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AS101 Lecture 4

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Wilfrid Laurier University
Patrick Mc Graw

Lecture 4 1/16/2013 12:22:00 PM The View from here, Part II Reading: Ch. 2.1-2.3 Recap: mapping the sky:  When we talk about mapping the sky, e are thinking about objects (stars, planets, etc.) as if they were on the surface of an imaginary sphere, the celestial sphere that surrounds the earth.  You can make maps of the celestial sphere much as you can make maps of the earth’s surface.  Astronomers have divided the celestial sphere into 88 constellations with well-defined boundaries.  There are several ways to describe the location of something in the sky. One is by using landmarks and directions. We can also describe in which constellation something is located.  Every star is in one and only one of the 88 constellations  Astronomers distinguish constellations (regions of the celestial sphere) from asterisms, which are patterns or groups of stars, more like landmarks.  A given star may be part of more than one asterism. For example, Rigel is part of the “foot” of Orion and is also part of the Winter Circle.  One of the things a map should tell us about is the distances between places. What do you mean when we talk about distances in the sky or on the celestial sphere? Angular Distance vs. Actual (Physical) Distance:  Stars in a constellation or asterism might not be physically close to each other.  What they have in common is that they lie in approximately the same direction from Earth. Angular Size and Angular Distance:  Since we cannot accurately judge how far objects in the sky are, we CANNOT tell their true size. However, we CAN talk about the angular size of an object or the angular distance between two objects.  Angular size or distance is measured in degrees.  Try to estimate the angular size of an object for yourself o Full circle = 360˚ o 1˚ = 60’ (arcminutes) o 1’ = 60” (arcseconds) Angular Size and Physical Size:  An objects angular size appears smaller if it is farther away. This relationship is very useful for calculating actual distances or size. o Angular size = physical size x 360˚ 2 π x distance.  If you double the distance, you cut the angular size in half.  EXAMPLE: o Jupiter if 4.2 au from Earth at the closest. o 4.2 au x (1.5x10^8km/au)=6.3x10^10 km. o Jupiter’s diameter = 1.4x10^5km o Angular size = physical size x 360˚ o 2 π x distance. o 1.4 x 10^5 km x Some approximate Angular Sizes for Comparison  Width of a finger at arm’s length: 1 degree  Size of moon or sun 0.5 degree  Width of “bowl” f Big Dipper: 10 degrees  Smallest feature most human eyes can distinguish  1arcminute (= 1/60 of a degree).  Jupiter (from earth, now): about 0.8 arcminute  F the sun were only 1 ly away, its angular size would be about 9x10^-6 degrees or 003 arcsecond  We can almost see planets’ sizes with unaided eyes, but stars ar
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