The superior planets, those beyond the Earth's orbit, are best seen at opposition, when the planet is closest to Earth and directly opposite the Sun; it then rises at sunset and culminates at midnight. During subsequent weeks it moves into the evening sky. The phase of an superior planet is always full or gibbous (more than half of the disk illuminated); the gibbous phase is quite pronounced on Mars, but less so on the more distant planets. As the Earth moves around the Sun faster than an outer planet, it slowly overtakes the planet. The result is a loop in the apparent path against the background stars. This apparent reversal in a planet's movement is known as retrograde motion (Figure 2a).
The inferior planets, Mercury and Venus, go through a complete cycle of phases like the Moon. When an inferior planet is at the greatest apparent distance from the Sun, it is said to be at greatest elongation. At greatest eastern elongation, the planet is visible in the evening sky after sunset; at greatest western elongation, it is visible in the morning sky before sunrise (Figure 2b). The apparent diameter of an inferior planet changes appreciably with its position in its orbit (Figure 4); it is largest at inferior conjunction, but the planet cannot be seen. The change in brightness is illustrated in Figure 3.
[FIGURE 2a OMITTED]
[FIGURE 2b OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The Dwarf Planets
The discovery of 2003 [UB.sub.313], now known as Eris, by Michael Brown in January 2005 created an urgent need for a redefinition of planets. Eris is 5 percent larger than Pluto, so if Pluto is a planet, shouldn't Eris be one too? The outcome of the International Astronomical Union resolution (see page 70) was that Pluto, Ceres and Eris would in future be recognised as "Dwarf Planets". Pluto would also be recognised as the prototype for a new class of trans-Neptunian objects (TNO) to be called "Plutoids".
Pluto was discovered on February 18 1930 by Tombaugh. It has an equatorial diameter of 2250km with a very low mass of 0.002 (on a scale where Earth has a mass of 1) which indicates that its mean density is less than 1. Thus Pluto is a huge iceberg, consisting mainly of water ice and possessing a crust of methane ice. Since water ice is geologically a very weak rock, gravitational forces would mould it to a sphere.
The first asteroid ever discovered, on January 1 1801, was Ceres. It lies in the middle of the main asteroid belt 2.77 AU from the Sun and is nearly spherical with a diameter of 1023km. The mass of Ceres has been determined to be 5.9 x [10.sup.-10] that of the Sun, which means that Ceres constitutes about 30 percent of the mass of the main asteroid belt.
On July 11 2008, the Working Group for Planetary System Nomenclature classified Makemake as a dwarf planet. On 17 September of the same year, Haumea (formally 136108) was also accepted as a dwarf planet by the IAU and named after Haumea, the Hawaiian goddess of childbirth. Its mass is one-third the mass of Pluto and calculations from its light curve suggest that it is an ellipsoid with its greatest axis twice as long as its shortest axis. Nonetheless its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the definition of a dwarf planet and making it the fifth dwarf planet in the solar system.
As seen from Earth, the occultation of one planet by another happens about once every 33 years. It seems, however, that we are not living in average times: the previous mutual occultation was in 1818 (Venus occulting Jupiter) and the next event will be in 2065 (also Venus and Jupiter) an interval of 247 years (by far the longest interval between events in the period -1000 BCE to +6000 CE). The two closest approaches of two planets in 2011 occur on March 27 and on May 01. All pairings closer than 4[degrees] are listed in Table 4. A planetary trio occurred on December 10 2006, when Mercury, Mars and Jupiter were within a circle of diameter 59 arcminutes. This was the tightest grouping in the period 1980-2050.
There are 23 stars, brighter than magnitude 3.5, that could possibly be occulted by a planet. During 1900-2100, only 14 such occultations occur. The last event was the 1984 occultation of X Sgr by Venus; the next event is in 2015 of 0 Ophiuchi (a magnitude 3.2 star) by Mercury.
[FIGURE 5 OMITTED]
Rise-set time corrections
Approximate time corrections (in hours and minutes) to be applied to rise and set times in Figure 5 (opposite) are given in Table 5. Locations in time zones other than SAST will have to adjust by the relevant whole number of hours. The first column is for the Sun at the summer solstice, the second for the Sun at the equinoxes and the third for the Sun at the winter solstice. The same corrections are approximately valid for Mercury and Venus, while for Mars the third column applies from January to June, the second from July to October, and the first for November and December. Corrections for Jupiter and Uranus are approximately those given in the second column throughout the year, while for Saturn, Neptune and Pluto the corrections are approximately those in the third column throughout 2011.
