![[image]](img/calc.jpg)
I N S T R U C T I O N S F O R
U S E
Access to the ephemerides service.
On the home page of the ephemerides service of the
natural planetary satellites, you will find links to several ephemerides.
First of them is MULTI-SAT .
You will find the following types of ephemerides with options:
- Ephemerides for given dates
- Ephemerides tabulated with a constant time step
- Ephemerides for a given series of dates allowing the calculation of (O-C) if data are provided
- Graphical representation of the satellites systems configurations
- Times of apparent events
- Times of maximum elongations of satellites
- Mutual occultations and eclipses and eplipses of satellites by planet
These tools use the same basis for the modeling of satellite motion
and differ only in the way in which input data are entered and
the results are displayed.
Working with the different tools is similar.
Selecting your choices in the following menus:
- the model of satellite motion to be used for the calculations
- the satellites for which the ephemerides will be made.
- the celestial body relative to which ephemerides will be calculated.
This may be another satellite or the center of the planet. If you select "geo-equator
and equinox", the satellite's absolute coordinates, right ascension and
declination, will be calculated.
Note that it is also possible to calculate the position of the
center of mass of the system relatively to the center of mass of the planet.
- the ephemeris for the planet. If
absolute coordinates of the satellite are required, this choice is significant
since errors on coordinates of the planet are completely included into
the errors on the satellite positions. When calculating satellite relative
coordinates, the influence of errors of planetary coordinates is significantly less.
- the observing site for which topocentric satellite coordinates must be calculated.
The number of the observatory should be entered manually. To find out the number,
use the corresponding list to which a link is given. If the number is 500,
the satellite's geocentric coordinates will be calculated.
- the epoch of equator and equinox of the reference system. The variant
Jan 1st means the selection of the mean coordinate system of the beginning of the
year to which belong the ephemerides dates.
- the time scale of ephemerides (TT: Terrestrial Time formerly Ephemerides Time;
or UTC: Universal Time, mostly used by the observers).
- the format for the starting date or of the given series of dates.
- the starting date or a series of dates associated or not
to observed positions in order to get (O-C)s.
Specific choice for ephemerides tabulated with a constant time step:
the unit for the time step, the value of the time step in this unit and the number of steps
the type of ephemerides data that you wish in output
Specific choice for ephemerides for a series of dates
with optionnaly the calculation of (O-C) if observed data
are provided
the type of observational data that you wish to submit in input if so and
which will be in output if not.
please indicate then if you will enter observational data following the dates or if you
enter only a series of dates.
enter your set of data in the windows designed for this purpose
Specific choice for the graphical representation
of the satellites systems configurations
choose the satellite which will be cross-marked
choose the satellite (or planet center) which will be put at the center of field
choose the type of coordinates that you wish for the satellites
positions displayed on the screen
choose an initial scale for the field displayed on the screen
Specific choice for the search for times of maximum elongations
of satellites
choose the type of max. elongation :
max. elongations from planet axe
max. elongations from orbit axe
max. and min. distance
max. elongations in X tangential equatorial coordinate
Specific choice for the search for mutual occultations
and eclipses and eplipses of the satellites by the planet
the time interval during which you look
for events (time unit and number of units)
the time limit of calculation in minutes (less than 5 minutes)
the type of events :
mutual events of the main satellites only
mutual events of the main and small eclipsed
mutual events of the main and small
eclipses by planet of main satellites only
eclipses by planet of main and small
Note that the web-server itself limits the time of calculation
usually up to 5 minutes.
How to start the calculation of the ephemerides
Calculations start after clicking on
the "Calculate" (or "Show") button.
A new page will appear providing the results of the ephemerides.
These results may be copied in the user's file through Windows.
Data in output
This ephemerides server offers numerous possibilities of
ephemerides data in output. To know which ones,
please click here.
Features of the ephemerides for a given series of
dates and calculation of (O-C)s from observed positions.
This tool is used for solving two problems.
- First, it is possible to enter,
in tabular form, a series of dates for which ephemerides must be calculated.
The format is "Year, month, day, hour, minute and second" or "Year, month, day
and fraction of day
- Second, it is possible to enter into the window a series of lines with observational
data followong the dates and to obtain the residuals (O-C) for these observations
from the ephemerides for all the
observations at once as well as some statistics.
The choice between these two possibilities is made by the user by means
of radio buttons on the Web page.
Formats of input dates and observed values are selected from
the corresponding menus. Then, on each line of the window, the date of
observation (or for ephemerides) is entered and, if necessary, observational data
in right ascension and declination
or in any other type of coordinates proposed as ephemerides output.
All the entered values are to be separated by one
or more blank spaces.
To enter data into the window it is convenient to use the
clipboard.
At last the user will click on the "Calculate" button.
A new page will appear displaying the (O-C) results or the calculated ephemerides.
The results may be copied into user's files using the clipboard.
The units of (O-C) results are always arcseconds.
The table with the results is followed by the mean values
of the (O-C) in right ascension and declination
or other coordinates, mean and root-mean-square values
of (O-C) between observed and calculated positions
of the satellite.
