Motions in the Sky
Download
Report
Transcript Motions in the Sky
Motions in the Sky
Watchers and Timekeepers
The
Milky
Way
over
Utah
The First Science
Unless you were a troglodyte* thousands
of years ago, you couldn’t help but notice
that the sky changed, hour by hour, day
by day, month by month, year by year
*someone who lives underground
The Sun rose and set and sometimes
even disappeared;
The Moon also rose and set, changed
shape, and occasionally turned red;
There were patterns in the night sky that
came into view at different times of the
year;
There were stars that defied the patterns
and marched recklessly across the sky;
And there were often visitors in the
heavens.
Astrologers: precursors to
Astronomers*
As in any population, there were a few
geeks who took great interest in what
was happening in the sky
These people never knew the cause of
the events and phenomena they
observed, but they did keep track of their
periodicities
And they were good storytellers!
*Discussed in another ppt.
Great mysticism was attached to the
goings-on in the sky
The people who recognized the
repetition of celestial events and could
make predictions of the next occurrence
were therefore seen as shamans
Shamans in a society wield great power
The ability to predict phenomena in the
sky extended back down to Earth and
the supposed ability to foresee the future
Often for personal gain
Time
Since celestial events happen with a
regularity, the shaman astrologers could
use this periodicity to construct
calendars (coming up in a minute)
Even today our calendars are closely
attached to Astronomical phenomena
But it took millennia to iron out all the
kinks
Months and Years
The most obvious cyclic events in the
sky are the rising and setting of the Sun
and the Moon
Important to realize is the fact that these
rising and settings vary from day to day
Also critical is to understand that the Sun
and the Moon cycles are not connected
The Sun rises in a different place every
day
These four special days mark out many
calendars
They are: Winter Solstice, Vernal Equinox,
Summer Solstice, Autumnal Equinox
Northern Hemisphericocentric*
The Southern Hemisphere has
equivalent days
The Sun’s rising position changes over a
year, from south of East in December to
directly east in March to north of East in
June and back to East in September
And all points in between
Same all year long on the equator
*not a real word
Effects
This changing rise position is caused by
the tilt of the Earth relative to the Sun
and by the motion of the Earth around
the Sun
The tilt is what causes the Seasons*
Ancient astrologers did not know about
the tilt or the orbit, but they knew the
cycle repeated after about 365 days
*discussed in another ppt
Analemma
Not a regular path in
the sky
Left: each picture
was shot on a
different day but at
the same time over a
year
Important Days
The four Astronomically important days
delineated the four Seasons, although
some cultures like the ancient Egyptians
recognized only three
Please realize that ancient peoples did
not necessarily have a December,
March, etc., but the two solstices and the
two equinoxes were recognized
nonetheless
The Moon
Less ‘well-behaved’ than the Sun
Rises in different places and much
different times every night, if it was
visible at all
Changed shape as well, unlike the
unchanging Sun
Phases
The Moon appears to
change shape because
of its position relative to
the Earth and the Sun
In the bottom right there
seems to be a problem;
the Moon is there
however!
The cycle takes about 29
days, an interval known
to the ancients
Briefly;
Patterns in the Night
Constellations: “stars together”
Different for different cultures (following
slides)
Fanciful at best, but a good mnemonic to
find things
Useful for calendars
Special Constellations
Zodiacal
12 Astrological signs
Misleading: the sign
is not up at night
during its reign
Circumpolar
Never set
Useful for navigation
Two Western Constellations
Orion is facing backwards for use with a celestial sphere
Egyptian
Sky
(interpretation of
Greek view)
African Sky
Used the sky to explain myths
Chinese
star map
from the
Tang
Dynasty
Calendars (snapshots, only for
comparison)
Egyptian
360 + 5 days
Sothis
3 seasons
3 10-day weeks
Babylonian sexigesimal system
360 days, 12-30 day months, 4 seasons
Begins with crescent Moon
Greek
4th C. BC
Months 1-4 36 days, 5-10 35 days each
Roman
12-month 355 day year
D days per month
Mensis Intercalaris month of 27 days
Chinese
12 Lunar months with alternating names
Mayan
Many variants that synchronized, e.g. 260 day
Tzolkin with 365 day Haab
Also lunar and Venusian cycles
Modern Western
Established by Pope Gregory 1582
Threw out lunar influence (almost)
365.24 days
Leap year rules
Still corrected for atomic clocks
Days of the Week
Like calendars, each culture had
different names for the days of the week,
number of days in a week, and number
of hours in a day
Western names derive from celestial
objects
English names span several languages
Sundials
The oldest known instruments for
keeping track of time
First attempt at marking time ~ 5000 BC
2500 BC: Babylonian and Egyptian
obelisks
By 250BC the Greeks produced complex
and more accurate sundials using their
knowledge of geometry
Examples
Visitors in the Sky
The First UFOs!
Unpredictable,
irregular
Shooting Stars
Not! Small grains of
dust
Comets
“Bad Star”: Dis-Aster
Harbingers of
catastrophe
Why Astrology is Not a
Science
Vague, untestable
No account for time delay
Wide constellations
Twins