Transcript 04 Lines in the Sky

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.
1
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.
2
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.
3
Earth’s Spin Axis
• The imaginary line
connecting Earth’s North and
South Poles is called the
Earth’s 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 Earth’s Equator.
4
Earth’s Spin Axis and Polaris
• Everything in the sky appears
to move around the Celestial
Polaris, the North Star
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.
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How to find the North Star
Pointer stars
• 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.
6
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.
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Altitude and Azimuth
• We can measure the
location of objects in the
sky using pairs of angles.
Here’s one way.
• Altitude is the angle
above the horizon.
• Azimuth is the angle
around the horizon
clockwise from North.
North is 12:00, East is
3:00
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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)
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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.
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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.
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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.
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