This Web tool also allows to compare ephemerides by entering
a set of calculated positions as observed positions.
You may enter your own calculations or compare
the ephemerides proposed here by entering the results
of calculation for an ephemeris as observed positions
to be compared with another ephemerides.
Features of the graphical representation of the satellites
systems configurations
After clicking on the "Show" button a new page will appear
providing the graphical representation
of the satellites systems configurations. The
comments and menus are similar to those on the parent page.
Choosing new values of the configuration parameters, you can
refresh the image to get the new configuration.
To see the relative coordinates and magnitudes
of satellites you may click on the relevant link on the page of
the graphical representation.
Features of the search for the times
of maximum elongations of satellites.
The home page proposes a link to this option. Then you have
to choose a system of satellites then some parameters
as explaned above for the search of events. Choosing the times
of maximum elongations of satellites, the table on the output
page presents the times followed by the coordinates of the
satellite measured in the apparent planet centered coordinate
system that you choose for the elongations.
The Y-axis (denoted B) is towards the pole of the planet
if you choose "planet-axis", of the orbit if you choose
"orbit-axis" and of the Earth pole if you choose "in X".
If you choose elongations in distance, the positions provided
will be distance and position-angle in Earth-equatorial system.
The X-axis (denoted A) is at 90 degrees in counterclockwise
direction. The position angle of the B axis is also given
under the denomination Tilt.
Features of the search for mutual occultations and eclipses
and eplipses of satellites by the planet.
The home page proposes a link to this option. Then you have
to choose a system of satellites and enter some parameters
as explaned above to get the moments of the start and the end
of the period of events. After clicking of the "Calculate"
button the computation starts running up to the time limit
expiration. A table on the output page shows the parameters
and circumstances of each detected event.
For a mutual occultation or eclipse they are the date
and moment of the start and the moment of the end of the event.
Then follow type, duration, impact factor, combined magnitude
of the satellites, maximum magnitude drop, apparent distance
of the involved satellite from the planet limb and apparent
distance between the satellites in the case of a mutual eclipse.
The type of the event is given in the form "nEm" or "nOm"
where n is the number of eclipsing or occulting satellite,
m is the number of eclipsed or occulted one.
"E" denotes an eclipse and "O" denotes an occultation.
The impact factor is equal to zero if this is a central passage
of satellite through the shadow or through the apparent disk
of another satellite.
The impact factor is equal to unit 1 in the case of a grazing
event. Negative value of the distance from the planet limb
occurs if the occulted or eclipsed satellite is on or under
the planetary disk. After the table providing the ephemerides,
the last date tested for an event is provided.
You may use this value to restart the search.
In the case of search for eclipses of the satellites by the planet
the planet is denoted "0" and the time is shown in brackets
for the event starting or ending invisible from the Earth.
In this case the distance from the planet limb is omitted also.
Note that in the case of mutual eclipses the start of the event
is assumed as the first contact of the eclipsed satellite
with the penumbra of the eclipsing one, and the end of the event
is assumed as the last contact.
In the case of the eclipses by the planet
the penumbra from the planet with the corresponding limiting cone
and complete shadow with the corresponding cone are taken into account.
The shape of the planet is approximated by an ellipsoid
with known equatorial and polar radii.
The beginning of the eclipse is at first touch
of the penumbra by the satellite.
The end of the eclipse is the last touch
of the penumbra by the satellite.
The time of "last speck"
when the trailing limb of the satellite enters the shadow cone,
and the time of "first speck"
when the leading limb of the satellite first exits the shadow cone
are considered also.
The time of "last speck" and the time of "first speck"
are added in the output of the server (two last column).
Note that the brightness of the satellite is changed only
between the beginning of the eclipse and "last speck"
and between "first speck" and the end of the eclipse.
Intervalle of time where ephemerides are calculated.
The calculation of the ephemerides
is limited in general to the
intervalle 01/01/01 - 27/08/3026
If you wish to calculate absolute coordinates of satellite
(right ascension and declination) some planetary ephemerides
will be involved directly in the satellite coordinates.
A set of planetary ephemerides
(INPOP06, DE200, DE405, DE406, VSOP87)
is available on the relevant time intervals
smaller than the interval proposed above
(see the section 'sources' of the Guide).
Outside these intervals, the calculations will be made using
an approximation for the ephemerides of the planet.
For the relative satellites coordinates this approximation
is sufficient.
Using different theories of the motion of the satellite,
the time interval of the ephemerides of reasonable accuracy
will be limited by the features of the used theory.
Ephemerides of all outer satellites are calculated
using our original numerical models built for given time intervals.
If the user enters a date
out of the interval of our numerical models,
the calculating program uses approximated Keplerian orbits which
give some representation of the orbit but
do not provide a good precision of satellite coordinates.
For different satellites, the intervals of the numerical models
are different.
|
|
Dates of the first and the last observations
used for ephemerides,
intervals of the nimerical ephemerides of the satellites
of . . .
|
|
|
Jupiter
|
|
|
Saturne
|
|
|
Uranus
|
|
|
Neptune
|
|
|
![[image]](img/tag_p3.jpg)