Additional Factors To Planets' Visibility

Additional factors may be taken in account when it comes to observing the planets

Opposition and Conjunction (Outer Planets Only)
Perihelion and Aphelion (Outer Planets Only)
North or South Of the Ecliptic (Outer Planets Only)
Mars Cycle of Martian Oppositions
Mutual Phenomena of Jupiter's Galileans
The Rings, A Factor Specific to Saturn
Ranges of Apparent Diameter and Magnitudes (All Planets)
The Inclination of the Ecliptic (All Planets)

Opposition and Conjunction (Outer Planets Only)

"Outer planets" means that these faraway worlds are located beyond Earth. At the opposite "inferior planets" are located between the Earth and Sun. The rules of celestial mechanics makes that the outer planets come into typical configurations with Earth along their orbits. Such typical configurations are the opposition and the conjunction (for more about outer and inner planets' apparent motion, see the tutorial "Planets Apparent Motion". When an outer planets is at opposition, this means it is in line with Sun and Earth, beyond Earth. Particularly this means that it is at that period that the outer planet is nearest to us, being at its maximum brightness and at its maximum apparent diameter. On the other hand this means too that it is observable all night long as it is rising on the eastern horizon when Sun sets. All these factors enhance the observation of the outer planet. At the opposite the conjunction is the moment when the outer planet is in line with Earth and Sun, on the other side of the Sun relative to Earth. Approaching this period and at this period, the outer planet becomes unobservable. A marginal, rare effect is produced when Sun, Earth, and the exterior planet are perfectly aligned at opposition -that is that seen from the planet, Earth would be seen transiting the solar disk (just like a Venus or Mercury transit). Such a perfect alignment much enhances the planet's (and possible moons) brightness due to the minimal phase angle (sunlight hits and is reflected at the least angle possible). For Saturn, for example, this last took place in 1990; next will be in 2020. Jupiter, more generally, has its best oppositions in the northern hemisphere between November and January and the least May-July

The 'opposition effect' is the -- sometimes dramatic -- increase in reflected sunlight that occurs from a celestial body at the opposition. The angle between Sun and the Earth as seen from the planet is called the solar phase angle, or simply the phase angle. The opposition effect is the increase in brightness observed as the phase angle decreases to zero. The effect at Saturn also affects the rings. The opposition effect is the product of two phenomena. Particle shadow hiding results from that particles at a planet or a ring can hide their own shadows and contribute to an increase in brightness, as coherent backscatter additionnally from incident (or incoming) rays of sunlight interfering constructively with sunlight reflected from the surface. Planets or moons with a atmospheres do not exhibit dramatic opposition effects

Perihelion and Aphelion (Outer Planets Only)

Planets' orbits are elliptical, that is they are not a perfect circle but better a kind of egg-shaped. Hence a point of the orbit is found nearer to the Sun and a point is found further. First is called the "perihelion" (from two Greek words: "peri", "near"; "helios", "Sun"); second is termed the "aphelion". This is another factor to the outer planets visibility. When the planet is at its perihelion, these are years when the planet is treated with a bonus of luminosity and apparent diameter. for more about orbits see the tutorial "Orbits"

North or South Of the Ecliptic (Outer Planets Only)

A last point of interest is about the position of the planet relative to the ecliptic. The ecliptic is this zone of the sky along which the planets' apparent motion seems to take place. The orbital plane of the planets' orbit is slightly tilted relative to the orbital plane of Earth's orbit. This means that at some moments of their orbits the outer planets are seen North of the ecliptic as at some others South. This is of importance mainly for Saturn and Neptune orbit inclination of which is 2.5 and 1.8 respectively. When Saturn is above the ecliptic this brings a surplus of visibility to the planet, as it is visible in better conditions. The position relative to the ecliptic is important too at the moment of the oppositions of Mars. The higher the Red Planet, the less atmospheric haze its image has to cross. see orbital inclinations at "Solar System Figures"

