# Astronomical Observations

## Presentation on theme: "Astronomical Observations"— Presentation transcript:

Astronomical Observations
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: º Conversational Distance Distance: 3' 6" Angle: º Across the Room Distance: 31' 6" Angle: º A football field Distance: 100 yds Angle: ' Distance: 3.5 miles Angle: " 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
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 Π Distance Angular Size = True Size Distance = Degrees Radians

Examples Lunar Angular Diameter: Solar Angular Diameter:
Km Π Km = 0.517° 31.2 arcminutes Solar Angular Diameter: 149.6x106 Km Angular Diameter = x106 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. Note that this is what allows solar eclipses

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) Distances in the ‘local neighborhood’ 1 pc = 3.26 Ly Megaparsec (Mpc) Distances to distant galaxies 1 Mpc = 1,000,000 pc

Review: Brightness Apparent Magnitude Absolute 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) 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 ירושלים القدس 北京

Location, Location, Location
Let's take a look at: The shape of the Earth Zenith & Nadir Meridian Equator Latitude Longitude

The Shape of the Earth Flat? A sphere?
Flat? A disk? Where's the elephants and the great turtle? 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: Zenith … The point above your head Nadir … The point beneath your feet 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 º 58’N º 43’E Москва º 45’N º 27’E القدس ירושלים 31º 47’N º 13’E 北京 º 55’N º 24’E New York º 40’N º 56’W London º 30’ 26N º 07’ 39”W

Finding our way in the Night Sky
The Celestial Sphere A projection of the Earth's coordinates onto the sky The poles are extended to become the celestial poles The equator is projected to become the celestial equator The Latitude lines (parallels) are projected onto the celestial sphere and given the name 'Declination' 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 Measured in degrees '+' or '-' from the celestial equator Right Ascension Measured in hours, minutes and seconds From 0h 0m 0s to 23h 59m s

Celestial Coordinates
For convenience, stars are assumed to be fixed to the celestial sphere and can be located on the coordinate chart: 6 12 18 Right Ascension -45 +45 Declination +90 -90

Using the Coordinates Dubhe 11h 03m 55s +61º 43' 58"
Merak 11h 02m 01s º 21' 52" Alkaid 13h 47m 42s º 17' 20"

Meridian & Right Ascension