Wandering Stars:

The Movements and Visibility Cycles

of the Naked Eye Planets

by Martin J. Powell

"How much richer it is to measure, evaluate, and treasure your life not just in the dry numbers of mechanical clocks and paper calendars but also in the multifarious moving lights in the sky and their interrelations."

- Fred Schaaf, The Starry Room

The planets each have their own movement, brightness and colour characteristics. The ancient Greeks referred to them as 'planetes asteres' ('wandering stars') from which the word 'planet' is derived. The planets undergo a regular appearance cycle and several of them can be seen with the naked eye for much of the year, apart from short periods of time when they are too close to the Sun to observe.

The planets can be distinguished from the stars because they change their position slightly against the background stars from one night to the next, and their brightness varies in a regular cycle over a period of time. The changing positions of the planets which are closest to the Earth - i.e. Mercury, Venus and Mars - can be followed easily with the naked eye over only a matter of days. The motion of the more distant planets against the stars can be seen with the naked eye over longer periods of time, although a pair of binoculars will help to reveal their motion over a much shorter time span.

In addition to their shifting positions against the background stars, the brighter planets also do not appear to twinkle (or scintillate) as readily as the stars. Rather, when they are well above the horizon, the planets shine with a more steady light.

The Movements of the Planets in the Night Sky

The Sun, Moon and planets are seen to move along a fairly narrow band of the night sky which passes through the twelve commonly-known zodiac constellations - namely Pisces (the Fishes), Aries (the Ram), Taurus (the Bull), Gemini (the Twins), Cancer (the Crab), Leo (the Lion), Virgo (the Virgin), Libra (the Balance or Scales), Scorpius (or Scorpio, the Scorpion), Sagittarius (the Archer), Capricornus (or Capricorn, the Sea Goat) and Aquarius (the Water Carrier). In addition, the Sun, Moon and planets also move through a thirteenth, lesser-known constellation - Ophiuchus (the Serpent Holder), the Southernmost part of which lies between Scorpius and Sagittarius. Any bright 'star' which is seen in any of these thirteen constellations that does not appear on a star chart is very probably a planet.

The name zodiac derives from the ancient Greek term 'zoidiakos kyklos' - the 'circle of animals' - which these twelve constellations represent (apart from Libra, that is, whose stars originally formed 'the claws' of the Scorpion; the Romans later separated these stars from Scorpius and re-named them Libra).

Viewed from the orbiting Earth, the Sun appears to move against the background stars, i.e. the stars which form the twelve zodiac constellations and Ophiuchus. The apparent path along which the Sun moves during the year is called the ecliptic (from the ancient Greek term 'ekleiptikos', meaning 'the path along which eclipses occur'). If we were able to stand at the centre of the Sun (!) and observe the Earth orbiting around us, we would see that, throughout the course of its orbit, the Earth described a path against the background zodiac constellations - the same path that we see the Sun describe, but viewed from the opposite direction. Therefore, as well as being the Sun's apparent path against the background stars, the ecliptic can also be considered as the plane of the Earth's orbit projected out into space.

Whilst the Sun's apparent movement defines the line of the ecliptic, the Moon and planets do not strictly follow this line because their orbits are inclined at slightly differing angles to it (this is referred to as their orbital inclination). For much of the time, the planets deviate above or below the ecliptic by only a few degrees during the course of their orbit, but the Moon, Mercury, Venus and Mars can deviate by up to several degrees; consequently, they do occasionally wander into adjacent non-zodiac constellations (e.g. Cetus, Orion, Sextans) for short periods of time. Pluto has the largest orbital inclination of all the planets - it moves up to 17º away from the ecliptic - so it can also move well outside the zodiacal band.

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The Visibility of the Planets

The length of time a planet is visible on any given night - or whether it is visible at all - depends upon the relative orbital positions of the planet and the Earth with respect to the Sun on that particular night. The duration of twilight at the observer's location is also an important factor because, of course, if the sky is too light the planet will not be visible!

Viewed from the orbiting Earth, the planets undergo a regular appearance cycle - known as an apparition - over a given period of time. The precise nature of this cycle depends upon whether it is an inferior planet (closer to the Sun than the Earth, i.e. Mercury and Venus) or a superior planet (further out than the Earth, i.e. Mars and beyond).

