Published on November 15, 2007
Discussion : Discussion How do we measure positions on the Earth? The celestial sphere : The celestial sphere If we imagine the sky as the inside surface of a sphere, we can specify any point on this sphere using two angles. Discussion: Discussion How high above the north horizon point is the north celestial pole as viewed from Earth’s north pole? Discussion: Discussion How high above the north horizon point is the north celestial pole as viewed from Earth’s equator? Discussion: Discussion How high above the north horizon point is the north celestial pole as viewed from a latitude of 45 degrees north? Discussion: Discussion If Polaris (the North star) appears 36 degrees above your north horizon, what is you latitude? Discussion: Discussion As viewed from a latitude of 42 degrees north, how high above the southern horizon point is the celestial equator? Diurnal (daily) motion: Diurnal (daily) motion The Sun, Moon, stars and planets rise in the east and set in the west crossing the meridian each day. AM – ante meridian PM – post meridian Circumpolar stars: Circumpolar stars Some stars always remain above the horizon. Discussion: Discussion Is there a place on Earth where all the stars are visible and none are circumpolar? Discussion: Discussion Is there a place on Earth where all the stars visible are circumpolar? Earth’s sidereal rotation : Earth’s sidereal rotation The stars take 23 hours 56 minutes from one meridian crossing to the next This is Earth’s actual rotation period. The annual motion of the Sun: The annual motion of the Sun The Sun moves on average 4 minutes eastward each day relative to the stars, staying in the sky 4 minutes longer each day than a star at the same declination. Earth’s orbit speed is not constant: Earth’s orbit speed is not constant The Earth moves faster when it is closer to the Sun (in January) and slower when it is further away (in July). This means that the length of the day is not a constant 24 hours during the year. Discussion: Discussion If the Earth moved faster in its orbit would the length of the day be longer or shorter? Why? Annual motion of the stars: Annual motion of the stars The same stars are not visible all year long. Any given non-circumpolar star will set 4 minutes early each day relative to the Sun, until it becomes lost in the glare of the setting Sun. The ecliptic: The ecliptic The ecliptic is the annual path through the sky that the Sun appears to take. The ecliptic is also the plane of Earth’s orbit projected onto the stars. Constellations of the Zodiac: Constellations of the Zodiac The ecliptic passes through 12 constellation during the year. This are know as the zodiacal constellations. Discussion: Discussion In what zodiacal constellation is the Sun in right now? What zodiacal constellation is visible near the meridian at midnight? The Seasons: The Seasons The seasons are caused by the 23.5 degree tilt of the Earth’s rotation axis relative to its perpendicular of orbital plane. Why is summer warmer? : Why is summer warmer? The Sun, being above the celestial equator remains in the sky longer during the summer, the longest daylight time occurring on the summer solstice. The Sun rays hit the earth more directly during the summer months, i.e. the summer hemisphere receives more energy per square meter. Discussion: Discussion What is the highest the Sun can get in the sky at a latitude of 42 degrees? The Earth’s rotation: The Earth’s rotation The speed at which points on the Earth’s equator are moving is larger than points on the Earth at higher latitudes. Slide41: This extra velocity at the equator pushes the matter out at the equator. The Earth is not a perfect sphere: The Earth is not a perfect sphere The Earth bulges out at its equator, that is, its diameter measured along its equator is 43 km larger than its diameter from the north to the south pole. Discussion: Discussion Why is Kennedy Space Center in Florida and not in Maine? Conservation of angular momentum : Conservation of angular momentum Anything that spins on an axis or revolves around another object has angular momentum. Conservation of angular momentum requires that the axis of rotation of any spinning object remains in a fixed direction in space. Slide45: The Earth’s rotation axis remains in a fixed direction as Earth orbits the Sun. The Sun and Moon apply a torque to the Earth: Because the Earth is not a perfect sphere and its rotation axis is tilted 23.5 degrees from its orbital plane, the Sun and the Moon pull on the extra mass in Earth’s equatorial bulge and try to straighten out the tilt. The Sun and Moon apply a torque to the Earth Discussion: Discussion The Great Pyramid at Giza has a tunnel which points toward the north celestial pole. At the time the pyramid was built, around 2600 BCE, toward which star did it point? Precession of the Equinoxes : Precession of the Equinoxes If the celestial poles change with time, so to must the celestial equator, which is 90 degrees away from the poles. If the celestial equator changes with time, than the intersection of ecliptic and the celestial equator will also change with time. Discussion: Discussion Do you think the ancient Egyptians, unlike modern astrologers, were aware of precession?