Presentation on theme: "Notes on Stonehenge and Seasons"— Presentation transcript:
1Notes on Stonehenge and Seasons 1Figure 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.)
22b. The Ecliptic15The Babylonians determined the exact path of the sun through the zodiac constellations
32b.1 Ecliptic is the dashed line on your Starwheel 16Its NOT the same as the equator!
42b.2 Obliquity of the Ecliptic 17The Ecliptic is tilted 23½° to the equator (“obliquity”)
52b.3 Obliquity of the Ecliptic 18This 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.
62b.4 From Earth’s point of view 19Plane of theEarth’s orbitAround the sunFigure 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 equatorBlack is eclipticYellow is equatorBlue is eclipticFig 1-6, p.24
7Ascending Node of Sun (blue) is start of spring 2b.5 Ecliptic on Mercator Map20Ascending Node of Sun (blue) is start of spring
82c.1 Ecliptic Longitude21Ecliptic Longitude is measured eastward along the ecliptic, starting at 0 degrees at the First Point of Aries.Solstitial ColureEquinoctial ColureSolstitial ColureEquinoctial Colure90°0°180°270°
92c.2 Ecliptic Longitude on Polar Map 22The sun moves about 1 degree east along the ecliptic each day.0° Spring Equinox90° Summer Solstice180° Fall Equinox270° Winter SolsticeEquinoctial ColureSolstitial Colure0°90°270°North Ecliptic Pole180°
103a. The Seasons, and what causes them 40The Earth’s axis of rotation is tilted 23 with respectto 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 thesame as the Earth revolves around the SunFig 3-4, p.64
11C.1b Local Horizon53Figure 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
12C.1c Local Horizon System 54Prime Meridian is line fromNorth to South through Zenith
14C.2b The Equinoctial Sun56Spring (and Fall) Equinox, the sun is on the equatorSunrise is due EastSunset is due WestTransit is when sun crosses prime meridianSun Transits at “local noon”, at 52 above the horizon
15C.2b The Summer Sun57Sun is on Tropic of Cancer, highest declination 23.5°Sunrise is in North-EastSunset is in the North-WestTransit is at 52+23=75 altitude angle (above horizon)Length of day is around 15 hoursTropic of Cancer
16C.2b The Winter Sun58Sun is on Tropic of Capricorn, lowest declination -23.5°Sunrise is in South-EastSunset is in South-WestTransit is at 52-23=29 altitude angle (above horizon)Length of day is about 9 hoursTropic of Capricorn
182c.2 Transit Times60Note 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
192c.3 Sun is a poor timekeeper 61Sun moves further in Right Ascension near solstices than at equinoxes, makes sun get behind clock after both solsticesAlso 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
203. 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
213a.1 Rising and Setting Points 63Ancient 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
223b.1 Stonehenge 3100 BC The stone circle was added 1000 years later! 65The stone circle was added 1000 years later!
233b.2 “the avenue” points towards summer sunrise 66