The Inferior Planets

Because the orbits of the inferior planets are contained within the Earth's orbit, they can only appear within a limited angular distance (or solar elongation) from the Sun. As a result, they are only visible for a limited period of time, either before sunrise or after sunset. In the case of Mercury, the maximum solar elongation (called simply the greatest elongation) varies between 18º and 28º and in the case of Venus, it is between 45º and 47º. A planet's solar elongation is normally referred to as being either East or West of the Sun; East of the Sun for Evening appearances and West of the Sun for Morning appearances.

The appearance cycle of an inferior planet is as follows. The planet first passes between the Earth and the Sun, at a point called inferior conjunction. The planet cannot be seen from the Earth at this time, nor for about a week on either side of it. Seen from the Earth, the planet usually passes slightly to the North or South of the Sun, though on rare occasions an inferior planet can pass directly in front of the Sun in an event known as a transit (on these occasions, when using safe solar filters or telescopic projection, the planet can be seen as a small black dot moving across the Sun's disk).

Following inferior conjunction, the planet emerges in the dawn sky as a 'morning star', rising shortly before the Sun. It moves further away from the Sun as each day passes (i.e. its solar elongation slowly increases westwards) and the planet gradually brightens as it does so. Eventually the planet reaches its greatest western elongation. When Mercury reaches this point in its orbit - and depending upon the observer's latitude - it rises up to an hour (and sometimes up to 1½ hours) before sunrise; when Venus reaches this point, it rises up to three hours before sunrise. Mercury brightens all the way through to greatest elongation, but Venus shines at its greatest brilliance (maximum apparent magnitude) when it reaches a point about 40º West of the Sun, when its apparent size (its angular size when seen from the Earth) and its phase - a 28% illuminated crescent - combine to best effect. Venus then dims slightly as it approaches greatest elongation, but it remains brilliant nonetheless.

After greatest western elongation, the planet slowly begins to move back in towards the Sun (its solar elongation slowly decreases). Mercury continues to brighten but Venus continues to fade slightly, both planets becoming increasingly difficult to see in the morning twilight. The planet then disappears from view and passes behind the Sun, reaching a point called superior conjunction; Mercury cannot be seen from the Earth for a couple of weeks - and Venus for several weeks - on either side of superior conjunction.

Our inferior planet then re-emerges in the evening sky as an 'evening star', setting shortly after the Sun and moving further away from it as each day passes (solar elongation increasing eastwards). Mercury slowly dims during this period but Venus brightens, both planets eventually reaching their greatest eastern elongation. Depending upon the observer's latitude, Mercury then sets up to an hour (and sometimes up to 1½ hours) after sunset; Venus sets up to three hours after sunset. The planet then moves back in towards the Sun, Mercury fading and Venus brightening, the latter reaching its second greatest brilliance point at around 40º East of the Sun, after which it slightly fades. Our inferior planet then sinks into the evening twilight and becomes lost from view. Soon afterwards it returns to inferior conjunction and the cycle begins again.

An animation showing a typical evening apparition of Venus can be seen here.

Note that Venus is brightest when it is on the near side of its orbit to the Earth, at a point on either side of inferior conjunction when it shows a crescent phase. Mercury, on the other hand, is brightest on the far side of its orbit - shortly before and after superior conjunction - when it shows a gibbous phase. Paradoxically, when Mercury is brightest it is virtually impossible to see, because it is then so close to the Sun.

The length of time elapsed between one inferior conjunction and the next inferior conjunction (one complete appearance cycle) is about 116 days (16½ weeks) for Mercury and 584 days (just over 1½ years) for Venus. The time elapsed between any two of the same planetary aspects (or configurations) when seen from the Earth is called the synodic period; these periods are listed for all of the planets in the Orbital Data Table below.

The visibility of an inferior planet from any given location on Earth is heavily dependent upon its brightness, the observer's latitude and the season in which the planet is observed. As previously mentioned, the length of local twilight is one factor, but equally important is the angle of the ecliptic to the observer's horizon at the point where the planet is rising (morning sky) or setting (evening sky).

