AST 101 Weeks 3 and 4 Notes
AST 101 Weeks 3 and 4 Notes 101
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This 8 page Class Notes was uploaded by Bethany Marsfelder on Wednesday September 21, 2016. The Class Notes belongs to 101 at Syracuse University taught by Professor Walter Freeman in Fall 2016. Since its upload, it has received 148 views. For similar materials see Our Corner of the Universe in Astronomy at Syracuse University.
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Date Created: 09/21/16
September 13, 2016 AST 101 – Professor Freeman Lecture: Understanding the seasons Textbook Pages: 32-38 Lecture Tutorial: 93-98 Brief Tangent: Extraterrestrial life? o Exists and common? o We look for extraterrestrial life by looking for light; both visible and electromagnetic (ex: radio waves, x-rays, infrared) The tilt of Earth’s axis o The Earth’s axis of rotation is not lined up with its orbital axis. o It is tilted by 23.4 degrees. o The axis of rotation changes only very slowly (over millennia.) o Earth’s axis points towards the North Star. Astrology and the Zodiac o Since Earth’s axis wobbles, astrology is right, but was right a very long time ago. o The people who are Capricorns actually were born with their sun in Scorpio. What causes the seasons? o Either we can think of the Earth itself as crooked, or the earth’s orbit is crooked: either way, the slight inclination towards and away from direct sunlight is what causes the seasons. o The axes of revolution and rotation are also tilted. The Sun at different times of year o The Sun is higher or lower in the sky depending on the time of year o The Sun moves westward with respect to the stars: Every solar day, the Sun’s east/west position (azimuth) stays fixed, but the stars move East Every sidereal day, the stars’ position stays fixed, but the Sun moves West There are special names to the points/times in Earth’s orbit where the Earth’s axis is tilted directly toward/away from the Sun: o Summer Solstice/Midsummer (Northern Hemisphere)/June Solstice (Northern Hemisphere) – where the day is longest in the Northern Hemisphere; where the Northern Hemisphere is tipped most directly towards the Sun o Autumnal Equinox/September Equinox – where day and night are of equivalent length; where the Sun shines equally on both hemispheres o Winter Solstice/December Solstice (Northern Hemisphere) – where the day is shortest; where the Southern Hemisphere is tipped most directly towards the Sun o Vernal Equinox/March Equinox – where day and night are of equivalent length; where the Sun shines equally on both hemispheres Generally speaking: o Solstice – where the earth is pointing directly towards or away from the Sun o Equinox – where the earth is pointing neither toward or away from the Sun, directly on the side The tropics – The regions on Earth where the Sun alternates between the northern sky and the southern sky. o Northern boundary – Tropic of Cancer o Southern boundary – Tropic of Capricorn o These occur at 23.4 N/S latitude. o On the June solstice, the Sun reaches its zenith along the Tropic of Cancer. o On the December solstice, the sun reaches the zenith along the Tropic of Capricorn. The Arctic (north) and Antarctic (south) – The regions where the sun either never rises or never sets during part of the year. o North of the Arctic Circle, the sun never rises on the December solstice, and never sets on the June solstice. o South of the Antarctic Circle, the sun never sets on the December solstice, and never rises on the June solstice. o These occur at 66.6 N/S latitude (reached by 90-23.4) September 15, 2016 AST 101 – Professor Freeman Lecture: The Moon and its phases Textbook Pages: 39-42 Lecture Tutorial: 81-88 We now understand motion of the stars, and the combined effects of Earth’s axial tilt, rotation, and orbit have on the seasons. What’s left? o The Moon o The planets o Oddities: comets, meteors, novas, eclipses… The Moon has often been a simple of change, but of regular change: every 29.5 days, the pattern of phases repeats. o This is orderly enough that it is the basis of many calendars: Hebrew calendar Traditional Chinese calendar Babylonian calendar Thai calendar Chinese lunar calendar o But not the traditional calendars of Europe. Everything else in the sky seems to be a constant size and shape, but the Moon waxes and wanes. o Wax – getting bigger o Wane – getting smaller o This is because the Moon does not make its own light; half the Moon is always lit by the Sun, and the “dark side of the moon” is the half not lit by the Sun. Our perspective changes how much of the lit half we see, however. The moon orbits the earth counterclockwise 400,000 kilometers away every 29 days The Moon’s Phases o New Moon – nothing is visible o Crescent Moon – less than a quarter is visible o First Quarter – about a quarter is visible o Half-moon – half is visible o Gibbous/Third Quarter – approximately three quarters is visible o Full moon – all is visible Helpful Diagrams o The below is courtesy of “hmxearthscience.com” o The below is courtesy of “moonphases.info” Summary o The phase of the Moon depends on its position relative to the Sun as it orbits Earth. o The Moon’s phase affects not only its appearance, but also its rise and set times. Rise and Set Times o New Moon Rise: 6 AM Highest: Noon Set: 6 PM o Waxing