Download Astronomy 1010 Week 1 (1/24-28/11) Group 14 The earth`s orbit

Astronomy 1010
Week 1 (1/24-28/11)
Group 14
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The earth’s orbit defines a plane. The point where the plane intersects the celestial sphere is the
ecliptic. There is also a plane of the moon going around the earth which is off from the earth’s orbit
by 5 degrees
(3rd edition – Figure 2.29)
The Moon
How long does it take the moon to go around the earth?
o Sidereal period – 27.3 days
 how long it takes for the moon to travel around the earth and be lined up with a
certain star again.
 ‘side’ – star – based on the stars
o Synodic period – 29.5 days
 how long it takes for the moon to come back to be lined up with the sun.
 ‘syn’ – sun – based on the sun
In order to tell time you must pick a reference point: either a star or the sun
Siderial day – 23hrs 56 min
Synodic day – 24 hours
o We use mean (average) “solar time” for our clocks.
The moon takes about 28 days to orbit around the earth.
The moon always keeps the same face towards the earth.
(3rd edition - Figure 2.18)
The moon IS rotating – it rotates around its axix once a day, the same time it takes to orbit around
the earth
o Why? Because of synchronous rotation (caused by tidal forces)
We always see the same face of the moon, but because of libration we get to see more than 50%,
about 59% of the moon’s surface
o Libration: the “nodding” of the moon because of the complicated gravitational pull of other
celestial bodies
“the dark side of the moon” really is just the side that we don’t see… call it the back side or the far
side because it’s not always dark on the side that we can’t see – when the moon is between us and
the sun the “dark/back side” is facing the sun and thus fully lit up.
Eclipses
Don’t happen every month because the earth and the moon don’t have the same orbital plane – the
orbit of the moon is 5 degrees off from the orbit of the earth.
Conditions for a solar eclipse:
o E-M-S: the moon must be between the earth and the sun
o They must be lined up along the line of nodes, with the line pointing towards the sun
 Line of nodes: where the orbital planes of the earth and the moon intersect
Astronomy 1010
Week 1 (1/24-28/11)
Group 14
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This line is also slowly precessing, meaning eclipses’ don’t occur in the same month
every year.
Conditions for a lunar eclipse:
o S-E-M: the earth must be between the sun and moon
o Lined up along the line of nodes, with the line pointing to the sun
Once you know how many days it takes for each condition to be met (how often they occur) you can
predict eclipses years in advance… you just have to know the pattern.
o
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Ex. the moon is between the earth
and the sun every 30 days and the
line of nodes is lined up every 200
days
Not only do these two conditions have to be
met, but you also have to pass through the
right spot of the shadow to actually see an
eclipse.
(3rd edition – Figure 2.22)
Condition 1 (E-M-S)
Yes
30
60
90
120
150
180
210
…
600 – YES
Condition 2 (line of nodes)
Yes
X
X
X
X
X
X
X
…
600 - YES
Geocentric vs. Heliocentric models
 3rd century BC…
o Aristarchus: made calculations that proved the sun was the center
o Eratosthenes: determined that the earth was a sphere and calculated its size
o (3rd edition – math tools 2.1)
 They knew all of this information in the 3rd century BC but the information was forgotten throughout
the Roman Conquests and the Dark Ages.
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The Calendar
The year is not exactly 365 days… it’s closer to 356.242
If you base a year on 365 and never change it then the dates of known events (particularly the
equinoxes) will slowly shift (the vernal equinox – the start of spring – march 25 – will end up in
December)
The Julian Calendar
o In 46 BC, Julius Caesar added several months to the year, making 46BC 445 days long, in
order to shift the dates back to where they were supposed to be. This was known as “the
year of great confusion”. Then he instated the “leap year” once every four years.
BUT… 365.25 is still not exact. Over the centuries the vernal equinox shifted forward by 10 days.
The Gregorian Calendar
o In 1582, Pope Gregory XIII took off 10 days (decreed that the day after October 4th was
October 15th) and made the vernal equinox March 21st.
Astronomy 1010
Week 1 (1/24-28/11)
Group 14
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These two calendars co-existed until 1752 when parliament decreed another change from Sept. 2,
1752 to the next day being Sept. 13, 1752 and then everyone was on the same record… (except for
the Eastern Orthodox Church who kept their own calendar until after WWI)
The leap year system helps but everything still keeps shifting so the solution for this is that every
beginning of a century that is not divisible by 4 is not a leap year.
o 1700, 1800, 1900 are NOT leap years
o 1600 and 2000 ARE leap years
This means the length of the year is now 365.2425… which is much closer to 365.2422 and the
calendar won’t shift noticeably for centuries.
