Start Celestia

[dmz]

Hello!
I began recently to work with Celestia,
but have not found an answer to the following simple question;
- would be grateful, if anyone could help me with my problem:
- ref -
Menu 'Navigation' - Window 'Go-To-Object':
Values 'LAT, LONG'
- question -
After having started Celestia on 2005/08/26, 20:33h(CET)
I realized the 'axis' of the Earth tilted with
LAT= +9,06745 - against the sun apparently in the background - and with
LONG= -95,06198 - representing the degree of longitude,
where the Earth was situated at the time mentioned.
:::
I could identify the LONG-value with the help of an Atlas,
but I have no idea, how to explain the LAT-value (:axis of earth tilted 23.45degrees).
What is the astronomical meaning of the above-mentioned LAT-value
immediately after having started Celestia ?

[selden]

Long = Longitude. It measures the distance on the Earth's surface from the Prime Meridan, parallel to the Equator.

Lat = Latitude. It measures the distance on the Earth's surface from the Equator toward one of the Poles, parallel to the Prime Meridian.

See http://jwocky.gsfc.nasa.gov/teacher/latlonarchive.html

[dmz]

@selden
Thank you for your reply.
:::
However, - and that should have been the proper content of my question -,
could you please additionally explain to me:
"what is the astronomical meaning of the value (LAT=) +9,06745 degrees ?" .....
:::
P.S.: [LAT, LONG] in degrees -
18/08/2005 21:33h LAT= +11,53287 LONG= -108,65313
::: Denver,USA (LONG ca.-105)
18/08/2005 23:33h LAT= +11,50763 LONG= -138,77985
19/08/2005 02:33h LAT= +11,46984 LONG= +176,17975
::: Wellington,Newsealand (LONG.ca.+175)
19/08/2005 08:33h LAT= +11,39416 LONG= +_86,06764
19/08/2005 15:33h LAT= +11,30578 LONG= -_18,86965
19/08/2005 20:33h LAT= +11,24252 LONG= -_93,90244
::: NewOrleans,USA,Mississippi-Delta(LONG.ca.-90)
19/08/2005 21:33h LAT= +11,22987 LONG= -108,87741
::: Denver,USA (LONG ca.-105)
23/08/2005 20:33h LAT= +10,01091 LONG= -_94,55700
::: NewOrleans,USA,Mississippi-Delta(LONG.ca.-90)
24/08/2005 21:33h LAT= +_9,68515 LONG= -109,71662
26/08/2005 20:33h LAT= +_9,06745 LONG= -_95,06198
:::
The axis of the earth is tilted 23.45 degrees.
Why the values of +11,53287 /OR/ +_9,06745 ?

[selden]

Latitude and Longitude are on the surface of the Earth. They are measured relative to the Equator and the Poles. The tilt of the Earth does not affect latitude.

A latitude of 0 is on the Equator.

A latitude of +9 is slightly north of the Equator.

A latitude of +90 is at the North Pole.

A latitude of -90 is at the South Pole.

[t00fri]

Latitude and Longitude are on the surface of the Earth. They are measured relative to the Equator and the Poles. The tilt of the Earth does not affect latitude.

A latitude of 0 is on the Equator.

A latitude of +9 is slightly north of the Equator.

A latitude of +90 is at the North Pole.

A latitude of -90 is at the South Pole.

dmz,

just try the following excercise to familiarize yourself with Long/Lat in Celestia: activate Markers from the options menue. In any atlas you will find that Hamburg is about Longitude ~9.93 degrees East, Latitude ~ 53.6 degrees North.

Put yourself above Earth (30000Km, say) and enter these numbers into the GotoLongLat dialog. After pushing RETURN you will see the Marker go close to where Hamburg is located.

