makes Northern Hemisphere climate warmer and this is
the location where most of the land masses are that would
have glaciers
The Hypothesis –

Milankovitch believed
the more elliptical
Earth’s orbit became
the warmer the
temperatures would
be which would
prevent accumulation
of snow and ice that
are needed to form
glaciers, which would
lead to a warming of
our climate
st
1
motion
The Hypothesis – 2nd motion
Precession causes Earth’s axis to sweep
around a cone with a period of 26,000 years,
and that changes the location of the seasons
around Earth’s orbit
 Northern winters now occur when Earth is
1.7% closer to the sun, but in 13,000
northern winters will occur on the other
side of Earth’s orbit where Earth is farther
from the sun

 Northern winters will then be colder, and glaciers may grow
The Hypothesis – 2nd motion
This diagram helps show how our axis
changes over time and can show how our
planet’s tilt will effect climate in the future
The Hypothesis –
rd
3
motion
The inclination of Earth’s equator to its
orbit is changing
 The current tilt on our axis is 23.5˚, this
angle varies from 22˚ to 24˚ with a period
of roughly 41,000 years

 When this inclination is
greater our planet has
more severe seasons
Homework

Page 29-30 Review Questions
◦ 2, 5-6, 9-10, 14

Pg 30 Problems
◦ 3-4