asteroid occultation

Hi all,

1. Further to last email, the asteroid (773 Irmintraud) is moving from SSE to NNW, so the track actually enters Ireland at the SE Coast, and exits at Sligo/Mayo.

The exact predicted central track over Ireland is as follows, with times in UT. Add 1 hour for BST ! ! !

Longitude Latitude Time (UT)
Long. 5 deg 0m 0s W; Lat. 51 deg 14m 03s N; 01h 19m 03s.
Long. 6 deg 0m 0s W; Lat. 52 deg 03m 33s N; 01h 19m 11s.
Long. 7 deg 0m 0s W; Lat. 52 deg 49m 54s N; 01h 19m 19s.
Long. 8 deg 0m 0s W; Lat. 53 deg 33m 23s N; 01h 19m 25s.
Long. 9 deg 0m 0s W; Lat. 54 deg 14m 14s N; 01h 19m 32s.

The nominal track width is 112km, with an uncertainty (1 sigma) of about 40km either side of the track.

The following towns are near the centre of the track (SE to NW): Wexford, Enniscorthy, Carlow, Tullamore, Moate, Roscommon, Charlestown, & Ballycastle (Mayo).

Dungarvan & Galway lie just W of the predicted limit, and Dublin & Sligo lie just E of the predicted limit, but all four cities are within the uncertainty limits, so it's still worth observing from there.

For those of you who have never observed a similar event (which is most of you!), you WON'T see the asteroid at all, unless you have a very big telescope, or CCD imaging - it's magnitude 13.8, and the sky isn't totally dark at that time! So it will be an 'invisible' object, moving across the sky, which will, if predictions are correct, pass in front of a 6m.3 magnitude star, making it disappear. If you are on the centre of the track, the star will suddenly disappear for up to 10 seconds (if our assumptions of the diameter of Irmintraud are correct). If you are off centre, the duration will be less, down to just a fraction of a second if you are right on the edge of the track.

So you just keep observing, or imaging, the star, starting preferably 10 minutes before the predicted time, and observe/image continuously until 10 minutes after the predicted time. If you can't manage that length of time, concentrate on the predicted time +/- 5 minutes.

Record the time of disappearance & re-appearance as accurately as possible. Be prepared for the possibility of a 'double disappearance & re-appearance', which could happen if the asteroid is a close binary, or even hour-glass shaped and you lie a bit off the centre of the track. This is where continuous imaging is best - it will record all fluctuations in light. If you just have one stopwatch, you can only record 2 times (use the 'stop' & the 'elapsed' or 'lap' time functions - and practice beforehand!).

If we get a series of accurate timing from various locations we can determine the size & shape of the asteroid to a high degree of accuracy. Do have a go - after all, the star will be visible even in binoculars! If you are using binocs, it will be MUCH easier if you mount them on a tripod. The star is also designated 31 Piscium, and it lies 0.5 deg N of 32 Psc (mag 5m.7), which itself lies 1.8 deg NNE of Omega Psc (mag 4).

REMEMBER - it's at about 02.19 BST. Be ready to observe no later than 02.10 BST!

2. Huge Sunspots! Wow! Do look at the Sun if you can do so safely - if you are not sure, DON'T! There are 4 large spot groups, and other small ones, now on the disc. One is just past the meridian, two are just approaching it, and the biggest of all has just come round the East limb. It's a whopper - mainly a single large very dark umbral spot & good penumbra - The umbra alone is comparable in size to Venus during its recent transit! Use a PROPER solar filter if you have one, otherwise project the image through the telescope onto a piece of white card held about 1 foot behind a low-power eyepiece. DON'T Look at it directly, or through any other sort of filter.

Good luck to all.

Terry Moseley

1. OCCULTATION, night of 20/21 July.
I have now done a more accurate plot of the track, using the exact latitudes & longitudes, rather than just using the rather small scale & foreshortened map on that website.

The central line runs from "The Raven Point" which is the promontory jutting out on the N side of Wexford Harbour, to Kilglass (just N of Inniscrone, Mayo) exiting near the NE corner of Killala Bay.
The following towns are within 1-2 km of the centre line: Enniscorthy, Portlaoise & Tobercurry.
Roscommon & Ballaghadereen are within 5km to the SW of the line, and Carlow, Mountmellick, Tullamore, Clara, Moate, Frenchpark & Easky are within 10km of the NE of the line, and Wexford, Athlone & Castlerea are within 10 km SE of the line.

Correction to earlier email: using the more accurate track above, Sligo is now within the overall predicted track limits, though near the NE edge.

As a very rough guide, the SW limit runs from Tramore (Co Waterford) to Westport and the Mullet peninsula, and the NE limit from Wicklow to Bundoran, but you may still see an occultation if you are outside these limits, given the uncertainty in the track. Concommitantly, you might NOT see an occultation even if you ARE within the predicted limits, as the track might actually pass a bit to either side of the predicted limits.

