Mars post-opposition 2003

The media attention given recently to Mars has prompted some questions from the general public.

The impression must have been given that the recent close encounter offered Earth-based observers an opportunity scarcely enjoyed in thousands of years. Of course this is far from the case. The 2003 opposition (distance 0.3728 AU) was only marginally closer than at other oppositions past and in the relatively near future.

For example, in 1956 (0.3791 AU) Mars, with its higher altitude, was better placed for northern observers. Yet the apparent diameter (24".760 was comparable to 2003 (25."11). There were also good oppositions in 1971 and 1988, the latter being the most favourable for northern observers.

Looking to the future; the opposition of 2050 Mars will be almost as close, and therefore as bright, as it was this year but will be much lower in our northern skies. The opposition of 2082, on the other hand, will see Mars very close to the distance in 2003 and at a higher altitude from northern latitudes.

This brings me to the other question I have been asked, namely, how do we know that the 2003 distance was the lowest in 60,000 years (the figure quoted by some authorities)?

Strictly speaking we cannot know for certain since the data upon which the calculations are based make some assumptions concerning the continuity within the Solar System environment over that period. We would only be fully justified in making such a claim if accurate positional observations went back that far. Precision astrometry is a comparatively recent activity originating from the time of Flamsteed (1646 - 1719) with the harnessing of the refracting telescope to measuring instruments.

I hesitate to draw comparisons with the sort of claims made by many cosmologists in relation to the age and formation of what is loosely called "The Universe". To say we "know" something about something is to imply certain knowledge of the matter. It is wiser, in my opinion, to accept cosmological theories for what they are, namely, exercises in human ingenuity.

Saturn 2003 -2004

Saturn is of particular interest to northern observers and will be treated in greater detail shortly. We are currently enjoying this planet close to its maximum northern declination at a time when the ring system is wide open. This autumn and winter will see Saturn high in the sky in the constellation Gemini. Already the planet is a conspicuous object in the early hours above the eastern horizon rising at 00 15 BST on September 1st and at 22 32 BST on the 30th.

J.V.
29/08

Mars in 2003: Opposition August 28

The planet Mars will be a prominent object low down in the southern sky throughout the late summer nights of July and August. During September the planet will gain slowly in altitude but will fade in brightness rapidly from its splendour at the beginning of the month.

Dia 1: The Orbits of Mars and Earth The longitude of the Earth on the 28th is within two days of the longitude of perihelion.

Data for 2003.0 - Mars longitude of perihelion 336 degs; heliocentric longitude of Earth (Aug 28) 334 degs.

From dia. 1, it will be seen that this year the time of opposition falls very close indeed to the longitude of perihelion. The orbit of Mars is relatively eccentric at 0.0934 (compare Venus 0.00677). This means the distance from Earth at different oppositions can vary considerably.

On August 28, 2003, Mars will have an apparent diameter of 25.11 arc seconds; at the unfavourable opposition in February, 2027, the diameter will be 71% of this value at 17.8 arc seconds. At the previous opposition in June, 2001 (dia. 1) Mars had an apparent diameter of 20.8 arc seconds.

At this year's opposition Mars will be as bright as Jupiter at his brightest, visual magnitude -2.9; in February 20027 it will be approximately one third as bright.

For northern observers such favourable oppositions are more than offset by the planet's low altitude: on August 28 Mars will rise to no more than a little over 15 degrees above the southern horizon. Seeing conditions are seldom of sufficient quality to allow high powers on an object at this altitude. Instead, one has to look to oppositions of Mars when the planet's altitude is double this value. The next opposition in 2005, when Mars will be between Taurus and Aries, will offer northern telescopic observers a better opportunity. For all that, it is seldom that one has the chance to see Mars with the naked eye looking as spectacular as this year. The next comparable oppositions occur in September 2035, August 2050, and September 2082.

Telescopic appearance of Mars.

Most people unfamiliar with astronomical observing are surprised by the small size presented by Mars in the telescope even at a favourable opposition. Much the same response applies to all the planets with the possible exception of Jupiter and Venus (near inferior conjunction). In contrast the Moon requires only a low power to impress since the craters themselves are just at the threshold of naked eye visibility.

To put things into perspective, consider the Moon at full in November this year when its diameter will be almost exactly one half degree arc. The planets in contrast all have apparent diameters below one arc minute, once again with the exception of Venus when it may be a little above this value. (See the notes for the transit of Venus in 2004 in a forthcoming article.) In other words, since the Moon has a diameter of around 1800 arc seconds we are generally dealing with planetary images of around one sixtieth of this, and in the case of Mars in August/September this year its diameter will be only 1.4% of the Moon's diameter on November 9.

