Transcript pptx
Motions of the Earth and Sky Part I Outline • • • • • • • History: flat vs. spherical earth Map of the sky Constellations Diurnal and Yearly Motion The seasons Precession Phases of the Moon Flat or Spherical Earth? • Proposed spherical earth: Aristotle (384-322 B.C.) • Evidence: curved shadow earth casts on the Moon • Additional evidence: new stars seen when sailing south • Spherical earth widely believed since time of Aristotle • Columbus set out to prove the world is round? Myth created by fictional biography of Columbus by Washington Irving Constellations • Constellations are patterns of stars that have a historical reference to a mythological figure, animal, or object. • The stars are usually not near each other in space; they just lie in the same direction. 101 LY ~80 LY 124 LY Ursa Major, The Big Bear (Big Dipper) The Big Dipper is an asterism within the constellation of Ursa Major. An asterism is a prominent pattern of stars that is smaller than a constellation. The meridian, zenith, and horizon The meridian is the line in the sky passing overhead from north to south. The zenith is the point directly overhead. Noon is defined as the time for a local observer when the Sun crosses the meridian. A.M. = Ante Meridian (before the Sun crosses the meridian) P.M. = Post Meridian (after the Sun crosses the meridian) The Celestial Sphere = map of the sky The celestial poles The North celestial pole is the point in the sky above Earth’s north pole. There happens to be a bright star near it, which is called Polaris, or the North Star. Similarly, there’s a South celestial pole, but it’s not near a bright star. If you are at the north pole, the north celestial pole is directly overhead. At the equator, the north & south celestial poles appear on the horizon. Diurnal Motion Each day, the earth rotates once (counterclockwise) on its axis. As a result, objects in space appear to move across the sky over the course of the day. This daily motion of the Sun, Moon, stars, and planets is called diurnal motion. Diurnal Motion During diurnal motion, stars in the northern half of the sky appear to move in circles around the north celestial pole (and southern stars circle the south celestial pole). Yearly Motion In addition to rotating, Earth also revolves around the Sun. As Earth orbits the Sun over the course of a year, the Sun appears to move across the stars in our sky. This path of the Sun in the sky is called the ecliptic. The constellations that the Sun moves through are the zodiac constellations. The Sun’s path in the sky is the intersection of the plane of Earth’s orbit with the stars, so that plane is also called the ecliptic. Yearly Motion Because of the Earth’s orbit around the Sun, some constellations (particularly the Zodiac) are in the sky at night for only a part of the year. The Seasons • Earth’s spin axis is tilted 23.5° from the plane of Earth’s orbit. • Because of this tilt, the “directness” of sunlight changes during the year. • northern hemisphere tilted toward the Sun summer solstice • southern hemisphere tilted toward the Sun winter solstice • neither hemisphere is tilted toward the Sun vernal and autumnal equinox • Seasons not caused by earth changing its distance from the Sun, or the fact that one hemisphere is physically close to the Sun than the other When one hemisphere is tilted toward the Sun (summer in that hemisphere), the Sun reaches higher in the sky and is in the sky longer (i.e., more hours of daylight) than at the same latitude in the other hemisphere (which is in winter). At higher latitudes (closer to the poles), the Sun doesn’t reach as high in the sky as at lower latitudes. For instance, at the poles, the Sun never moves higher than 23.5 degrees above the horizon. At higher latitudes in a hemisphere experiencing summer, although the Sun doesn’t reach high in the sky, it remains in the sky longer than at lower latitudes (i.e., more hours of daylight). At higher latitudes in a hemisphere in winter, there are fewer hours of daylight than at low latitudes. So the amount of daylight is more extreme at high latitudes (up to 24 hours of daylight in summer, up to 24 hours of darkness in winter). Precession In addition to its rotation and revolution, the earth’s axis also precesses (wobbles) like a top. The angle between the spin axis and the earth’s orbital plane remains at 23.5°, but the direction of the spin axis changes. It takes 26,000 years for the spin axis to complete one full wobble. Precession Because of precession: • Polaris won’t always be the North Star • The season during which a constellation appears at night changes very slowly over time http://www.csulb.edu/~gpickett/ps112_ems.html http://www.astro.psu.edu/users/kluhman/a5/Lunar_Nav.swf Phases of the Moon The Moon experiences phases because during its orbit around the Earth, the relative positions of the Moon and Sun change. As a result, we see different amounts of the illuminated and dark halves of the Moon. It takes roughly 28 days for the Moon to orbit the Earth once, and hence this is the time between one full moon and the next full moon. Moon’s phase Moon’s position in sky Local time Given 2 of these, you can deduce the 3rd 3rd quarter sunrise noon midnight sunset new full 1st quarter Opposition and Conjunction A celestial object is in opposition if it appears on the opposite side of the sky from the Sun. For instance, the Moon is in opposition when it is full. An object is in conjunction when it is near the Sun in the sky. The Moon is in conjunction when it is new. Opposition/conjunction refer to the sky from where one is standing. This is usually on Earth, but one can also consider opposition/conjunction as viewed from other places in the solar system.