Activity 1: The Rotating Earth

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Activity 1 The Rotating Earth
Module 3 The Celestial Sphere
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Summary

• In this Activity, we will investigate
• (a) day and night the Earths rotation,
• (b) star trails,
• (c) the celestial sphere celestial poles, and
• (d) sidereal and mean solar time.

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(a) Day and night the Earths rotation

• As the Earth rotates on its axis from west to
  east, the Sun appears to rise in the east and set
  in the west.

Locations on the Earths surface alternate
between sunlight and darkness -

that is, day and night.
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• Here we show four frames of the Earth rotating,
  showing Australia move from day to night. The
  Sun in on the left

animations Swinburne
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(b) Star trails
Because of the Earths rotation, the stars appear
to slowly move across the night sky as the hours
go by.
(The stars also appear to slowly shift in
position each night - so that you will see
different stars overhead each night at, say,
midnight. This is due to the changing position
of the Earth in its orbit around the Sun, and
means that we see different zodiacal
constellations through the course of a year.)
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If a camera is left outside with its shutter open
for several hours on a clear night, it will
photograph star trails, recorded on the film
due to the apparent motion of stars across the
night sky.

• Star trails photographed in the southwest,
  towards the dome of the Anglo-Australian
  Telescope (AAT)

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To make this picture, David Malin of the
Anglo-Australian Observatory pointed a camera
towards the dome of the Anglo Australian
Telescope at Siding Spring Mountain in New South
Wales, Australia.

• Most stars rise set in our sky - the star
  trails here are made by stars settingin the
  southwestern sky.

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(c) The celestial sphere celestial poles
Some stars never set. Their trails form complete
circles around points in the sky called (c) the
celestial sphere celestial poles
the south celestial pole (for southern
hemisphere viewers) and the north celestial pol
e (for northern hemisphere viewers).

• Star trails around the south celestial pole,
  towards the dome of the Anglo-Australian
  Telescope (AAT)

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• We can explain this apparent motion if we
  recognize that it is caused by the Earths daily
  rotation on its axis.

Almost all stars appear to follow circular
paths, but most are partly obscured below the
horizon.

South CelestialPole
south east
south west
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• Only stars on a direct extension of the Earths
  rotation axis appear to stay stationary during
  the night.

Observers in the northern hemisphere see
Polaris, the North Star, as stationary - it
happens to be located almost at the North
Celestial Pole.
North CelestialPole
Polaris
north west
north east
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• There is no bright star at the South Celestial
  Pole.

South CelestialPole
south east
south west
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north celestial pole

• Although stars are actually at widely varying
  distances from Earth, we can picture these
  apparent motions as happening on an imaginary
  celestial sphere

south celestial pole
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Although alt and az are easy coordinate systems
to use, they depend on where the observer is
(i.e. where the horizon is located).
We can use the idea of the celestial sphere to
define another celestial coordinate system. This
one is the same for all Earth observers.
To be reminded of how altitude (alt) and azimuth
(az) are defined, review the Activity on Star
Patterns.
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north celestial pole

• We can imagine the celestial sphere as having a
  celestial equator

south celestial pole
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north celestial pole

• We can imagine the celestial sphere as having a
  celestial equator

which is anextension of theEarths equator.
south celestial pole
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north celestial pole

• We can also project the Earths imaginary
  longitude

and latitude lines onto the celestial sphere
The correspondingcelestial coordinatesare
Longitude ? right ascension (RA)
Latitude ? declination (dec)
south celestial pole
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Declination is measured in degrees, arcminutes
and arcseconds above or below the celestial
equator - so, for example, stars near the north
celestial pole have declinations close to 90,
and stars close to the south celestial pole have
declinations close to - 90.
(1 degree 60 arcmin, 1 arcmin 60 arcsec)
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Right ascension is measured in hours, minutes and
seconds, because it takes approx. 1 day for a
star to reappear at the same point in the sky.
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So a stars coordinates might look something
like 125203,
473443 which means RA 12 hours 52 min 3 sec,
dec - 47 degrees 34 arcm
in 43 arcsec
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north celestial pole

An observer on the Earthssurface
sees the night skyabove the horizon
but not below.
So this observer cansee the North Celestial
Pole and much of the sky (as the Earth
rotates),but not the southern-mostsky near the
South Celestial Pole
observers horizon
south celestial pole
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• Looking up, this northern hemisphere observer
  will see

Polaris, at thenorth celestialpole
N
Note the relativepositions of Eastand West on
thissky chart.
E
W
Their order mayseem odd, but remember thatthey
apply to anobservers view whenlooking directly
up.
horizon
S
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N
The imaginary line across the sky from the most
northern point on the horizon, through the
zenith, to the most southern point on the
horizon,
W
is called the celestial meridian.
zenith
horizon
S
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We can superimposelines of constant right
ascension (RA)
N
and declination(dec)
E
W
horizon
S
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• An observer in the southern hemisphere will see

N
RA lines
E
W
dec lines
x
horizon
the south celestial pole
S
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(d) Sidereal Mean Solar Time
The period of rotation of the Earth itself (the
day) depends on whether one defines it as
relative to the position of the Sun or relative
to the fixed stars.
The time interval between when any particular
(far distant) star is on the celestial meridian,
from one day to the next, is the sidereal day.
The average time interval from when the Sun is
at celestial meridian from one day to the next is
called the mean solar day.

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Because the Earth moves a small distance along
its orbit during one day, the Sun shifts its
position in the sky slightly eastwards each day.
Because of this, it takes a little longer for the
Sun to returnto the meridian each day than it
does for a distant star. Therefore the mean sol
ar day is slightly longer than the sidereal day -
by about 4 minutes (or, more exactly, 3m
55.51s!).
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• If we start counting a day when the Sun and some
  distant star are directly overhead, after the
  Earth has turned far enough for the stars to
  return to the same apparent position in the sky,
  the Earth must still move an extra 1/365 of 24
  hours (or about 4mins) for the Sun to return to
  your meridian.

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Local standard time (the time we set our clocks
to) is derived from mean solar time, but stars
rise accordingto sidereal time. This is why
stars appear to rise about 4 minutes earlier
each night.
This is why astronomers prefer to use sidereal
time to record their observations.
If you visit the control room of a research tele
scope, you are likely to find clocks displaying
local sidereal time, local standard time and
Universal Time (otherwise known as Greenwich
mean time).
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Image Credits

• NASA View of India and Saudi Arabia, taken by
  the Clementine spacecraft
  
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/c
  lem_india_saudi.jpg
  
• NASA Photo p-41508c Image of the Earth and Moon
  from Galileo (cropped)
  
• http//nssdc.gsfc.nasa.gov/image/planetary/earth/g
  al_earth_moon.jpg
  
• AAO, David Malin Image reference AAT 5
• Star trails in the southwest ( reproduced with
  permission)
  
• http//www.aao.gov.au/local/www/dfm/aat005.html
• AAO, David Malin Image reference AAT 6
• Star trails around the south celestial pole (
  reproduced with permission)
  
• http//www.aao.gov.au/local/www/dfm/aat006.html

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• Now return to the Module home page, and read more
  about day night and the celestial sphere in the
  Textbook Readings.

Hit the Esc key (escape) to return to the Module
3 Home Page
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