Introduction To Astronomy Lesson 1

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Introduction To AstronomyLesson 1

• Stanford Astronomical Society
• 11/30/07

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Topics for this Lesson

• Earth Based Coordinates
• The Celestial Sphere and Sky Coordinates
• The North Star
• Measuring Distances on the Sky
• The Motion of Objects in the Sky
• The Zodiac
• The Seasons

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A Reminder About Earth

• The earth is almost a sphere
• We locate points on the sphere with 3 coordinates
• Latitude (90º S -- 90º N)
• Longitude (180º W -- 180º E)
• Altitude (m above/below sea level)
• These are referred to as Terrestrial Coordinates

images taken from http//nationalatlas.gov/articl
es/mapping/a_latlong.html
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Reference Points/Lines on Earth
North Pole

• Latitude
• North Pole (90º or 90º N)
• South Pole (-90º or 90º S)
• Equator (0º N/S)
• Tropic of Capricorn (23º 26 22 N)
• Tropic of Cancer (23º 26 22 N)

Equator
South Pole

• Longitude
• Prime Meridian (0º E/W)
• International Dateline (180º E/W)
• Prime Meridian passes through
• Royal Greenwich Observatory in London

Int. Dateline
Prime Meridian
images taken from http//nationalatlas.gov/articl
es/mapping/a_latlong.html
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Side Note Sub-Degree Precision

• A Circle is 360 Degrees
• A Degree is 60 Arcminutes
• An Arcminute is 60 Arcseconds
• Thus, a Circle has

90º
90º
90º
90º
360º x 60' x 60'' 1,296,000'' 1 º 1'
On earths surface (at the equator) 1º 69
miles 1 1.15 miles 1' 100 feet
SYMBOLS º Degree ' Arcminute ''
ArcSecond
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Our Grid is Tilted and Rotates!

• The Earths Axis (imaginary line through north
  and south poles) is tilted by ? 23º with
  respect to its axis of revolution about the sun
• Tilt gives rise to seasons (earth is closer to
  sun in winter than in summer!)
• Revolution gives rise to a changing night sky

?
Summer Night Sky
Spring
Summer
Winter Night Sky
Winter
Fall

• Axis of rotation
• 1 rotation 1 day

• Axis of revolution
• 1 revolution 365 days

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Earth Rotation Sky Rotation
Polaris - The North Star

• Circumpolar Star- Around the Pole star
• Does not set below horizon in the
• sky at a particular latitude

• Noncircumpolar Star
• Rises and sets at some time during
• the day or night

• The stars in our sky are actually fixed
• They appear to move because of the earths
  rotation
• - Earth rotates east
• - Stars appear to move west
• They trace out circles around the axis of
  rotation
• This image was taken at Mauna Kea by leaving the
  shutter open for a few hours star trail pictures
  are easy to do for any anybody with a nice
  camera!

Horizon - where the sky meets the earth
image taken from http//apod.nasa.gov/apod/ap0512
20.html
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Finding Polaris the North Star

• Polaris is natures compass
• Draw a line straight from Polaris to the horizon
  with your finger and you will be pointing toward
  geographic north
• To find Polaris
• 1) Start at the two end stars of the big dipper
  ladle 1. Merak and 2. Dubhe
• 2) Multiply their distance by 4 and follow the
  line from 1 to 2 that distance until you hit 3
  Polaris

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2
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Note - Geographic North actually deviates
slightly from Magnetic North, the direction of
the north magnetic pole of the earth. Dont be
confused if your compass doesnt point exactly
towards the north star. - The discrepancy varies
with latitude.
Only works if you can see the big dipper
Image taken from http//en.wikibooks.org/wiki/Adve
ntist_Youth_Honors_Answer_Book/Recreation/Backpack
ing
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Another Polaris Trick

• Polaris can be used to determine your latitude!
• Angle from Polaris to horizon is your latitude

Image taken from http//lasp.colorado.edu/bagenal
/1010/images/latitude.jpg
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Measuring Angles on the Sky

• We can measure angles on the sky using our hands
  and fingers

.5º
Moon is about .5º in angular diameter
Image taken from http//www.astro.virginia.edu/cla
ss/oconnell/astr121/im/fingerangles.jpg
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Position of Polaris
For an observer located at some latitude and
longitude

• Polaris stays in same spot throughout the night
• We say it has the same Altitude and Azimuth
  throughout the night
• We can refer to all stars and objects with two
  angles (Altitude, Azimuth)
• These are Local Coordinates
• In Palo Alto, Polaris stays at (37.43, 0)

