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Notes on Stonehenge and Seasons
1 Figure 3.11 Part of Stonehenge This ancient monument was built between 2800 and 1500 BC, and used to keep track of the motions of the Sun and Moon. Today, heedless tourists and vandals have disturbed and chipped away at the stones to such a degree that the site is now fenced in and entry is restricted. (David Morrison) Stonehenge (2800 – 1500 B.C.)
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2b. The Ecliptic 15 The Babylonians determined the exact path of the sun through the zodiac constellations
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2b.1 Ecliptic is the dashed line on your Starwheel
16 Its NOT the same as the equator!
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2b.2 Obliquity of the Ecliptic
17 The Ecliptic is tilted 23½° to the equator (“obliquity”)
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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.
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2b.4 From Earth’s point of view
19 Plane of the Earth’s orbit Around the sun Figure 1.6 The Celestial Tilt The celestial equator is tilted by 23° to the ecliptic. As a result, North Americans and Europeans see the Sun north of the celestial equator and high in our sky in June, and south of the celestial equator and low in the sky in December. Red is equator Black is ecliptic Yellow is equator Blue is ecliptic Fig 1-6, p.24
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Ascending Node of Sun (blue) is start of spring
2b.5 Ecliptic on Mercator Map 20 Ascending Node of Sun (blue) is start of spring
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2c.1 Ecliptic Longitude 21 Ecliptic Longitude is measured eastward along the ecliptic, starting at 0 degrees at the First Point of Aries. Solstitial Colure Equinoctial Colure Solstitial Colure Equinoctial Colure 90° 0° 180° 270°
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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 Equinoctial Colure Solstitial Colure 0° 90° 270° North Ecliptic Pole 180°
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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. Figure 3.4 Seasons We see the Earth at different seasons as it circles the Sun. During our winter in the north, the Southern Hemisphere “leans into” the Sun and is illuminated more directly. In summer, it is the Northern Hemisphere that is leaning into the Sun and has longer days. In spring and autumn, the two hemispheres receive more equal shares of sunlight. The orientation of the tilted axis remains the same as the Earth revolves around the Sun Fig 3-4, p.64
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C.1b Local Horizon 53 Figure 1.1 The Sky Around Us The dome of the sky, as it appears to a naive observer. The horizon is where the sky meets the ground, and the observer's zenith is the point directly overhead. Fig 1-1, p.20
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C.1c Local Horizon System
54 Prime Meridian is line from North to South through Zenith
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C.2a Daily Path of Sun 55
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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
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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
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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
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59 2c.1 The Analemma
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2c.2 Transit Times 60 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
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2c.3 Sun is a poor timekeeper
61 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
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3. Archeoastronomy 62 Stonehenge (2800 – 1500 B.C.) Fig 3-11, p.70
Figure 3.11 Part of Stonehenge This ancient monument was built between 2800 and 1500 BC, and used to keep track of the motions of the Sun and Moon. Today, heedless tourists and vandals have disturbed and chipped away at the stones to such a degree that the site is now fenced in and entry is restricted. (David Morrison) Stonehenge (2800 – 1500 B.C.) Fig 3-11, p.70
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3a.1 Rising and Setting Points
63 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
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3b.1 Stonehenge 3100 BC The stone circle was added 1000 years later!
65 The stone circle was added 1000 years later!
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3b.2 “the avenue” points towards summer sunrise
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3b.3 Heelstone in the Avenue
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3b.4 Summer Solstice Sunrise
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b). Stone Circles 17 Stone circles often have 29 stones + 1 xtra one off to side. Originally there were 30 “sarson stone” in the outer ring of Stonehenge
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3b. Lunar Standstill 59 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!
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