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General Astronomy Astronomical Observations 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 Also, 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) Getting a grip on Angles Size of person is 5' 6" Angle: 90º Distance: Conversational Distance 3' 6" Angle: 10º Distance: Across the Room 31' 6" Angle: 1º Distance: A football field 100 yds Angle: 1' Distance: 3.5 miles Angle: 1" Distance: 215 miles Angular Separation These two stars have an angular separation of 11' 49" Being able to see both stars is a test of "perfect" vision 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 Two knuckles Fist Extended hand 1° 2° 10° 25° Rules of 'Thumb' Angular Size Instruments for Angles 16th century Quadrant used in navigation British Navy Sextant Circa 1840 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 True Size Angular Size = Distance Radians = 180 Π True Size Distance Degrees Examples Lunar Angular Diameter: Angular Diameter = 180 Π 3475 Km = 0.517° = 31.2 arcminutes 385000 Km Solar Angular Diameter: 180 Angular Diameter = Π 1.39x106 Km = 0.532° = 31.9 arcminutes 6 149.6x10 Km Even though the Sun is much larger than the moon the distances are such that they subtend nearly the same angle. • • • 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) • Megaparsec (Mpc) Distances in the ‘local neighborhood’ 1 pc = 3.26 Ly Distances to distant galaxies 1 Mpc = 1,000,000 pc 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 (MV) or photographic (MB) 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? AQHNA (This may not help much) Москва ירושלים القدس 北京 We need to precisely define our position on the surface of the earth (airplanes and submarines also need position with respect to the surface) Location, Location, Location Let's 1. 2. 3. 4. 5. 6. take a look at: The shape of the Earth Zenith & Nadir Meridian Equator Latitude Longitude The Shape of the Earth Flat? A disk? Where's the elephants and the great turtle? http://fxb.worth1000.com/entries/396292/great-a-tuin A sphere? This is close, but it's really more 'pear shaped' 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 The Earth Reference System L 39° 33' 09“ N 074 ° 29' 08“ W So Where is this? The Taylor Observatory Latitude 39° 33' 09“ N Longitude 074 ° 29' 08“ W Finding our way Now it’s easy to see that there are two the same… AQHNA 37º 58’N 023º 43’E Москва 55º 45’N 037º 27’E ירושלים 31º 47’N 035º 13’E القدس 31º 47’N 035º 13’E 北京 39º 55’N 116º 24’E New York 40º 40’N 073º 56’W London 51º 000º 07’ 39”W 30’ 26N 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' 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 The Celestial Sphere Star Trails Polar The motion of the stars as seen from the North Pole Star Trails: Equatorial The motion of the stars as seen from the Equator The Celestial Coordinates • Declination 1. Measured in degrees 2. '+' or '-' from the celestial equator • Right Ascension – – Measured in hours, minutes and seconds From 0h 0m 0s to 23h 59m 59.999s Celestial Coordinates For convenience, stars are assumed to be fixed to the celestial sphere and can be located on the coordinate chart: +90 +45 Declination 0 -45 -90 18 12 Right Ascension 6 0 Using the Coordinates Dubhe 11h 03m 55s Merak 11h 02m 01s +61º 43' 58" +56º 21' 52" Alkaid 13h 47m 42s +49º 17' 20" Meridian & Right Ascension