Mercury, Venus & Mars,
17 March 1985
On Sunday 17 March 1985, there occurred both the greatest eastern elongation of Mercury, and an occultation by Mars of the 6th magnitude star ZC299 (HD12140, SA092739). These events together with the proximity of the brilliant Venus made it a good opportunity for some early evening observations of the three planets.
The occultation by Mars was predicted to occur at 18:32 UT. During the day there had been scudding cloud and some rain; however as the sun set the western sky became beautifully clear and Venus, shining brilliantly, was particularly prominent. I had opened up my observatory at about 18:20 UT and tried to find Mars. Unfortunately the brightness of the sky was sufficient to render the planet invisible to the unaided eye and, even with binoculars, it took me about 15 minutes to find it. By this time I had missed the occultation (I will have to get setting circles fitted to my telescope!) and had noticed Mercury low down in the west.
I decided to give up Mars for the time being and take a quick look at Mercury. Using an Erfle eyepiece, giving about 90x magnification on my 25 cm reflector, I could make out a small shimmering half-illuminated disk. Turbulence made it difficult to determine the phase accurately but it appeared to be approximately 50%. Increasing magnification to 160x did not help to estimate the phase. The bright side appeared uniformly illuminated and had a faint pinkish tinge which could have been due to atmospheric absorption.
It was now 19:10 UT and I moved the telescope on to Venus. The appearance was of a brilliant thin crescent but again there was considerable turbulence affecting the image. There was a fairly bright star (I guessed magnitude 5 to 6) about 40" to the north-west of the planetary disk. I then went on to Mars but later (about 19:50 UT) came back to Venus and estimated that the distance to the star had closed to about 30-35". I wondered if there might be an occultation and continued to observe Venus until it disappeared, about ten minutes later, below the fence that is behind my observatory. I later plotted the position of Venus on a star map and determined that the star must have been ZC168 (HD6966, SA092288), a 6.3 magnitude star in Pisces.
I first studied Mars with the 25 cm telescope at approximately 19:30 UT, about one hour after the occultation. The red disk of the planet was very small but clearly discernible with 160x magnification. I was expecting to see a fairly faint star very close to the planet but could only find a star at least one minute of arc away to the south-west. This must have been the correct star but at this stage I found it difficult to believe there could have been an occultation only one hour previously. When I checked later I found that Mars was actually moving across the stellar background at about 110 arcsec per hour so that the observed position at 19:30 UT was consistent with the predicted time of occultation. The sketches below show views of the three planets as observed on 17 March; the drawings are approximately to the same scale.
David Payne