So if you observe, and DON'T see any occultation, and you are fairly sure of that, then REPORT THAT! That will set a limit to the edge of the track, and hence to the maximum diameter of the asteroid.

Ideally, it would be good if every amateur astronomer between the lines joining Youghal to Clifden, and Balbriggan to Falcarragh, could observe that star for at least 5 minutes before & after 02.19 BST on 21 July! I know it's late, but you can go back to sleep again afterwards. ("Time for bed", said Zebedee. "OK" said Irmintraud. Any Magic Roundabout fans out there?)

For all reports, give your exact position: i.e. map reference, lat & long, or some other unambiguous & accurate description. Also state what instrument & timing method you used, your time reference source (the most convenient might be the speaking clock, tel 1191) the observing conditions, and your confidence in your observation. If you see a definite disappearance lasting for 5-10 secs that's fine; but if you only see a short disappearance lasting for a second or less, say how sure you are that it was real. One way to check that any disappearance is 'real' is to check the visibility of 32 Piscium, just 1/2 degree to the South - if it remains visible throughout, then any disappearance of the other star is probably real.

PERSONAL EQUATION: As Tolis has pointed out, you also need to allow for your reaction time if using a stopwatch, i.e. the short delay between you seeing the event, and your pressing the button on the stopwatch. You can practice reacting quickly by getting a friend to switch a small torch on & off at random. Try to estimate your reaction time, and say whether you have, or have not, applied it to your actual reported recorded times.

There are at least two, and maybe three, 'proper' amateur observatories within the predicted limits, so it would be good if they could participate (I'll not embarass them by naming them!).

I would be glad to receive all reports, both positive & negative (or even if you tried, but were clouded out), but please also copy them all to Tolis Christou at Armagh Observatory <aac@arm.ac.uk>

2. Giant sunspot. That big spot coming over the E limb has now been split into two by a very bright prominent 'bridge', running approximately N-S. It was obvious even using low power on my 70mm Lidl refractor! It's worth keeping an eye on (safely, of course!)

Clear Skies,

Terry Moseley

I've uploaded a map showing the occultation track over Ireland. This is based off Terrys info and includes a margin of about 30 miles each side. It's about 300K, so shouldn't be too big for 56k-ers.

www.irishastronomy.org/user_resources/fi...-occultation-map.jpg

Hi all,

The latest astrometry shifts the path of the occultation by Irmintraud a bit to the SW. The width & times are the same as before, but the centre line now runs from Carnsore Point to Belmullet. This brings Galway City just to the very edge of the actual predicted path.

But the wider uncertainty limits (2 sigma) cover the whole of Ireland, so if it's clear, why not have a look?

Good Luck to all,

Clear skies,

Terry Moseley

tell you what. everyone go out and observe it and terry can make up the patch _afterwards_

Original email

Hi all,

1. Bright star to be occulted by asteroid, 20/21 July. The track crosses
Ireland diagonally from NW to SE, entering at Counties Mayo & Sligo, and exiting
at Wicklow, Wexford & E Waterford. It occurs at about 01.19 UT - that's 02.19
BST!
It will be a very interesting observation if you see it, and if you time
it accurately (to the nearest second, or better - preferably to 0.2 or even
0.1sec) you can do some real useful science!
The star is magnitude 6m.4, and will be occulted by 773 Irmintraud,
magnitude 13.8, so the drop in combined brightness will be 7.4 magnitudes - in
effect the star will totally disappear for up to 10 seconds or so! You''ll need to
be set up about 20 mts before the event, with the star (HIP 186, RA 0h 2m
24.167 secs, Dec +8 deg 57m 24.55 sec) clearly identified, your tracking on, your
timing ready, stopwatch or camera timer checked against an accurate time
source, etc. Time both disappearance & reappearance as accurately as possible. It's
worth looking for up to 15 mts before & after the nominal time, in case there
is a secondary occultation by a satellite of Irmintraud!
The following details were sent to me by Tolis Christo at Armagh
Observatory, who will co-ordinate any observations <aac.@arm.ac.uk>

"Hi Terry, I don't know whether you've made any plans for the Irmintraud
occultation on the night of the 20th July.
We here intend to undertake a little expedition and travel to the centre
of the track; the exact location is to be decided by the weather at the last
minute. For a 6th mag star, I think it is worth it.
I was wondering whether you know of any amateurs that might be willing to
observe from within the nominal track. Check the track map at
www.asteroidoccultation.com/2004_07/0721_773_1842_MapE.gif
If we get enough chords, we should be able to do a shape determination
(best-fit ellipsoid) with Irish-only data.
Thanks, Tolis."
CCD or Digital video camera observations are best, but even visual timings
with an accurate stopwatch will be useful if they are done properly.

I updated the track map for the path over Ireland.

Get it here: www.irishastronomy.org/user_resources/fi...ultation-newpath.jpg

Oy Terry, any chance of moving the path so it passes over my house? ah go on!

I suppose on the plus side, the path is now mostly over dark skys.