Being the innermost planet, Mercury is usually near the Sun, and can only be seen low in the east just before sunrise, or low in the west just after sunset (about the time of beginning and end of civil twilight). It is visible in the mornings between the following approximate dates: 1 January to 13 February, 18 April to 5 June, 25 August to 19 September and 10 December to 31 December. The planet is brighter at the end of each period, (the best viewing conditions in the southern latitudes are from mid April to the end of the third week of May). It is visible in the evenings between the following approximate dates: 7 March to 2 April, 20 June to 9 August and 12 October to 28 November. The planet is brighter at the beginning of each period, (the best viewing conditions for the southern latitudes occur in July from the beginning of the second week). Binoculars show it as a bright "star", yellowish or pale orange in colour. A telescope reveals a small disk, never larger than 11.97 arcseconds across, and like the Moon and Venus, it goes through a sequence of phases. Mercury, like the Moon, is faintest when in crescent phase and brightest when full. A 20-cm or larger telescope is usually required to see any detail on the disk, which shows vague streaks and patches similar to those of Mars, only much fainter. The planet peaks in brightness on 26 February (magnitude -1.6), 18 June (magnitude -1.6) and again on 24 September (magnitude -1.5).
Venus is a brilliant object in the morning sky from the beginning of the year until near the end of the second week of July when it becomes too close to the Sun for observation. During the second half of September it reappears in the evening sky where it stays until the end of the year. Venus is in conjunction with Jupiter on 11 May and with Mars on 22 May. Venus is quite often referred to as 'the morning or evening star' even though it is not a star. This could be because Venus is often mistaken as the first or last 'star or star-like object' one can see in the evening or morning sky. It would be more accurate to refer to Venus as the first or last 'starlike' object we see in the evening or morning sky. Venus can be either the 'morning star' OR the 'evening star' but not both at the same time. Venus is the brightest starlike object in the night sky, and can even be seen in broad daylight with the naked eye. In binoculars Venus is a dazzlingly bright sight (sometimes reported as a UFO). A telescope does not show any surface features, because the planet is shrouded in heavy cloud.
Like the Moon, Venus goes through a sequence of phases which can be followed with highpower binoculars or a small telescope. Unlike the Moon, Venus is brightest when in crescent phase and faintest when full. This is because when Venus is 'full' it is furthest from Earth and the illuminated surface (area) appears less than when it is 'new' and nearest the Earth and we see a greater illuminated area (area). It starts the year at magnitude -4.5 and grows in brightness to magnitude -3.9 in June through to November after which it dims slightly to magnitude -4.0 in December
The Earth's atmosphere creates a number of optical phenomena that the astronomer may encounter. Possibly the most familiar is the rainbow, which is always seen in the sky opposite the Sun and is caused by refraction and reflection of light in water drops. At sunrise or sunset, a sun pillar may be noticed, a column of scattered light rising upward above the Sun. The halo, a coloured ring centred around the Moon, usually 44[degrees] across, is due to light refraction by ice crystals. Halos also occur around the Sun and may be accompanied by a pair of bright spots flanking the Sun, called sundogs.
On any clear night it is possible to see many artificial satellites moving across the sky. Looking like slow-moving stars, sometimes steady but more often variable or sometimes as bright as the brightest stars, they take about 10 -15 minutes to travel from horizon to horizon. They can appear in any direction moving either in a west-east, north-south or a south-north direction. (There are some satellites that move towards the west but they are too faint to be seen with the unaided eye).
Satellites are most prominent in the first and last 90 minutes of the night. Since the satellites shine by reflected sunlight they have to clear the earth's shadow and are brightest in the direction opposite the sun. In the evening face east and scan from about halfway above the horizon to overhead (i.e. with your back to where the sun set), similarly in the morning face west with your back to where the sun will rise.
The vast majority of satellites will appear as single objects but from time to time it is possible to see groups of two or three. These are military satellites flying in formation and often give rise to reports of UFO's. The brightness of the satellite depends on the size of the satellite or rocket casing and can vary from very bright (e.g. the International Space Station) to satellites barely visible with the unaided eye. There are roughly 250-300 satellites bright enough to be easily seen under good conditions and with slight optical aid, such as 7 x 50mm binoculars, the number can increase several times. Armed with predictions which tell you when and where to look one could expect to see several dozen satellites within a short period.
