Chapter 2 - Discovering the Universe for Yourself

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Astronomy & Astrophysics
Michael Reid

Notes – 2 2.1 Patterns in the Night Sky Universe from Earth - More than 2000 stars are visible - The Milky Way is visible - As well we can observe patterns of stars Constellations - This is a region of the sky with well-defined borders - 88 official constellations cover the celestial sphere - Every star belongs to a constellation - The IAU members who chose the names for the constellations were mainly American or European, therefore the constellation names in the Northern Hemisphere: ancient Middle Eastern and the constellation names in the Southern Hemisphere: 17 century European explorers Celestial Sphere - Stars in constellation are farther apart than they appear as they are located at various distances from Earth - This illusion occurs because we lack depth perception as the stars are so far away - The Greeks believed this illusion for reality and imagined all the stars and constellations to lie on the great Celestial Sphere that surrounded earth. - Although it is an illusion it is very useful as its allows us to map the sky as seen from earth - Now we can identify these: The North Celestial Pole – the point directly over Earth’s North Pole The South Celestial Pole – the point directly over Earth’s South Pole The Celestial Equator – a projection of Earth’s equator into space, makes a complete circle around the celestial sphere The Ecliptic – the path which the sun follows as it appear to circle around the celestial sphere once each year (crosses equator at a 23 ½ angle. The Milky Way - Band of light, circles all the way around the celestial sphere - When we look out of the galactic plane we have a clear view of the universe - When we look in any direction into the galactic plane, we see the stars and interstellar clouds that make up the Milky Way The local sky - The sky as seen from wherever you happen to be standing - Appears to take the shape of a hemisphere of dome - This dome shape arises from the fact that we can only see half of the celestial sphere at any given time from any particular location – the ground blocks the other half from view. - How we can describe the position of an object in the sky  Horizon – boundary between Earth and the sky  Zenith – point directly overhead  Meridian – imaginary half circle stretching from the horizon due South, through the zenith, to the horizon due North.  We can pinpoint an objects position by stating its direction along the horizon and its altitude above the horizon. Angular Sizes and Distances - Our lack of depth perception – we have no way to judge the true sizes or separations of objects in the sky - However we can describe the angular sizes and separations without knowing how far they are - The angular size – the angle it appears to span in your field of view - Angular size does not tell us an objects true size because this depends on the distances - The angular distance – the angle that appears to separate two objects in the sky Why do stars rise and set? - The Earth rotates daily (not the rest of the universe despite popular ancient belief) - Although it appears the stars are rotating around us, the motion is more complex in the local sky, because the horizon cuts the celestial sphere in half - Rules: a. Stars near the north celestial pole are circumpolar: they remain perpetually above the horizon, circling (counterclockwise) around the north celestial pole each day b. Stars near the south celestial pole never rise above the horizon at all c. All other stars have daily circles that are partly above the horizon and partly below it, which means they appear to rise in the east and set in the west Why do the constellations we see depend on latitude and time of year? Variation with Latitude - Latitude: measures north-south position on Earth (0 degrees at equator) - Longitude: measures east-west position (0 degrees along the prime meridian) - Stating both latitude and longitude pinpoints a location on earth - Latitude (but not longitude) affects the constellations we see because it affects the location of the horizon and zenith relative to the celestial sphere. - The altitude of the celestial pole in your sky is equal to your altitude - ????? Variation with Time of Year * - The night sky also changes due to earths changing position around the sun - The suns apparent location along the ecliptic determines which constellations we see at night 2.2 The Reason for Seasons The combination of earth’s rotation and orbit also leads to the progression of the seasons What causes the seasons? - The tilt of earth’s axis causes the season. The axis points in the same direction throughout the year, so as Earth orbits the Sun, sunlight hits different parts of the Earth more directly as different times of year. Solstices and Equinoxes *review figure 2.16 and 2.17 - To help mark the changing seasons, we have determined four special moments in the year each corresponding to one of the four special positions in Earth’s orbit - Exact date and times of these vary every year* 1. Summer (June) solstice When the Northern Hemisphere is tipped most directly toward the Sun and receives the most direct sunlight 2. Winter (December) solstice When the Northern Hemisphere receives the least direct sunlight 3. Spring (March) equinox The moment when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly towards the Sun 4. Fall (September) equinox When the Northern Hemisphere first starts to be tipped away from the Sun Why Orbital Distance Doesn’t Affect Our Season - Seasons aren’t caused by Earth’s varying distance from the Sun - Still we may expect the varying distance to play at least a small role - E.g. the Northern Hemisphere has winter when Earth is closer to the Sun and summer when earth is farther – thus we may expect the Northern Hemisphere to have more moderate climates - But…it does not – rather the seasons are more extreme in the Northern Hemisphere due to the fact that it has less bodies of water than the Southern Hemisphere (Bodies of water is much harder to heat than land) - Although distance from the sun doesn’t play a role in terms of Earth, it does in terms of other planets How does the orientation of Earth’s axis change with time? Precession – a gradual wobble that alters the orientation of the Earth’s axis in space Caused: by gravity’s effect on a titled, rotating object (the gravitational tugs from the Sun and Moon)
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