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AS101 Lecture Notes - Generic Point, Celestial Pole, Declination

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Patrick Mc Graw

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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
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

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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.
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 are
way too tiny in angular size.
Mapping the Sky: your local sky:
Zenith: the point directly overhead
Horizon: all points 90˚ away from the zenith
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