Orbital Elements question

In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound right?

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000. I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

I'm hoping someone will set me straight if my assumptions are flawed.

Regards,

--
Hop David
http://clowder.net/hop/index.html

Hop David writes:

In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound right?

That depends. I will note that the precision in your spreadsheet
corresponds to about 3 minutes, whereas the precision provided by
Horizons is about 4 milliseconds (half of the least significant
digit shown).

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000.

Not a good assumption. Besselian years have gone out of style, but
it's possible your spreadsheet uses them. Julian years are in vogue,
but a Julian year is 365.25 days long. Or it's possible your
spreadsheet uses something else.

I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

Try 365.25 days for all years.

I'm hoping someone will set me straight if my assumptions are flawed.

It's hard to be certain, because you didn't say anything about the
source of your spreadsheet.

"Hop David" wrote in message
...
In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound
right?

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000. I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

I'm hoping someone will set me straight if my assumptions are flawed.

You should check to see if your spreadsheet is assuming
Julian years (365.25 days each), or taking into account
leap years and providing day number as a fraction of the
"true" year.

Are the elements you looked at from Horizons the
osculating elements for a particular epoch?

wrote:
Hop David writes:


In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound right?


That depends. I will note that the precision in your spreadsheet
corresponds to about 3 minutes, whereas the precision provided by
Horizons is about 4 milliseconds (half of the least significant
digit shown).


I'm assuming that 2004-Jan-01.00000 equates to 2004.00000.


Not a good assumption. Besselian years have gone out of style, but
it's possible your spreadsheet uses them. Julian years are in vogue,
but a Julian year is 365.25 days long. Or it's possible your
spreadsheet uses something else.


I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).


Try 365.25 days for all years.


I'm hoping someone will set me straight if my assumptions are flawed.


It's hard to be certain, because you didn't say anything about the
source of your spreadsheet.


I'm making my own spreadsheets in Microsoft Excel. I'll try to give an
example of a use. 2004 FN8 has a period of 1.26148 years. I'd like to
know when the perihelion will be 4 periods after the 2004-Mar-04.6315122
perihelion. I add (4 * 1.26148) + 2004.17386 = 2009.21968.

My regular year spreadsheet looks something like this:
Date #days fraction year
....
19-Mar 77 0.210958904
20-Mar 78 0.21369863
21-Mar 79 0.216438356
22-Mar 80 0.219178082
23-Mar 81 0.221917808

So by this chart I make 2009.21968 to be about 4:20 a.m. on March 22, 2009.

But, thinking about it, I believe it'd be better to use day increments
of 1/365.25 on both my regular year and leap year spreadsheets.


--
Hop David
http://clowder.net/hop/index.html

Greg Neill wrote:
"Hop David" wrote in message
...

In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound

right?

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000. I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

I'm hoping someone will set me straight if my assumptions are flawed.


You should check to see if your spreadsheet is assuming
Julian years (365.25 days each), or taking into account
leap years and providing day number as a fraction of the
"true" year.

I'll go for year/365.25 = 1 day.


Are the elements you looked at from Horizons the
osculating elements for a particular epoch?


I don't know what "osculating" means. I'll cut and paste what I'm
looking at:

HORIZONS Body Information

************************************************** *****************************
JPL/HORIZONS (2004 FN8) 2004-Mar-31
11:33:32
Rec #:143727 (+COV) Soln.date: 2004-Mar-26_00:53:07 # obs: 18
(2 days)

FK5/J2000.0 helio. ecliptic osc. elements (AU, DAYS, DEG, period=Julian
yrs):

EPOCH= 2453089.5 ! 2004-Mar-25.00 (CT) Residual RMS= .49615

EC= .1446108213962278 QR= .9986463372847678 TP= 2453069.131512152

OM= 4.366709836239084 W= 158.4642600792634 IN= 5.332522707958341

A= 1.167476000707456 MA= 15.91442381701383 ADIST=
1.336305664130144
PER= 1.26148 N= .781325739 ANGMOM= .018391461

DAN= 1.32072 DDN= 1.00753 L= 162.9156899

B= 1.9550252 TP=
2004-Mar-04.6315122

Physical parameters (KM, SEC, rotational period in hours):
GM= n.a. RAD= n.a. ROTPER= n.a.

H= 27.034 G= .150 B-V= n.a.

ALBEDO= n.a. STYP= n.a.


ASTEROID comments:
1: soln ref.= JPL#2, 2004 FN8
2:
************************************************** *****************************

Another question:

By taking PER = A^3/2
QR = A * (1 - EC)
Adist = A * (1 + EC)
I get answers that match the Horizon figures almost exactly.

However when I add OM + w to get L, my answer usually differs
substantially (For example for 2004 FN8 I get 162.8310 while Horizons
has 162.9156899 degrees)

I have more questions but I think I'll wait a little while as I don't
want to exhaust your generousity.