Satellites can appear as steady points of light but most will show some variation, especially if the satellite is tumbling, sometimes quite rapidly and regularly, or slowly and irregularly. Some satellites can momentarily flare up as bright as magnitude--8 and can be seen in daylight if you know where to look.
There are several areas in which the amateur can play an important role, ranging from just observing a satellite (noting its "optical" characteristics such as brightness, whether it appears steady or variable and if possible determining the period of variability), to the specialized art of determining the position of the satellite, using a stopwatch and the position of the satellite at that instant using stars as reference points. If the observations are accurate enough it is sometimes possible to derive the orbit.
Currently there are about 200 satellites that have no orbital data from official sources--many of these objects are classified military launches but a fair percentage are lost rocket casings or debris of one kind or another. A handful of amateur space "detectives" have located, identified and keep regular tabs on such objects. Surprisingly, many are bright naked eye objects.
With modest optical aids and a simple, cheap webcam it is possible to see and record the superficial structure of some of the larger satellites, for example, the International Space Station has an apparent size larger than that of Jupiter. It is even possible to view the larger satellites crossing the face of the Moon or Sun. Video technology is playing an ever-increasing role and is invaluable in positional work or determining optical characteristics.
If one likes mathematics one could generate one's own prediction, but for beginners the best site for orbital predictions is [http://www.heavens-above.com] where predictions for the brighter satellites, including Iridium flares, can be found for your geographical location. You can even download a star chart showing the satellite track against a star background. An excellent start to your search for information on tracking satellites can be found at [http://canopus.saao. ac.za/~wpk].
Mars is too close to the Sun for observation until mid-April when it appears in the morning sky in the constellation of Pisces. Its westward elongation gradually increases as it passes through Aries, Taurus (passing 5[degrees]N of Aldebaran on 6 July), Gemini (passing 6[degrees]S of Pollux on 10 September), Cancer and Leo (passing 1.4[degrees]N of Regulus on 10 November). Mars is in conjunction with Mercury on 19 April and 20 May, with Jupiter on 1 May and with Venus on 22 May. To the naked eye, Mars has a distinct orange-red colour, which is more pronounced in binoculars. With a 5-cm telescope you should be able to see one of the white polar caps, which vary with the seasons, while at least a 15-cm telescope is needed to see the dark markings. In general, the features observed on Mars are not topographic, but are rather superficial dust shadings. The two moons of Mars, Phobos and Deimos, are bright enough to be seen in a small telescope but they are almost hidden by the planet's glare and a telescope of at least 20-cm aperture is required. They are most easily seen when at greatest elongation from the planet (see Diary for dates).
Figure 5a Aspects of Mars January 01 Dia = 3.93" Mag = 1.2 June 04 Dia 4.11" Mag = 1.3 December 31 Dia = 8.93" Mag = 0.2
[FIGURE 5b OMITTED]
Jupiter can be seen at the beginning of the year in the evening sky in Pisces, moves into Cetus in late February and back into Pisces in early March. From late March it becomes too close to the Sun for observation. It reappears in the morning sky in the second half of April and moves into Aries in early June. Its westward elongation gradually increases and from the beginning of August it can be seen for more than half the night. It is at opposition on 29 October when it is visible throughout the night. Its eastward elongation gradually decreases and at the end of the first week of December it passes once more into Pisces. Jupiter is in conjunction with Mercury on 16 March and 10 May, with Mars on 1 May and with Venus on 11 May.
[FIGURE 6 OMITTED]
Binoculars will show the planet as a tiny pale yellow disk along with the four brightest satellites (the Galilean moons). A small (5-cm) telescope will show some features on the disk, usually two dark bands (called equatorial belts). With a 12-cm or larger telescope, the disk can be seen as being clearly elliptical and displaying a wealth of detail. Because Jupiter rotates very rapidly on its axis, one Jovian day being less than 10 hours long, details on the planet's surface will move noticeably in a period as short as 10 minutes. These markings are storm features in Jupiter's upper atmosphere and consist of dark brownish strips interspersed by brighter zones.
The light-coloured zones are strong winds blowing the direction of the planet's eastward rotation, while the darker belts are winds in the opposite direction. The South Equatorial Belt (SEB), often double, varies in prominence; located within it is the Great Red Spot, a cyclonic storm that has been raging for over 300 years.
[FIGURE 7 OMITTED]
The four Galilean moons -lo, Europa, Ganymede and Callisto are often arranged in interesting configurations as they orbit the planet. Diagrams in the Diary section (e.g. p 5) illustrate this motion. For each month, time increases downward in the diagram; the disk of Jupiter is stretched to make the central column, while the horizontal tick-marks represent hours 00:00.
