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The planets: "My very educated mother just sent us nougat".

The dance of the planets

The Ave (classic) naked-eye planets--Mercury, Venus, Mars, Jupiter, and Saturn--are probably the most popular observing targets for amateurs. They are often the brightest objects in the sky, and as they move in their orbits around the Sun they are at times arranged in attractive groupings with each other or the Moon (see the sky maps in the monthly calendar).

The outer planets, those beyond the Earth's orbit around the Sun, are best seen at opposition. During 2019, Jupiter reaches opposition on June 10, Saturn on July 9, Uranus on October 28, Neptune on September 10, and dwarf planet Pluto on July 14.

Opposition occurs 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 outer planet is always full or gibbous (more than half but not all of the disc 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.

The inner planets, Mercury and Venus, go through a complete cycle of phases like the Moon. When an inner 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.

The apparent diameter of an inner planet changes appreciably with its position in its orbit; it is largest at inferior conjunction, but since it is then between the Sun and the Earth it cannot be seen. The changing brightness and apparent diameters of the planets during 2019 are illustrated in Figures 5 and 6.

During the year, the planets appear in the 12 zodiac constellations, as well as in Cetus (Mercury and Venus) and Ophiuchus (Mercury, Venus and Jupiter). Of the zodiac constellations, Scorpius sees the least planetary action during the year, hosting only Mercury (for three days) and Venus (for 12 days). Sagittarius is the best-populated for the year, hosting Mercury for 26 days, Venus for 55 days, Jupiter for 45 days and Saturn for the entire year.

As seen from Earth, the occultation of one planet by another happens, on average, once every 33 years. The previous mutual occultation was on 1818 January 03 (Venus occulting Jupiter) and the next event will be on 2065 November 22 (also Venus and Jupiter). Close planetary pairings for 2019 are listed in Table 2 (p 73).

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 next event is in 2035, when Venus occults [pi] Sgr. In 2044, Venus will occult Regulus.

Visibility of the planets

The rise and set time of the planets is illustrated in Figure 7 opposite. The figure also shows the time of sunrise and sunset, and the beginning and end of astronomical twilight. Various geocentric and heliocentric phenomena of the planets (listed in the monthly almanac), as well as close planetary pairings (Table 2), can be related to this figure with some study.

Mercury

Being the innermost planet, Mercury is near the Sun, and can only be seen low in the east just before sunrise, or low in the west just after sunset. Mercury is most prominent in the morning sky during April (greatest western elongation is on April 11). It can be seen at dawn from the start of the year until mid-January, from late-March until mid-May, end-July until late-August, and from mid-November until late-December. Mercury is most prominent in the evening sky during October (greatest eastern elongation is on October 20). It can be seen during evening twilight from mid-February until the first week of March, late-May until mid-July, and mid-September until the first week of November.

Binoculars show it as a bright star, yellowish or pale orange in colour. A telescope reveals a small disc, never larger than 12.3" across, that, like the Moon and Venus, goes through a sequence of phases. Like the Moon, Mercury 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 disc, which shows vague streaks and patches like those on Mars, only much fainter.

There are about 13 transits of Mercury each century; all fall within several days of May 08 and November 10. On November 11, from 14:35 to 20:04, Mercury will transit the Sun. See p55 for details of this rare event. The next transit will occur on 2032 November 13.

Venus

Venus is the brightest starlike object in the night sky, and can even be seen in broad daylight with the naked eye. The best time to see it during the day is when the planet is near greatest elongation, and relatively near the Moon, which serves as a visual anchor. See the monthly almanac for dates when Venus is near the Moon at noon and sufficiently distant from the Sun to be seen.

Through 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 high-power binoculars or a telescope. Unlike the Moon, Venus is brightest when in crescent phase and faintest when full. Its maximum apparent angular diameter is 63.0", which occurs when Venus is at minimum geocentric distance.

Venus starts the year as the brilliant Morning Star, remaining visible until the first week of July, after which it moves too close to the Sun to be visible. lt reappears in late September as Evening Star and is prominent after sunset until year-end. Venus is best-placed for observing during January, rising more than three hours before the Sun.

