In the topic today that I am going to take
I am taking one of such topic which is used
Very commonly in our field and i.e. Celestial Sphere
I am going to explain you first
what actually I've drawn over here
This interior circle which I've drawn is the earth's sphere
and the exterior one is obviously celestial sphere
Now, Since the celestial sphere is obtained by the
expansion or the surface expansion of the earth sphere
hence, there are a lot of similarities between these two
For e.g., the north pole on the earth sphere
becomes the north celestial pole here
South pole extension becomes the south celestial pole
The equator expansion becomes the equinoctial
as I told this surface expansion of the earth
was the one by which we obtained the celestial sphere
hence the centre of the earth and
celestial sphere remains the same
Now the meridian that we call up here
on the expansion turns up to celestial meridian on celestial sphere
Now the declination circle that we are seeing
They are basically the expansion of the latitude circles
This split circle that you are seeing on this picture
the tilted one. This is known as the ecliptic
What is ecliptic? Ecliptic is basically
the annual apparent path of the Sun
Now by the annual apparent path I mean
It is basically being formed by the expansion
of the earth's orbit
and now earth on its orbit revolves
in a period of one year which is 365 days
hence it is also known as annual apparent path of sun
Now when I say apparent, it clearly means virtual motion
A thing which is not permanent but
it is in observance with respect to the observer on the earth
This intersection point that you are seeing at this place
This is intersection of the ecliptic and the equinoctial
There are two intersections basically, one is down and one is here
Since I told you this is the path on which sun apparently moves
so when it moves from south to north
when it crosses the equinoctial from south to north
This intersection point is known as first point of Aries
When it crosses from north to south
This intersection point is known as first point of libra
Now libra is not of navigational important to us
We only consider Aries
This is the observer meridian
and exterior one is the Greenwich meridian
so the extension of this observer meridian on the
celestial sphere becomes the celestial meridian of the observer
and the extension of the Greenwich meridian on celestial sphere
becomes celestial meridian of Greenwich.
Now this is the celestial body
Now this celestial body, the meridian passing through it
becomes the celestial meridian of the body
Celestial meridian passing through first point of Aries
is known as celestial meridian of the first point of Aries
before going further what I
was actually going to take up now
how you define these different things
which I had written in green, specifically
the Declination, SHA, LHA and GHA.
one of the most common definitions in our field used in day to day basis
especially while sight taking and sight calculations
but lot of time we tend to forget the actual meaning
so to remember them easily is what my basic topic was today
and that is what I am going to come through
if you see the left hand side of board what I have written
over here is PART and SEQUENCES to define anything
These are the three main sequences that .
we follow to define any of the definitions
The first one is Arc of, the second one is angle subtended
and third one written over here is contained between.
These three in the same sequence they are written
are being followed to define for any definition.
Before moving forward with this I would
like to brief you about how to use them
Suppose this is earth,
this obviously equator, suppose there’s a point called A
and this will be parallel of latitude passing through it
now when we will define latitude suppose in the form
which I had written over here obviously
the first thing we need is ARC.
We will come back to this side
I have clearly written here
Hints to remeber
in the column of hints to remember I have clearly written
you have to check one of these two things first
whether the angle which we were going to define
is being formed between vertically running things
or between horizontally running things
When I say vertical running things it simply means meridians
because meridians are vertically running and when I say
horizontally running it simply means latitude circles.
so when I talk of latitude obviously latitude's angle
is always formed between horizontally running things
I come back again on this side. When I say
Iam fine with one of these two things
whichever we have to define
it is either between horizontally running or vertically running
Then we will move forward
If I identify this in case of latitude it is formed
between horizontally running things then
I have clearly written in this case arc will be formed
of meridian in case of earth
and if we are defining something on the celestial sphere
it will be celestial meridian
and similarly next point was angle subtended
and in this case, angle will be subtended
at the centre of the earth
This is as I said the parallel of the latitude
and suppose we have to define it
This is the meridian passing through it
Suppose this is the centre of the plane of the earth
and this is the centre
this angle we have written 20 deg. here
now, this 20 deg. basically, this angle which
is being formed at the centre of the earth
by this arc, now this arc obviously is of the meridian
So going back to part
where I had written how to define any definition
and consulting it with this figure which I had drawn
I can clearly see this latitude can be defined as
the arc of the meridian and angle subtended
on the centre of earth
contained, now third part was contained between
contained between the equator
and parallel of latitude passing through that place
so by now you must have realised it is
one of the simplest ways to remember anything
If you can analyze this
factors to remember which I have mentioned here
If you can relate this with these sequences
you can easily define anything
So, I will move forward with this, as I told you
latitude's replication on the celestial sphere is declination
now this also means the defination of
declination will be very similar to that of latitude
the only difference would be where I called equator here
that would be called equinoctial there
where I called meridian here would be called as celestial meridian there
and the parallel of latitude on the earth would be
defined as the parallel of declination on celestial sphere
So keeping these thoughts in mind
declination can simply be defined with
these sequences again
Arc in this case it will be of celestial meridian,
the angle subtended as I told centre is the same
so it will definilty be at the centre of the earth
arc of the celestial meridian
of angle subtended at the centre of the earth, i.e.
this angle
Arc of celestial meridian, or angle subtended at centre of the earth
third point was contained between
contained between the equinoctial and parallel
of declination passing through the body.
Simple definition of the declinatin. I hope
you can remember it very easily:
I will move on with this, I will move on to now SHA, LHA and GHA
Sequentially they are called as Sidereal Hour angle,
Local Hour angle and Greenwich Hour angle.
Before going into the definitions for them I would like
you to remember two basic points for them
the first one is from where they start off
so you can clearly see SHA starts from First Point of Aries
LHA starts from the celestial meridian of the Observer
and GHA starts from celestial meridian of Greenwich.
And for that matter they all will end at celestial meridian of body
because for that matter celestial sphere was being
formed for the study of the body itself
Second point to remember in these definitions
is they all are measured westerly
from the point, they are starting starting
So SHA also westerly, LHA also westerly
and GHA also westerly as I have shown by the arrows also
After we remember these points we will again
go back to our main basic which I started of from
I will define them in the same sequesnce again
these angles as we can see clearly are being formed
between vertically running things which are obviously the
celestial meridian in this case
so vertical running things as clearly written- arc of this
will always be of equator in case of earth
and equinoctial in case of celestial sphere
and angle substantial again subsequentially for equator
it will be at poles and for equinoctial, it will be at celestial poles
for SHA, LHA, GHA they in celestial sphere
we have to go with these two
I can start off again with this
In case of SHA suppose the Arc of equinoctial
or angle subtended at the celestial poles
now third point again contained between
we can read out from here itself
contained between the celestial meridian of first point of aries
and that of the body measured westward from aries
I will move on to LHAagain
similar kind of definition- Arc of equinoctial or angle subtended
at celestial poles
third point again contained between
this is starting from the celestial meridian of the observer
and that of the body again last measured westward from the observer
Now GHA again, its name only clears out the half part
i.e. Greenwich, obviously it will be starting from Greenwich
So GHA can be defined again as “Arc of the equinoctial
or angle at the celestial poles contained between
the third point celestial meridian of greenwich
and that of the body measured westwardly from Greenwich
I hope
this part which I explained in the video
have cleared up a lot of doubts regarding how to remember the definitions
how to remember them correctly
If you follow this process you can never ever forget
any definition, whenever you need you can just
on the side of a page you can draw this diagram
and easily explain it to anybody.