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Notes on Stonehenge and Seasons 1 Stonehenge (2800 – 1500 B.C.)

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Presentation on theme: "Notes on Stonehenge and Seasons 1 Stonehenge (2800 – 1500 B.C.)"— Presentation transcript:

1 Notes on Stonehenge and Seasons 1 Stonehenge (2800 – 1500 B.C.)

2 The Babylonians determined the exact path of the sun through the zodiac constellations 15 2b. The Ecliptic

3 2b.1 Ecliptic is the dashed line on your Starwheel 16 Its NOT the same as the equator!

4 2b.2 Obliquity of the Ecliptic 17 The Ecliptic is tilted 23½° to the equator (“obliquity”)

5 2b.3 Obliquity of the Ecliptic 18 This is because the earth’s axis of rotation is tilted by 23½ degrees relative to the axis of its orbital revolution around the sun. This is what gives us seasons.

6 Fig 1-6, p.24 2b.4 From Earth’s point of view 19 Yellow is equator Blue is ecliptic Plane of the Earth’s orbit Around the sun Red is equator Black is ecliptic

7 2b.5 Ecliptic on Mercator Map Ascending Node of Sun (blue) is start of spring 20

8 2c.1 Ecliptic Longitude 21 Ecliptic Longitude is measured eastward along the ecliptic, starting at 0 degrees at the First Point of Aries. 0°0° 90 ° 270 ° 180 ° Equinoctial Colure Solstitial Colure

9 2c.2 Ecliptic Longitude on Polar Map 22 The sun moves about 1 degree east along the ecliptic each day. 0 ° Spring Equinox 90° Summer Solstice 180° Fall Equinox 270° Winter Solstice 0°0° 90 ° 270 ° 180 ° Equinoctial Colure Solstitial Colure North Ecliptic Pole

10 Fig 3-4, p.64 The Earth’s axis of rotation is tilted 23  with respect to the Earth’s orbital plane. The orientation of the tilted axis remains the same as the Earth revolves around the Sun 3a. The Seasons, and what causes them 40

11 C.1b Local Horizon Fig 1-1, p.20 53

12 C.1c Local Horizon System 54 Prime Meridian is line from North to South through Zenith

13 C.2a Daily Path of Sun 55

14 C.2b The Equinoctial Sun 56 Spring (and Fall) Equinox, the sun is on the equator Sunrise is due East Sunset is due West Transit is when sun crosses prime meridian Sun Transits at “local noon”, at 52  above the horizon

15 C.2b The Summer Sun 57 Sun is on Tropic of Cancer, highest declination 23.5 ° Sunrise is in North-East Sunset is in the North-West Transit is at 52+23=75  altitude angle (above horizon) Length of day is around 15 hours Tropic of Cancer

16 C.2b The Winter Sun 58 Sun is on Tropic of Capricorn, lowest declination -23.5° Sunrise is in South-East Sunset is in South-West Transit is at 52-23=29  altitude angle (above horizon) Length of day is about 9 hours Tropic of Capricorn

17 2c.1 The Analemma 59

18 2c.2 Transit Times Note Sun transits 12:08 pm on average at Santa Clara, because we are 8 minutes west of the center of the pacific time zone. Equation of Time: Sun is as much as 20 minutes early/late due to elliptical orbit of earth, and obliquity of ecliptic. Analemma: is the figure 8 plot of declination of sun vs equation of time 60

19 2c.3 Sun is a poor timekeeper Sun moves further in Right Ascension near solstices than at equinoxes, makes sun get behind clock after both solstices Also the day is longer than 24 hours when we are near the perihelion (sun moves faster on ecliptic). This is why the lower loop of the figure 8 is bigger in the analemma 61

20 Fig 3-11, p.70 Stonehenge (2800 – 1500 B.C.) 3. Archeoastronomy 62

21 3a.1 Rising and Setting Points Ancient astronomers would naturally put a rock on the ground to mark the extreme points on the horizon where the sun rises/sets each summer and winter 63

22 3b.1 Stonehenge 3100 BC The stone circle was added 1000 years later! 65

23 3b.2 “the avenue” points towards summer sunrise 66

24 3b.3 Heelstone in the Avenue 67

25 3b.4 Summer Solstice Sunrise 68

26 b). Stone Circles Stone circles often have 29 stones + 1 xtra one off to side. Originally there were 30 “sarson stone” in the outer ring of Stonehenge 17

27 3b. Lunar Standstill Winter Full moon at major standstill will rise one arch to the north of the where the sun rises at summer solstice At “Minor Standstill” it will rise in the arch to the right! 59

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