1. Lecture 2:The Sky The Origins ofAstronomy
SCI238 W08
Lecture 2:The Sky
The Origins ofAstronomy
Sun pillar atTahoe
L2-Jan10/08
Early Astronomy

2. The Sky/Origins of Astronomy
Today’s Lecture
this week’s events
Moon
naked-eye astronomy
seasons, time and the calendar, precession
origins of astronomy: prehistory (Stone Age)
origins: Greek (beginnings of “science”?)
Pythagoras
Aristotle
Hipparchus
Ptolemy
Eratosthenes
origins: Chinese, Hindu, Arabic contributions to astronomy
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The Sky/Origins of Astronomy

3. The Sky/Origins of Astronomy
This week’s events:
the Moon: First Quarter Jan 15
Venus: visible low in east before sunrise; brightest “morning” star
Mars: is visible all night, rises at sunset
Jupiter: not visible
Saturn: rises at 10pm
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The Sky/Origins of Astronomy

4. The Sky/Origins of Astronomy
Ecliptic = Sun’s path through the Sky (actually the “reflex” or negative or opposite of the Earth’s motion around the Sun); it is not in the same plane as the equator – which is set by the rotation of the Earth. These two planes (ecliptic and equator) are inclined to each other by 23 ½ degrees
Why?
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The Sky/Origins of Astronomy

5. The Sky/Origins of Astronomy
it is this inclination of Earth’s rotation axis to its orbital plane that is the primary cause of our seasons
Why?
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The Sky/Origins of Astronomy

6. The Sky/Origins of Astronomy
Sunlight: direct/indirect
total hours above the horizon
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7. The Sky/Origins of Astronomy
Time of day and the seasons: Dec 22
North Pole
Arctic Circle
Tropics
noon
Equator
3 pm
Antarctic Circle
6 pm
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The Sky/Origins of Astronomy

8. Time of day and the seasons: Mar. 22
North Pole
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9. The Sky/Origins of Astronomy
Time Measurement
one second is now defined by a “hyperfine transition of cesium”… it has a frequency of 9, ,779 per second (ATOMIC TIME)
one day was originally defined by the Sun’s position (e.g. noon to the next noon) and this defined the second: 24x60x60=86400 seconds = one synodic or solar day
Got here (did not cover this slide) before I stopped for a film. Lecture time to this point = 55 minutes
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The Sky/Origins of Astronomy

10. The Sky/Origins of Astronomy
Earth travels ~1o/day around its orbit
So…one solar day requires ~361o of rotation
solar day
vs.
sidereal day
…and a solar day is ~4 min longer than a sidereal day
it’s all in
the orbit!
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The Sky/Origins of Astronomy

11. The Sky/Origins of Astronomy
many kinds of day, year…
measuring by the stars: one sidereal day = 23 hours, 56 min., sec. which is seconds
Apparent Solar Time = time measured by Sun’s position in the sky (a local time)
Mean Solar Time = average length of Solar day
Standard Time is the system where all places on the Earth in a 15 degree wide (in longitude) Time Zone use the same Mean Solar Time
Sidereal Year (time for Earth to complete one orbit against the stars) is Mean Solar Days
Tropical Year (time from one spring equinox to the next – i.e. against the Sun) is Mean Solar Days
Got here (did not cover this slide) before I stopped for a film. Lecture time to this point = 55 minutes
L2-January 10/08
The Sky/Origins of Astronomy

12. Twenty full Moons: May 05-Dec06
a month?
full Moon to full...
differences in?
angular diameter
“aspect”
caused by?
more on scale and distance later…
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The Sky/Origins of Astronomy

13. The Sky/Origins of Astronomy
The Calendar
synodic month: days (12x29.53=354 days)
sidereal month: days
Roman calendar: 13th month every 3rd year
Julian calendar: 12 months,30 or 31 days/month
leap year every 4th year (year is then 365 ¼ days)
started 1 January 45 BC
added 3 extra months to 46 BC
actual year = 11 min, 14 sec < days
by 1582 the calendar was 10 days “short”
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The Sky/Origins of Astronomy