Bye Fridger

[dmz]

@selden, t00fri
Thank you for your reply which confirmed my correct practice in this context.
I have excercised your statements, and use them often to manipulate the earth's view.
:::
The purpose of my (initial) posting is however, to clear up the specification of the LAT-value,
which is given in the navigation window *'Go-To-Object' immediately after start of Celestia.
I have only the above-mentioned LAT-values so far, and lack the experience of a complete year.
So I would like to ask a further question:
:::
Could it be *right that these initial LAT-values, - stated in the navigation window immediately after start of Celestia -,
are related to the position of the sun:
they are values between the tropics +23.45 > 0 > -23.45 degrees ?
Correct or wrong ?
:::
PS: Values of 2005/08/28, 22:48h(CET)
LAT= +8.39978 LONG= -129.27716,
Position of SUN in 'Celestia': LAT= ca.+8. LONG= ca.10:30h, tendency > +7./11:h
-
*corr.2005/08/29

[t00fri]

@selden, t00fri
Thank you for your reply which confirmed my correct practice in this context.
I have excercised your statements, and use them often to manipulate the earth's view.
:::
The purpose of my (initial) posting is however, to clear up the specification of the LAT-value,
which is given in the navigation window immediately after start of Celestia.
I have only the above-mentioned LAT-values so far, and lack the experience of a complete year.
So I would like to ask a further question:
:::
Could it be that these initial LAT-values, - stated in the navigation window immediately after start of Celestia -,
are related to the position of the sun:
they are values between the tropics +23.45 > 0 > -23.45 degrees ?
Correct or wrong ?
:::
PS: Values of 2005/08/28, 22:48h(CET)
LAT= +8.39978 LONG= -129.27716,
Position of SUN in 'Celestia': LAT= ca.+8. LONG= ca.10:30h, tendency > +7./11:h

dmz,

the initial LONG/LAT values are those under the /Marker/ that is centered in the image. Move the earth such that the Marker points e.g. to Hamburg. Then open the Long/Lat dialog and you will recognize the values.

Bye Fridger



such that the Marker is right there

[dmz]

Hello t00fri !
Thank you for your reply.
However your answer does not meet properly with my question.
I have understood, how to manipulate the LAT-/LONG-values.
:::
But start 'Celestia' and try to explain the astronomical spezification of the initial LAT-value,
stated in the navigation window 'Go-To-Object'.
:::
I am expecting LAT=0 on Sept 21. - the beginning of Autumn.

[t00fri]

Hello t00fri !
Thank you for your reply.
However your answer does not meet properly with my question.
I have understood, how to manipulate the LAT-/LONG-values.
:::
But start 'Celestia' and try to explain the astronomical spezification of the initial LAT-value,
stated in the navigation window 'Go-To-Object'.
:::
I am expecting LAT=0 on Sept 21. - the beginning of Autumn.

Sorry this I don't understand at all.

When you open that dialog, you find a pair of LONG-LAT values. These have NO astronomical implicitations besides specifying points on the earth: they are 2d curvilinear coordinates along the Earth's surface. They are naturally independent of the time of the year!
Hamburg has LONG=9.96 deg E and LAT=53.6 N at all times.

You seem to be mixing up something completely: the coordinate frame for LONG-LAT is located on Earth...its axes are not straight lines, but great circles following the earth surface: one = (LONG=0) goes through the north and south poles and Greenwhich/GB observatory. The other (LAT = 0) follows the equator. Locations of the same LONG all lie on circles just like locations sharing the same LAT also do.

Bye Fridger

[Andy74]

dmz,

maybe you talk about ecliptical coordinates.
They are much like Right ascension and declination, but with respect to the ecliptic rather than to the equator. And they are called "longitude" and "latitude".
But don't mix them up with terrestrial coordinates.

Andy

[jestr]

dmz,when you start Celestia ,I'm pretty sure it is designed so as it goes to Earth on the daylight side always (thats why it goes to the sun first),not sure about the Latitude (0,I guess).Jestr

[dmz]

Hello t00fr, Andy74, jestr !

When you open that dialog, you find a pair of LONG-LAT values.
These have NO astronomical implicitations besides specifying points on the earth.

