General Astronomy
Astronomical Observations
Angles and Angular Measurement
Remember there are:360° in a circle 60' in a degree 60" in a minuteOr 2Π radians in a circle
60 min to an Hour60 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,
Getting a grip on AnglesSize of person is 5' 6"
Angle: 90º Conversational DistanceDistance: 3' 6"
Angle: 10º Across the RoomDistance: 31' 6"
Angle: 1º A football fieldDistance: 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 1°Two knuckles 2°Fist 10°Extended hand 25°
Rules of 'Thumb'
Angular Size
Instruments for Angles
British Navy SextantCirca 1840
16th century Quadrant used in navigation
True versus Apparent SizeThe 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 SizeDistance
Radians
=180 True Size
Π Distance
Degrees
ExamplesLunar Angular Diameter:
Angular Diameter = 180 3475 Km
Π 385000 Km= 0.517° =31.2 arcminutes
Solar Angular Diameter:
149.6x106 KmAngular Diameter = 180 1.39x106 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.
Review: Measuring Distance• Miles/Kilometers
Distances on the surface of a object
• Astronomical Unit (AU)Distances within the Solar System1 AU = 93,000,000 Miles
• Lightyear (Ly)Distances to nearby stars and other objects1 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 galaxies1 Mpc = 1,000,000 pc
Review: Brightness• Apparent Magnitude
How bright does it appear?
• Absolute MagnitudeHow bright would it appear from 10 pc?
• Sometimes either magnitude may be further identified as visual (MV) or photographic (MB)
Finding our wayBefore 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)
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
ירושליםالقد北京س
Location, Location, Location
Let's take a look at:1. The shape of the Earth2. Zenith & Nadir3. Meridian4. Equator5. Latitude6. Longitude
http://fxb.worth1000.com/entries/396292/great-a-tuin
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?
Defining the Earth
• The North and South Poles• The Parallels of Latitude
The Equator
• The Meridians of LongitudeThe Prime MeridianThe International Date Line
• Your position:1. Zenith … The point above your head2. Nadir … The point beneath your feet3. 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 ObservatoryLatitude 39° 33' 09“ NLongitude 074 ° 29' 08“ W
北京 39º 55’N 116º 24’E
AQHNA 37º 58’N 023º 43’E
Москва 55º 45’N 037º 27’E
Finding our way
القدس
ירושלים
31º 47’N 035º 13’E
31º 47’N 035º 13’E
New York 40º 40’N 073º 56’W
London 51º 30’ 26N 000º 07’ 39”W
Now it’s easy to see that there are two the same…
Finding our way in the Night Sky
The Celestial Sphere1.A projection of the Earth's coordinates onto
the sky2.The poles are extended to become the
celestial poles3.The equator is projected to become the
celestial equator4.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
StarTrails
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 degrees2. '+' or '-' from the celestial equator
• Right Ascension– Measured in hours, minutes and
seconds– From 0h 0m 0s to 23h 59m 59.999s
Celestial Coordinates
0
061218
Right Ascension
-45
+45
Declin
atio
n
+90
-90
For convenience, stars are assumed to be fixed to the celestial sphere and can be located on the coordinatechart:
Using the Coordinates
Alkaid 13h 47m 42s +49º 17' 20"
Merak 11h 02m 01s +56º 21' 52"
Dubhe 11h 03m 55s +61º 43' 58"
Meridian & Right Ascension