As is the case with Mercury, Venus can also transit the Sun. Such transits are among the rarest of planetary alignments. Only eight such events have occurred since the invention of the telescope, the most recent occurring in 2012. Transits of Venus are possible only during early December and early June, and recur at intervals of 8, 121.5, 8 and 105.5 years. The next pair of Venus transits occur on 2117 December 11 and 2125 December 08.

Earth

The Earth is at perihelion on January 03 and at aphelion on July 06. The equinoxes are on March 20 (23:58) and September 23 (09:50), and the solstices on June 21 (17:54) and December 22 (06:19).

Atmospheric phenomena

Light from the Sun that reaches the Earth's surface consists of photons with a wide range of energies, of which the human eye can perceive only a limited range. We can sense light with a wavelength from about 720nm (red) to about 380nm (purple), which defines the visible part of the spectrum. Three types of chemical receptors in the retina of the eye respond most strongly to red, green and blue wavelengths, which in combination give us colour vision.

As sunlight passes through the Earth's atmosphere, it encounters molecules of oxygen and nitrogen in the air along the way. These molecules scatter the light, and the degree of scattering depends on the wavelength, or colour, of the light: the shorter the wavelength, the more pronounced the scattering effect. Blue light is scattered about 10 times more than is red light. This explains why the day-time sky is blue: no matter where you look in a clear sky, there will always be blue light scattered in your direction. However, purple is an even shorter wavelength, so shouldn't the day-time sky be purple instead (after all, a rainbow shows indigo and violet hues beyond the blue)? Three factors contribute to conspire against purple: the Sun's output isn't constant at all wavelengths; some of the purple light is absorbed high in the atmosphere; and the chemical receptors in the eye that detect blue light are stimulated more strongly.

A clear-sky sunset will appear yellow, as the sunlight reaching the eye has had some of its blue light scattered away. lf there are small particles in the air, the sunset will appear redder. A sunset over the ocean may appear orange as light is scattered by suspended salt particles. During a forest fire or a volcanic eruption, particles thrown up are typically the right size to scatter red light more effectively. Incoming sunlight may also encounter water droplets in the air. Under the right conditions, these droplets will refract and reflect the light, causing a rainbow. Each observer sees their own rainbow, always located in the sky directly opposite the Sun. lf the water droplets are frozen, the ice crystals may refract incoming light into a coloured ring, usually 44[degrees] across, centred around the Sun. Such a halo may also be seen at night around the Moon.

Artificial satellites

On any clear night, a dozen or so artificial satellites can be seen moving slowly across the sky. Looking like slow-moving stars, sometimes as bright as magnitude +2, they take about 10 minutes to travel from horizon to horizon. Satellites are most prominent in the first and last hours of the night, since they are then still clear of the Earth's shadow. Sometimes they fade suddenly and disappear as the satellite enters the shadow. Generally, Russian satellites tend to orbit south-north or north-south, as they are launched from reasonably high-latitude sites.

Some satellites, such as the Hubble Space Telescope and the lnternational Space Station are distinctly brighter than others. The lridium communication satellites can produce spectacular flashes (briefly shining at -8-mag.) when the Sun glints off their shiny surfaces. Other satellites, visible to the naked eye, move together in groups of two or three, sometimes giving rise to UFO reports.

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 magnitude, whether it appeared steady or was tumbling), to the specialized art of determining the position of a satellite against the celestial background so that an accurate orbit can be calculated. Approximately 350 satellites have no orbital data from official sources, as they are classified military launches. A handful of amateur space detectives have located and identified such objects and keep tabs on them. South Africa has always been a location of importance to the satellite tracking community on account of it being in the southern hemisphere and also under the flight path of many satellites during the first few hours in orbit.

Mars

Mars is visible in the evening sky, amongst the stars of Pisces, at the beginning of the year (see Figure 11 overleaf). lt remains an evening sky object until mid-July, when it lies too near the Sun to be visible (solar conjunction is on September 02). lt reappears, in the morning sky, during the third week of October, and remains in the morning sky until year-end.

Mars gradually fades throughout the year, dropping below mag. +1 in mid-February. The planet's disc is usually small, so it is best seen when at opposition. Mars was at opposition on 2018 July 27 and will be so again on 2020 October 13. During 2019 it remains angularly small, starting the year at 7.4 arcseconds. From July 27 until November 08 it appears smaller than Uranus (see p 71).

