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## ASTR100 (Spring 2006) Introduction to Astronomy Discovering the Universe

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### ASTR100 (Spring 2006) Introduction to Astronomy Discovering the Universe Prof. D.C. Richardson Sections 0101-0106 – PowerPoint PPT presentation

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Title: ASTR100 (Spring 2006) Introduction to Astronomy Discovering the Universe

1
ASTR100 (Spring 2006) Introduction to
AstronomyDiscovering the Universe
• Prof. D.C. Richardson
• Sections 0101-0106

2
Celestial Sphere
Horizon Where the sky meets the
ground
3
Celestial Sphere
North Celestial Pole Point on celestial
sphere above North Pole
Celestial Equator Line on celestial
sphere above Equator
4
What is the arrow pointing to?A. the zenithB.
the north celestial poleC. the celestial equator
5
What is the arrow pointing to?A. the zenithB.
the north celestial poleC. the celestial equator
6
Review Coordinates on the Earth
• Latitude position north or south of equator
• Longitude position east or west of prime
meridian (runs through Greenwich, England)

7
Altitude of the celestial pole your latitude
8
The North Star (Polaris) is 50 above your
horizon, due north. Where are you?
1. You are on the equator.
2. You are at the North Pole.
3. You are at latitude 50N.
4. You are at longitude 50E.

9
The North Star (Polaris) is 50 above your
horizon, due north. Where are you?
1. You are on the equator.
2. You are at the North Pole.
3. You are at latitude 50N.
4. You are at longitude 50E.

10
The sky varies as Earth orbits the Sun
• As the Earth orbits the Sun, the Sun appears to
move eastward along the ecliptic.

11
Why do the constellations we see depend on
latitude and time of year?
• They depend on latitude because your position on
Earth determines which constellations remain
below the horizon.
• They depend on time of year because Earths orbit
changes the apparent location of the Sun among
the stars.

12
The Seasons
TRUE OR FALSE? Earth is closer to the Sun in
summer and farther from the Sun in
winter.
13
TRUE OR FALSE? Earth is closer to the Sun in
summer and farther from the Sun in
winter.
Hint When it is summer in the U.S., it is
winter in Australia.
14
TRUE OR FALSE! Earth is closer to the Sun in
summer and farther from the Sun in
winter.
• Seasons are opposite in the N and S
hemispheres, so distance cannot be the reason.
• The real reason for seasons involves Earths
axis tilt.

15
What Causes the Seasons?
Seasons depend on how Earths axis affects
directness of sunlight.
16
Axis tilt changes directness of sunlight during
the year.
17
SummaryThe Real Reason for Seasons
• Orientation of Earths axis relative to the Sun
changes as Earth orbits Sun.
• Summer occurs in your hemisphere when sunlight
hits it more directly winter occurs when the
sunlight is less direct. Spring and fall are in
between.
• AXIS TILT is the key to the seasons without it,
we would not have seasons on Earth!

18
Why doesnt distance matter?
• Earths orbit nearly circular anyway.
• Note more ocean, less land means less extreme
seasons in the southern hemisphere.
• Note Some planets have greater distance
variation that DOES affect their seasons, e.g.,
Mars, Pluto.

19
How do we mark the progression of the seasons?
• We define four special points
• summer solstice winter solstice
• spring (vernal) equinox fall (autumnal) equinox

20
We can recognize solstices and equinoxes by Suns
path across sky
Summer solstice highest path, rise and set at
most extreme north of due east. Winter solstice
lowest path, rise and set at most extreme south
of due east. Equinoxes Sun rises precisely due
east and sets precisely due west.
21
Seasonal changes are more extreme at high
latitudes
• Path of the Sun on the summer solstice at the
Arctic Circle

22
Does the orientation of Earths axis change with
time?
• Although the axis seems fixed on human time
scales, it actually precesses over about 26,000
years.
• Polaris wont always be the North Star!
• Positions of equinoxes/solstices move.

Earths axis precesses like the axis of a
spinning top.
23
Phases of the Moon
24
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25
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26
Why do we see phases of the Moon?
• Half the Moon illuminated by Sun and half dark.
• We see some combination of the bright and dark
faces.

27
Phases of the Moon
28
Moon Rise/Set by Phase
29
Phases of the Moon 29.5-day cycle
new crescent first quarter gibbous full gibbous la
st quarter crescent
• waxing
• Moon visible in afternoon/evening.
• Gets fuller and rises later each day.

• waning
• Moon visible in late night/morning.
• Gets less and sets later each day.

30
Its 9 am. You look up in the sky and see a moon
with half its face bright and half dark. What
phase is it?
1. First quarter
2. Waxing gibbous
3. Third quarter
4. Half moon

31
Its 9 am. You look up in the sky and see a moon
with half its face bright and half dark. What
phase is it?
1. First quarter
2. Waxing gibbous
3. Third quarter
4. Half moon

32
We see only one side of the Moon
Synchronous rotation The Moon rotates exactly
once with each orbit. This is why only one side
is visible from Earth.
33
What causes eclipses?
• The Earth and Moon cast shadows.
• When either passes through the others shadow, we
have an eclipse.

34
When can eclipses occur?
• Lunar eclipses can occur only at full moon.
• Lunar eclipses can be penumbral, partial, or
total.

35
When can eclipses occur?
• Solar eclipses can occur only at new moon.
• Solar eclipses can be partial, total, or annular.

36
• Why dont we have an eclipse at every new and
full moon?
• The Moons orbit is tilted 5 to ecliptic plane
• So we have about two eclipse seasons each year,
with a lunar eclipse at full moon and solar
eclipse at new moon.

37
Another look
38
Summary Two conditions must be met to have an
eclipse
1. It must be full moon (for a lunar eclipse) or new
moon (for a solar eclipse).
2. The Moon must be at or near one of the two points
in its orbit where it crosses the ecliptic plane
(its nodes).

AND
39
Predicting Eclipses
• Eclipses recur with the 18 yr, 11 1/3 day saros
cycle, but type (e.g., partial, total) and
location may vary.

40
Ended After a Few More Slides (history of astro)
Feb 5/08
• (see class04)