The ecliptic can be envisaged as a 'celestial highway'; a giant, invisible sinusoidal curve in the sky, only half of which is above the observer's horizon at any given time. Its most Northerly point is in Gemini and its most Southerly point is directly opposite, in Sagittarius; at these points, the ecliptic 'flattens out' as the sinusoidal curve reaches its upper and lower limits (i.e. at the Sun's midsummer and midwinter positions). The ecliptic is inclined to the celestial equator at an angle of 23º.5 (this is known as the obliquity of the ecliptic) and in the night sky this angle is most evident where it cuts across the celestial equator. This happens at two points along the ecliptic: near the head of Pisces (where the Sun heads Northwards at the Vernal equinox) and at the opposite side of the sky near the head of Virgo (where the Sun heads Southwards at the Autumnal equinox).

Consequently, the ecliptic presents a variety of angles to the local horizon at its rising or setting point, depending upon the observer's latitude, the time of day and the season of the year. If Mercury or Venus is positioned in a constellation in which the ecliptic presents a shallow angle to the horizon when it is rising or setting, the planet will be very low down in the twilight and it will only be visible for a short while (if it is visible at all). On the other hand, a steep ecliptic angle to the horizon means the planet will be seen higher up in the twilight, resulting in a longer period of visibility. Hence an inferior planet, even when it reaches greatest elongation, can appear low down in the twilight in certain seasons, but high up in other seasons, either before sunrise or after sunset. Mercury is particularly prone to visibility problems in this respect, since it is never very far from the Sun.

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The Superior Planets

The superior planets, unlike the inferior planets, can appear at any angular distance from the Sun (up to 180º away) so they are often visible for much of the night.

The yearly cycle of a superior planet is as follows. After passing behind the Sun (at superior conjunction, at which time it is not visible) the planet emerges in the dawn sky as a 'morning star', rising shortly before sunrise. It slowly moves away from the Sun, the planet rising earlier and earlier on each successive day (typically, several minutes earlier each day) its solar elongation slowly increasing westwards. Rising well after local midnight, it becomes visible into the early hours of the morning. When it reaches a point 90º West of the Sun called western quadrature (solar elongation = 90º West), the planet rises around local midnight. It continues to rise earlier each day and eventually reaches a point called opposition, when it is directly opposite the Sun in the sky (solar elongation = 180º). It is then visible all night long, rising opposite the Sun around sunset and setting opposite the Sun around sunrise. In the Northern Hemisphere, the planet is seen due South around local midnight; in the Southern Hemisphere it is seen due North around local midnight.

A superior planet's opposition is an important time for planetary observers using telescopes, for several reasons. Firstly, the Earth is then at its closest point to the planet for the whole year. Secondly, the planet appears largest in the night sky for the whole year (i.e. it attains its greatest apparent size). Thirdly, the planet's fully illuminated side is then facing towards the Earth (i.e. the planet's phase = 100%). The full illumination, combined with the planet's large apparent size, means that the planet then shines at its greatest apparent magnitude (brightness) for the year. To the observer using only the unaided eye, a planet's brightness is its most outstanding feature when it reaches opposition; indeed, seeing a bright planet rising directly opposite the Sun after sunset is one of the highlights of the naked eye planet watcher's year.

Because of the eccentricity (non-circularity) of the planets' orbits, some oppositions are more favourable than others, the planets then shining brighter than at other oppositions. This is particularly true for Mars, whose orbit is quite eccentric, the result being that its distance from the Earth varies considerably from one opposition to the next. A planet's closest point to the Sun in its orbit is known as its perihelion, and its furthest point from the Sun is called its aphelion. Whenever a planet's opposition occurs close to - or at - its perihelion or aphelion point, it is sometimes referred to as a perihelic or aphelic opposition. Mars' best oppositions (perihelic) occur at approximately 15-year intervals when the planet is in Aquarius, oppositions taking place in August or September (this last occurred in 2003 and will next occur in 2018 and 2035). Its worst oppositions (aphelic) take place in Leo (which will next occur in 2012). Jupiter's best oppositions take place when it is in either Pisces or Aries, when oppositions take place in September or October (this will next happen in 2010 and 2011). Saturn has two best opposition times - on opposite sides of its orbit - when its rings are displayed in their full splendor. These are in Taurus or Gemini (oppositions in December-January) and in Scorpius, Ophiuchus or Sagittarius (oppositions in June-July). Saturn's last brightest opposition took place in late 2002, and its next brightest will be in 2017 (for more details, see the Saturn orbit page).

