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Midterm

AS 101 Midterm Review Notes.docx

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Department
Astronomy
Course
AS101
Professor
Victor Arora
Semester
Winter

Description
What is our place in the Cosmos? The Cosmos  Mostly vacuum (empty space)  Everything we can possibly see is a tiny fraction  Why does it seem like there are so many objects in space The Sun is Our Star  Massive ball of glowing gas that generates energy through nuclear fusion  100x as wide as the Earth  Source of almost all energy on Earth Planets are Less Massive than Stars  Non-luminous and spherical  In orbit around a star  “cleared the neighbourhood” of other objects Exoplanets  Over 600 planets confirmed to be orbiting other stars  Thousands of “candidate” exoplanets, observed by the Kepler Space Telescope Some Planets have Satellites  An object in orbit around a planet  Natural satellites are also known as “moons” Other Objects in the Solar System  Asteroid: a small, rocky, object orbiting a star  Comet: a small, icy object that orbits the sun Galaxies: “Cities” of the Cosmos  A large system of stars, dark matter, gas, and dust, all bound together by their combined gravity Nebulae  Clouds of gas and/or “dust”  Raw materials for new stars from previous generations Star Clusters  Open clusters: 1000s of stars  Globular Clusters: 100000s of stars  Our Sun probably formed in an open star cluster but has since “moved out” Galaxy Groups and Clusters  A group of galaxies (a few dozen up to thousands) all held together gravitationally  Many clusters form a supercluster which make up the cosmic web Models in Science  In general, a model is something that represents reality  A scientific model is a hypothesis that describes reality and has withstood observational or experimental tests  We can also develop conceptual models to help us think about how nature works  Scale Models: miniature or shrunk down versions o How to the sizes of the planets compare to the distances between them? o What if we shrunk the solar system by a factor of about 60 billion?  Sun -> radius=12mm  Earth -> radius=0.1mm ; at this scale Earth is 3m away  Jupiter -> radius=1mm ; at this scale Jupiter is 17m away  Neptune -> radius=0.4mm ; at this scale Neptune is 100m away o The sizes of the planets are tiny compared to the distances between them The Astronomical Unit  We can specify distances in the solar system by comparing them to the average Earth-Sun distance o 1 AU= 150, 000, 000 km OR 150, 000,000,000 m  A planet twice as far from the Sun would be 2 AU away Representing Large Numbers  The Earth is on average 150,000,000,000m from the Sun  In Scientific Notation we would write this as a coefficient between one and ten, multiplied by the appropriate power of ten  Hint: just count the number of places the decimal would move… o 150, 000, 000, 000 m= 1.5 x 10 m Doing Math with Exponents  To multiply two (or more) numbers written in scientific notation, just o Multiply the coefficients (if there are any) o Add the exponents How far away are the Stars? 13  The nearest star system is Alpha Centauri: 4.1 x 10 km  Using our scale from our model solar system: 60 billion actual km=1 scale km…puts Alpha Centauri at 683 km away, in NYC Light-Travel and Look-Back Time  BUT light has a finite speed: 300,000 km/s  In other words, this star is so far away that we are seeing it as it looked in the past  The further an object appears to be in space, the further we are looking back in time 13  A light-year, is the distance light travels in one year, about 10 km (Ten trillion km)  So Alpha Centauri is about 4 light years away Some Distances in the Universe  The Sun: about 8 light minutes  Alpha Centauri: 4 light-years  Andromeda Galaxy: 2 million light-years  Observable Universe: 13.7 billion light-years Putting it in Perspective  What if the entire age of the Universe were one calendar year? o All of recorded history happens on the last day of the year, 30 seconds before midnight o The Egyptian pyramids were built about 11 seconds ago o Copernicus and others convinced humanity that the Earth orbits the Sun about 1 second ago o You were born about 0.04 seconds ago (assuming your age is 18) How the Earth is Moving  Two basic motions: o Revolves around (“orbits”) the Sun o Rotates on its axis  Which motion is responsible for rising and setting of the Sun? o Rotation is responsible for the rising and setting of the Sun The Rotating Earth and You!  