14. The Sky/Origins of Astronomy
The Calendar
Solution???
the Gregorian calendar:
4 October 1582 was followed by 15 October 1582 (in Catholic countries)
century years were not leap years unless divisible by 400
in England and North America 11 days were added in 1752
when is Washington’s birthday??
Napoleon’s victory at Austerlitz in 1805
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The Sky/Origins of Astronomy

15. The Sky/Origins of Astronomy
Precession
the Earth spins on its rotation axis, which is set off at some random angle relative to its orbital motion
the other planets, especially Jupiter, exert a “sideways” force on the spinning Earth
any spinning object that is pushed sideways responds by moving at right angles to the “expected” direction
the Earth (its rotation axis) changes direction of spin because of the forces on it from other bodies
→ Precession
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The Sky/Origins of Astronomy

16. The Sky/Origins of Astronomy
Precession affects the orientation of a spinning object’s axis, but not its tilt
Axial rotation period = 24hr; precession period ~26,000yr
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The Sky/Origins of Astronomy

17. The Sky/Origins of Astronomy
Earth’s rotational axis is always inclined 23.5o to its orbital plane – but slowly precession changes the direction in the sky that marks north…
Currently NCP ~ Polaris; in ~13,000yr ~ Vega
Currently NCP ~ Polaris
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The Sky/Origins of Astronomy

18. The Sky/Origins of Astronomy
Eclipses
Lunar Eclipse: when the moon passes through the Earth’s shadow (the Moon disappears, or “is eclipsed”)
can happen only at Full Moon
Solar Eclipse: when the Earth passes through the Moon’s shadow (the Sun disappears)
can happen only at New Moon
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The Sky/Origins of Astronomy

19. The Sky/Origins of Astronomy
Lunar Eclipse
Earth is between the Sun and Moon
blocks sunlight from illuminating the Moon
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20. The Sky/Origins of Astronomy
sunlight completely blocked in umbra,
partially blocked in penumbra
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21. The Sky/Origins of Astronomy
Types of
Solar Eclipses
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22. The Sky/Origins of Astronomy
Solar eclipses
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23. The Sky/Origins of Astronomy
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24. The Sky/Origins of Astronomy
Eclipse “Seasons”
in order for an eclipse to happen, the Earth, Sun, and Moon must all line up.
but, while the Sun follows the ecliptic, the Moon follows a different path around the sky; because the Earth/Moon orbital plane is inclined ~5o to the ecliptic
these planes cross only twice each year => two eclipse seasons/year
orientation of Moon’s orbit constantly changes: the “line of nodes” (where lunar path crosses the ecliptic) makes a complete circuit every 18.6 years!
eclipse seasons occur ~20 days earlier each year.
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The Sky/Origins of Astronomy

25. The Sky/Origins of Astronomy
eclipse seasons illustrated
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26. The Sky/Origins of Astronomy
orbital planes…ecliptic and lunar
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27. The Sky/Origins of Astronomy
paths of total solar eclipses
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one total eclipse in 2008; one annular

28. The Sky/Origins of Astronomy
Lunar eclipses:
viewable in NA
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29. “Stone-Age” Astronomy
Primarily to mark seasons, special times of year … the world’s largest calendars…
Stonehenge “stone circles”, and standing stones
pyramids
Mayan, Peru
Medicine Wheels
also: sundials to give time of day
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30. The Sky/Origins of Astronomy
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31. Sunrise over Stonehenge: June 21, 2005
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32. The Sky/Origins of Astronomy
sun dagger at summer solstice:
noon in Chaco Canyon, NM
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33. The Sky/Origins of Astronomy
Giant “sundial” in St. Peter’s Square
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34. Crescent Moon “predicts” rainfall in central Nigeria
cause? -> variation in relative positions of Sun and Moon along the ecliptic throughout the year
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35. Ancient Greek Astronomy
planet = “wanderer” – there were 7 planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn)
Pythagoras (~ BC)
Aristotle ( BC)
Aristarchus of Samos ( BC)
Eratosthenes ( BC)
Hipparchus (~ BC)
Ptolemy (~140AD)
about to start this - begin here on 15th Jan. 2008
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The Sky/Origins of Astronomy