From the above, a telescope working at a power of x72 would produce an image of Mars at this opposition comparable in size to the full Moon as seen with the naked eye. This sounds encouraging but we have to bear in mind that we are also magnifying the unsteadiness in the atmosphere and that the resulting image will be degraded as a result.

I can well recall an occasion in the 60s when I was demonstrating the 8 inch Cooke refractor at the University of London Observatory, Mill Hill. The contrast between the impressive size of the telescope and the little image displayed by Mars had some visitors wondering what all the fuss was about. In common with all skills, observation requires practice and above all, patience.

Mars 1988 October 2, 21.30 22.30 UT, drawing: Richard Baum 115 mm OG x186, by kind permission.

Mars 1988 October 20, 17.20 - 18.30 UT, drawing: Richard Baum 115 mm OG x186, by kind permission.

One of the problems today is the fact that telescopes of considerable power and sophistication are available at prices well within the reach of many people. Why a disadvantage? Simply because it is a case of improving practice as an observer. An experienced observer will see more with a given instrument than an inexperienced observer. The two drawings of Mars in Fig. 1 & 2 were made by Richard Baum (former director of the Mercury and Venus Section of the British Astronomical Association) with many years observing experience behind him. Note the power of x186 would not be considered excessive for a good telescope of 115 mm aperture, yet the detail recorded is quite remarkable. As Richard described it to me, this was one of those rare occasions where, for a few brief moments, the seeing excelled itself, making very fine detail readily apparent.

Modern CCD (charge-coupled device) photography enables high resolution images to be produced through digital techniques using relatively small telescopes. This is possible because exposures are much shorter compared to those necessary for conventional film photography. Amateurs are now producing results which are comparable to the work of professional observatories of a few decades back. Some quite outstanding photographs have even been achieved when Mars has had diameters of less than 10 arc seconds. The work of Damian Peach using a 305 mm SCT (Schmidt Cassegrain Telescope) has shown that Martian detail may be recorded with the apparent disc half this value. A series of photographs by Peach in February 2002 when Mars had a diameter of around 5 arc seconds were reproduced in the JBAA Vol 112 No. 3.

Although not a seasoned planetary observer myself, I used to take "time off" from my work on double stars in order to use large refractors in the daytime on the planets. Contrary to popular belief, I found that at times close to sunrise and sunset, seeing can often be superior than at any other time of day or night. Orkney is by no means an ideal location for observing the planets but I found in late February 2002, close to sunset, it was possible to use powers of x360 (178 mm Maksutov) on Mars and to glimpse the more prominent features shown on Damian Peach's photographs.

Table 1 Mars in August/September 2003

               
                     Distance  Diam  Mag  Phase
                     (in AU)     "        (1.0 = 100%)
                              1 Aug 0.4187837 22.35 -2.3 0.960 
               5     0.4064069 23.03 -2.4 0.969 
               10    0.3934371 23.79 -2.6 0.979 
               15    0.3834297 24.41 -2.7 0.987 
               20    0.3765936 24.85 -2.8 0.994 
               25    0.3731334 25.08 -2.9 0.997 
               30    0.3731993 25.08 -2.9 0.998 
               4 Sep 0.3768322 24.84 -2.8 0.996 
               9     0.3839464 24.38 -2.7 0.991 
               14    0.3943983 23.73 -2.6 0.984 
               19    0.4080328 22.94 -2.5 0.975 
               24    0.4246754 22.04 -2.3 0.965 
               29    0.4441175 21.08 -2.2 0.954 

Table 2 The rising, setting and transit of Mars, July 15 - Sep. 26, 2003
(For the location of Kirkwall.)

               Date   Rise     Set      Transit
               15 Jul 22:51:45 08:00:56 03:27:57 
               21     22:31:57 07:41:02 03:08:11 
               27     22:11:34 07:18:15 02:46:41 
               
               02 Aug 21:50:29 06:52:28 02:23:21 
               08     21:28:31 06:23:52 01:58:10 
               14     21:05:33 05:52:50 01:31:17 
               20     20:41:28 05:20:00 01:02:55 
               26     20:16:13 04:46:14 00:33:30 
               
               01 Sep 19:49:51 04:12:42 00:03:36 
               07     19:22:35 03:40:35 23:29:03 
               14     18:49:59 03:06:07 22:55:45 
               20     18:21:48 02:39:48 22:28:42 
               26     17:53:43 02:16:45 22:03:22 
               
               02 Oct 17:26:02 01:57:00 21:39:53 
               08     16:58:57 01:40:20 21:18:12 
               14     16:32:31 01:26:23 20:58:12 
               20     16:06:48 01:14:47 20:39:41 
               26     15:41:45 01:05:13 20:22:31 
               
               01 Nov 15:17:20 00:57:25 20:06:31 

J.V. 11/07/03