Astronomical Society of Coonabarabran

Planetary Grouping In The West (September 2008)

by Harry Roberts

Alerted by the Sydney Observatory Blog that three planets were gathering in the western sky, the 4" Maksutov was set up as dusk loomed. Binoculars located Venus in the bright sky. The Mak with a 4.8mm Nagler gave a nice view of it at 208x. Venus was distinctly gibbous and close to maximum brightness - visually dominating the western sky. Where were the others?

Sweeping revealed Mercury at the same altitude as Venus, but several degrees to the south. Mercury showed a nice sharp crescent, about 40% illuminated - and it was dim and brownish in colour. The size of Mercury and its altitude showed this is a favourable elongation for southern observers. First magnitude Spica (Alpha Virginis) was swept up while looking for Mercury - it was whiter than Venus.

Mars was lower in the dusk sky and harder to find. It showed a tiny disc about half the size of Mercury. It was a dim red-orange colour and seemed slightly gibbous. It was clearly a long way off - presumably on the opposite side of the Sun.

Once located, it was possible to sweep from planet to planet and compare sizes, phases, brightness and colours. Fitting a cross-hair eyepiece at 160x permitted transit timings, but poor seeing made the timings difficult. Venus' large disc averaged ~0.75 sec in diameter, or ~11 arc seconds. Mercury was about half the apparent size of Venus or ~7 arc sec. Mars was mostly guesswork - but being about half of Mercury, it is around 3 to 4 arc sec.

While making the timings it became clear that Mercury was strongly inclined to the hour circle, a position angle (PA) that closely matched the current PA of the Sun. Mercury's apparent PA, like the Sun's, is presumably caused by Earth's tilted axis. Re-examining Venus showed it too was tilted similar to Mercury. Mars' was too small to tell if it was tilted or not.

Interpretation of the planetary alignment proved tricky until I found a diagram by Nick Lomb of Sydney Observatory that makes clear how the planets are located in their orbits. I have adapted his diagram to show the observed phases of the planets. Of course the diagram is not to scale and the planets are shown much larger for clarity.

The three planets are roughly in line from Mercury the closest, to Mars the most distant. Venus is about half way between them, but not yet at the point of maximum elongation - so we see a little of its unlit side. Mercury by contrast shows mostly its unlit face and a little of its yellowish-brown surface. What term is best to describe this planetary alignment? My ancient "Norton's Star Atlas" suggests "appulse"

"The apparent close approach of one celestial object to another, although they may be far apart in actual distance."

My first guess was that we were seeing a "planetary conjunction", but "Norton's" tells me that would be the wrong use of the term. Get your small 'scopes out and enjoy the appulse in the western sky.