To the naked eye, Mars has a distinct orange-red colour, which is more pronounced through binoculars. With a 5-cm telescope you should be able to see one of the white polar caps, which vary with the Martian season, while at least a 15-cm telescope is needed to see the dark markings, some of which were famously reported as "canals" by some late-19th-century observers. ln general, the features observed on Mars are not topographic, but are rather superficial dust shadings. In larger telescopes, the gigantic volcano Olympus Mons (three times as tall as Mount Everest) can be seen as a prominent white spot. lt is the tallest known volcano in the solar system. The start of autumn in the Martian southern hemisphere is on March 23, while winter starts on October 08.

The two moons of Mars, Phobos ("Fear") and Deimos ("Dread"), are bright enough (+11.3and +12.4-mag., respectively) to be seen in a small telescope, but because they are almost hidden in the planet's glare, a telescope of at least 20-cm aperture is needed.

Jupiter

Jupiter is one of the brightest objects in the night sky, outshone only by the Moon and Venus, and sometimes Mars. Jupiter is visible in the morning sky at the start of the year, located in Ophiuchus (see Figure 14). By mid-March it is visible for more than half the night. It is at opposition on June 10 and is visible throughout the night. By early September it can be seen only in the evening sky. From mid-December it lies too near the Sun to be visible.

Binoculars will show the planet as a tiny, pale yellow disc. Its four brightest satellites (the Galilean moons, so-called because they were discovered by Galileo in 1610) can also be seen. A small (5-cm) telescope will show some features on the disc, usually two dark belts. With a 12-cm or larger telescope a wealth of detail can be observed. Figure 12 shows the planet's main belts and zones; their appearance is variable so the illustration should be regarded as a guide only.

Because Jupiter rotates very rapidly, one Jovian day lasting less than 10 hours, details on the planet's surface will move noticeably in a period as short as 10 minutes. These markings are storm features in its upper atmosphere and consist of dark brownish strips interspersed by brighter zones. The light-coloured zones are strong winds blowing in 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.

The four Galilean moons--Io, Europa, Ganymede and Callisto--are often arranged in interesting configurations as they orbit the planet. An eclipse occurs when the satellite moves through the shadow of Jupiter (see Figure 13), 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, as seen from the Earth, 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 disc. The shadow may be seen through a telescope as a westward-moving small black dot on the planet's surface. Eclipses, occultations and transits, visible from southern Africa, are listed in the almanac. Triple shadow transits always involve Callisto and two of the inner moons. ln the period 1900 - 2100, 32 such events take place; the next event is on 2021 August 15. There are no mutual eclipses or occultations of the moons during 2019; the next event is an occultation of Europa by lo on 2020 May 16.

Jupiter will appear moonless on 2019 November 09 when lo and Ganymede are in transit, Europa is occulted and Callisto is eclipsed.

The Galilean moons are bright enough to be seen without optical aid, but are usually masked by the glare from Jupiter. At greatest elongation, Callisto can be up to 10.7 arcminutes from Jupiter, and should be visible with the naked eye if light from the planet is blocked off. Dates of maximum separation during 2019 are listed in the almanac.

Saturn

Saturn starts the year hidden by the glare of the Sun. In late January it becomes visible shortly before sunrise and can be seen in the morning sky until July. By mid-April it can be seen for more than half the night. It is at opposition on July 09, when it can be seen throughout the night. From early October until late December it can be seen only in the evening sky.

Always brighter than 2nd magnitude, Saturn 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 moon in the solar system. The table on p 9 lists the dates and approximate times when Titan is at maximum angular separation from Saturn.

A small (5-cm) telescope will clearly show the famous rings as two bright elliptical bands with a thin gap (Cassini's Division). A larger telescope is needed to see the C (Crepe) Ring and the Encke Division (a narrow, high-contrast feature located near the outer edge of the A Ring). Saturn's ring opening angle (relative to Earth) varies between 23.46[degrees] and 25.45[degrees] during 2019.

Like Jupiter, Saturn is oval-shaped, but its surface markings are much less prominent. At minimum geocentric distance, Saturn's disc is 20.7 arcseconds across. At opposition, it shines at magnitude +0.7 (rings are closed) or -0.2 (with the rings fully open).

