Motions in the Sky

Watchers and Timekeepers

TheMilkyWayoverUtah

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 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