An eclipse occurs when the satellite moves through the shadow of Jupiter (see Figure 7), and an occultation when the satellite moves behind the planet. Satellites always disappear into occultation at the west side of Jupiter and come out from behind the planet at the east side. A satellite transits when it moves in front of Jupiter, travelling east to west across the face of the planet. When in transit, lo, Ganymede and Europa are usually seen as bright, sharply-defined spots when near Jupiter's limb, but are darker when near the centre of the planet (Europa is then usually invisible). Callisto is dark during almost all of its transits and is often mistaken for a shadow. A shadow transit occurs when the shadow of a satellite, cast by the Sun, moves across Jupiter's disk. The shadow may be seen through a telescope as a westward-moving small black dot on the planet's surface. Tables in the Diary give the dates and times for mid-event of these phenomena during 2011. Triple-shadow conjunctions, when the shadows of three moons appear on Jupiter simultaneously, always involve Callisto and two of the inner moons. In the period 1900-2100, 32 such events take place. The last event occurred in 2004 (lo, Ganymede & Callisto) and the next event will take place in 2013 (lo, Europa and Callisto). The Galilean moons are bright enough to be seen without optical aid, but are usually masked by the glare from Jupiter. At greatest elongation from Jupiter in September, Callisto reaches its maximum diameter of 10.92 arcminutes. At magnitude 5.41 it should be visible with the naked eye if light from the planet is blocked off, perhaps by occulting it behind a building or a tree branch. Dates of maximum elongation of Callisto appear in the Diary.
Saturn rises shortly after midnight at the beginning of the year in Virgo and remains in this constellation throughout the year (passing 5[degrees]N of Spica on 31 October). Saturn is at opposition on 4 April when it can be seen throughout the night, and from early July until late September it is visible only in the evening sky. It then becomes too close to the Sun for observation until the end of October, after which it can be seen in the morning sky for the rest of the year. Always brighter than magnitude 1.1, it is readily visible to the naked eye. Binoculars show a pale yellow "star", perhaps slightly elongated. A careful search will reveal 8th magnitude Titan, Saturn's brightest moon and the second-largest satellite in the solar system. Diagrams in the Diary section show the motion of Titan around Saturn. A 20-cm telescope will clearly show the famous rings as two bright elliptical bands (Rings A and B) with a thin gap (Cassini's Division). A larger telescope is needed to see Ring C (Crepe Ring) and the gap in the middle of Ring A (the Encke Division). The rings are becoming more distinct as Saturn moves from its 2009 position when it was edgeon to our line of sight. The fact that Saturn's opposition distance will increase by more than 1.5 AU over the next seven years, means that the planet's overall brightness will be decreasing. Including Titan, Saturn has nine large moons. A 7.5-cm telescope will show Rhea and sometimes Lapetus (which varies in brightness because it has a large dark surface feature), while a slightly larger telescope shows Enceladus, Tethys and Dione. A 20-cm telescope is needed for Hyperion and Phoebe.
Uranus is visible at the beginning of the year in the evening sky in Pisces and remains in this constellation throughout the year. From the beginning of March it becomes too close to the Sun for observation and reappears in the second week of April in the morning sky. Uranus is at opposition on 26 September. Its eastward elongation gradually decreases and from late December it can only be seen in the evening sky. A finder chart for Uranus is given on P69.
Uranus reaches magnitude 5.7 at opposition so under dark skies it can be seen with the naked eye. Easily seen as a starlike point in binoculars, a telescope reveals its greenish-blue 3.63 arcsecond disk. Even in large telescopes, the disk is essentially featureless. A 40-cm telescope is required to spot the two largest moons, Titania (mv=+13.7) and Oberon (mv=+14.1). Miranda and Ariel are the next two largest moons after Titana and Oberon.
Neptune is visible at the beginning of the year in the evening sky in Capricornus, moves into Aquarius in the second half of January and remains in this constellation for the rest of the year. In late January it becomes too close to the Sun for observation and reappears in the second week of March in the morning sky. Neptune is at opposition on 22 August and from late November can be seen only in the evening sky.