Saturn has nine large moons. A 7.5-cm telescope will show Rhea and sometimes Iapetus (which varies in brightness because it has a large, dark surface feature), while a slightly larger telescope shows Enceladus, Tethys and Dione.

Uranus

Uranus starts the year visible in the evening sky, where it remains until early April, when it is lost in the solar glare. It is at conjunction on April 23. Uranus reappears in mid-May in the morning sky. Until late October (opposition on October 28) it remains in the morning sky; thereafter it is visible for more than half the night until the end of the year.

Uranus reaches magnitude +5.5 at opposition, so under dark skies it can be seen with the naked eye. During 2019 its magnitude varies between +5.7 and +5.9 (see Figure 5, p 71). Easily seen as a starlike point through binoculars, a telescope reveals its greenish-blue 4-arcsecond disc. Even in large telescopes, the disc is essentially featureless. A 20-cm telescope will show the two largest moons, Titania (13.7-mag.) and Oberon (14.1-mag.).

Neptune

Neptune starts the year visible in the evening sky, where it remains until mid-February, when it is lost in the solar glare. It is at conjunction on March 07. Neptune reappears in late March in the morning sky. Until early September (opposition on September 10) it remains in the morning sky; thereafter it is visible for more than half the night. From early December it can be seen only in the evening sky.

Neptune is too faint to be seen with the naked eye, but binoculars show it readily as a dim "star", about magnitude +8. A small telescope will show it as a bluish "star", its tiny 2.4-arcsecond disc requiring a larger telescope (and higher magnification) to resolve. Elusive white spots may develop on the planet's surface and may be observed with a 25-cm or larger telescope. Neptune's brightest satellite, Triton (13.5-mag.) is visible in a 20-cm telescope.

Dwarf planets

A dwarf planet is a category of solar system object as defined by the International Astronomical Union (IAU) in 2006. A dwarf planet is in direct orbit of the Sun, is neither a planet nor a natural satellite, has sufficient mass for its self-gravity to cause it to assume a nearly round shape, and has not cleared the neighbourhood around its orbit.

To date, the IAU recognizes Ave dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris (see Table 5). Many dwarf planets are thought to exist in the solar system (more than 10 000 by some estimates) and the study of small solar system bodies is ongoing. Several candidate dwarf planets are listed in Table 5.

Pluto, previously thought of as a planet, was reclassified as a dwarf planet in 2006 August and is recognized as the prototype of the Plutoids class of trans-Neptunian objects (see Figure 18). Pluto is at opposition on July 14, reaching magnitude +14.4. Located in Sagittarius, towards the Galactic Centre, it lies within a crowded star field and requires care to locate.

Ceres, previously thought of as an asteroid, is the only dwarf planet in the inner solar system. lt is bright enough to be seen through binoculars; during 2019 its magnitude ranges between +7.0 and +9.2. Ceres is at opposition on May 28 (magnitude +6.9) and is stationary on April 08 and July 19 (see p 61). lt does not reach solar conjunction during the year. On August 15 at 22:51, Ceres occults UCAC5 344-074929, a magnitude +12.2 star in Scorpius.

Haumea, Makemake and Eris are distant bodies never brighter than magnitude +17.

Caption: Two views of Jupiter, taken on 2018 June 18 at 19:26 (left) and 2018 April 20 at 03:02 (right). In the first image, the Jovian moon Ganymede is perched on the giant planet's edge. The moon spans a mere 1.57-arcseconds. The second image shows the Great Red Spot, which is about 6-arcseconds, or some 16 000 km, wide. Imaged by Clyde Foster from Centurion with a 14-inch Celestron Edge HD SCT.

Caption: Figure 5. Visual magnitudes of the planets and Ceres in 2019. Brighter objects have negative magnitude values. The gaps in the curves for Venus and Mercury correspond to their periods at conjunction.

Caption: Figure 6. Angular diameters (arcseconds) of the planets in 2019. Mercury is at inferior conjunction on March 15, July 21, and November 11. Saturn is at opposition on July 9 and Jupiter on June 10.

