Pluto, 15-18 May 1986
The middle of May 1986 brought some good, clear skies that were ideal for searching for that most elusive planet, Pluto1. I started the search on the night of Thursday 15 May. I began with binoculars and easily located the magnitude 6.5 star2 near the centre of the BAA finder chart shown below; this star is approximately 2° west of the naked eye star 109 Virginis and is easy to find with the aid of Norton's Star Atlas or Sky Atlas 2000. I centred my 250 mm reflector on the magnitude 6.5 star. Using a low magnification eyepiece (x80 Erfle) I was able to identify the fainter stars in the area surrounding the position of Pluto. The Moon was approaching first quarter and proved troublesome so, having located the area of sky where I thought that Pluto was located, I increased the magnification to darken the sky background and increase contrast. I used a range of magnifications from x80 to x280 to draw the star chart below. Unfortunately, because I had convinced myself that I knew the position of Pluto, I drew only the stars close to the position of interest and was wrong! I had thought that Pluto was close to the star marked "A" and because I could see several stars in addition to those on the finder chart I thought that one of them must be Pluto. So, preconception and careless plotting meant that I did not observe Pluto on this occasion; it was in fact located a few minutes of arc further west. Unfortunately I didn't discover my error until the following day when I re-checked the position of Pluto.
The following night, Friday 16 May, was also clear, but the Moon was older and its light more troublesome. I quickly found the appropriate area of the sky and again using various magnifications was able to extend my star chart westwards. I found several stars visible in the telescope which did not appear on the finder chart: one of them must be Pluto!
The next night was cloudy and I was unable to confirm the observation. One night later still, Sunday 18 May, was again clear. The Moon was a problem but with high magnification I could discern the faint stars seen on previous nights (albeit with great difficulty!) and one of them had moved - this had to be Pluto!
The drawing below records my sketches of the star field made over the three nights 15, 16 and 18 May. The rectangle shown on the finder chart corresponds approximately to the area shown on the star chart. During this period, OASI member Mike Harlow took a photograph on 15 May of the star field around Pluto which recorded the planet. Unfortunately, the photograph would not reproduce well so is not shown below.
BAA finder chart for Pluto, 1986.
Sketch of the star field around Pluto, 15, 16 and 18 May 1986.
||At the time of the observations, the solar system was understood to contain nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. It was on 24 August 2006 that the IAU redefined the term planet in a way which excluded Pluto and reclassified the latter as a dwarf planet.
||Approximately a decade after David's observations, the star was catalogued as Hipparcos 71510.