AST101 - Chapter 2 notes

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Department
Astronomy & Astrophysics
Course Code
AST101H1
Professor
Michael Reid

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2.1 Patterns in the sky Constellations - names given to pattern of stars - it is a region of the sky with well-defined borders - every point in the sky belongs to some constellation - 88 official constellations - stars and constellations appear to lie on a celestial sphere that surrounds Earth, but this is an illusion created by our lack of depth perception in space, a consequence of the fact that the stars are so far away Four special Points and circle on the celestial sphere  North celestial pole: directly above the North Pole  South celestial pole: directly above the South Pole  Celestial Equator: the projection of Earth’s Equator into space  Ecliptic: Sun’s apparent annual path around the celestial sphere Milky Way - Band of light, shaped like a thin pancake with a bulge in the middle - circles all the way around the celestial sphere, passing through more than a dozen constellations - most visible in the southern Hemisphere - traces our galaxy’s disk of stars (the galactic plane) as it appears from our location in the outskirts of the galaxy - the dark lanes that run down the center of the Milky Way contain the densest clouds, and they appear dark obscuring our view of the stars behind them - these clouds generally prevent us from seeing more than a few thousand light-years years into our galaxy’s disks - As a result, much of our own galaxy remained hidden from view until just a few decades ago, when new technologies allowed us to peer through the clouds by observing forms of light that are invisible to our eyes (such as radio waves and x rays). The Local Sky Local sky: the sky as seen from wherever you happen to be standing Horizon: the boundary between Earth and the sky Zenith: the point directly overhead (altitude at 90 degrees, has no direction) Meridian: imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due North - we see only half of the celestial sphere at any particular moment - can pinpoint the position of any object in the local sky by stating its direction along the horizon - from any place on Earth, the local sky looks like a dome (hemisphere) -we can pinpoint the position of any object in the local sky by stating its direction along the horizon (sometimes stated as azimuth, which is degrees clockwise from due north) and its altitude above the horizon Angular Sizes and Distances Angular Size - is the angle it appears to span in your field of view - lack of depth perception means we can’t judge the true sizes of the objects we see in the sky - but we can describe the angular sizes or separations of objects without knowing how far away they are - we measure angular sizes or angular distances, rather than actual sizes or distances, when we look at objects in the sky - for more precise astronomical measurements, we subdivide each degree into 60 arcminutes (‘) and subdivide each arcminute into 60 arcseconds (“) Ex. The Sun o the Sun is about 400 times larger in diameter than the Moon, but it has the same angular size in our sky because it is also about 400 times farther away Why do stars rise and set? - Because Earth rotates from west to east (counter clockwise as viewed from above the North Pole), the stars appear to rise in the east and set in the west Stars relatively near the north celestial pole - remain perpetually above the horizon - never rise or set but instead make daily counter clockwise circles around the north celestial pole o We say that such stars are circumpolar - stars relatively near the south celestial pole never rise above the horizon at all -all other stars have daily circles that are partly above the horizon and partly below it. Why do the constellations we see depend on latitude and time of year? -we can locate any place on Earth’s surface by its latitude and longitude -northern hemisphere: the sky appears to turn counter clockwise around the north celestial pole -southern hemisphere: sky appears to turn clockwise around the south celestial pole Latitude - measures north-south of the equator, 0 degrees at the equator, 90 degrees and the North and South Pole - lines of latitude are actually circles running parallel to the equator - affects the constellations we see because it affects the locations of the horizon and zenith relative to the celestial sphere - people along the same latitude see the same set of constellations at night - the sky varies with latitude o note that although the sky varies with latitude, it does not vary with longitude - the altitude of the celestial pole that is visible in your sky is always equal to your latitude Longitude - measures east-west of the prime meridian, 0 degrees along the prime meridian - lines of longitude are semicircles (half-circles) extending from the North to the South Pole - The prime meridian (longitude = 0 degrees) passes through Greenwich, England Variation with time of year -the night sky changes throughout the year because of Earth’s changing position in its orbit around the Sun -the Sun moves eastward along the ecliptic, completing one circuit each year -the constellations along the ecliptic are called the constellations of the zodiac (tradition says there are 12, but officially there are 13) -the Sun’s apparent location along the ecliptic determines which constellations we see at night 2.2 The reason for seasons What causes the Seasons? - the combination of Earth’s rotation and orbit leads to the progression of seasons - seasons on Earth are caused only by the axis tilt and not by any change in Earth’s distance from the Sun - although Earth’s orbital distance varies over the course of each year, the variation is fairly small - the tilt of Earth’s axis causes sunlight to fall differently on Earth at different times a year - Earth’s axis remains pointed in the same direction in space (toward Polaris) throughout the year - when sunlight hits a hemisphere at a steeper angle, it makes it summer in that hemisphere o as the steeper angle means more concentrated sunlight o the steeper angle means the Sun follows a longer and higher path through the sky, giving that hemisphere more hours of daylight Solstices and Equinoxes  Summer (June) solstice: when Northern Hemisphere is tipped most directly toward the