Caption: Figure 7. Times of rising (solid lines) and setting (dashed lines) of the Sun and planets. The beginning and end of astronomical twilight is also shown (shaded region). A line drawn vertically and followed upwards indicates the succession of events during the course of one night. The times are accurate for longitude 30[degrees]E and latitude 30[degrees]S. Approximate time corrections for other locations are given in Table 3 (opposite).

Caption: Figure 8. Venus on 2018 August 22, four days after greatest eastern elongation. The planet's diameter was 23 arcseconds. Image by Clyde Foster.

Caption: Figure 9. The nSight-1 satellite shortly after release from the lnternational Space Station as part of the QB50 Mission. The satellite was designed and built by SCS Space, a subsidiary of the SCS Aerospace Group, South Africa's largest privately owned group of satellite companies. lmage: SCS (supplied).

Caption: Figure 10. Mars on 2018 June 19, when it was just 6 arcseconds in diameter. Image by Clyde Foster.

Caption: Figure 11. During the year Mars moves through Pisces (Jan 01-Feb 13), Aries (Feb 14-Mar 23), Taurus (Mar 24-May 16), Gemini (May 17-Jun 28), Cancer (Jun 29-Jul 30), Leo (Jul 31-Sep 24), Virgo (Sep 25-Dec 01) and Libra (Dec 02-Dec 31). Tick marks show its position at the start of each month.

Caption: Figure 12. Jupiter's belts (B) and zones (Z). N: north, S: south, E: equatorial, T: temperate, Tr: tropical, PR: polar region, GRS: Great Red Spot.

Caption: Figure 13. Example of various phenomena of Jupiter's moons: lo is shown at the start of transit, Europa is being occulted, Ganymede is coming out of eclipse and Callisto's shadow is falling on Jupiter.

Caption: Figure 14. Jupiter moves through Ophiuchus (Jan 01 - Nov 16) and Sagittarius (Nov 17-Dec 31) during 2019. It is stationary on April 10 and August 11. Tick marks show its position at the start of each month.

Caption: Figure 15. Saturn's rings.

Caption: Figure 16. Saturn is in Sagittarius throughout 2019. lt is stationary on April 30 and September 18. Tick marks show its position at the start of each month.

Caption: Figure 17. The paths of Uranus (top) and Neptune (bottom) during 2019. Uranus is stationary on January 7 and August 12, while Neptune is stationary on June 22 and November 27.

Caption: Figure 18. Venn diagram of solar system bodies.

Caption: Figure 19. Discovery circumstances of confirmed exoplanets (as at 2018 September), showing their declination on the sky and discovery technique.
Table 2. Close planetary pairings

Date     Planets and separation

Feb 13   Mars--Uranus 58.6' (65[degrees] E of Sun)
Feb 19   Mercury--Neptune 40.3' (15[degrees] E of Sun)
Apr 02   Mercury--Neptune 23' (26[degrees] W of Sun)
Apr 10   Venus--Neptune 17.1' (33[degrees] W of Sun)
Jun 18   Mercury--Mars 13.2' (25[degrees] E of Sun)
Aug 24   Venus--Mars 17.4' (3[degrees] E of Sun)
Sep 03   Mercury--Mars 38.5' (2[degrees] E of Sun)
Sep 13   Mercury--Venus 17.1' (9[degrees] E of Sun)

Table 3. Rise-set time corrections

Location        Sun         Sun         Sun
                at summer   at the      at winter
                solstice    equinoxes   solstice

Antananarivo    -0h 46m     -1h 10m     -1h 34m
Bloemfontein    +0 17       +0 15       +0 13
Cape Town       +0 36       +0 46       +0 56
Cederberg       +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
Gaborone        +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          -0 11       -0 43       -1 15
Nairobi         +0 29       -0 27       -1 23
Port Louis      -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

Approximate time corrections to be applied to the rise and set
times from Figure 7 (opposite). Use column 1 for Mercury, Venus and
the Sun at the summer solstice and for Mars at the beginning of the
year. Use column 2 for Mercury, Venus and the Sun at the equinoxes,
for Mars in the middle of the year, and for Jupiter. Use column 3
for Mercury, Venus and the Sun at the winter solstice, for Mars
towards the end of the year, and for Saturn. Locations in other
time zones will have to adjust by the relevant whole number of
hours.

