Notes on Stonehenge and Seasons

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

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

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

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

5. 18 2b.3Obliquity of the Ecliptic 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. 2b.4From Earth’s point of view 19 Plane of the Earth’s orbit Around the sun Yellow is equator Blue is ecliptic Red is equator Black is ecliptic Fig 1-6, p.24

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

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

9. 22 Equinoctial Colure Solstitial Colure North Ecliptic Pole 2c.2 Ecliptic Longitude on Polar Map 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° 90° 270° 180°

10. 3a. The Seasons, and what causes them 40 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 Fig 3-4, p.64

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

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

13. 55 C.2a Daily Path of Sun

14. 56 C.2b The Equinoctial Sun • 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. 57 Tropic of Cancer C.2b The Summer Sun • 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

16. 58 Tropic of Capricorn C.2b The Winter Sun • 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

17. 59 2c.1 The Analemma

18. 60 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

19. 61 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

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

21. 63 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

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

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

24. 67 3b.3 Heelstone in the Avenue

25. 68 3b.4 Summer Solstice Sunrise

26. 17 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

27. 59 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!