Download 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