After opposition, a superior planet becomes an 'evening star', rising before sunset. As it continues to rise earlier each day, its solar elongation - having reached a maximum of 180º at opposition - begins to reduce as it slowly closes in on the Sun. When it is 90º East of the Sun - at eastern quadrature - the planet sets around local midnight. As it continues closing in on the Sun, the planet remains visible after sunset, but now only for a few hours. It eventually sets only a short while after sunset, before finally disappearing into the twilight and passing behind the Sun, when it again returns to superior conjunction. The planet is not visible for a couple of weeks or so on either side of superior conjunction. The yearly cycle then begins again.

When the superior planets are emerging in the dawn sky (after superior conjunction) or disappearing into the dusk sky (before superior conjunction), they are subject to the same visibility problems as the inferior planets, due to the effects of twilight and the angle of the ecliptic to the local horizon. However, in the case of the superior planets, this is of lesser concern since they can be seen well at most other times of the year, and in any case they are more distant - and therefore not seen at their best - when they are in the vicinity of the Sun.

The synodic periods of all the superior planets except Mars are shorter than those of the inferior planets. Mars is a special case since it is the closest superior planet to the Earth and it moves more swiftly through the night sky than the other superior planets. Indeed, Mars has the longest synodic period of all the planets - 780 days (2 years and 7 weeks) - so it is only seen at its best (around opposition) at these intervals. In between oppositions, Mars spends much of its time dim and distant, on the far side of its orbit, loitering in the vicinity of the Sun. At these times the planet can often be difficult to distinguish from an ordinary, ruddy-colored star, shining little brighter than Polaris (the Pole star, apparent magnitude +2.0). The dramatic differences in Mars' brightness over time, coupled with its unique colouration, make it one of the more remarkable sights in the night sky.

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Wandering Stars

The planets are continually moving in their orbits, giving us an ever-changing variety of visual celestial events. The angular distance moved by a planet through its orbit in any given period of time depends upon the planet's distance from the Sun; the further away the planet is from the Sun, the slower the planet moves in its orbit and the less distance it is seen to move against the background stars. For example, nearby Mars moves a substantial 191º - over half of its orbit - in the course of one Earth year, Jupiter moves about 30º in a year, whilst distant Neptune moves just 2º - about four Full Moon widths - per year (the Moon itself moves Eastwards through the zodiac by about 12º per day). Consequently, some superior planets can spend several years in one constellation, whilst the faster-moving inferior planets can traverse the same constellation in only a matter of days. A complete list of the planets' mean angular motions (both daily and yearly) is shown in the Orbital Data Table below.

Because the planets move around the Sun in an anti-clockwise direction when seen from above the Earth's North Pole, all of the superior planets are seen to drift Eastwards through the zodiac at their successive oppositions. If we assume that one zodiac constellation spans 30º across (i.e. 360º / 12 constellations = 30º) we find that Jupiter appears in each successive zodiac constellation from one opposition to the next, and on average, Saturn spends two oppositions in each zodiac constellation. Mars is seen to drift one or two constellations Eastwards at successive oppositions.

The Waltzing Planets

During the course of the year, all the planets appear to describe intricate 'loop formations' against the background stars; this is a line-of-sight effect caused by the changing relative positions of the Earth and the planets as they move at differing speeds in their orbits. During a loop formation, the planets are seen to move direct or prograde (from West to East) and retrograde (from East to West, or 'backwards') with short periods in between when they appear stationary. A superior planet is seen to move retrograde because the Earth, being closer to the Sun and moving at a faster orbital speed, effectively 'overtakes' the superior planet, the result being that the planet appears to move backwards against the distant star background (although of course, both planets are moving in the same direction). The situation can be likened to two cars travelling in the same direction along a highway, the faster car (i.e. the Earth) overtaking the slower car (i.e. the planet). A passenger seated in the faster car sees the slower car move 'backwards' in relation to its background - although of course, both cars are moving in the same direction.

A superior planet typically remains at its first stationary point - at the Eastern end of the loop - for several days before accelerating into the retrograde section. Half-way through the retrograde section, the planet reaches opposition, at which point - since the Earth is then closest to it - the planet moves at its fastest apparent speed of the whole loop formation. Jupiter takes about 4 months to complete its retrograde path and Saturn takes about 4½ months, but Mars, being much closer to Earth, takes just 2½ months. The planet then decelerates as it approaches its second (Western) stationary point, where it again 'hovers' for several days before accelerating into direct (Eastward) motion. Half-way along this Eastward stretch (i.e. half-way between successive loops) the planet passes its superior conjunction point (when it is furthest away from the Earth and dimmest for the year). Half of a superior planet's loop formation takes place when it is a 'morning star' (after superior conjunction but before opposition) and the other half when it is an 'evening star' (after opposition but before superior conjunction).