36. Ancient Greek Astronomy
Philolaus: non-geocentric cosmology: Earth moves around central fire (not the Sun!) & “counter Earth” on opposite side of this fire
Pythagoras: the “heavenly bodies” are spherical
Aristotle
How can the Earth move? → Geocentric Universe
Correct explanations of phases of Moon and of eclipses
Sun much more distant than the Moon
Earth spherical
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The Sky/Origins of Astronomy

37. Ancient Greek Astronomy
Aristarchus of Samos
Measured the relative distances of the Moon and Sun and found the Sun was times further away then the Moon
Determined relative sizes of Earth, Moon, and Sun from lunar eclipse data (Moon diameter= 1/3 × Earth, Sun diameter = 7 × Earth)
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The Sky/Origins of Astronomy

38. Aristarchus’s method of distances
Determine the length of time from third quarter (at bottom) to first quarter (at top) = t
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39. Aristarchus’s method for determining sizes
Moon and Sun are the same (angular) size in the sky (1/2 deg.) => they must fit within the same (1/2 deg) pair of lines: so their relative distances are known.
From eclipse timing it was known that the Earth’s shadow is 8/3 the size of the moon. Combine these to get relative sizes of Earth, Moon, Sun
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40. angular scale, distance, and diameter
And so…
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41. The Sky/Origins of Astronomy
Eratosthenes measured the size of the Earth from “shadows”: sun angles
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42. Eratosthenes measured Earth’s diameter
if dA-S=5000stadia
=> circumference of Earth = 250,000stadia
or… ~42,000km
(true ~40,000km)
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43. Ancient Greek Astronomy
Hipparchus
invented trigonometry
measured positions and brightnesses of 850 stars – high precision, valuable today
celestial coordinate system, magnitude system, discovered precession
estimated Moon’s size and distance
measured length of year (to within 6 minutes of correct value!)
explained eclipses
“invented” epicycles
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44. The Sky/Origins of Astronomy
Retrograde Motion
tracking planetary positions had shown that some – most notably Mars – did not follow simple paths across the sky
superior planets make a loop in the sky once every year
the part of their path when a planet is moving “backward” in the sky is called “retrograde motion”
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45. The Sky/Origins of Astronomy
Retrograde Motion
normally planets appear to move from west to east in the sky
but, at times, superior planets reverse direction briefly before resuming EW motion
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46. The Sky/Origins of Astronomy
retrograde motion:
is easily explained in a Sun-centred SS
planets more distant from the Sun orbit more slowly
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47. Mars’ retrograde motion
Feb 2006
July 2005
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48. The Sky/Origins of Astronomy
epicycles “invented” to explain retrograde motion in a geocentric SS
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49. Ancient Greek Astronomy
Ptolemy (140BC)
circles are perfect
→ Epicyclic Theory of Solar System
Needed epicycles on epicycles
→ complicated model
Used parallax to measure distance to Moon
Published a 13 volume summary of all astronomical knowledge: the “Almagest”
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50. The Sky/Origins of Astronomy
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51. The Sky/Origins of Astronomy
Greek Model: Earth is at the centre of the SS/universe; all objects, including the stars, have their own spheres
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52. Parallax: Hipparchus and Ptolemy
First, measure the angle to the centre of the Moon from two positions on the Earth at the same time. Here the Moon is overhead at one place and at angle α from another place.
In the triangle formed with centres of both Moon and Earth and the position of the “other place” we know all of the angles (we know where we are on the Earth – therefore know β), and the length of one side is the radius of the Earth…
This is enough to get the distance to the Moon! True value =60!
Solar distance was too small by ~10
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The Sky/Origins of Astronomy

53. Pre-scientific Astronomy in “the rest of the World”
Hindus
measurements of Moon
systems of numbers
Islamic
algebra
records of positions of planets
eclipse records
Babylonians, Assyrians, Egyptians: calendar, time-keeping, surveying
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54. Pre-scientific Astronomy in “the rest of the World”
Chinese
Oldest records: 2159BC - two astronomers executed for calendar errors
good records of comets, meteorites and supernova (“new stars”)
350BC: Shih Shen makes first star catalogue with 800 entries
Old astronomical records are invaluable!
Normally things change in the sky so slowly that the timescale is >> than a person’s lifetime
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The Sky/Origins of Astronomy