sun  Winter (December) solstice: when Northern Hemisphere is tipped most directly away from the Sun  Spring (March) equinox: moment the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slight toward the Sun  Fall (September) equinox: the moment when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly toward the Sun -the exact dates and times of the solstices and equinoxes vary yearly -the equinoxes occur on the only two days of the year on which the Sun rises precisely due east and sets precisely due west. These are also the only two days when sunlight falls equally on both hemispheres First day of seasons -We usually say that each equinox and solstice marks the first day of a season, but it is actually the midpoint Although the choice of summer solstice as the first day of summer is arbitrary, it makes sense in at least two ways 1) It was much easier for the ancient people to identify the days on which the Sun reached extreme positions in the sky-such as when it reached its highest point on the summer solstice 2) We usually think of the seasons in terms of weather, and the solstices and equinoxes correspond quite well with the beginnings of seasonal weather patterns Solstices - although the Sun’s path through the Northern Hemisphere sky is longest and highest around the time of the summer solstice, the warmest days tend to come 1 to 2 months later, because it takes some time for sunlight to heat the ground and oceans - seasonal variations around the times of solstices are more extreme at high latitudes - at the Arctic Circle, the Sun remains above the horizon all day long on the summer solstice o the most extreme cases occur at the North and South Poles, where the Sun remains above the horizon for 6 months in the Summer and below the horizon for 6 months in the winter Equinoxes - the Earth’s equator gets it most direct sunlight on the two equinoxes and its least direct sunlight on the solstices o so people living near the equator don’t experience four seasons in the same way as people living at mid-latitudes o instead, equatorial regions generally having rainy and dry seasons, with rainy seasons coming when the Sun is higher in the sky Why orbital distance doesn’t affect our seasons Seasons - more extreme in the Northern Hemisphere because most of the Earth’s land lies in the North with far more ocean in the Southern Hemisphere - water takes longer to heat or cool than soil or rock (largely because sunlight heats bodies of water to a depth of many meters while heating only the very top layer of land) - although distance from the Sun plays no role in Earth’s seasons, the same is not always true for other planets, especially if they have significantly greater distance variations, such as Mars, even though it has the same axis tilt as Earth How does the orientation of Earth’s axis change with time? Precession: a gradual wobble that alters the orientation of Earth’s axis in space - caused by gravity’s effect on a tilted, rotating object that is not a perfect sphere - each cycle of Earth’s precession takes about 26,000 years - does not change axis tilt but changes the solstices and equinoxes - thus, the constellations associated with the solstices and equinoxes gradually change Ex: Spinning Top - spinning top wobbles (or processes) more slowly than it spins - it stays upright because rotating objects tend to keep spinning around the same rotation axis (law of conservation of angular momentum) - this tendency prevents gravity from immediately pulling the spinning top over, instead, gravity succeeds only in making the axis trace circles of precession - the spinning (rotating) Earth precesses because of gravitational tugs from the Sun and Moon - the Earth is not a sphere, instead bulging slightly at its equator - the gravitational attractions of the Sun and Moon try to pull the equatorial bulge into the ecliptic plane, effectively trying to straighten out Earth’s axis tilt - however, like a spinning top, Earth tends to keep rotating around the same axis. - Gravity thus does not succeed in changing Earth’s axis tilt but only makes the axis precess 2.3 The Moon, our Constant Companion Phases of the Moon - Depends on its position relative to the Sun as it orbits Earth - Half the Moon is always illuminated by the Sun, but the amount of this illuminated half that we see from Earth depends on the Moon’s position in its orbit - Each complete cycle of phases from one new moon to the next, takes about 29 ½ days (which is about 2 days longer than the Moon’s actual orbital period because of Earth’s motion around the Sun during the time the Moon is orbiting around Earth) - Moon at highest point at midnight - Waxing (increasing): phases from new to full moon - Waning (decreasing): phases from full to new moon - Half-moon: no phase, instead we see half the Moon’s face at first-quarter and third-quarter, these phases mark the times when the Moon is one-quarter or three-quarters of the way through the monthly cycle (which begins at new moon) - Crescent: phases just before and after full moon - Gibbous (hump-back): just before and after full moon, which is essentially the opposite of a crescent moon The Moon’s Synchronous Rotation - while we see many phases of the Moon, we do not see many faces - from Earth we always see the same face of the Moon because of synchronous rotation: when the Moon rotates on its axis in the same amount of time that it takes to orbit Earth - the Moon’s synchronous rotation is not a coincidence, rather, it is a consequence of Earth’s gravity affecting the Moon in much the same way that the Moon’s gravity causes tides on Earth - the fact that we always see the same face of the Moon means that the Moon must rotate once in the same amount of time that it takes to orbit Earth once The View from the Moon  New Moon: Moon is between the Sun and Earth -the dark portion of the lunar face is not totally dark  Ashen light or earthshine enables us to see the outline of the full face of the Moon even when the Moon is not full  because Earth is much larger than the Moon, the full earth is much bigger and brighter in the lunar sky than the full moon is in the Earth’s sky  this reflected light from Earth faintly illuminates the dark portions of the Moon’s face What causes eclipses? Eclipses: when the Moon and Earth cast shadows in sunlight, occurs whe
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