Hello Roger,


Guide does not need the time of perigee. It asks for the epoch of the
elements. This epoch is in your mail (52 400), in a form corresponding
to a modified julian day (MJD) or a kind of :
variable stars' observers often use
MJD = JD - 2 400 000.5
but maybe the epoch given at JPL follows the convention
"MJD" = JD - 2 400 000.0
Check on their website. Of course JD is the julian date.

The easiest way to find the corresponding date is to enter the JD
(not MJD) in the time menu of Guide and to watch the day, month,
year, hour, minute, seocond.

Assuming the "JPL convention" (52 400 = JD - 2 400 000) then
for the perihelion the JD is 2 452 400 and the date is (from the
time menu of Guide :-)
5 May 2002 12 h (5.5 May 2002).


Perhaps you was wondering how to compute the _date of perihelion_ ?
Here are some explanations.

The orbital elements you refer to are osculating or instantaneous ones.
They would accurately characterize the orbit in the case the asteroid
undergoes no perturbation. It is not the case. But they are good
enough for short time spans. As a consequence there is no simple
way to find the date of perigee accurately.

But simple maths allow to find a good _approximation_ if the
asteroid's motion has weak perturbations and has therefore a nearly
keplerian motion.

1. From the semi-major axis (a), compute the period (P) using
the 3rd law of Kepler and the mean motion (n):
(n^2) * (a^3) = (0.985 607 6686)^2 * 1^3
(i.e. (n^2)*(a^3) for the Earth).
Hence
n = 0.985 607 6686 / (a^1.5).
This result is in degrees / day.
-> n = 0.985 607 6686 / (2.040 6306^1.5) = 0.338 109 5843

2. Compute the time of last perihelion (T) before the epoch of
the elements, for which the mean anomaly (M) is given:
T = t - (M / n)
-> T = 52 400 - (308.613 5465 / 0.338 109 5843) = 51 487,2381

3. See above the remark about MJD.
Assuming "MJD" = JD - 2 400 000
-> T = 2 451 487.2381
-> T = 4.7381 Nov. 1999

4. If necessary add or substract an integer number of periods.
The period (P) has to be computed first:
P = 360 / n
-> P = 360 / 0.338 109 5843 = 1064,7436
-> T' = T + P = 2 451 487.2381 + 1064.7436
= 2 452 551,9817
= 4.4817 Oct. 2002

Keep in mind that these are only approximations because
there are perturbations. The true date of perihelion is likely to be
off by several days (the farthest from the epoch the worse).
I hope this will help you :-)


Laurent Zimmermann
http://users.skynet.be/zmn/cab

> I went on the internet and got the orbital elements for 2002 NY40, the
> asteroid that will make a close pass to the Earth on Aug. 17.
> I did CTRL+K to enter the elements. Here are the elements that were on
> the JPL website:
>
> 2002 NY40
> epoch = 52400
> a = 2.0406306
> e = 0.71027602
> i = 5.77204
> w = 268.27294
> Node = 146.86145
> M = 308.6135465
> H = 18.86
> G = 0.15
>
> The date of perigee is not given in the JPL o.e.'s. Can anyone tell me
> how to compute it or how to input it the elements into Guide without it?