It means, the initial LAT value - immediately after start of 'Celestia' - is a result by accident ?
I have recorded LATs after starting 'Celestia' (see posting above)
since 08/18 with LAT= +11..... --- 08/23LAT= +10.... --- 08/26LAT= +9....---
08/28LAT= +8....---08/30LAT= +7....,
which promise regularity (+LAT>0>-LAT).
So I cannot believe that the LATs are results by accident.
I am not quite sure, but thinking about it I suppose the following:
:::
Having started 'Celestia' the Sun appears for a moment before the Earth is eventually shown.
This is the visual translation that a bundel of sunbeams has hit the Earth's surface,
the center of the bundel creating a right angel with the surface
and marking theoretically a point on the Earth's surface.
This point corresponds with a certain LAT on Earth and is stated in the navigation window 'Go-To-Object'.
As the Earth revolves round the Sun,
while her axis remains each degree in the same parallel plane,
the point (LATvalue) will be changed continuously between the tropics of max.+23.45N > 0 > min.-23.45*S degrees -
with LAT=0 crossing the equator.
Sunbeams hitting the Earth's surface beyond LAT>/23.45/?° never create a right angel to the surface.
In other words:
LAT= +23.45?°, June 21.((9h)>summertime) / LAT= -23.45?°, Dec.21.((3h)>wintertime) -
- earth's axis and sun in the same plane.
LAT= 0?°, Sep.21.((6h)>autumn) / LAT= 0?°, March 21.((12h)>spring) -
- earth's axis and sun in different but parallel planes.
:::
Correct or wrong ?
-
*corr.09/01, 2005

[t00fri]

Hello t00fr, Andy74, jestr !

When you open that dialog, you find a pair of LONG-LAT values.
These have NO astronomical implicitations besides specifying points on the earth.

It means, the initial LAT value - immediately after start of 'Celestia' - is a result by accident ?
I have recorded LATs after starting 'Celestia' (see posting above)
since 08/18 with LAT= +11..... --- 08/23LAT= +10.... --- 08/26LAT= +9....---
08/28LAT= +8....---08/30LAT= +7....,
which are promissing regularity (+LAT>0>-LAT).
So I cannot believe that the LATs are results by accident.
I am not quite sure, but thinking about it I suppose the following:
:::
Having started 'Celestia' the Sun appears for a moment before the Earth is eventually shown.
This is the visual translation that a bundel of sunbeams has hit the Earth's surface,
the center of the bundel creating a right angel with the surface
and marking theoretically a point on the Earth's surface.
This point corresponds with a certain LAT on Earth and is stated in the navigation window 'Go-To-Object'.
As the Earth revolves round the Sun,
while her axis remains each degree in the same parallel plane,
the LAT value will be changed continuously between the tropics of max.+23.45N > 0 > min.-23.45E degrees -
with LAT=0 crossing the equator.
Sunbeams hitting the Earth's surface beyond LAT>/23.45/?° never create a right angel to the surface.
In other words:
LAT= +23.45?°, June 21.((9h)>summertime) / LAT= -23.45?°, Dec.21.((3h)>wintertime) -
- earth's axis and sun in the same plane.
LAT= 0?°, Sep.21.((6h)>autumn) / LAT= 0?°, March 21.((12h)>spring).

:::
Correct or wrong ?

dmz,

"Du siehst den Wald vor lauter Baeumen nicht!"

Your conclusion that given my explanation, the LONG-LAT values in the dialog must be accidental is just incorrect.

I have explained it all above, but you did not read carefully. I am a developer and co-author of Celestia and thus happen to know the code!

The (activated) red marker in the /center/ of the Celestia canvas initially points to a particular spot on the Earth surface. That spot on Earth (LONG, LAT) to which the marker points, of course depends on the DATE when you switch on Celestia, since the Earth rotates under the marker!!! NOTE: Celestia uses the actual time when you switch it on.

Here is another experiment to prove that I am right.

Go to 'set time' in the menue and set some local time, hit 'apply'. The earth moves to some position exposing a certain location under the red marker. Read off LONG-LAT from the Goto LONG-LAT dialog and estimate the LONG-LAT of the marker spot from some atlas. The two pairs of data should ~ agree qualitatively!

Set another time , several hours earlier or later, hit 'apply'. The Earth rotates to a new position with the red marker in the center now pointing to another place. Compare again the values in the Goto LONG-LAT dialog with values from an atlas.

In the view that is displayed initially, the marker points to the place where the sun is precisely overhead at the particular time of the day. You can notice the sun's reflection in the sea! So it's always a daylight view.

And so on. Can't be THAT hard ....


Bye Fridger

PS: Here is the 'start.cel' script that is executed after startup:

Code: Select all

  preloadtex { object "Sol/Earth" }
  preloadtex { object "Sol/Earth/Moon" }

  select {object "Sol"}
  goto   {time 3.0 distance 30}
  wait   {duration 3.0}

  select {object "Sol/Earth"}
  follow {}             <====================
  goto   {time 3.0  distance 6.0}
  wait   {duration 2.0}