The Sun… rises in the east; and sets in the west  To a moving observer, stationary objects appear to move in the opposite direction as the observer’s motion  The Earth rotates from west to east (rotates counterclockwise when viewed from the North Pole) Celestial Sphere: A Conceptual Model of the Sky  An imaginary sphere of very large radius surrounding the Earth  Objects appear to be attached to the rotating celestial sphere  Are the stars really moving? o No the planets are moving making it seem like the stars are moving  We can see exactly one-half of the entire sky at any given time, from horizon up to the zenith (directly overhead)  The North and South Celestial Poles are directly above the Earth’s North and South poles  The Meridian is the line passing through the North and South points on the horizon, and the Zenith Starry Night Simulation  In the Northern Hemisphere, stars that never appear to set are located near the ___________. These are called circumpolar stars because they appear to circle the pole  The North Celestial Pole is near the star Polaris  Stars reach their highest points when they cross the ___________ Terrestrial Coordinates  We specify locations on Earth using latitude and longitude  Longitude: in degrees East or West of the Prime Meridian  Latitude: in degrees North of South of the Equator  On Earth, intersecting lines of longitude and latitude form a grid  Imagine this grid projected outward onto the celestial sphere Celestial Coordinates  We specify these positions in the sky using Right Ascension (RA) and Declination (Dec)  Right Ascension: in hours East of the Vernal Equinox (from 0h to 24h)  Declination: in degrees North (+) or South (-) or the Celestial Equator (from +90 to -90 ) Latitude Affects What You Can See  Example: you are sitting at the North Pole  Recall: the NCP is directly above the North Pole o So at the North Pole the NCP is at the Zenith and has an altitude of 90 N o This is the same as the latitude of the observer  Example: you are sitting on the equator o The altitude of the NCP is equal to your latitude, so it is on the horizon (what about the SCP?) o Precession: A Slow Wobble  Precession: a slow but regular wobble of the Earth’s axis  The Sun and Moon are unevenly pulling on the slight (43 km) bulge of around the Earth’s equator  This causes a very slow but regular wobble of the Earth’s axis o The orientation of the axis changes but it remains tilted at 23.5 Precession changes the NCP  This very slowly changes the position of the NCP, and all the other coordinates  Catalogues of stellar coordinates are slightly adjusted only every 50 years The Constellations: Then & Now  Then: the constellations are arbitrary patterns of stars invented, mostly, by ancient cultures  Today: astronomers officially recognize 88 constellations  The sky is divided into parcels that each contain the constellation and other stars or objects in the same part of the sky Stars in the Constellations  The stars in a constellation are not at the same distance from the Earth, just in the same direction o Analogy: sometimes you see a light in the sky and you believe it is a star but as it gets closer you realize it’s a plane Measuring Angular Distances  You can measure the angular sizes and separations on the celestial sphere using degrees o Angular size of an object deoreases the farther away you are separated from it  The Meridian spans a distance of 180  Rule of pinky: pinky held at arms length=1 o  Rule of fist: fist held at armlength= 10 Measuring Apparent Brightness  Astronomers sometimes specify how bright an object would appear to the naked eye using the magnitude scale  For historical reasons, the scale runs backwards  Our eyes are non-linear detectors: each step is a factor of 2.5x brighter  Professional astronomers measure the ”flux” (rate of light energy collected per unit area)  If we don’t know how far away the star is, its apparent brightness does not tell us about its true (absolute) power output  Sirius is the brightest star in the night sky. Venus is the brightest object I the sky, however it is a planet not a star. Polaris is the 48 brightest star in the sky Sidereal and Solar Days  Recall: the two basic motions of the Earth are revolution and rotation  There are two kinds of days: o Solar Day- the average time between successive Meridian crossings for the Sun (24h) o Sidereal Day- the time between successive Meridian crossings for any other star ( 23h 56m)  The Earth has moved along its orbit so its takes a bit longer for the observer to line up with the Sun again Yearly Variation  The slight difference means that the stars rise about 4 min earlier each night  Over the course of a year, the constellations we see at night time change  The underlying cause of the change is the Earth’s revolution around the Sun Clarification: Altitude vs. Declination  The angular distance of a star from the horizon is its altitude o 0 at horizon, +90 at the Zenith o A star’s altitude varies as the Earth rotates  The Celestial Sphere (and the stars) appear to revolve around the Earth **The Zenith, Horizon, and Meridian do not move for the observer  The angular