On Friday night December 21st 2007 the waxing gibbous Moon would occult the northern section of the famous Pleiades (Messier 45 or seven sisters) during the late evening. Mid occultation would occur after 22.00 UT when it would occult a number of the cluster's fainter members but most importantly the naked eye star Maia would vanish behind the lunar disk. Since the Moon was in waxing gibbous phase it would present a dark terminator as it approached the star, then snuff the star out from existence like clock work. Occultations like this make for very pleasant viewing and show the observer that the solar system is a very dynamic place.
Since this event would occur only four days before Christmas I was eager to observe it for a little festive atmosphere. Above the Earth's atmosphere the Universe was behaving however the weather below was not. For the last few days my location has been covered in a severe frost and dense banks of fog which even in day time seemed to hang over the sky making the Sun's disk an easy and comfortable sight with the naked eye. After sunset all I could see was the Moon and nothing else so I was having doubts about my planned observing session.
Fog and frost over the car park in Cookstown outside Argos. The fog and frost were much worse than they appear on this image!
Soon after a situation arose. My family was planning on doing some Christmas shopping and I was asked if I wanted to go. I had a double take. If I went, I risked missing the event, however if I stayed I might be fogged out all night and miss a nice evening among festive shoppers. I decided to have the best of both worlds and went Christmas shopping with my camera and tripod with me in the backseat of the car. After leaving the exit at Magherafelt roundabout the fog thinned and things were looking good. 30 minutes later we where in the car park outside Argos in Cookstown so I set up my camera and began imaging the Moon while my mum and sister did their thing. I was getting some very strange looks from passing shoppers and one guy who was sitting in his car (obviously waiting for his wife to return from the shops) looked very intrigued and fascinated by what I was doing. He kept starring up at the Moon and back to me and at one stage I thought he was going to come over but he never did. Pity though, as I had already prepared a quick speach in my head about the occultation.
I could not see M45 with the naked eye but after a few shots with the camera I could clearly see the the cluster on the LCD screen close-by to the SE of the Moon and the gap was closing fast. Taking an image of this event was an extreme challenge as you have two subjects with such a phenomenal range in magnitude. Naturally the Moon would have to be overexposed in order to pick up the individual stars however too much of an exposure would cause a digital white-out. I experimented with the exposure and closed the aperture down low which seemed to get a decent result, even through the fog and frost. I had captured the first stage so things were good. The car park was full of shoppers and a dense fog drifted over the ground with an eerie blue-green colour from the sodium lights.
When my family returned we then drove to the local ASDA shopping centre. I went inside and took part in the shopping then when I returned I set up the camera again for another photo session which got me more strange looks. This time when I previewed the image I could see that the cluster did not look its usual self. Bright Maia was gone from the sky and now behind the eastward creeping disk of the Moon. The frost was severe now and the ground and car rooftops where caked with the stuff which I liked to see!.
On the way back we stopped in Magherafelt for a takeaway and while our food was being prepared I was out in the street again getting more captures. A chip shop was an unusual place for imaging occultations!. By now the Moon had moved rapidly to the east grazing the northern sector of the cluster which was now partially swallowed by the disk. At last we where on our way home and my goodness what a treacherous drive it was. The visibility was down to ten meters with very think moonlit fog and the frost was bright and dangerous. If it wasn't for the cats eyes I am certain we would have ended up in field. At one stage we almost veered into the bridge over the Moyola River. It wasn't just us as other drivers had the same trouble, some of them were on our side of the road as they approached us!. I haven't seen road conditions this bad in a long time.
Back in Maghera I was able to watch the end of the show from my back garden. I had a good visual look through the 8.5" F/7 reflector and 10x50s. A few more images showed the cluster to the south of the lunar limb however Maia and her fainter companions where still behind the disk. I never seen the star emerge again as more dense fog followed by cloud arrived. I was quite pleased with the results though. I had images showing various stages of the occultation taken through fog and frost - all in between festive periods of Christmas shopping. You can't get better than that. What a great way to spend the Winter solstice!...clear skies.
Moon and Aldebaran above 'Moto Bites' takeaway in Magherafelt. M45 was deeply occulted at this stage.
Martin Mc Kenna