FORTHCOMING 2001 MARS
(3)

De: Central latitude     Diam: Apparent diameter

Mars in 1954, 1969, 1986 and 2001
	Opposition	Closest approach	Period where diameter is larger than 10"
1954	24 June 184°Ls, De=1°N	02 June 189°Ls, De=2°N, Diam=21.9"	5 Apr (141°Ls) ~ 24 Oct (259°Ls)
1969	31 May 164°Ls, De=7°N	09 June 170°Ls, De=9°N Diam=19.5"	21 Mar (129°Ls) ~ 23 Sept (233°Ls)
1986	10 July 203°Ls, De=6°S	16 July 206°Ls, De=5°S; Diam=23.2"	10 Apr (151°Ls) ~ 07 Nov (278°Ls)
2001	13 June 177°Ls, De=3°N	21 June 182°Ls, De=4°N; Diam=20.8"	28 Mar (137°Ls) ~ 10 Oct (250°Ls)

  Since the eccentricity of Mars is peculiarly larger, this implies, the apparent diameters at opposition in 1986, 1954, 2001 and 1969 stand thus in order of size. The maximal diameter when the planet was closest to the Earth in 1986 was 23.2 seconds of arc while it was only 19.5 seconds in 1969. Figure 1 shows how the apparent diameter varied in 1969, 1954, and 1986 and will vary in 2001. It also proves that the season each apparition covers slightly differs each other. The 1969 Mars made closest approach at 170°Ls, while the 1986 Mars at 206°Ls, and so they differ by 35 degrees of Ls. So if we observe four similar apparitions, the observable range of the season will be much widened: The apparent diameter proved larger than 15 seconds of arc from 148°Ls (28 Apr 1969) to 247°Ls (20 Sept 1986). Mostly they overlapped and the season 180°Ls was (and will be) observable every time even if we admit that Diam should not be less than 15". This is a characteristic of this series of apparitions.

  It should however be remarked that the same Ls does not imply the same central latitude De nor the same Martian surface at a station. Figure 2 shows how the tilt of the poles of Mars move toward or away from the Earth in 1954, 1969, 1986 and 2001 (previously on the page 2778, #233).

  The variations of De look similar before 130°Ls and after 250°Ls, but in the important range near 180°Ls, the vernal equinox of the southern hemisphere, they quite differently vary. As is expected, in the 1986 case where the opposition occurred closer to the perihelion, the Martian southern hemisphere much more faced towards us, while the 1969 case occurred farther from the perihelion and so the northern hemisphere were better seen from the Earth.

  The 2001 case is intermediate as was the case in 1954, but more similar to the 1969 case. As one of us (Mn) remembers, in 1969 it was difficult to watch the south-polar region to discriminate the south polar cap (spc) from the hood. In 1969, the central latitude was at De=11.4°N when the season was at 180°Ls (on 27 June 1969). It was even hard to observe a definite bright spc in mid-July when 190°Ls reached. On the other hand in 1986, De=10.5°S at 180°Ls (on 1 June 1986) so that the spc appeared very disclosed from the outset. For instance, according to Mn's observations at Taipei in 1986, it was possible to detect brighter spots and shadowy areas inside the spc already at 176°Ls (on 24 May 1986) when De=11°S, (and LCM=097°W). As remarked at the Yokohama Meeting in September, the MGS obtained an image of the spc at the same season in 1999 in good accordance with the result in 1977 revealed by the Viking Obiter 2 (cf CMO #231 p2736). In 1986, the central
area of the spc so rapidly melted then that the spc appeared soon to show a dip at the top limb part of the spc, as was noticed by several observers.

  This was never witnessed in 1969 because of the tilt of the south pole away from us. In the case of 2001 apparition it will not be so convenient; even then it will be much easier to watch the south-polar region than in 1969. At 180°Ls, the tilt of the south pole is a bit away, while the De lies on the southern hemisphere 140°Ls to 170°Ls and hence this is a good chance to observe the final stage of the south polar hood. Furthermore, we should recall that at 176°Ls the apparent diameter reaches Diam=20.4" in 2001 while Diam was only 18.9" in 1986 at the same season.

  The observation of the dispersive process of the south polar hood (sph) before and after the southern vernal equinox had better be made when the surface showing Hellas is facing to us. At Yokohama, we cited the case of S EBISAWA in 1986 who compared the surface at a similar LMT at 136°Ls, 165°Ls and 187°Ls (Fig 8 in l'Astronomie, juillet-août 1987 p410). The same LCM repeats to return after about 40 days at a stationary location, and so we can compare the different seasons every 20°Ls. One would be able thus to discriminate the stages where Hellas and the spr are covered by the mist, where Hellas is cleared while the sph is visible, and finally where the sph has been dissipated to reveal the real spc. In 2001, from Japan we can observe the concerned surface of Hellas around 3 Mar 2001 (124°Ls) at 18:00GMT, and then around 10 Apr 2001 (143°Ls) at 18:00GMT. According to the classical Smith-Smith result, the Hellas haze will begin to decay at 150°Ls and dissipate at 170°Ls. Then a bump follows around at 260°Ls. These are averaged aspects, and for example, Morimasa NAKAJIMA (Nk) notices that Hellas looked hazed at 200°Ls ~ 220°Ls in 1973, and a white cloud was observed in the northern Hellas at 250°Ls in 1971 and so on. These are to be on defence in 2001 as well as in 2003 and 2005.

  Two supplementary remarks are in order: First, the 1986 observation showed that the spc had began to thaw at around 187°Ls, and so the period from 130°Ls to 190°Ls should be densely spent to concentrate on the spr. Secondly, as shown in Fig 2, the tilt moves to the southern hemisphere from 245°Ls when the 1956 great dust storm occurred at Noachis (corresponding to the beginning of October 2001), and so there is no other obstacle to watch than the smaller diameter and the lower declination. In 1997, the MGS detected a large dust cloud at Noachis at 224°Ls (on 26 Nov 1997); and so we should be on alert from the end of August in 2001. As to this dust, refer to
http://www.msss.com/mars_images/moc/science_paper/f7c/index.html

  We finally comment that similar apparitions before 1954 are found if back to 1937, 1922, 1907, 1890, 1875 and so on. As far as these years are concerned, no case exceeds the 1986 southern hemisphere type (that is, no oppositions are closer to the perihelion than the 1986 case). The 1907 Mars is quite similar to the 1986 Mars (the periodicity of 79 years). On the other hand, the 1937 Mars was more remote from the perihelion than 1969, and the maximal apparent diameter was no larger than 18.4 seconds of arc, though it shined highest in the sky at opposition seen from the Northern Hemisphere (App Decl was -20° 40'). The 1986 case will be exceeded 79 years after in 2069 according to Jean MEEUS.

"it (declination) will go down to -5° at the end of 2000"
should be read
"it will go down to nearly -12° at the end of 2000"