# AS101 Lecture Notes - Lecture 4: Eastern Hemisphere, 50Th Parallel North, Dailymotion

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11 Feb 2016
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Astronomy: Lecture #4
The Local Sky
- An onject’s altitude (above horizon) and direction (along horizon) specifies its location
-Zenith = altitude is 90 degrees; the point directly overhead.
-Horizon = altitude is 0 degrees; all points 90 degrees away from zenith.
-Meridian = line passing through zenith and connecting N and S points on horizon.
- Is the zenith the same as the north celestial pole?
Not always – right now, we are not standing on the North pole – if we were standing on
the North pole, looking up, then the zenith would be the same as the North pole.
Specifying Locations on Earth
- When we describe the sky, we must specify where on earth we are observing the sky
from.
- We specify our location or earth using latitude and longitude.
- 0 degrees = equator, 90 degrees = north pole.
Specifying Locations on Earth
- Imagine the Earth as a transparent sphere (actually the because of Earth's rotation, its
equator bulges out a little).
- Through the transparent Earth we can see its equatorial plane.
- The point O is the center of the Earth.
- The angular elevation of a point P north or south of the equatorial plane is its latitude.
- All points of equal elevation lie on the same circle of latitude.
- The longitude of a point is determined by the meridian passing through it.
- Meridians of longitude are semicircles going through the north pole, south pole and
crossing the equator.
- The meridian passing through Greenwich, England is historically chosen to be 0 degrees.
It is called the Prime Meridian.
- The other longitudes are designated by the angle east or west of the equator where they
cross the equator.
- The longitude of a point is determined by the meridian passing through it.
- All points with longitude between 0 and 180 degrees west of the equator lie in the
Western Hemisphere.
- All points with longitude between 0 and 180 degrees east of the equator lie in the Eastern
Hemisphere
-Latitude: position on Earth north or south of equator
-Longitude: position on Earth east or west of prime meridian (runs through Greenwich,
England)
-http://www.fourmilab.ch/cgi-bin/Earth
- Longitude helps us determine universal and local time.
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