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Chapter 2

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John Lester

Astronomy Chapter 2 Discovering the Universe for Yourself Constellations: region in the sky with well defined borders; the familiar patterns of stars merely help us locate these constellations.  88 names were chosen by the International Astronomical Union.  Every point in the sky belongs to some constellation.  They look to be like they are close to each other, but the truth is that they are at very different distances (we lack depth perception when we look into space. o Ancient Greeks took this illusion for reality  imagined that the stars and constellations lie on a celestial sphere. Celestial Sphere – the imaginary sphere in which objects in the sky appear to reside when observed from earth.  Allows us to mp the sky as seen from Earth.  Identify the following special features on the celestial sphere: o North Celestial Sphere – point directly over Earth’s North Pole. o South Celestial Sphere – point directly over Earth’s South Pole. o Celestial Equator – projection of Earth’s equator into space, makes a complete circle around the celestial sphere. o Ecliptic – the path the Sun follows as it appears to circle around the celestial sphere once each year. It crosses the celestial equator at 23.5 degree, due to the tilt of Earth’s axis. The Milky Way: - Band of Light – circles around the celestial sphere, passing through more than a dozen constellations, and bears an important relationship to the Milky way Galaxy:  It traces our galaxy’s disk of stars – the galactic plane – as it appears from our location in the outskirts of the galaxy. - In all directions when we look within the pancake, we see countless stars and clouds that make the milky way at night; that is why the band of light makes a full circle around our sky.  The milky way looks somewhat wider in the direction of Sagittarius because when we look that way, we are looking at the galaxies central bulge.  When we look away from the galactic plane (direction where there are few stars and clouds) we see that we have a clear view of the distant universe. - The dark lanes of the Milky way contain the densest clouds  obscuring our view of stars behind them.  Generally preventing us from seeing more than a few thousand of light years into the galaxies disk.  Most of our galaxy was hidden until new technology allowed us to peer though the clouds. Astronomy Chapter 2 Discovering the Universe for Yourself The local sky: the sky as seen from wherever you are standing – appears to be in the shape of a hemisphere or a dome. - The dome shape rises from the fact that we see only half of the celestial sphere at any particular moment from any particular location, while the ground blocks the other half of our view. - Features of the local sky:  Horizon – boundary between the Earth and sky (altitude = 0 degrees)  Zenith – point directly overhead (altitude = 90 degrees).  Meridian – imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north. o We can pinpoint position of any object in the local sky by stating its direction along the horizon and its altitude above the horizon. o Zenith has altitude of 90 degrees but no direction because it is straight overhead. Angular Sizes and Distances: Angular size – the angle of an object as it appears to span in your field of view.  The angular size alone doesn’t tell us the object’s true size, because angular size also depends of the distance.  The farther away an object, the smaller it’s angular size. Angular Distance – distance between a pair of objects in the sky is the angle that appears to separate them. Why do stars rise and set? - Ancients had it all wrong (universe revolves around Earth): The Earth rotates, not the rest of the universe, which is why the Sun, Moon, Planets and stars all move across our sky each day. - Celestial sphere makes a full circle around earth each day, however, the motion looks more complex in the local sky, because the horizon cuts the celestial sphere in half. - Key facts about the paths of various stars through the local sky:  Stars near the north celestial pole do not rise or set, rather they remain above the horizon, and make counterclockwise circles around the north celestial pole. We say that stars are circumpolar.  Stars 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. Because Earth rotates from west to east (counterclockwise as viewed from above the North Pole), these stars appear to rise in the east and set in the west. - The north celestial pole lies at the center of these of the circumpolar stars. The circles grow farther from the north celestial pole , and if they are large enough, the circles cross the horizon and rise in the east and set in the west.  Vice Versa for south celestial pole. Astronomy Chapter 2 Discovering the Universe for Yourself Variation with Latitude: Latitude – measures north-south position on Earth Longitude – measures the east-west position. - Latitude can be defined as 0 degrees from equator and goes up to 90 degrees North at the north pole. - Longitude is defined to be passing by Greenwich, England.  