--
Hop David
http://clowder.net/hop/index.html

Hop David writes:

In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound right?

That depends. I will note that the precision in your spreadsheet
corresponds to about 3 minutes, whereas the precision provided by
Horizons is about 4 milliseconds (half of the least significant
digit shown).

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000.

Not a good assumption. Besselian years have gone out of style, but
it's possible your spreadsheet uses them. Julian years are in vogue,
but a Julian year is 365.25 days long. Or it's possible your
spreadsheet uses something else.

I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

Try 365.25 days for all years.

I'm hoping someone will set me straight if my assumptions are flawed.

It's hard to be certain, because you didn't say anything about the
source of your spreadsheet.

I'm making my own spreadsheets in Microsoft Excel. I'll try to give an
example of a use. 2004 FN8 has a period of 1.26148 years.

Tropical years? Sidereal years? Julian years? Besselian years?
Anomalistic years?

I'd like to
know when the perihelion will be 4 periods after the 2004-Mar-04.6315122
perihelion. I add (4 * 1.26148) + 2004.17386 = 2009.21968.

Does that mean you now know "when" that perihelion occurs relative to
the calendar we use?

My regular year spreadsheet looks something like this:
Date #days fraction year
....
19-Mar 77 0.210958904
20-Mar 78 0.21369863
21-Mar 79 0.216438356
22-Mar 80 0.219178082
23-Mar 81 0.221917808

So by this chart I make 2009.21968 to be about 4:20 a.m. on March 22, 2009.

But our calendar is based on the tropical year. It's not at all clear
whether the period you used is also in tropical years.

But, thinking about it, I believe it'd be better to use day increments
of 1/365.25 on both my regular year and leap year spreadsheets.

Well, astronomers decided to switch to using the Julian year instead of
the Besselian year, even though it will introduce a drift.

Hop David writes:

I don't know what "osculating" means.

"Osculating" refers to the orbit corresponding to the instantaneous
position and velocity at the epoch of osculation. Gravitational
perturbations by the other planets constantly change the orbit,
therefore the resulting orbit is strictly valid for only an instant
in time, the epoch of osculation.

EPOCH= 2453089.5 ! 2004-Mar-25.00 (CT) Residual RMS= .49615

That looks like the epoch of osculation right there, though it's a
non-standard date.

wrote:
Hop David writes:

Tropical years? Sidereal years? Julian years? Besselian years?
Anomalistic years?

I'm sad to confess I wouldn't know an anomalistic or a tropical year if
they bit me on the ass. I will Google these.

I am hoping the year units given in the Horizon ephemeris is the time it
takes Earth to complete a 360 degree circuit about the sun.


--
Hop David
http://clowder.net/hop/index.html

Hop David writes:

Tropical years? Sidereal years? Julian years? Besselian years?
Anomalistic years?

I'm sad to confess I wouldn't know an anomalistic or a tropical year if
they bit me on the ass. I will Google these.

I am hoping the year units given in the Horizon ephemeris is the time it
takes Earth to complete a 360 degree circuit about the sun.

Well, 360 degrees of true anomaly is an anomalistic year. 360 degrees
relative to a fixed reference frame is a sidereal year. But 360 degrees
from vernal equinox to vernal equinox is a tropical year. They're all
different.

wrote:

Hop David writes:

In my asteroid spread sheets I've been using years and astronomical
units as my units of measurement.

In the Horizon ephemeris it will give time perihelion like this
TP= 2004-Mar-04.6315122
(for asteroid 2004 FN8)

In my 2004 FN8 spreadsheet I have this as 2004.17386. Does that sound right?

That depends. I will note that the precision in your spreadsheet
corresponds to about 3 minutes, whereas the precision provided by
Horizons is about 4 milliseconds (half of the least significant
digit shown).

I'm assuming that 2004-Jan-01.00000 equates to 2004.00000.

Not a good assumption. Besselian years have gone out of style, but
it's possible your spreadsheet uses them. Julian years are in vogue,
but a Julian year is 365.25 days long. Or it's possible your
spreadsheet uses something else.


Idiot! Julian years are a bad choice. Some asstronomer you pretend to be.


I'm also
calling leap year days 1/366 of a year (.00273224) and reg year days
1/365 of a year (.00273973).

Try 365.25 days for all years.


Idiot! 365.2422 is a better approximation. The number of days in a year
are roughly 365 days, 5 hours, 48 mins and 46.08 secs. The average Julian
year has 365.25 days and is 1 day out after 128 years.

More info -

The Gregorian calendar is better and is out 1 day after 3333 years.

In 1923, the Soviet Union introduced a better calendar based on a modulo
9 method with remainders of 2 and 6 being leap years. This is only a day
out after 45,000 years.


I'm hoping someone will set me straight if my assumptions are flawed.

It's hard to be certain, because you didn't say anything about the
source of your spreadsheet.


What we can confirm, however, is that Tholen is an idiot. Only a fool
like Tholen would make such an elementary mistake.

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