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General Astronomy Astronomical Observations

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Angles and Angular Measurement Remember there are: 360° in a circle 60' in a degree 60" in a minute Or 2Π radians in a circle 60 min to an Hour 60 sec to a minute To try to keep confusion to a minimum, sometimes seconds refering to angular measurement is designated as arcsec (arc – seconds) Also,

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Getting a grip on Angles Size of person is 5' 6" Angle: 90º Conversational Distance Distance:3' 6" Angle: 10º Across the Room Distance:31' 6" Angle: 1º A football field Distance:100 yds Angle: 1' Distance:3.5 miles Angle: 1" Distance:215 miles

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Angular Separation These two stars have an angular separation of 11' 49" Being able to see both stars is a test of "perfect" vision

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Rules of 'Thumb' It is difficult to measure the distances to the stars – as we will see later on in the course, but it is relatively easy to measure the angles between objects and between the horizon and an object. Even when 'just stargazing' it is common to hear directions such as "find the first two stars in ---- then go 30° to find ---" Some rough estimates are: Using an outstreached arm, Thumb 1° Two knuckles 2° Fist10° Extended hand 25°

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Rules of 'Thumb'

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Angular Size

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Instruments for Angles British Navy Sextant Circa th century Quadrant used in navigation

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True versus Apparent Size The Sun has an angular size of about 30' of arc. And it appears to be about the size of a quarter as we view it. We can relate the angular size of an object to its true size if we know the distance to the object Angular Size = True Size Distance Radians = 180 True Size ΠDistance Degrees

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Examples Lunar Angular Diameter: Angular Diameter = Km Π Km =0.517 ° =31.2 arcminutes Solar Angular Diameter: 149.6x10 6 Km Angular Diameter = x10 6 Km Π =0.532 ° =31.9 arcminutes Even though the Sun is much larger than the moon the distances are such that they subtend nearly the same angle.

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Review: Measuring Distance Miles/Kilometers Distances on the surface of a object Astronomical Unit (AU) Distances within the Solar System 1 AU = 93,000,000 Miles Lightyear (Ly) Distances to nearby stars and other objects 1 Ly = 65,000 AU = 6,000,000,000,000 Miles Parsec (pc) Distances in the local neighborhood 1 pc = 3.26 Ly Megaparsec (Mpc) Distances to distant galaxies 1 Mpc = 1,000,000 pc

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Review: Brightness Apparent Magnitude How bright does it appear? Absolute Magnitude How bright would it appear from 10 pc? Sometimes either magnitude may be further identified as visual (M V ) or photographic (M B )

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Finding our way Before we can find our way amongst the stars, it would be good to find our way here on Earth. Where are you? This may not help much) We need to precisely define our position on the surface of the earth (airplanes and submarines also need position with respect to the surface) Москва ירושלים القدس

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Location, Location, Location Let's take a look at: 1.The shape of the Earth 2.Zenith & Nadir 3.Meridian 4.Equator 5.Latitude 6.Longitude

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The Shape of the Earth This is close, but it's really more 'pear shaped' Flat? A disk? Where's the elephants and the great turtle? A sphere?

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Defining the Earth The North and South Poles The Parallels of Latitude The Equator The Meridians of Longitude The Prime Meridian The International Date Line Your position: 1.Zenith … The point above your head 2.Nadir … The point beneath your feet 3.Meridian … The line over your head and the poles

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The Earth Reference System L 39° 33' 09 N 074 ° 29' 08 W

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So Where is this? The Taylor Observatory Latitude 39° 33' 09 N Longitude 074 ° 29' 08 W

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39º 55N 116º 24E 37º 58N 023º 43E Москва 55º 45N 037º 27E Finding our way القدس ירושלים 31 º 47N 035 º 13E New York 40º 40N 073º 56W London 51 º 30 26N 000º 07 39W Now its easy to see that there are two the same…

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Finding our way in the Night Sky The Celestial Sphere 1.A projection of the Earth's coordinates onto the sky 2.The poles are extended to become the celestial poles 3.The equator is projected to become the celestial equator 4.The Latitude lines (parallels) are projected onto the celestial sphere and given the name 'Declination' 5.The Longitude lines (meridians) are projected out and are now called 'Right Ascension'

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The Celestial Sphere The North Celestial Pole appears to be near a star, Polaris. As the evening passes, the stars appear to rotate clockwise about Polaris. For a given latitude of an observer, some stars never set - these are known as circumpolar stars If you were at the North Pole, Polaris would be nearly on your zenith and the motion of the stars would be parallel to the horizon. If you were at the Equator, Polaris would be on the horizon; The stars would appear to move vertically: "up" to the East, "down" to the West

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The Celestial Sphere

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Star Trails Polar

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The motion of the stars as seen from the North Pole

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Star Trails: Equatorial

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The motion of the stars as seen from the Equator

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The Celestial Coordinates Declination 1.Measured in degrees 2.'+' or '-' from the celestial equator Right Ascension –Measured in hours, minutes and seconds –From 0 h 0 m 0 s to 23 h 59 m s

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Celestial Coordinates Right Ascension Declination For convenience, stars are assumed to be fixed to the celestial sphere and can be located on the coordinate chart:

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Using the Coordinates Alkaid 13 h 47 m 42 s +49º 17' 20" Merak 11 h 02 m 01 s +56º 21' 52" Dubhe 11 h 03 m 55 s +61º 43' 58"

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Meridian & Right Ascension

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