(1) back after an opposition occurred Earth is performing one orbit. When it comes back to where the opposition took place the outer planet moved further on its orbit hence Earth has to perform an additional move to reach the next opposition. This interval is called such or such planet's synodic year
(2) back as seen from Earth, outer planets are seen moving eastwards. Due to both relative motions of the outer planet and Earth however, the outer planet seems to reverse its motion at a moment and moves westwards. This motion is termed "retrograde motion". The outer planet eventually comes back to its eastward motion (which is termed "direct motion"). The opposition takes place when the outer planet is at the middle of its retrograde motion

Mars Cycle of Martian Oppositions

The observation of Mars by amateur astronomers who specialize into is introducting to the concept of 'Mars Observation Campaign', or 'Martian Observation Campaign,' which a time spanning either side of the opposition of the Red planet, generally from the time Mars is reaching 6" of apparent diameter to the time it is reaching it back. Mars usually is best seen during such a campaign as due to the relative proximity of Mars of the Earth, a Martian opposition occurs about each 26 months only and not yearly like for any superior planet, those planets which are located beyond Earth relative to Sun. for more details about Mars' conditions of observation during a Mars Observation Campaign, check at our Mars Oppositions tutorial!

Mutual Phenomena of Jupiter's Galileans

'Mutual phenomena of Jupiter's Galileans' is the fact that the four main Jupiter's satellites, or the Galilean satellites, are mutually occulting and eclipsing eachother every 6 years, at the time of Jupiter's equinox as the Sun then is lying with the equatorial plane of the gas giant. The timing of such events is so accurate that their accuracy is reaching to the measurements of some planetary probes. The next season of mutual phenomena will occur with the Feb. 5, 2015 equinox, which itself occurs about Jupiter' yearly opposition, with a timespan October 2014-June 2015. Amateur astronomers can work about those phenomena in term of photometry. A configuration of the main Jovian satellites for a given time may be found at the French Bureau des Longitudes (site in French); of note that predictions of the mutual phenomena of Jupiter's Galileans are not very accurate as a predicted time may differ of several minutes of the actual event

The Rings, A Factor Specific to Saturn

Saturn's rings are a factor specific to Saturn. The rings are in the equatorial plane. As the planet axis is tilted 26.7° the rings are seen from Earth at a varying angle along Saturn's orbit. That angle value and the duration of Saturn's revolution of 29.4 years determine a cycle of how Saturn's rings are seen. About each 13 to 15 years the rings are seen edge-on. Between such passages they reach their maximum tilt (26.7°). This variation of the ring aspect impact on the magnitude of the planet and renders impossible the observation of Mimas the innermost observable Saturn's moon. When rings are seen edge-on on the other hand, the rings also appear darker than usual

(1) back due to celestial passages next edge-on passages theoretically occur in 2009 and 2025. Saturn then will be 10° apart of the Sun only however, making these passages unobservable
(2) back see note (1)
(3) back after rings reached their maximum tilt, they are seen gradually decreasing over a time span of 7 years down to the edge-on period. And the reverse: after each edge-on passage, rings tilt is seen reappearing and increasing up to the next maximum tilt period

Ranges of Apparent Diameter and Magnitudes (All Planets)

how Jupiter's apparent diameter is varying along one year; the example of the year 2009. picture with Celestia

(1) back first set of values was taken from a site on the Internet; second set is taken from the "Planetary Fact Sheets" at NASA's NSSDC
(2) back inferior conjunction for inferior planets (Mercury and Venus), opposition for superior planets (Mars to Pluto)
(3) back figures for Saturn do not include rings. Acceptable values for ring may be 35"-49"
(4) back a set of values at the American Association of Amateur Astronomers (AAAA) gives 0.16"-0.28"
(5) back Jupiter in 2010 came 75 million km closer to Earth than usual, a occurrence which won't occur again before in 2022

The Inclination of the Ecliptic (All Planets)

According to the seasons, the observer's latitude, and the observation's time, the ecliptic has a varying inclination relative to the horizons of the observers. The nearer to the terrestrial equator, the more vertical the ecliptic is able to be. Such data further are combining themselves with the time at which the observation is performed (which itself, function of the season during which the observation is performed, may yield a dark night early for a given day). see sketches showing the ecliptic inclination at various moments and horizons for mid-northern latitudes, equator, and mid-southern latitudes