Crescent Rise: 9 AM Highest: 3 PM Set: 9 PM o First Quarter Rise: noon Highest: 6 PM Set: midnight o Waxing Gibbous Rise: 3 PM Highest: 9 PM Set: 3 AM o Full Moon Rise: 6 PM Highest: midnight Set: 6 AM o Waning Gibbous Rise: 9 PM Highest: 3 AM Set: 9 AM o Third Quarter Rise: midnight Highest: 6 AM Set: noon o Waning Crescent Rise: 3 AM Highest: 9 AM Set: 3 PM o New Moon Rise: 6 AM Highest: noon Set: 6 PM September 20, 2016 AST 101 – Professor Freeman Lecture: Oddballs in the sky: comets, planets, eclipses Textbook Pages: 42-48 Lecture Tutorial: None Example Problem o “What time of day is it when a waxing crescent moon is highest in the sky?” Things to consider: Where is the moon? Where is the observer? What time is it? So far, we’ve talked about the Sun, the Moon, and the stars. What about… o Planets o Comets o Eclipses o Meteors o Novas (to be returned to later) Sometimes some planets appear to go backwards (“retrograde motion”). o This tells us that the celestial sphere model can’t be literally true. Why does it work for everything else? The celestial sphere model works if things appear to only rotate around the Earth? The stars are so far away that only the Earth’s rotation matters The Earth orbits the Sun, so we just pretend that the Sun is on a different sphere turning a bit slower, taking into account both our revolution around it and our rotation The Moon orbits the Earth, so we again put the Moon on a different sphere, turning slower? …but how can we get a sphere to go forwards and backwards, like Jupiter and Mercury do? o The celestial sphere model gets the motion of the planets badly wrong. We’ve long used the immutability of the sky as a symbol for constancy. The cycles of the Sun, Moons, and stars don’t ever change, but some things do o Now we know why these unexpected things happen. Eclipses o During a new moon, the Moon lies roughly between the Earth and the Sun. o However, the moon’s orbit is tilted a bit (5 degrees) so it usually goes over and under the Sun o If the moon passes in front of the sun, you get a solar eclipse o During a full moon, the Earth lies roughly between the Moon and the Sun. o Usually the Earth’s shadow misses the Moon. Sometimes it doesn’t, and you get a lunar eclipse! Some light is refracted by the atmosphere. The blue component is scattered away by the atmosphere; the red component bends and hit the Moon Meteors o Orbits of things in the Solar System are not always circular (or close to it). There are lots of small things in the Solar System, many of which have elongated orbits that sometimes cross ours o Little rocky or metallic bits of matter that orbit the Sun are called meteors o Sometimes they get to Earth and glow as atmospheric heats them o Sometimes they hit the surface and we get chunks of space-slag o Historical cultures sometimes used them as easy access to metal Comets o “Dirty snowballs” whose orbits are highly elongated o Mostly made of ice o When they get close to the Sun, the heat melts bits off of them o This stream of stuff reflects sunlight and makes the comet’s tails o They’re just space-snowballs, but were treated as omens of doom. Terms o Nodes – the two points in each orbit at which the Moon crosses the surface o Umbra – where sunlight is completely bright o Penumbra – surrounds umbra, where sunlight is only partially blocked o Lunar eclipse – occurs when Earth lies directly between the Sun and the Moon, so Earth’s shadow falls on the Moon Total lunar eclipse – if the Sun, the Earth, and the Moon are nearly perfectly aligned, the Moon will pass through the Earth’s umbra. Also called a blood moon. Totality – when the Moon is entirely engulfed in the umbra, usually lasting about an hour. Partial lunar eclipse – if the alignment is somewhat less perfect, only part of the full moon will pass through the umbra, with the rest in the penumbra. Penumbral lunar eclipse – the moon passes only through Earth’s penumbra. o Solar eclipse – occurs when the Mthe eclipse occurs when the Moon is in a part oon lies directly between the Sun and Earth, so the Moon’s shadow falls on Earth Total solar eclipse – if a solar eclipse occurs when the Moon is in a part of its orbit where it is relatively close to Earth, the Moon’s umbra can cover a small area of Earth’s surface. Annular eclipse – when the Moon is in a part of its orbit that puts it farther from Earth, the umbra may not reach Earth’s surface, leading to a ring of sunlight surrounding the Moon in the small region of Earth directly behind the umbra. Partial solar eclipse – in which only part of the Sun is blocked from view. o Eclipse seasons – the two periods in which the nodes of the Moon’s orbit are closely aligned with the Sun. Saros cycle – cycle of about 18 years, 11 1/3 days, caused by the Moon’s lunar phase cycle, in which eclipses recur. o Stellar parallax – the apparent shifting of objects depending on distance, with nearer objects exhibiting greater parallax than more distant objects. Times When Planets Appear o Mercury – infrequently, and only just after sunset or just before sunrise o Venus – often shines brightly in the early evening in the west or before dawn in the east o Mars – often recognizable by its reddish color o Jupiter – when visible, is brightest object in the sky besides the Moon and Venus o Saturn – easy to see but is as bright as many other stars