Study Guides (248,019)
Canada (121,233)
Astronomy (142)
AS101 (137)

AS 101 Midterm Review Notes.docx

13 Pages
Unlock Document

Victor Arora

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
More Less

Related notes for AS101

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.