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Finding Your Way Around the Night Sky. Constellations & The Celestial Sphere. Every night there are an abundance of interesting objects to look at in the sky Whether you are an amateur astronomer or a professional, you have to know how to find objects from one night to the next.
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Finding Your Way Around the Night Sky Constellations & The Celestial Sphere Every night there are an abundance of interesting objects to look at in the sky Whether you are an amateur astronomer or a professional, you have to know how to find objects from one night to the next
Relative Positions - The Constellations • Constellations are simply groups of stars in the sky that appear to form a pattern • Ancient cultures recognized these patterns from season to season • Many constellations have various names from different cultures • The Big Dipper is also known as: • The Wagon or Plough (Western Europe) • The Great Bear (Greek) • The Leg of an Ox (Egyptian) • A Stag (Siberian) • A Funeral Procession (Native American)
Orion - Visible in Winter Betelgeuse Rigel
Constellations • Constellations serve many purposes • They mark the seasons • Orion in winter, the Summer Triangle in summer • They serve as relative landmarks for finding stars • Analogy: Which is easier? • Joe's house is in Atlanta • Joe's house right next to the Big Chicken in Marietta • Same applies to constellations • Betelgeuse is that red star ... right there (points) • Betelgeuse is the star that makes Orion's right shoulder • They make for good storytelling!
Constellations • There are a total of 88 constellations in the sky (most of them relatively unknown) • The twelve signs of the Zodiac are important constellations because they lie in the path of the Sun through the sky (more on this in a sec) • The brightest stars in the sky are given names based on their constellation • The brightest star in a constellation is (usually) called Alpha _____, the second brightest Beta ______, and so on • For example, Betelgeuse is also known as Alpha Orionis, Rigel is also known as Beta Orionis • Remember Alpha Centauri, the nearest star system? It is the brightest star in the constellation Centauris
Constellations and The Celestial Sphere • Always remember that how we see the stars on the sky has nothing to do with how they are grouped in space • We have no depth perception when looking at the sky Stars in a constellation that appear to be right next to each other on the sky can actually be 100's or even 1,000's of light years away from each other.
The Celestial Sphere • The Celestial Sphere is a simplistic way of thinking about the stars in the sky • Looking up at the stars over the course of a night, it appears simply as a bunch of fixed points of light, moving uniformly across the sky • One can think of this as the Earth being in the middle of a huge ball or sphere with lots of little lights painted on the inside • As the sphere spins, we see the stars move across the sky
Reality Check • ALWAYS REMEMBER: it is the Earth's rotation that makes the stars, Sun, Moon, and planets move from east to west in the sky • ALWAYS REMEMBER: each point of light is actually a huge star, all of which are at varying distances from us • The Celestial Sphere is just a very handy way of thinking about the sky and is very useful for finding a specific object in the sky
Earth Coordinates Review • Lines of longitude are measured east-west around the globe relative to the prime meridian (Greenwich, England) • 0-180o east or west of the prime meridian • Lines of latitude are measured north-south on the globe relative to the equator • 0-90o north or south of the equator • For example • Atlanta is at: • 84o 23' 16.8" W • 33o 44' 56.4" S
Celestial Coordinates • Right ascension (RA) measures east-west on the celestial sphere • Measured in hours, minutes, seconds since one rotation of the celestial sphere is equal to one sidereal day • Remember, the four minute difference between sidereal and solar day will cause stars to not appear at the same exact time and place every night • Declination (DEC) measures north-south of the celestial equator • Measured in degrees, minutes, seconds like latitude • For example, Betelgeuse is at: • RA: 05h 55m 10.3s • DEC: 07o 24' 25.4" RA: 00h 00m 00s to 23h 59m 59s DEC: +90o (north) to -90o (south)
The Ecliptic • Because of the tilt of the Earth's rotation axis, the path of the Sun is tilted relative to the celestial equator • The path that the Sun takes through the sky is called the ecliptic • All of the planets (except for Pluto) also follow the ecliptic through the sky
Solstices & Equinoxes • There are four special positions along the ecliptic • The summer solstice is when the sun is highest in the sky for the northern hemisphere • The winter solstice happens when the sun is lowest in the sky for the northern hemisphere • The vernal (spring) and autumnal (fall) equinox occur when the sun is crossing the celestial equator
What's Your Sign? - The Zodiac • The ecliptic (and therefore the Sun) passes through the twelve signs of the Zodiac • Your birth sign roughly corresponds to the when the Sun is located in your constellation • Notice you cannot see your sign when it is your birth month, since the Sun outshines everything else • This is about all astrology and astronomy have in common, no matter what my mom thinks
Observing the Heavens • At any given time, half of the celestial sphere is visible to you • The point directly overhead in the sky is called the zenith • The edge of the visible sky (where the sky meets the ground) is called the horizon • An object's altitude is the vertical angle between the horizon and the object • The zenith distance is the angle between the zenith and the object • Note: 90o - altitude = zenith distance
Observing the Heavens • All objects in the sky (the Sun, the Moon, the planets, the star, everything) RISE IN THE EAST and SET IN THE WEST • Not all objects stay in the sky as long as others, due to the tilt of the Earth's axis and your position on the surface • Circumpolar stars are stars near the north celestial pole that neither rise or set