90º Zenith

Altitude
Azimuth
0º Horizon
Comparison to Terrestrial Coordinates Latitude Al
titude (0º to 90º - is below
horizon) Longitude Azimuth (0º is N, CW to
180ºS)
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Limitations of Local Coordinates

• For an observer, all stars and objects in sky
  besides North and South Pole stars have Altitude
  and Azimuth that are constantly changing
• Two people at different spots on earth will
  disagree about Alt., Az. coordinates even if they
  are looking at the same object at the same time

Solution Use a coordinate system that is glued
to the fixed stars rather than a spot on earth
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The Celestial Sphere

• The Celestial Sphere is an ancient concept
  dating back to the time when we thought the earth
  was the center of universe
• It is a gigantic sphere glued to the stars
• Stars appear to rotate because sphere is rotating
• Sun, Planets, Comets, Asteroids all move on
  surface of sphere

Images taken from http//zebu.uoregon.edu/imamura
/121/images/sphere.gif
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Coordinates on the Sphere

• Just like Terrestrial Coordinates on earth, we
  define Celestial Coordinates for the Celestial
  Sphere
• Stars and other things outside our solar system
  have a particular Right Ascension and
  Declination or RA and DEC.
• Earths Equator, North Pole, and South Pole line
  up with the Equator and North Pole, and South
  Pole, of the Celestial Sphere

Coordinate System North/South East/West
Terrestrial Latitude Longitude
Celestial Declination Right Ascension
Ecliptic - extremely important line along which
planets and the sun appear to move as viewed from
earth
Image taken from http//www.onr.navy.mil/focus/spa
cesciences/images/observingsky/celestialsphere.jpg
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More about Celestial Coordinates

• Right Ascension (RA or ?)
• - Sometimes Referred to in Hours
• - 1 Hour 15º
• - 0 - 360º or 0 - 24 Hours (h)
• - Why Hours?
• Because the sky rotates 360º in 24 hours of
  time. So in 1 hour of time, sky appears to rotate
  by 15º.

Declination (DEC or ?) - 0º is Celestial
Equator - 90º is Celestial North Pole - -90º
is Celestial South Pole
Note We will not cover how to convert between
local and celestial coordinates. You can either
do the trigonometry or use an online calculator.
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How it looks on the Sky
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Right Ascension and the Zodiac

• The Ecliptic is
• The apparent path of the sun across the heavens
• The Zodiac is
• The ecliptic divided into 12 segments
• Each segment subtends 30º or 2 h of RA
• Your zodiac sign is the constellation that the
  sun was in when you were born

12 h RA - Libra the Scales
30º
0 h RA - Aries the Ram
Note Due to precession of Earths axis, the
position of the sun in the zodiac constellations
is actually shifting
Image taken from http//www.astrologyclub.org/arti
cles/ecliptic/ecliptic2.gif
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The Zodiac and Seasons

• The plane of the Ecliptic is tilted by about 23º
  with respect to the Celestial Equator (Guess
  Why?)
• They intersect at two points
• 1. Vernal Equinox (Pisces 0 h RA)
• 2. Autumnal Equinox (Virgo 12 h RA)
• The Sun has its maximum declination at
• 3. Summer Solstice (Gemini 0 h RA)
• The Sun has its minimum declination at
• 4. Winter Solstice (Sagitarrius 0 h RA)
• Below is a table showing the position of the sun
  at each of the 4 points

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1
3
2
Image taken from http//www.astro.uiuc.edu/kaler
/celsph.html
Spring Equinox (March 21) Summer Solstice (June 21) Autumn Equinox (Sept. 21) Winter Solstice (Dec. 21)
RA 0 h 6 h 12 h 18 h
DEC 0º 23º27' 0º -23º27'
Very useful to know since we cannot see
constellations when theyre behind the sun!!
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Now we should be able to

• Find the North Star
• Measure distances on the sky with our hands and
  use this to find objects
• Use Altitude and Azimuth to point out an object
  to somebody
• Understand Seasons and know what constellation
  the sun is blocking on a given date

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Next Lesson

• Solar/Sidereal Time
• Observing Conditions
• Catching an object high in the sky
• New/Full Moon
• Atmospheric turbulence and seeing
• Night/Averted Vision
• Magnitude/Brightness
• Some popular stars