Chapter 2: Discovering the Universe for YourselfConstellationsMonday, January 14, yChapter 2: Discovering the Universe for YourselfConstellations-Constellations- patterns of stars and were described by many civilizations-Astronomers define Constellations as specific regions of the sky• Constellations fill the entire sky, much like the 48 continental states, fill the entire continental US.-any point in the sky is within a particular constellation• Stars in a constellation may actually be quite far away from each other-Ex: the brightest stars in orion are not the same distance• Because we can’t tell the distances to stars it often looks like the stars are points of light on a sphere surrounding the Earth-Celestial sphere- an imaginary sphere surrounding the Earth on which all celes-tial objects appear to lie• The 88 official constellations cover the celestial sphere-these are fixed in the celestial sphere they never move. Only the Earth and plan-ets move in relation to the sun and each other -’s axis of rotation is inclined 23.5 degrees relative to its orbital axis • The other 7 planets in our solar system orbit the sun in roughly the same plane the does• Sometimes the “axial tilt” or “obliquity of the ecliptic”• current value of axial tilt 23.41Monday, January 14, y-gravitational effects of other planets cause this.-What causes the seasons• This means that the planes defined by the equator and the Earth’s orbit are inclined• The Ecliptic is the plane defined byEarth’s orbit-The sun lies along the ecliptic as well as the planets• Time of day depends on the earth’s rotation at your location (obviously)• look at image on power point for ch 2-it is inclined by 23.5 degrees relative to the celestial equator• The planes defined by the equator and Earth’s orbit are inclined • Ecliptic is the plane defined by Earth’s orbit-The sun lies along the ecliptic and planets-Inclined 23.5 degrees relative to the celestial equator-Reference points for your local sky• Zenith: the point directly overhead• Horizon: all points 90 degrees away from Zenith• Altitude: Angle above horizon• Don’t need to know Meridian• The Height of the NCP above your horizon equals your N latitude-Separations and sizes on the celestial sphere can only be measured using angles2Monday, January 14, y• We don’t know the distances between some of these things • You would need to know the distance to get true size• EX: the sun and moon have nearly the same angular size (think of eclipses• Units for angular measurements-1 degree is 60 arc min-60 arc seconds in every arc min-What you can see depends on where you are on Earth-Earth rotates West to East so stars appear to circle from East to West-apparent motion (stars) is not due to Star’s motion, but Earth’s-Stars appear to travel on circular paths around the NCP or SCP not your zenith• What you can see depends on your latitude and what time of year it is (you can’t see stars in the direction of the sun) You can only see the stars that face away from the sun• During a year. the Sun moves along the ecliptic. The Zodiac is the ring of constel-lations that lines the ecliptic• In astrology (which is not a science), The location of the sun when you were born determines your “sign”-All are off by about 2 constellations!• Seasons are not caused by the earth being closer to the sun-Seasons are caused by the tilt of the Earth’s axis 3Monday, January 14, y• Seasons depend on the directness of sunlight or how directly overhead the sunis.• Why does directness mater?-When not over head the sun’s energy is spread over a larger area• If sun is coming directly above it is coming in at 342 watts per square meter• if the sun is coming in at a forty five degree angle, it is coming in at 242 watts per square meter• Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct-autumnal equinox• Neither hemisphere pointed toward Sun, moving toward northern winter-summer solstice• Northern hemisphere pointed toward the sun-vernal equinox• Neither hemisphere pointed toward Sun, Moving toward Northern summer-winter solstice• Northern hemisphere pointed away from the sun• The axis of the earth is not perfectly fixed-Earth’s axis precesses like the axis of a spinning top• precession- the slow bobble that alters the orientation of Earth’s axis4Monday, January 14, y• precesses over the course of about 26,000 years-our North star, will no longer be polaris in 6,500 years, it will be VegaPhases of the Moon-Why do we see phases of the moon?• you can only see the side of the moon the Sun is shining on.• The Moon’s location in it’s orbit around Earth affects how much of that illuminated side you can see• Cycle of phases is 29.5 daysWhat causes eclipses?-The Earth and moon cast shadows• When either passes through the other’s shadow, we have an eclipse.• Lunar Eclipse- Earth casts a shadow falls preventing us from being able to see the moon.- Only occur at a Full Moon• Can be Partial (only part in shadow)• Can be Total (completely in shadow)-a totally eclipsed moon often looks eerily red aka. a “blood moon”• More red light can pass throughout eh atmosphere than blue right. This is why sunsets are red• the light that passes through the atmosphere is refracted, much like how a telescope lens of your eye can bend light.5Monday, January 14, y• Solar Eclipse- Moon casts a shadow, preventing us from being able to see the sun-Folks in middle of the shadowed region on Earth, would see the Sun and Moon above (ex above)- Only occur at a New Moon• Can be Partial, Total or Annular-because Moon’s orbit is elliptical and sometimes the moon is further away orcloser• Why aren’t there eclipses at every new and full moon?• Because the moon’s orbit is tilted 5 degrees to ecliptic planeThe ancient mystery of the Planets-The word “planet” means “wandering star” in Greek-5 Naked Eye Planets• first planets known• Mercury-Difficult to see; always close to sun in sky-Mercury is hardest to se because it never gets from the Sun -it’s small and has a darks surface so it doesn’t reflect much light)-exhibits phases• Venus -Very bright when visible morning or evening “star”6Monday, January 14, y-exhibits phases• Mars-Noticeably red• Jupiter -Very Bright• Saturn-Moderately bright-Why was motion