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Observation Theory
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Planetary Basics
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| Mercury Venus Mars Jupiter | Saturn Uranus, Neptune, and Pluto |
Mercury- Power, instrument: 200x to 250x power are minimum; 100 mm instruments are a minimum. Physical data of importance: being an inferior planet, and nearest to Sun, Mercury appears only as a morning or an evening star. It is often low on horizon. This means poor atmosphere conditons. As Mercury is small and far, it displays a small apparent diameter. Features are subtle but may be seen however. Advanced observers may use filters. These are useful too against background sky glare. Miscellaneous: due to its slow rotation Mercury is the least mapped planet by space probes, and due to its proximity to Sun Hubble is never pointed to Mercury. Mercury appears in evening sky 12 days after its superior conjunction and in morning sky 5 days after its inferior conjunction. see a list of relief features names' meaning. see additional data about planets visibility
- Venus
- Power, instrument: 6' (150 mm) for reflectors, 3' (75 mm) for refractors is a minimum. Venus endures high magnification. Physical data of importance: Venus features are elusive. Venus features observation is hindered by planet's strong brilliance. Planet is best observed against a clear sky (contrast with sky is not so important and does not hamper low contrast Venus features). Some observers track Venus too well into daylight in the evening or morning, and 20° above horizon seems a limit to avoid bad seeing. Observing Venus is always a trade-off between poor seeing (due to planet being too low), observation in too much daylight, and too much contrast between planet's brightness and dark sky. Miscellaneous: Venus appears in evening sky 35 days after its superior conjunction and in morning sky 6 days after its inferior conjunction. see a list of relief features names' meaning. see additional data about planets visibility
- Mars
- Power, instrument: a 200x magnification is a minimum, 100 mm of aperture for refractors, 150 for reflectors are minimum. Physical data of importance: seasons are about twice Earth's seasons and day is slightly longer. Mars except near opposition always shows a gibbous phase. Features drift significantly in one hour or two. Each pole is capped by a polar cap which retreats in the hemisphere's spring and winter. Each cap, until local spring is overheaded by a thick cloud (termed the "North (or South) Polar Hood". Albedo features (the dark terrain regions) endure seasonal and secular changes. Miscellaneous: many observers use color filters. Mars is a fine object to observe. see a list of relief features names' meaning. see additional data about planets visibility
click to a map of Mars. ckeck the same map in black-and-white. map site "Amateur Astronomy" based on material courtesy NASA/JPL/Malin Space Science Systems - Jupiter
- Power, instrument: -na-. Physical data of importance: Jupiter is characterized by a series of bands, zones and belts, parallel to the equator, nomenclature of which is to be used in observation reports. "Great Red Spot" (GRS) is a cyclone twice Earth's size. It exists since at least one century and is located South of Jupiter' equator. Its color
faded since about 1975 (it is usual and just a periodical phenomenon). Satellites are also part of the show: they may transit in front of planet disk, be eclipsed by Jupiter's shadow or themselves cast shadows (like Moon is doing at Earth during a solar eclipse) on Jupiter. Double eclipses are relatively frequent and may occur in a row. Triple eclipses occur each decade, as quadruple eclipses never occur. Miscellaneous: Jupiter possesses three longitude systems (I, for visible equatorial features, II for visible mid-latitude to polar features, III for radio and professionals). Satellites magnitudes range for 5.1 for Ganymede to 6.2 for Callisto (Io is 5.5 and Europa 6.1). Due to an important apparent diameter, Jupiter is an easy target for amateurs. see additional data about planets visibility
click to a chart of Jupiter nomenclature (belts, zones) (left), to a chart of Jupiter nomenclature, smaller format (right) see a detailed list of belts and zones. South is up. pictures © site "Amateur Astronomy" with a picture Cassini-Huygens mission - Saturn
- Power, instrument: ideal are a 6' (152 mm) refractor or a 10' reflector. Physical data of importance: like Jupiter, Saturn is characterized by a series of zones and belts parallel to equator;
features are less impressive than Jupiter's. Rings are an object of observation too, as may be Saturn's satellites. Miscellaneous: two system of longitude are used (I -equatorial regions, II -north or south of previous). Zones and belts are given a specific nomenclature like for Jupiter. Saturn's ring is included in the nomenclature too. Satellites magnitudes are ranging like: Titan 8.4; Rhea 9.7; Tethys 10.3; Dione 10.4; Enceladus 11.8; Iapetus 10.1-11.9; Mimas 12.9. see additional data about planets visibility
click to a chart of Saturn nomenclature (belts, zones) (left), to ring features (center), to nomenclature-ring smaller format (right). South is up. see a detailed list of belts and zones. pictures © site "Amateur Astronomy" - Uranus, Neptune, and Pluto
- Power, instrument: -na-. Physical data of importance: Uranus, Neptune and Pluto are faraway worlds and all nead important apertures to be observed. Miscellaneous: Uranus axis is tilted on orbit beyond orbit's plane (97.8°). Planet presents us either its northern or southern hemisphere. Presently both are seen as Uranus is near one of its equinoxes due to occur in 2007. Last solstice (we were seeing southern part of the planet) took place in 1985, next one (we will see planet's northern part of the planet) will be in 2030. Titania and Oberon, Uranus' moons may be glimpsed with apertures as small as 8" as Umbriel and Ariel are much more difficult to observe. Neptune: Neptune is about midway its aphelion (which was in July 1959) and its perihelion. It will reach the latter on Oct 9 2042 only. Pluto: due to its orbit, Pluto is orbiting inside Neptune's orbit during 20 years; this last happened between 1979 and 1999. Next time Pluto will come back inside Neptune's orbit will be on April, 5th, 2231 only. see additional data about planets visibility
