Transcript General Astronomy - Stockton University

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