All the superior planets except Mars perform one loop formation each year, Mars doing so only every couple of years. Planetary loop formations take on a variety of shapes; they can be 'closed-loops', 'zig-zag' shapes (i.e. a 'Z-shape' or 'S-curve' - effectively an 'open' loop) or a combination of the two. The exact nature of the loop depends upon whether the planet is above, below, or crossing the ecliptic at the time we observe it. Closed loops are the most common form and zigzags are the least common, the latter only taking place at two points in the planet's orbit: when it is crossing the ecliptic heading Northwards, and on the opposite side of its orbit when it is crossing the ecliptic heading Southwards (these orbital points are technically referred to as the ascending node and the descending node). Mars's zig-zag formations take place when the planet is in Aries (planet crossing the ecliptic heading Northwards) and Libra (crossing the ecliptic heading Southwards). Jupiter zigzags when it is in Gemini (heading Northwards) and Sagittarius (heading Southwards), and Saturn zig-zags when the planet is just a little further along - on the Gemini/Cancer border (heading North) and on the Sagittarius/Capricornus border (heading South). Zig-zag formations are easy to detect for Mars since it is so close, but they can be more difficult to discern for the more distant planets, because their loops are much smaller and flatter.

Mars and Saturn both performed zig-zag formations from late 2005 into 2006; Mars in Aries and Saturn in Cancer. Mars will next zig-zag in 2016 (on the Libra/Scorpius border, as it moves South of the ecliptic) whilst Saturn moves slowly enough along the ecliptic to perform a few zig-zag formations in succession. Its 2006-7 zig-zag was the last of three, the first having been in Gemini in 2003-4; the next one will take place in 2019, when the planet is in Sagittarius. Jupiter, having zig-zagged in Gemini from 2001-2, next zig-zags in 2008, also when it is in Sagittarius.

The angular size of the loop described by a planet (i.e. the angular distance from Eastern to Western stationary points) depends upon its distance from the Earth; the further away the planet, the smaller its loop. Mars' loops average about 15º across, Jupiter's loops are about 10º across, Saturn's loops are about 7º across whilst remote Pluto has a loop formation of less than 3º across. The time interval between successive loops is the same as the planet's synodic period; hence Jupiter and Saturn perform their loops at intervals of just over a year, in which time they cover an overall distance in the night sky approximately equal to its mean annual motion (see the Orbital Data Table below). Mars, on the other hand, makes a complete circuit of the zodiac between loops, taking over two years (780 days) to do so.

Note that the angular width of a planet's loop in the night sky is not the same angular width as its mean annual motion. The angular width of a loop is determined by two factors: firstly, the apparent shift in the planet's position against the background stars when seen from two opposite points in the Earth's orbit (an effect known as parallax) and secondly, the movement of the planet itself in its orbit. The mean annual motion of a planet, on the other hand, is determined solely by its orbital speed around the Sun, irrespective of the Earth's own motion. Uranus is an interesting case in point; the angular width of its loop (3º.9) is only a little less than its mean annual motion (4º.2), so there is only a narrow gap in between successive loops (see Path of Uranus, 2006-19). If a planet is distant enough, the angular width of its loop will exceed that of its mean annual motion, in which case the planet's looping formation will slightly overlap that of the previous year - as is the case for Neptune (see Path of Neptune, 2006-23) and Pluto (see Path of Pluto, 2006-22).

To witness a superior planet performing a loop formation, one needs to observe it at regular intervals (e.g. weekly) over the course of several months, and on each occasion note down its position in relation to the surrounding 'fixed' stars. This is a rewarding way in which to learn about the movements, brightness and colours of the planets in the sky, while perhaps learning to recognise some of the constellations in the process. Although modern planetarium software will quickly and easily simulate the planets' movements in the night sky, they can somewhat take away from the primal experience of witnessing the event for oneself over a long period of time - from a planet's first appearance at dawn, through its brightest period in the night sky and its slow dance among the stars, to its eventual disappearance into the dusk.

• For descriptions and images of the planets as they appear to the naked eye or with binoculars, see the accompanying artic