Orwell Astronomical Society (Ipswich)

Mars At Opposition, 01 October - 16 November 1988

Mars, the so called Red Planet, was at opposition on 28 September 1988 and has not been better placed for observations from the northern hemisphere since 1725. The planet attained magnitude -2.8, just brighter than Jupiter, and an angular diameter of 23.8". The opposition occurred when Mars was close to perihelion, and the angular diameter was very close to the largest value possible. The apparition was special because the large angular diameter was combined with a relatively high altitude (Mars was close to the celestial equator at opposition). This meant much better seeing than usually occurs with perihelic oppositions when Mars is generally well south of the celestial equator. Because of the special nature of the apparition, the British Astronomical Association (BAA), the Planetary Society, the Association of Lunar and Planetary Observers (ALPO), the Oriental Astronomical Association (OAA) and the NASA Jet Propulsion Laboratory (JPL) sponsored an International Mars Patrol (IMP). The IMP strongly recommended the use of a standard disk of 42 mm diameter for recording observations of Mars. OASI encouraged members to make observations of Mars during the apparition and to submit results on the standard disks.

The close opposition meant that even relatively small telescopes were capable of revealing the major features on the surface of Mars, particularly the polar caps and the dusky area Syrtis Major. However, in practice a 150 mm reflector was the smallest instrument necessary to observe fine detail and ideally an aperture of at least 250 mm was needed to do serious work. The 26 cm refractor at Orwell Park is capable of resolving fine detail under ideal conditions. Following opposition, as the nights darkened, I made some observations of Mars using my 25 cm Newtonian reflector at home and the 26 cm refractor at Orwell Park.

• 01 October 1988. My first observation was a promising start to the month. It was a rather hazy evening with some rapid turbulence that limited useful magnification to at most 200x with my 25 cm reflector. As the drawing below indicates, the most prominent feature was an elongated marking just south of the equator. Initially the feature appeared in solid colour, but in moments of improved seeing I could discern a brighter, reddish coloured central region that existed inside a fairly thin, dark perimeter. Further south, towards the small, bright polar cap, I could see a second dusky marking, which was, however, a lower contrast feature than the more northerly one.
• 05 October 1988. This was the next relatively clear evening that coincided with an opportunity to observe; I visited Orwell Park Observatory to use the refractor. It was actually a rather poor night with thin cloud regularly drifting across the disk of Mars and the features were less distinct than on 01 October. Because I was observing earlier in the evening (19:30 UT rather than 23:30 UT), and due to Mars' rotation speed being slower than that of the Earth, the visible aspect of the surface was approximately 90° further east than that visible during the observation on 01 October.
• 23 October 1988. Dismal weather during the middle of the month meant that it was not until late October that clear skies and opportunity to observe again coincided. Again there was thin cloud and hazy seeing which reduced contrast, but the atmosphere was much steadier than earlier in the month and I could see much more detail. I could see Syrtis Major on the western limb of the planet. The surface aspect visible was displaced approximately 180° from that seen on 05 October.
• 27 October 1988. My best observing evening during the month. It was a clear, bright night with fairly steady seeing although a small amplitude, rapid turbulence was present that obscured fine detail. I made an observation during early evening with Syrtis Major visible towards the eastern limb.
• 31 October 1988. The evening was very clear but with significant rapid and large-scale turbulence that impeded observations. However, there were occasional fairly steady periods during which significant detail was visible.
• 02 November 1988. My next observation was with the Orwell Park refractor. Again turbulence affected observations and there were only fleeting moments of steady seeing.
• 04 November 1988. The night offered fairly steady seeing although there was a slight haze. There was a hint of a dark border to the north polar cap: this feature was very difficult and uncertain. This was the first time during this apparition that I had glimpsed darker areas in the northern hemisphere.
• 13 November 1988. Poor weather delayed my next observation until 13 November. On that evening, the seeing was clear and again turbulence affected contrast but there were occasional good steady periods. There was a hint of very low contrast dusky areas in the northern hemisphere but these were difficult and uncertain. My observations did, however, suggest that there had likely been extensive dust storms in the northern hemisphere that might now be abating. By this stage during the apparition of Mars, the disk was becoming noticeably gibbous.
• 16 November 1988. My last observation was with the Orwell Park Refractor. The seeing was fairly good and steady and I could discern markings in the northern hemisphere although they appeared as low contrast features. Again, the disk of Mars appeared noticeably gibbous.

David Payne

Original: Newsletters September, December 1988; February 1989
Updated: 26 March 2023