Lines in the Sky

1
Lines in the Sky

• In order to use the sky to measure time you need
  to measure the location of objects in the sky. We
  will look at two methods of measuring locations
  in the sky.
• Both methods require measuring angles.
• These methods have long been used not only for
  timekeeping but for navigation as well.
• But first we need to define some terms

2
The Earth and Sky

• From where we stand Earth appears flat and the
  sky appears like a dome above us.
• The point where the dome of the sky meets the
  Earth is called the horizon.

3
The Earth and Sky

• If an object is above the horizon it is visible.
  If it is below the horizon it is blocked from
  view by the Earth
• Stars, the Sun, Moon and planets all appear to
  rise above the horizon in the East and set below
  it in the West.

4
Earths Spin Axis

• The imaginary line connecting Earths North and
  South Poles is called the Earths rotation axis.
  The line around which the Earth spins.
• Extending this line into the sky points to the
  North and South Celestial Poles.
• The Celestial Equator is the extension of Earths
  Equator.

5
Earths Spin Axis and Polaris

• Everything in the sky appears to move around the
  Celestial poles.
• The star Polaris lies very near the North
  Celestial Pole. It never rises or sets. It is
  always visible from the Northern Hemisphere.
• Polaris, the North Star, is never visible from
  the Southern Hemisphere.

Polaris, the North Star
6

• It is the spin of the Earth on its axis that
  makes the Sun and stars appear to move across the
  sky.

7
Motion of Stars at the North Pole
8
Motion of Stars at the Equator
9
How to find the North Star

• Polaris can be found using the Pointer stars of
  the Big Dipper
• The Big Dipper is part of a larger constellation
  of stars called Ursa Major (the Great Bear)
• Now you know how to find true North.

Pointer stars
10
Meridian, Altitude Zenith

• Another important point in the sky is the Zenith,
  the point directly overhead.
• The line that crosses from North to South Horizon
  and passes through the Zenith and Poles is the
  Local Meridian.

11
Altitude and Azimuth

• We can measure the location of objects in the sky
  using pairs of angles. Heres one way.
• Altitude is the angle above the horizon.
• Azimuth is the angle around the horizon clockwise
  from North. North is 1200, East is 300

12
Locations on Earth

• We identify positions on Earth using Latitude
  (Degrees North or South of the Equator) and
  Longitude (Degrees East or West of Greenwich,
  England).
• Greenwich is at the Prime Meridian (Longitude
  0)
• PGCC about (39 North, 77 West)

13
Navigation

• Measuring the altitude of the North Celestial
  Pole can give you your Latitude on Earth in the
  Northern Hemisphere.
• South of the Equator Polaris is never visible.
• The farther North you are the higher the North
  Celestial Pole appears in the sky.

14
Locations in the Sky

• We can measure locations in the sky in a similar
  way.
• Like Latitude, the Declination is the angular
  distance from the Celestial Equator.
• Now we have to decide on a Celestial Prime
  Meridian.
• Astronomers choose one of the points where the
  Ecliptic crosses the Equator.

15
Right Ascension Declination

• The Sun traveling along the Ecliptic crosses the
  Equator at two points, the Vernal Equinox and the
  Autumnal Equinox.
• The Right Ascension is the angular distance from
  the Vernal Equinox
• Each position can be identified by its Right
  Ascension and Declination.