Transcript Night and Day - Centre for Astrophysics and Supercomputing

Module 3:
The Celestial Sphere
Activity 1:
The Rotating Earth
Summary:
In this Activity, we will investigate
(a) day and night & the Earth’s rotation,
(b) star trails,
(c) the celestial sphere & celestial poles, and
(d) sidereal and mean solar time.
(a) Day and night & the Earth’s 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 Earth’s
surface alternate between
sunlight and darkness that is, day and night.
Here we show four frames of the Earth rotating, showing
Australia move from day to night. The Sun in on the left:
Sunlight
animations © Swinburne
(b) Star trails
Because of the Earth’s 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.)
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)
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 setting
in the southwestern sky.
(c) The celestial sphere & celestial poles
Some stars never set. Their
trails form complete circles
around points in the sky
called
(c) “south
the celestial
sphere
&
the
celestial
pole”
celestial
poleshemisphere
(for southern
viewers) and
the “north celestial pole”
(for northern hemisphere
viewers).
Star trails around the south celestial
pole, towards the dome of the AngloAustralian Telescope (AAT)
We can explain this apparent motion if we recognize
that it is caused by the Earth’s daily rotation on its axis.
Almost all stars appear
to follow circular
paths,
but most are
partly obscured
below the
horizon.
South Celestial
Pole
south east
south west
Only stars on a direct extension of the Earth’s 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 Celestial
Polaris
Pole
north west
north east
There is no bright star at the South Celestial Pole.
South Celestial
Pole
south east
south west
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
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.
north celestial pole
We can
imagine the
celestial
sphere as
having a
“celestial
equator”
south celestial pole
north celestial pole
We can
imagine the
celestial
sphere as
having a
“celestial
equator”
… which is an
extension of the
Earth’s equator.
south celestial pole
north celestial pole
We can also project the
Earth’s imaginary
longitude
and latitude lines
onto the celestial
sphere
The corresponding
celestial coordinates
are:
Longitude  right ascension (RA)
Latitude  declination (dec)
south celestial pole
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)
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.
So a star’s coordinates might look something like:
12:52:03, – 47:34:43
which means RA = 12 hours 52 min 3 sec,
dec = - 47 degrees 34 arcmin 43 arcsec
north celestial pole
An observer on the Earth’s
surface
sees the night sky
above the horizon
but not below.
So this observer can
see the North
Celestial Pole and
much of the sky (as
the Earth rotates),
but not the southern-most
sky near the South Celestial Pole
south celestial pole
Looking up, this northern
hemisphere
observer will see:
N
Polaris, at the
north celestial
pole
Note the relative
positions of East
and West on this
sky chart.
E
Their order may
seem odd, but
remember that
they apply to an
observer’s view when
looking directly up.
W
S
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,
N
W
is called the
“celestial meridian”.
zenith
S
We can
superimpose
lines of constant
right ascension
(RA)
and declination
(dec)
N
E
W
S
An observer in the
southern
hemisphere will
see:
RA
lines
dec
lines
N
E
W
x
S
the south
celestial pole
(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.
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 return
to the meridian each day than it does for a distant star.
Therefore the mean solar day is slightly longer than the
sidereal day - by about 4 minutes (or, more exactly,
3m 55.51s!).
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.
Day Zero: Sun
and distant star
overhead
distant star
Sun
4mins
Solar day: Sun
overhead again
(but now more
Sidereal day:
than one sidereal
distant star
overhead again day has passed)
(but not Sun)
Local standard time (the time we set our clocks to) is
derived from mean solar time, but stars rise according
to 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 telescope, you
are likely to find clocks displaying local sidereal time,
local standard time and Universal Time (otherwise
known as Greenwich mean time).
Image Credits
NASA: View of India and Saudi Arabia, taken by the Clementine spacecraft
http://nssdc.gsfc.nasa.gov/image/planetary/earth/clem_india_saudi.jpg
NASA Photo p-41508c: Image of the Earth and Moon from Galileo (cropped)
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_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
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