Neptune, at its brightest magnitude of 7.8, is too faint to be seen with the naked eye, but binoculars show it readily as a dim "star" (see chart on P69). A small telescope will show it as a bluish object, its tiny 2.31 arcsecond disk requiring a larger telescope (and higher magnification) to resolve.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Dwarf Planet Pluto
Pluto is in Sagittarius throughout the year, near the border of Serpens. At least a 40-cm telescope is needed to see this faint object. Observe it over a few nights to confirm its motion. At opposition on 28 June it shines feebly at magnitude +14.00 with a diameter of 0.13 arcseconds. Its satellite Charon is just within reach of the larger amateur telescopes (mag.=+16.8). The IAU Working Group for Planetary System Nomenclature has approved the following new designations and names of satellites of Pluto. Pluto II (Nix) and Pluto III (Hydra). A finder chart for Pluto is available from the ASSA web site at [http://saao.ac.za/assa/html/skyguide]
Definition of a Planet
Contemporary observations are changing our understanding of the Solar System, and it is important that our nomenclature for objects reflects our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries force us to create a new definition, which we can make using currently available scientific information.
At the international conference in Prague on August 24 2006, the International Astronomical Union (IAU) resolved that the planets and other Solar System bodies be defined into three distinct categories as follows.
IAU Resolution 5: Definition of a "Planet" in the Solar System
(1) A "planet (1)" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (2), and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (3), (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects (4) except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
IAU Resolution 6: Pluto
The IAU further resolves that: Pluto is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of Trans-Neptunian Objects (5).
(1.) The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
(2.) This generally applies to objects with mass above 5 x 1020 kg and a diameter greater than 800 Km.
(3.) An IAU process will be established to assign borderline objects into either dwarf planets or other categories.
(4.) These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. Any TNO that is also a dwarf planet, is now called a "Plutoid".
Did you know ?...
SATURN is an intense source of Radio waves generated by the interaction of solar wind with the magnetic field at the poles. Interestingly, one cannot 'hear' radio waves but if one converts the waveform to sound and slow the frequency down 44 times, one gets an eerie wailing or shrieking sound with a background clicking noise.
JUPITER's Great Red Spot is an area of high pressure that results from the planets internal heat source and is the largest anticyclone in the Solar System. The Great Red Spot has existed for over 300 years and covers an area of at least two times that of the planet Earth.
Did you know? ...
One weighs less at the equator than at the Poles. There are 2 main reasons why this should be so. Firstly the earth bulges by about 21km at the equator. This means that one is then just a little further from the Earth's centre of gravity and an object's weight will decrease by about 0.5%. Secondly the spin of the Earth causes a centrifugal effect that further reduces the pull of gravity by a further 0.3%. The result is that if a person who weighs 70kg moves from the poles to the equator, that person will lose about 600 grams. There are a lot easier ways for losing 600 grams than travelling a quarter of the way around the Earth--just leave the pies alone!