Table 4. Dwarf planets during 2019

Name       Designation                  Magnitude

Pluto      134340 Pluto                 14.2-14.4
Ceres      1 Ceres                      7.0-9.2
Eris       136199 Eris (2003 UB313)     18.7-18.8
Haumea     136108 Haumea (2003 EL61)    17.3-17.4
Makemake   136472 Makemake (2005 FY9)   17.1-17.2

Name       Constellation(s)     Geocentric (AU)

Pluto      Sgr                  32.82-34.91
Ceres      Lib, Oph, Sco, Sgr   1.75-3.88
Eris       Cet                  95.05-97.04
Haumea     Boo                  49.54-51.26
Makemake   Com                  51.67-53.45

Table 5. Solar system fact sheet

Orbital characteristics

Body      Perihelion        Aphelion    Eccentricity   Inclination
          ([10.sup.6] km)   (106 km)                   (degrees)

Mercury   46                69.8        0.21           7
Venus     107.5             108.9       0.01           3.4
Earth     147.1             152.1       0.02           0
Mars      206.7             249.2       0.09           1.9
Ceres     381.4             447.8       0.08           10.6
Jupiter   740.5             816.6       0.05           1.3
Saturn    1 352.6           1 514.5     0.06           2.5
Uranus    2 741.3           3 003.6     0.05           0.8
Neptune   4 452.9           4 553.9     0.01           1.8
Pluto     4 436.8           7 375.9     0.25           17.1
Makemake  5 760.8           7 939.7     0.16           29
Haumea    5 194             7 710       0.19           28.2
Eris      5 650             14 600      0.44           44.2

Body      Orbital period   Orb.velocity
          (days)           (km x [s.sup.-1])

Mercury   88               47.9
Venus     224.7            35
Earth     365.3            29.8
Mars      687              24.1
Ceres     1 679.8          17.9
Jupiter   4 331.6          13.1
Saturn    10 832.3         9.7
Uranus    30 799           6.8
Neptune   60 190           5.4
Pluto     90 613           4.7
Makemake  113 183          4.4
Haumea    103 468          4.5
Eris      203 600          3.4

The average distance from the Sun of the planets (top) and
dwarf planets (bottom) in the solar system.
Note that the horizontal scale is logarithmic.

Physical characteristics

Body      Mass        Radius   Density          Gravity
          (1023 kg)   (km)     (g/[cm.sup.3])   (m/[s.sup.2])

Mercury   3.302       2 440    5.427            3.701
Venus     48.685      6 052    5.204            8.87
Earth     59.736      6 378    5.515            9.78
Mars      6.4185      3 390    3.933            3.71
Ceres     0.00943     487      2.077            0.27
Jupiter   18981.3     71 492   1.326            24.79
Saturn    5683.19     60 268   0.687            10.44
Uranus    868.103     25 559   1.318            8.87
Neptune   1024.1      24 766   1.638            11.15
Pluto     0.1314      1 151    2.06             0.655
Makemake  0.03        717      1.7              0.37
Haumea    0.04        770      2.6              0.44
Eris      0.167       1 163    2.52             0.827

Body      Rotation     Albedo
          (hours)

Mercury   1 407.509    0.106
Venus     -5 832.444   0.65
Earth     23.934       0.367
Mars      24.623       0.15
Ceres     9.074        0.09
Jupiter   9.925        0.52
Saturn    10.656       0.47
Uranus    17.24        0.51
Neptune   16.11        0.41
Pluto     153.293      0.3
Makemake  7.771        0.81
Haumea    3.916        0.7
Eris      25.9         0.96

Candidate dwarf planets

Body        Discovery     Diam (km)   Moons   Perihelion (AU)

Orcus       2004 Feb 17   917           1     30.73
2002 MS4    2002 Jun 18   934           0     35.98
Salacia     2004 Sep 22   854           1     37.31
Quaoar      2002 Jun 04   1 110         1     41.97
2007 OR10   2007 Jul 17   1 535         1     33.18
Sedna       2003 Nov 14   995           0     76.05

Body        Aphelion   Period (yr)

Orcus       48.07      247.29
2002 MS4    47.77      271
Salacia     46.52      271.34
Quaoar      45.16      287.53
2007 OR10   101.1      550.19
Sedna       899.48     10772.7
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