Download Celestial Phenomena

Observing Project
Remember that for one of your observing projects you can
go to a star party (stargazing).
This is available at the Lawrence Hall of Science every 1st and
3rd Saturday of the month. For more information (be sure to call
ahead) see:
http://lawrencehallofscience.org/visit/exhibits/stargazing
Also: The Chabot Space & Science Center has stargazing
every Friday and Saturday 7:30-10:30. This can also count for
your star party observing project.
This Saturday, there will be a special lecture (James Allen
Telescope Array - ATA) and star party, starting at 7:30 pm.
Chabot Space & Science Center:
http://www.chabotspace.org/visit/observatories.aspx
East Bay Astronomical Society: http://www.eastbayastro.org/
Celestial Phenomena
Today’s Lecture:
• Celestial Phenomena (chapter 4, pages 68-89)
Solar and lunar eclipses
Positions of the constellations
Day/night and seasons
• The Solar System (chapter 5, pages 90-94)
Early history of the Solar System
Copernicus and Galileo
Phases of
the Moon
The position of
the Moon in the
sky is closely
related to the
phases!
Can you see a
Full Moon at 12
o’clock noon?
What would
someone see
for the phases
of the Earth
from the Moon?
Total Solar Eclipse
Sun
Moon
Earth
• The Sun’s diameter is 390 times bigger than the
Moon’s, but the Sun is also 390 times farther away!
• This means that they subtend (or cover) the same
angle in the sky (1/2°). This is an amazing coincidence!
Annular Eclipse
Sun
Moon
Earth
• The Moon’s orbit is NOT circular. It’s elliptical.
• This means sometimes the Moon closer or further
from the Earth.
• Because of this, sometimes the Moon does not
completely cover the Sun, making an annular
eclipse.
Annular eclipses from 2001 to 20025
General Relativity
• Important historical use of total solar
eclipses: provided the first test of Einstein’s
General Theory of Relativity (1916).
• Showed that mass warps space and time,
giving rise to what we call gravity.
Sun
Earth
Apparent
position
True
positions
Moon
Apparent
position
Arthur Eddington measured the apparent
positions of stars during a solar eclipse in 1919.
apparent
true
Eclipsed
Sun
• Results roughly agreed
with Einstein’s predictions
for his strange and new
theory, making him an
instant celebrity.
• The closer starlight
passes to the Sun, the
greater the apparent-true
separation.
Total Lunar Eclipse
Sun
Earth
Moon
• Sometimes the Earth passes between the Sun and the
Moon, causing a lunar eclispe.
• When this occurs, the Moon looks very red. Why
would this be?
Total Lunar Eclipses Look Reddish
The Earth’s atmosphere refracts
the red light, but the blue and
green light is preferentially
absorbed and scattered.
Green and blue
light scattered
Sunlight
R e d lig h t
ed
is r e f r a c t
Moon
Earth
Total Lunar Eclipse (cont.)
• If lots of dust is in the atmosphere, then not much
light gets through; get a dark eclipse instead.
• A total lunar eclipse is visible from the entire dark
hemisphere of the Earth (unlike a total solar eclipse)
• Eclipses don’t occur every month because of a 5°
angle between the Moon/Earth and the Earth/Sun
orbital planes. So instead, there are only a few per
year (mostly partial).
Why isn’t every new
Moon an eclipse?
Constellations: 88 groups of
stars (mostly of Greek origin)
Stars are actually at different distances,
and not clustered as they appear!
Cassiopeia
Our view of
the sky
• From any point on
Earth, you can see half
of the celestial sphere at
any given time.
• As the Earth rotates,
stars move across the
sky with circles
centered on the North
Celestial Pole and the
South Celestial Pole.
Stars appear fixed to a large
very distant celestial sphere.
Star circles
Changing
view of the
night sky
• In Dec, we see
Orion overhead at
midnight.
• In Mar, Orion is
overhead at sunset
(6 hours earlier)
• Thus each
successive night,
we see Orion
overhead (or
rising) about 4
minutes earlier.
6 hrs = 360 minutes
360 min / 90 days = 4 min / day
Seasons: because Earth tilts
The seasons are a consequence of the tilt of the Earth’s
axis of rotation (23.5° from perpendicular to the Earth’s
orbital plane).
Seasons: because Earth tilts
• In the spring/summer, the tilt causes the Sun to be higher
in the sky: there are more daylight hours and the sunlight is
more direct (more heating).
• Differences in the distance of the Earth to the Sun (either
orbital or due to tilt) are small. (Earth is actually closest to
the Sun during the northern hemisphere’s winter.)
There is less heating per unit area of the ground
if the Sun is low (the sunlight is less direct)
At places where the Earth is more tilted
from the Sun, the sunlight is less direct.
The Solar System
Early history of Solar System Studies (Greece)
• Planets wander slowly (over
many weeks) among the “fixed
stars.” (This is NOT to be
confused with the daily east-towest movement due to the
Earth’s rotation.)
Earth
• Usually the planets drift from
west to east.
• It is natural (but incorrect) to
think of the Earth as the center
of the Universe
• Spheres for each planet, the
Moon, the Sun, and the stars
seem to rotate around it.
S
Stars embedded in
the celestial sphere
Retrograde Motion
• Retrograde motion is a problem for the simplest geocentric models.
• Each year, planets drift from east to west for a while!
E
W
E
W
• Ptolemy (~140 AD) devised a new geocentric theory to explain this.
From Ptolemy to Copernicus
• Ptolemy’s geocentric system was very complicated, but also very
accurate. It lasted for nearly 1500 years!
• But most people still thought that the “perfect reality” was a bunch
of “nested spheres” as Aristotle originally suggested.
• Copernicus wrote about
heliocentric theory, first
published in 1543 (after
death)
• Galileo’s used a telescope
to see that Venus goes
through an entire set of
phases (like the Moon)
confirming the heliocentric
hypothesis (1610)
The Phases of Venus
Venus according to
Copernicus: all phases
Venus according to Ptolemy:
only crescent and new
Since Venus always appear close to the Sun in the sky, it could not
go through a complete set of phases in Ptolemy’s system, but it
should in Copernicus’ system.
• Galileo made many other
discoveries.
• 4 moons of Jupiter (The
Earth is not the only center of
orbiting bodies! Also, these
moons are not “left behind” as
Jupiter moves.)
• The Roman Catholic Church
was extremely uptight about
this: they put Galileo under
perpetual house arrest.
• Galileo also published his
earlier studies of the motion of
falling bodies (that he had
found that the time it takes an
object to fall from a given
height is independent of its
mass).
Galileo Galilei