Basic Observations

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Basic Observations

• Dr Bryce

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Class notices

• Beginning Chapter 2 today
• Recommend online tutorial Seasons (not graded)

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The Night Sky

• Time lapse photography shows star trails.
• Star trails are centered around the Pole Star.
• This photograph had about an 8 hour exposure, the
  shutter was open for 8 hours

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Constellations

• A constellation is a region of the sky not the
  brightest stars.

88 constellations fill the entire sky.
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The Constellation Orion
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The Celestial Sphere

• The imaginary sphere that surrounds the Earth.
• Allows us to Map the Sky.
• North Celestial Pole is directly over the Earths
  North Pole

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More Celestial Sphere

• The South Celestial Pole is directly over the
  Earths South Pole.
• The Celestial Equator is a projector of the
  Earths equator.
• The Ecliptic is the Suns path during the Year.
• The Zenith is the point directly overhead.

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The Celestial Sphere
The 88 official constellations cover the
celestial sphere.
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Coordinates on Earth
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Coordinates on the Celestial Sphere

• Like Latitude and Longitude on Earth we need a
  way of describing positions on the Sky.
• The Vernal equinox is the Suns position on March
  21st.
• Declination is analogous to Latitude
• Right Ascension is analogous to Longitude

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We measure the sky using angles
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Angular Measurements

• Full circle 360º
• 1º 60? (arcminutes)
• 1? 60? (arcseconds)

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Thought Question The angular size of your finger
at arms length is about 1. How many arcseconds
is this?

• 60 arcseconds
• 600 arcseconds
• 60 ? 60 3,600 arcseconds

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The angular size of your finger at arms length
is about 1. How many arcseconds is this?

• 60 arcseconds
• 600 arcseconds
• 60 ? 60 3,600 arcseconds

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The Motion of the Stars

• Stars rise and set
• Rise in the East and Set in the West.
• Due to the Earths rotation.
• Stars near the North Celestial pole dont rise or
  set Circumpolar
• We dont see stars during the daytime as the Sky
  is too bright.

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Why?

• Earth rotates west to east, so stars appear to
  circle from east to west.

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Our view from Earth

• Stars near the north celestial pole are
  circumpolar and never set.
• We cannot see stars near the south celestial
  pole.
• All other stars (and Sun, Moon, planets) rise in
  east and set in west.

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The sky varies with latitude but not longitude.
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The Pole Star

• Polaris
• Not the Brightest star in the Sky.
• Very close to the North Celestial Pole.
• Helps you navigate at night towards North.
• The altitude of the Pole star (how far above the
  horizon it is) gives you your latitude.

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Altitude of the celestial pole your latitude
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Thought Question The North Star (Polaris) is 50
above your horizon, due north. Where are you?

• You are on the equator.
• You are at the North Pole.
• You are at latitude 50N.
• You are at longitude 50E.
• You are at latitude 50N and longitude 50E.

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Thought Question The North Star (Polaris) is 50
above your horizon, due north. Where are you?

• You are on the equator.
• You are at the North Pole.
• You are at latitude 50N.
• You are at longitude 50E.
• You are at latitude 50N and longitude 50E.

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Seasons

• One of our most basic observations in Astronomy.
• In Summer the Sun is above the horizon for more
  time than it is in Winter.
• The days are longer and the nights are shorter.
• In Winter the Sun is above the horizon for less
  time and so it gets darker earlier.

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And the stars we can see change
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Seasons depend on how Earths axis affects the
directness of sunlight
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Summary The Real Reason for Seasons

• Earths axis points in the same direction (to
  Polaris) all year round, so its orientation
  relative to the Sun changes as Earth orbits the
  Sun.
• Summer occurs in your hemisphere when sunlight
  hits it more directly winter occurs when the
  sunlight is less direct.
• AXIS TILT is the key to the seasons without it,
  we would not have seasons on Earth.

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We can recognize solstices and equinoxes by Suns
path across sky
Summer solstice Highest path, rise and set at
most extreme north of due east. Winter solstice
Lowest path, rise and set at most extreme south
of due east. Equinoxes Sun rises precisely due
east and sets precisely due west.
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Dates to remember

• Sept 22, Autumnal Equinox, Sun will rise in the
  East and set in the West, the day and night will
  be of equal length.
• Dec 21, Winter Solstice, Sun rises in SE sets in
  SW, least amount of day
• March 21, Vernal Equinox
• June 21, Summer Solstice, Sun rises in NE, sets
  in NW, longest day

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Extreme days

• Path of the Sun on the summer solstice at the
  Arctic Circle