Table 4. Close planetary pairings Date Time Planets in Separation the pairing Feb 19 01:20 Venus-Pluto 2[degrees]20' Feb 20 00:10 Mercury-Mars 0[degrees]59' Feb 21 02:30 Mercury-Neptune 1[degrees]35' Feb 21 05:40 Mars-Neptune 0[degrees]35' Mar 27 03:30 Venus-Neptune 0[degrees]08' Apr 03 22:50 Mars-Uranus 0[degrees]13' Apr 19 20:20 Mercury-Mars 0[degrees]37' Apr 23 04:10 Venus-Uranus 0[degrees]51' May 01 06:20 Mars-Jupiter 0[degrees]22' May 11 22:00 Mercury-Jupiter 2[degrees]04' May 11 16:40 Venus-Jupiter 0[degrees]34' May 21 08:40 Mercury-Mars 2[degrees]08' May 23 10:10 Venus-Mars 0[degrees]59' Sep 30 01:20 Venus-Saturn 1[degrees]16' Table 5. Rise-set time corrections Bloemfontein + 0 17 + 0 15 + 0 13 Cape Town + 0 36 + 0 46 + 0 56 Cedarberg + 0 37 + 0 43 + 0 49 Dar es Salaam + 0 09 - 0 37 - 1 23 Durban - 0 03 - 0 04 - 0 04 East London + 0 01 + 0 08 + 0 16 Francistown + 0 29 + 0 10 - 0 09 Gaberone + 0 28 + 0 16 + 0 04 Harare + 0 22 - 0 04 - 0 30 Johannesburg + 0 17 + 0 08 - 0 01 Komatipoort + 1 02 + 0 52 + 0 42 Knysna + 0 18 + 0 28 + 0 38 Luanda + 1 50 + 1 07 + 0 25 Lusaka + 0 37 + 0 07 - 0 24 Nacala (Moz) - 0 11 - 0 43 - 1 15 Nairobi + 0 29 - 0 27 - 1 23 Port Louis (Mar) - 1 29 - 1 50 - 2 11 Pretoria + 0 17 + 0 07 - 0 02 Seychelles - 0 52 - 1 42 - 2 32 Sutherland + 0 31 + 0 37 + 0 43 Upington + 0 39 + 0 35 + 0 31 Victoria Falls + 0 42 + 0 17 - 0 09 Windhoek + 1 08 + 0 52 + 0 35 Table 6. Solar system fact sheet Parameter Sun Mercury Venus Earth Mars Mass (1024kg) 1989100 0.330 4.87 5.97 0.642 Diameter (km) 1392000 4879 12104 12756 6794 Gravity (m x 274.0 3.7 8.9 9.8 3.7 [s.sup.-2]) EV (km x [s.sup.-1]) 617.7 4.3 10.4 11.2 5.0 R.period (h) 609.12 1407.6 5832.5(R) 23.9 24.6 Perih. (106 km) -- 46.0 107.5 147.1 206.6 Aph. (106 km) -- 69.8 108.9 152.1 249.2 O.period (days) -- 88.0 224.7 365.2 687.0 O.velocity (km x -- 47.9 35.0 29.8 24.1 [s.sup.-1]) O.inclination -- 7.0 3.4 0.0 1.9 ([degrees]) O.eccentricity -- 0.205 0.007 0.017 0.094 Temp. ([degrees]C) 6000 167 464 15 -65 Num. satellites -- 0 0 1 2 Ring system? -- No No No No Max.angular diameter 12.3 63.0 -- 25.1 Parameter Jupiter Saturn Uranus Neptune Pluto Mass (1024kg) 1899 568 86.8 102 0.0125 Diameter (km) 142984 120536 51118 49528 2390 Gravity (m x 23.1 9.0 8.7 11.0 0.6 [s.sup.-2]) EV (km x [s.sup.-1]) 59.5 35.5 21.3 23.5 1.1 R.period (h) 9.9 10.7 -17.2 16.1 -153.3 Perih. (106 km) 740.5 1352.6 2741.3 4444.5 4435.0 Aph. (106 km) 816.6 1514.5 3003.6 4545.7 7304.3 O.period (days) 4331 10747 30589 59 800 90 588 O.velocity (km x 13.1 9.7 6.8 5.4 4.7 [s.sup.-1]) O.inclination 1.3 2.5 0.8 1.8 17.2 ([degrees]) O.eccentricity 0.049 0.057 0.046 0.011 0.244 Temp. ([degrees]C) -110 -140 -195 -200 -225 Num. satellites 64 60 27 13 3 Ring system? Yes Yes Yes Yes No Max.angular diameter 49.9 20.7 4.1 2.4 0.11 Key: EV: escape velocity; R.Period: rotation period with (R) denoting a retrograde motion; Perih: perihelion distance; Aph: aphelion distance; O.period, O.velocity etc: orbital period, velocity, etc. Temp: mean temperature. Num.satellites: number of known natural moons. The maximum angular diameter, given in arcseconds, is based on its equatorial diameter at minimum geocentric distance. Table 7.Visibility of Jupiter's Red Spot Date Time Date Time Date Time Jan 01 22:35 May 01 22:30 Aug 24 22:47 Jan 04 00:14 May 04 20:01 Aug 27 20:16 Jan 06 21:45 May 06 21:40 Aug 31 23:33 Jan 08 03:32 May 08 23:19 Sep 26 05:05 Jan 30 21:43 May 28 00:06 Sep 29 22:25 Feb 01 03:31 May 30 01:45 Oct 23 22:08 Feb 04 20:54 Jun 01 22:16 Oct 28 01:24 Feb 06 22:33 Jun 03 05:03 Oct 30 03:02 Feb 09 00:12 Jun 26 19:02 Nov 21 21:00 Feb 28 20:54 Jun 28 00:49 Nov 23 22:39 Mar 02 02:41 Jun 30 22:19 Nov 26 00:17 Mar 05 20:04 Jul 02 04:07 Nov 28 01:55 Mar 07 21:43 Jul 26 23:50 Dec 20 00:06 Mar 09 23:22 Jul 29 01:28 Dec 24 03:23 Apr 29 01:00 Jul 31 22:58 Dec 27 20:45 Apr 30 06:47 Aug 02 04:45 Dec 29 22:23 The red spot transits the meridian aproximately every 10 hours. The above table lists some of the more favourable evening viewing times for each month.