On the morning of Saturday March 20 NZ Daylight Saving Time (night of March 19, Universal time) asteroid (524) Fidelio will occult (pass in front of) the magnitude 3.6 naked eye star Tau Librae. The purpose of this page is to provide a quick overview of the event and how it may be observed. Please refer to other pages on this site for more detailed observing information.

INTRODUCTION

The result we hope to attain by observing Fidelio to pass in front of Tau Librae is an estimate of the diameter and shape of the asteroid.

This information can be derived if a large number of people - each at geographically separate locations - each observe the occultation. An "observation" in this sense means an accurate timing of the instant at which the star disappears and the instant at which it reappears as Fidelio passes in front of it. Because each observer will see the star hidden by a different part of the asteroid, if enough observers are distributed across the path of the asteroid's shadow, combining their observations will - at minimum - allow a good diameter estimate to be made for Fidelio. And they may even allow details on the asteroid's limb to be resolved.

WHY IS THIS OCCULTATION ESPECIALLY IMPORTANT?

Firstly, the star involved is a naked eye star with a magnituide of 3.6. It can be observed without optical aid. It is very unusual for a naked eye star to be occulted by a minor planet, let alone a star of this brightness. Because the star is so bright it provides an opportunity for many more people to become involved in this observation than would normally be the case. Secondly, as the adjacent map shows, the shadow of Fidelio is likely to pass across New Zealand. However be aware that the exact path shown on the map is only one estimate of the occultation's true shadow track, and it should not be taken too literally; the true path may well lie east or west of this estimated path. The direction of the path does however allow observers over the full length of New Zealand an opportunity to observe this event. The asteroid's shadow is travelling from north to south and tick marks on the map are at one minute intervals. The tick mark showing 03h 40m NZ Daylight Saving Time is shown. This allows you to work out the predicted time for your location. However see the notes below concerning when you should start and stop observing.

The Fidelio occultation may well be the best opportunity we have this year to observe a minor planet occultation.

Click this link for predictions computed by Edwin Goffin in Belgium. These predictions include charts to help you find the star. For information about interpreting the predictions click here.

Click this link for predictions computed using David Herald's OCCULT program.

HOW DO I OBSERVE THIS EVENT?

Because the star is so bright you can observe with your naked eye (if you are in a dark location and are able to focus continuously on the star). However we recommend that you use binoculars if you have them.

If an occultation takes place, the star will COMPLETELY DISAPPEAR FROM VIEW for anything up to 16.5 seconds. The event will probably take you by surprise so you need to be prepared. If using binoculars they need to be held very steady on some support so you can give maximum concentration to watching the star. If using a telescope, make sure you are sitting comfortably for the same reason. Also make sure you have correctly identified the star at least 30 minutes before the event.

To obtain accurate timings of the instant of disappearance and reappearance you will need an accurate timebase and some form of recording device. If you own a shortwave radio, the most common timebases available are the time signal shortwave stations VNG (on 5.0, 8.638, 12.984 and 16.0 MHz) and WWVH (on 5.0, 10.0, 15.0 and 20.0 MHz). Each station broadcasts time pips on the second. If you do not possess a shortwave radio, you can use the time pips from NZ National Radio as described below.

The best device to record your observation is a portable cassette tape recorder. Place your shortwave radio (or an AM radio tuned to NZ National Radio) close to the telescope and position the tape recorder to pick up both the radio signals and your voice comments. Yell "Off" or "Gone" when the star disappears; and "On" or "Back" when it reappears. Continue observing and recording for at least 10 minutes either side of the predicted time. This is important because of possible errors which may still exist within the predictions.

If you are recording NZ National Radio, make sure that you leave the tape recorder running until it records the 4.00 am time pips. You can then stop recording. Later, use a stopwatch to determine the length of time between your "Off" and "On" comments and the 4.00 am time signal, and thereby work out the time of the events.

If you have recorded VNG or WWVH you can easily replay the tape several times to work out the time of disappearance and reappearance relative to the time pips. Repeated playing of the tape will usually let you determine the time of your comments to within 0.2 seconds. (Remember to make sure that you know which minute beats are which! This is particularly true in the case of VNG which does not broadcast voice announcements at every minute beat).

More detailed information about recording techniques is available here.

Even if you don't obtain timings your negative observations are useful as they may help us refine the limits of Fidelio's occultation track. Details of your observation (positive or negative) should be forwarded as soon as possible to the RASNZ Occultation Section at P.O. Box 2241, Wellington, or by email to Graham.Blow@actrix.gen.nz. An email report form is available here.

SUMMARY OF EVENT DETAILS:

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