Pluto, 26 April 1984

Pluto, the outermost planet¹ of the solar system, came to opposition on Friday 20 April 1984. The apparent magnitude of the planet at opposition was 13.7, and it therefore promised to be visible in the 26 cm Orwell Park refractor. During the following week, we were blessed with excellent weather and clear evening skies making it an opportune time to search for this elusive world.

Using the excellent finder chart printed in Astronomy magazine I began, together with Stewart Dedman and Darren Payne, to search for Pluto on Wednesday 25 April at Orwell Park. We started at about 21:30 UT, first identifying with binoculars the pattern of brighter stars in the vicinity of Pluto, then using the 7.5 cm finder on the Orwell Park refractor to point the telescope to the predicted position of Pluto. We used an eyepiece giving x70 magnification and a field of view of around ½°.

It was difficult initially to identify the stars visible in the 26 cm refractor with the stars shown on the finder chart. The chart below shows the region around the predicted position of Pluto on Wednesday 25 April. We first identified the obvious asterism constituted by the triangle of stars (in the dotted box, below) and then "star hopped" to position the field of the refractor over the predicted position of Pluto. We then increased the magnification to x150 in order to darken the sky background. The circle on the chart below shows the resulting field of view. The seeing was clear and steady (although there was considerable glow from the lights at Felixstowe Docks). The star numbered 1 on the chart was the most easily seen, and the star numbered 2 was next most easily seen, together with the object marked A. The latter was not shown on the finder chart - so was it Pluto? Careful observation of the star field revealed the star numbered 3 but we were unable to see the star marked 1a on this occasion.

Although we were fairly certain that object A was Pluto (Stewart and Darren confirmed my view on this) it could only be confirmed positively by further observations on successive evenings, to see if it moved in the correct manner. Luckily the clear weather held and Thursday night (26 April) was clear. I made a further observation with my 25 cm reflector at home. I found the area of sky quickly, easily recognising the star patterns from the previous evening. I could easily see stars 1, 2, 3 and 1a. Object A had moved to the position B, confirming that it was indeed Pluto!

I was unable to observe again until Sunday night (29 April). By this time, Pluto had moved a considerable distance to the position marked C on the chart and now formed a faint, close pair with a star. Unfortunately the weather deteriorated on the evening of Monday 30 April (and remained poor for many days) and I was unable to confirm which of the two stars at position C was Pluto. I believe it to be the one nearest to the A and B positions but cannot be certain.

Pluto was clearly visible with direct vision using my 25 cm reflector on Thursday evening and would probably have been visible in a 20 cm reflector with averted vision.

Footnote

[1]

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.