There are still some cultures that keep their own calendars
o Muslims – lunar calendars (29.5 days a month) – their months do get out of sync but they
don’t care.
o Egyptians has 12 30-day months and a 5 day holiday period at the end of the year.
IS THE WORLD ENDING 12/21/12??
Mayan Calendar(s)
o Holy calendar – 260 days
o Year – 365 days
o “long count” – 5,125 years
o *they were obsessed with being detailed and accurate about their calendars*
o
Long Count: 5,125 years  over 3 million days!
 Each day had a name – 5 slots, each with a cycle of names. We’ll use numbers to
simplify the example:
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0-20
0-18
0-18
0-19
0-19
Day 1
0
0
0
0
0
Day 2
0
0
0
0
1
Day 20
0
0
0
0
19
Day 21
0
0
0
1
0
Converted to the current Gregorian calendar: they began Aug. 13, 2114 BC and the
last day that can be calculated using their calendar is December 21, 2012.
According to their mythology the world ends when the calendar flips over and the
world is simply remade and begins again the whole cycle the next day.
This is the 4th of such cycles (according to Mayan mythology).
They paid very careful attention to details. They didn’t have some special insight or
mystical powers that enabled them to do this… so don’t worry!
Chapter 3: The Copernican Revolution
Late 1400s – early 1700s – CHANGED THE WEST FOREVER
Astronomy 1010
Week 1 (1/24-28/11)
Group 14
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Root of the industrial revolution.
BIG DEAL IN THE HISTORY OF SCIENCE!
Copernicus
Tycho
Kepler
Galileo
Newton
Copernicus, Tycho and Kepler pushed for the change from geocentric to heliocentric model
*explained what’s happening
Galileo and Newton explained WHY this was happening, thus removing some of the mysticism of the
heavens.
Up until this point (from before the first astronomical observations through the middle ages) the
heavens were viewed as all-powerful and perfect and beyond human comprehension.
The idea of the Copernican Revolution is that if you can understand nature you can control it and if
you can control it you can dominate it and from that point the industrial revolution… and then
creation of h-bombs, etc. is just a step away.
Copernicus – brought Aristarchus’ heliocentric model back into discussion… he decided to scrap the
geocentric model because it didn’t work and was so complicated. He tried the heliocentric model
and liked the way it explained things very naturally.
So why did it take so long for someone to bring up Aristarchus’ heliocentric theory and why was it
not automatically accepted?
o Copernicus was just slightly before his time - the renaissance hadn’t gotten to him yet and
so he was still tied to the middle-age ideas of perfect circles and spheres in the heavens and
so he couldn’t explain the varying speed of the planets without the complicated system of
epicycles upon epicycles.
o It wasn’t until the idea of ellipses came into the picture that the heliocentric model was
simple enough to be understood and accepted.
What was the big deal about the earth vs. the sun at the center debate?
o Explaining the way the planets move in an attempt to understand what was happening in
the heavens (what the gods were doing) and to predict where the planets would be.
o The main goal was NOT originally about finding the “correct” model or making the
explanations simple.
5 “naked eye planets”
1. Mercury
2. Venus
3. Mars
4. Jupiter
5. Saturn
Astronomy 1010
Week 1 (1/24-28/11)
Group 14
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“planets: means ‘wanderer’ in the original Greek because they seemed to wander through
the sky
Mercury and Venus stayed very close to the sun (within 21 and 45 degrees of the sun) and
were only able to be seen right before sunrise or just after sunset
Mars, Jupiter and Saturn on the other hand were visible all night, moving westward through
the backdrop of the stars… except every once in a while they would go backwards for a little
bit and then return to their westward track (retrograde motion)
 Why is this? Because the earth moves faster and “passes” the outer planets for a
little while and then they catch up.
 http://www.bisque.com/help/Patterns/image/retrograde_motion_of_mars_wmf.gif
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In a geocentric model either…
 the motion of the orbit is twisted backwards for a while
o problem with this explanation is that the circles and the heavens are
perfect and so they wouldn’t do that
 OR the planets orbits were suspended in a sphere (crystalline “orb” of ether
– the quintessential 5th substance [on earth there are 4… earth, water, air
and fire, the 5th substance is perfect and found in the heavenlies]
Astronomy 1010
Week 1 (1/24-28/11)
Group 14
“quintessence” = 5th substance) and on that sphere is another sphere on
which the planet orbits at a different speed
o Problem is these things are all moving at different rates, so with the
geocentric model they’d each need multiple epicycles in order to
explain the retrograde motion and the phenomenon that the
planets move at varying speeds within their orbits.
o (this is the same problem Copernicus ran into again because of his
use of the circular orbits in his heliocentric model)
o http://cse.ssl.berkeley.edu/bmendez/ay10/2000/hw/fig1.3.jpg
o