Transcript class 3, S11 (ch. 2b 1-18-11)

Class 3: End of Ch 2 and Ch. 3
Last time:
2.1. Patterns in The Sky: Stars and constellations, TODAY:
celestial coordinates
2.2 Seasons,…Precession
2.2 (cont.). 2.3. The Moon and Eclipses
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Lunar Phases and Eclipses
2.4 Ancient Mystery of the Planets:
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
Apparent Retrograde motion of planets
Parallax
Ch 3 (Histrory of Astronomy)
The Celestial Sphere
North & South celestial poles
the points in the sky directly above the Earth’s
North and South poles
celestial equator
the extension of the Earth’s equator onto the
celestial sphere
ecliptic
the annual path of the Sun through the celestial sphere,
which is a projection of ecliptic plane
The Celestial Sphere
The Milky Way
You’ve probably seen this band of light across the sky.
What are we actually seeing?
The Milky Way
•Our Galaxy is shaped like a disk.
•Our solar system is in that disk.
•When we look at the Milky Way in the sky,
we are looking along that disk.
Measuring the Sky
We measure the sky in angles, not distances.
• Full circle = 360º
• 1º = 60 arcmin
• 1 arcmin = 60 arcsec
Measuring Angles in the Sky
The Local Sky
zenith
the point directly above you
horizon
all points 90° from the zenith
altitude
the angle above the horizon
meridian
due north horizon zenith due south horizon
Review: Coordinates on the Earth
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
Review: Coordinates on the Earth
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
In Celestial sphere the equivalent coordinate are
• Declination: (more later)
• Right Ascension: (more later)
The Daily Motion
• As the Earth rotates, the sky
appears to us to rotate in the
opposite direction.
• The sky appears to rotate around
the N (or S) celestial poles.
• If you are standing at the poles,
nothing rises or sets.
• If you are standing at the equator,
everything rises & sets 90 to the
horizon.
Time Exposure
Photograph:
Star Trails
The Daily Motion
daily circles --- CCW looking north, CW looking south
Annual Motion
• As the Earth orbits the Sun, the Sun appears to move
eastward with respect to the stars.
• The Sun circles the celestial sphere once every year.
2.3 Seasons
• What is the cause of the seasons on Earth?
Annual Motion
• The Earth’s axis is tilted 23.5° from being
perpendicular to the ecliptic plane.
• Therefore, the celestial equator is tilted 23.5°
to the ecliptic.
• As seen from Earth, the Sun spends 6 months
north of the celestial equator and 6 months
south of the celestial equator.
• Seasons are caused by the Earth’s axis tilt,
not the distance from the Earth to the Sun!
Annual Motion
ecliptic
the apparent path of the Sun through the sky
(also the plane of Earth’s orbit)
equinox
where the ecliptic (the Sun) intersects the celestial equator
solstice
where the ecliptic (the Sun) is farthest from the celestial equator
zodiac
the constellations which lie along the ecliptic
The Cause of the Seasons
Coordinates on the Celestial
Sphere (not in book)
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
• Declination: position north or south of celestial
equator (in degrees)
• Right Ascension: distance (in hours, 0 to 23h 59
min.) East of vernal equinox (where the sun
crosses the celestial equator going North)
Question 1
What point on the celestial sphere is defined by a Right
Ascension of zero hours and a Declination of zero
degrees?
Question 2
What point on the celestial sphere is defined by a Right
Ascension of 12 hours and a Declination of zero
degrees?
Question 3
What is an equinox?
Question 4
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of +23.5 degrees?
Question 5
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of minus 23.5
degrees (i.e. 23.5 degrees South)?
Axis tilt causes uneven heating by
sunlight throughout the year.
When is summer?
(in the Northern Hemisphere)
• The solstice which occurs around June 21 is
considered the first day of summer.
• However, it takes time for the more direct
sunlight to heat up the land and water.
• Therefore, July & August are typically hotter
than June.
2.4 Precession
• What is the Earth’s cycle of precession?
Thought Question
What is the arrow pointing to?
A. The zenith
B. The north celestial pole
C. The celestial equator
Thought Question
What is the arrow pointing to?
A. The zenith
B. The north celestial pole
C. The celestial equator
Precession
• The Earth’s axis precesses (wobbles) like a
top, once about every 26,000 years.
• Precession changes the positions in the sky of
the celestial poles and the equinoxes.
 Polaris won't always be the north star.
 However the tilt in the axis is the same
(23.5 degrees) as the Earth’s axis precesses
Question
A. Will Polaris be the North star in another 13,000
years?
Question
B. Will Polaris be the North star in another 26,000
years?
2.3 The Moon, Our Constant Companion
• Why do we see phases of the Moon?
• What conditions are necessary for an eclipse?
Lunar Motion
Phases of the Moon’s 29.5 day cycle
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•
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•
•
•
new
crescent
first quarter
gibbous
full
gibbous
last quarter
crescent
waxing
waning
Lunar phases and the month
• A cycle of lunar phases is approximately 30
days and is the reason why we divide the year
in 12 months
• Why do lunar phases occur on different
calendar dates from one year to the next?
Lunar phases and the month
• A cycle of lunar phases is approximately one
month and is the reason why we divide the year
in 12 months
• Why do lunar phases occur on different
calendar dates from one year to the next?
• Because one year (365 days) is not exactly 12
times the period of the Moon’s phases (its
actually 12.4 times).
Why do we see phases?
• Half the Moon
illuminated by Sun
and half dark
• We see some
combination of the
bright and dark
faces
Moon’s position at sunset for waxing phases
(numbers are days since new Moon)
Phases of the Moon
Why do we see the same side on our
Moon?
Rotation period = orbital period
Eclipses
• The Earth & Moon cast
shadows.
• When either passes
through the other’s
shadow, we have an
eclipse.
• Why don’t we have an
eclipse every full & new
Moon?
Eclipses
When the Moon’s orbit intersects the
ecliptic (node):
at new moon
solar eclipse
you must be in Moon’s shadow to see it
•
within umbra: total solar eclipse
•
within penumbra: partial solar eclipse
at full moon
lunar eclipse
• everyone on the nighttime side of Earth can see it
Solar Eclipse
Question
What will be the Right Ascension and Declination
of the Moon during a total solar eclipse on
September 21?
Question
What will be the Right Ascension and Declination
of the Moon during a total solar eclipse on
September 21?
Hint: where is the Sun on Sept 21?
Lunar Eclipse
What have we learned?
• Why do we see phases of the Moon?
• At any time, half the Moon is illuminated by the Sun
and half is in darkness. The face of the Moon that we
see is some combination of these two portions,
determined by the relative locations of the Sun, Earth,
and Moon.
• What conditions are necessary for an eclipse?
• An eclipse can occur only when the nodes of the
Moon’s orbit are nearly aligned with the Earth and
the Sun. When this condition is met, we can get a
solar eclipse at new moon and a lunar eclipse at full
moon.