Apparent/Actual Motions
Summary
• Daily motion of sun (and nightly motion of stars) is due to
Earth’s rotation.
– Local noon occurs for an observer on the Earth when the Sun
reaches its highest point in the sky during that day
• That occurs when the Sun crosses the observer’s meridian, the line
that runs due North to due South for the observer.
• Clock time 12pm matches local noon only for one assigned reference
meridian within a time-zone.
• The moon orbits the Earth; Earth and the planets orbit the
sun.
• Some constellations are not visible year-round
– As the Earth orbits the sun we can only see those constellations in
a direction away from the Sun.
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Apparent/Actual Motions
Summary
• The Sun, Moon, and planets appear to move
through different constellations throughout the
year.
• From year-to-year, the Sun, Moon and planets
appear to move through the same group of
constellations: the Zodiac
– The line on the sky they follow is called the ecliptic.
– They follow the same line because the planets,
including Earth orbit the Sun in about the same plane.
• We call that plane the ecliptic plane.
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Daylight Savings Time
• Daylight savings time
– Determined by society
– Days are longer than nights in summer
– Spring forward:
• Now: extra daylight after work in summer
• Then (World War I): save energy
See Essay 3 in text for more on DST, time-zones,
calendars, leap years.
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Apparent Retrograde Motion of the
Planets: Mars Example
• APOD June 13, 2010, Tunc Tezel
• Dr. Stephen J. Daunt's Astronomy 161 web
site at The University of Tennesee,
Knoxville.
• more animations
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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
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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.
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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.
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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.
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Earth’s Spin Axis and Polaris
• Everything in the sky
Polaris, the North Star 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.
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It is the spin
of the Earth
on its axis
that makes
the Sun and
stars appear
to move
across the
sky.
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Motion of Stars at the North Pole
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Motion of Stars at the Equator
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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.
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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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