distance of a star from the celestial equator is its declination o Declination of a star is fixed, like the latitude of a location on Earth, but its altitude is always changing o The stars do not appear to move relative to one another over the course of a night Latitude affects what you can see  Example: you are sitting at the North Pole  Recall: the NCP is directly above the North Pole o So at the North Pole, the NCP is at the Zenith and has an altitude of 90 N o This is the same as the latitude of the observer Consider the Orbit of the Earth  The Earth’s orbit is nearly-circular  The distance from the Sun varies by only a few percent through the year  Since the Earth’s orbit is pretty much a circle, distance from the Sun does not cause the seasons Annual Motion of the Earth  The Earth revolves once around the Sun in 365 days and remains tilted at 23.5  The ecliptic is the plane of the Earth’s orbit around the sun  The amount of solar energy received in either hemisphere varies through the year due to the tilt Northern Summer Solstice: 6/21I  Northern Hemisphere: sunlight hits the surface at a nearly perpendicular angle o More concentrated energy o Start of summer  Southern Hemisphere: sunlight hits the surface at a grazing angle o Energy is spread out o Start of winter Northern Winter Solstice: 12/21  Northern Hemisphere: sunlight hits the surface at a grazing angle o Energy spreads out o Start of winter  Southern Hemisphere: sunlight hits the surface at a nearly perpendicular angle o More concentrated energy o Start of summer The Equinoxes: 3/21 or 9/21  Days with roughly equal amounts of daylight and darkness, everywhere on Earth  Mark the beginning of Spring (Vernal) or Fall (Autumnal) Apparent Annual Motion of Sun  The Sun moves Eastward with respect to the “background stars”  We can also think of the ecliptic as the apparent yearly path of the Sun amongst the stars  The Sun spends half the year north of the Celestial Equator (CE) and half the year south of the CE o Vernal Equinox: Sun crosses CE heading North o Summer Solstice: Sun at furthest point north from CE o Autumnal Equinox: Sun crosses CE heading South o Winter Solstice: Sun at furthest point south from CE Length of Days  The Sun rises and sets at different times through the year  In the Northern Hemisphere, there are more daylight hours when the Sun is further North  More daylight hours in the summer means more energy from the Sun is received at the surface per day The Earth-Moon System  The Moon orbits the Earth in a slightly elliptical path once in approximately one month How does the Moon shine?  The Moon shines by reflected light from the Sun  Only half of the spherical Moon is illuminated at any given time (just like the Earth) The Moon Rotates  This must be the case since we only ever see the same side of the Moon o There is no permanently “dark side” of the Moon, only a near-side and a far-side  The Moon’s librations allow us to see 59% of the surface over the course of a lunar cycle The Inconstant Moon  A different fraction of the daytime half of the moon is visible on Earth at different points in its orbit-these are the phases of the moon Primary Phases  New Moon  First Quarter  Full Moon  Last Quarter Moonrise and Moonset  The Moon rises at the same time that the Sun sets-What is its phase? o New: rise and set with the Sun o Waxing: rise before sunset, set before sunrise o Full: rise at sunset, set at sunrise o Waning: rise after sunset, set after sunrise Earth-Moon System & the Sun  Both orbits lie in almost the same three-dimensional plane  Lunar orbit is inclined by 5 to the ecliptic Shadows of the Earth and Moon  The shadows of the Earth and Moon consist of an umbra and penumbra  If, however, the Earth, Moon, and Sun are precisely aligned, the results can be spectacular Lunar Eclipses  Moon and Sun are on opposite sides, with Earth in middle  The Earth’s shadow falls on the moon  Types: penumbral, partial umbral, or total o Total Solar Eclipse: the Moon is directly between Earth and Sun; happens to be same angular size as the Sun Solar Eclipses  The eclipse is visible where the shadow of the Moon falls on the Earth  The moon is on a slightly elliptical orbit -> when it is further from the Earth, the eclipse is annular  But all eclipses will be annual (or partial) in the distant future Frequency of Eclipses  Why aren’t there eclipses in every lunar phase cycle? o An eclipse can only occur when the Moon is crossing the ecliptic and has the right phase  Why is it so much easier to witness a lunar eclipse than a solar eclipse? o The shadow of the Earth on the Moon is much larger than the Moon on the Earth o A lunar eclipse is visible on the entire night-side of the Earth; a Solar Eclipse is visible along narrow tracks Scientific
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