Graze Occultations

GRAZE  OCCULTATIONS                           

     This talk was accompanied by a complex diagram, presumably projected via an epidiascope ]  


     I am not sure how often in recent years a graze occultation was observed by the society, but years ago at least one or two were observed, though the organisation always entailed a lot of work and commitment. Is anyone not familiar with what goes on at a graze occultation ?

    It occurs when, during its steady progress, the Moon appears to graze the top or bottom edge of a star, especially when it is no more than first quarter or after the last quarter. The star can wink several times, disappearing when it is hidden by mountains on the rim, and reappearing when the relief is lower.

   The principle will be shown on the drawing observing this event, which follows a very narrow cross country track, and takes a lot of keen observers, with cars and portable telescopes.  The track and time can be found in the BAA handbook, but a suitably spacious spot on the track, where everyone can park and set up, must be chosen and checked out earlier.

   On the night, the live weather forecast is rung up, using a confidential number. If clear everyone goes to the spot with telescopes, mounted binoculars, and stopwatches. A line of telescopes is set up at right angles to the track, and for each telescope the timings are noted of each disappearance and reappearance of the star grazed by the Moon. These events will vary according to where the observer is along the row of telescopes, extending over several hundred yards. When all the timed ‘ons’ and’ offs’ are collated you should get an accurate view on paper of the mountains and valleys of the Moon, seen edge on at the limb.

   While this is familiar territory to a number of us, what may be less familiar are some facts and figures best illustrated in the drawing. [These cannot easily be transcribed from the diagrams into text].

   Some weeks ago Rodney teased us with the question :How frequent is a double transit of Venus and Mercury?. So I thought how wide (fuzzy) and fast would be the Moon shadow cast by the star?.

    Assume all other stars, terrestial lighting, and the Moon light were  extinguished, so all you had was the graze star. This would cast a very faint shadow of the Moon’s profile, moving fast along the track of the Earth where the graze occultation can be seen. The Moon shadow is so faint we need the telescope line-up technique to observe it. Looking at the drawing, here is the big picture. I have chosen a typical star, about 1 solar diameter, 1 million kms in diameter, and this is conveniently located 10 parsecs (32.6 lightyears) away.

   Although too small for even the best telescopes, the star is a disc, not a point of light, so the Moon shadow it casts on the Earth is not razor sharp but has a very small penumbria making the shadow blurred.

    The question I would like you to consider is this. How wide is the fuzzy edge of the Moon shadow cast by the star, given the chosen size 1 distance as typical ?.

     If you could freeze the whole action, how far would someone have to walk so that they cross into the shadow, seeing the star at one point, walking across the shadow until the grazing star is invisible ?

    I won’t make you wait a fortnight. In fact it would be only a metre, the blurred edge is only a metre wide. If the star were of similar size, brighter, but 3,200 light years away, the blurred edge would be only one centimetre wide.

    I could ask how fast the Moon shadow moves along its track, but this is very variable, depending on whether the time is am or pm, the latitude, and the angle above the horizon of the Moon and star. The typical speed would be 20 to 150 kms per minute.

   Graze occultations cannot be successfully observed when the Moon is full or nearly so, because the sunlit limb drowns out the occulted star. However, such a full Moon graze would work if it took place during an eclipse, ideally total, but the right kind of partial would also do. How often would an eclipsed full Moon graze occultation occur? Assume one normal graze a year is close enough to be seen by the Bristol Astronomical Society, good lunar eclipses can be seen from the UK for perhaps a combined total of 4 hours a year, one sixth of a day.

   Therefore the probability of a graze occultation occurring locally during a suitable lunar eclipse is 1 X 365 X  1/6th., that is about once in every 2200 years.

                                      Thank You.