We can pinpoint location from these two (ex. Miami). - Latitude affects the constellations because it affects the locations for the horizon and the zenith relative to the celestial sphere. NOTE: the local sky varies with latitude, not longitude.  Example: Sydney, Australia. - At the North Pole, you can only see objects that lie on the northern half of the celestial sphere, and they are all circumpolar. That is why the Sun remains above the horizon for 6 months at the North Pole: The Sun lies north of the celestial equator for half of each year (see the yellow dots), so during these 6 months, it circles the sky at the North Pole just like a circumpolar star. - Altitude of the celestial pole is equal to the your latitude.  Finding the north celestial pole is fairly easy because it lies very close to the star Polaris (North Star). Southern Hemisphere, you can find the south celestial pole by the aid of the Southern Cross. Variation with Time of Year: - Night sky changes throughout the year because of Earth’s changing position in it’s orbit around the Sun. As earth orbits, the Sun appears to move steadily eastward along the ecliptic, with the stars of different constellations in the background at different times of year.  Constellations along the ecliptic make up what we call the zodiac. - The Sun’s apparent location on the ecliptic determines what constellations we see at night.  For example, the Sun appears to be in Leo in late August. We therefore cannot see Leo at this time (because it is in our daytime sky), but we can see Aquarius all night long because of its location opposite Leo on the celestial sphere. Six months later, in February, we see Leo at night while Aquarius is above the horizon only in the daytime. Causes for seasons: Why do we have seasons?  The tilt of Earth’s axis causes sunlight to fall differently on Earth at different times of year.  In figure: it shows the earth is tipped towards Polaris throughout the year. As a result the orientation of the axis relative to the Sun changes over the course of each orbit. Astronomy Chapter 2 Discovering the Universe for Yourself o The Northern Hemisphere is tipped towards the Sun in June and away from the Sun in December  vice versa for Southern Hemisphere, experience opposite seasons). o Step 2 shows Earth in June, when axis tilt causes sunlight to strike the Northern Hemisphere at a steeper angle and the Southern Hemisphere at a shallower angle. The steeper sunlight angle makes it summer in the Northern Hemisphere for two reasons. First, as shown in the zoom-out, the steeper angle means that the sunlight hitting Earth is more concentrated, which tends to make it warmer. Second, if you visualize what happens as Earth rotates each day, you’ll see that the steeper angle also means that the Sun follows a longer and higher path through the sky, giving the Northern Hemisphere more hours of daylight during which it is warmed by the Sun. The opposite is true for the Southern Hemisphere at this time: The shallower sunlight angle makes it winter there because sunlight is less concentrated and the Sun follows a shorter, lower path through the sky. o Step 3 shows that both hemispheres are illuminated equally in March and September. It is therefore spring for the hemisphere that is on the way from winter to summer, and fall for the hemisphere on the way from summer to winter. - Earth is only 3% farther from the sun at its farthest point than at its nearest. - If Earth did not have an axis tilt, we would have no seasons. Four Special Positions in Earth’s Orbit: Solstice – when sunlight becomes extreme for the two hemispheres. Equinox – when the hemispheres are equally illuminated. - Summer (June) solstice: when the northern hemisphere is tipped most directly towards the sun and southern hemisphere is tipped most directly away from it. - Winter (December) solstice: when the Northern hemisphere is tipped most directly away from the sun and the southern hemisphere is tipped most directly towards it. - Spring (March) equinox: when the Northern hemisphere goes from being tipped slightly away from the sun to slightly towards the sun. - Fall (September) equinox: when the Northern hemisphere first starts to be tipped away from the sun.  Modern calendar includes leap years to keep solstices and equinoxes around the same dates (add a day in February) every fourth year.  Ancient structures such as Stonehenge in England and The Sun Dagger in New Mexico were used to observe solstices and equinoxes.  The sun rises precisely due east and sets west only on days of the spring and fall equinoxes. Astronomy Chapter 2 Discovering the Universe for Yourself  Summer and Winter solstices – occurs on day when the sun follows the longest and highest path through the northern hemisphere sky (more than any other day of the year). First day of the Seasons: - Summer solstices occur when
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