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The Naked Eye Planets
in the Night Sky
(and how to identify them)
by Martin J. Powell
"This morning, the ultimate dream of a planet watcher was come true: all of the planets condensed before me into one straightforward, nowhere peripheral view. It struck me like nine-branched lightning, held me still as a (deeply breathing) statue. All the planets at once."
- Fred Schaaf, The Starry Room
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(Neptune) |
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(Pluto) |
The five brightest planets - Mercury, Venus, Mars, Jupiter and Saturn - have been known since ancient times and can easily be seen with the naked eye if one knows when and where to look. They are visible for much of the year, except for short periods of time when they are too close to the Sun to observe. All of the planets will not normally be visible on a single night, however.
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Jupiter, Libra & Scorpius At First Light Jupiter was in the constellation of Libra when this photo was taken at dawn in late January 2006. Jupiter - the bright white 'star' visible towards the upper right - then shone at magnitude -2.0, outshining all of the stars in its vicinity. Near the horizon, the clouds glow orange, reflecting the streetlights of a distant town (Move your cursor over the image to identify the constellations, and click for a full-screen picture). Jupiter has since moved further South-eastwards along the ecliptic (the apparent path along which the Sun, Moon and planets move) and for much of 2007 it was positioned near the orange-red star Antares - a name derived from the ancient Greek meaning 'rival of Mars' due to its Mars-like coloration. |
The planets can be distinguished from the stars because their position changes slightly against the background stars from one night to the next, and their brightness varies in a regular cycle over a period of time. Stars are so distant that they appear as points of light in the night sky, and they appear to twinkle (or scintillate) because of turbulence in the Earth's atmosphere. Even in the largest Earthbound telescopes, a star appears as little more than a point of light. The naked eye planets, on the other hand, are close enough to the Earth to form a sizeable (but nonetheless tiny) disk in the night sky and consequently, when they are well above the horizon, the planets shine with a more steady light than the stars. Of the brighter planets, only Saturn and Mars are ever likely to be mistaken for ordinary stars - but only during their dimmer, more distant periods.
The stars generate their own light by internal nuclear fusion reactions, however the planets do not produce any light of their own; instead, they shine by reflecting a proportion of the sunlight they receive back out into space. The proportion of sunlight they reflect (known as the albedo) depends upon the planet's size, the amount of cloud cover it has and - where there is little or no cloud cover - the reflectivity of the features on its surface. The brightness of a planet to the naked eye depends upon a number of factors; its distance from the Sun, its apparent size (i.e. its angular size when seen from the Earth) and the relative positions of the planet and the Earth in their orbits.
The period of time during which any given planet (or any other celestial body) can be seen is known as an apparition. For Mercury and Venus, this is the period during which the planet is well seen in either the morning sky or the evening sky. For Mars and beyond, an apparition is measured from the time the planet becomes visible in the dawn sky (rising shortly before the Sun) to the time it disappears into the dusk sky (setting shortly after the Sun). The length of a planet's apparition depends upon its orbital period (i.e. the time it takes the planet to orbit the Sun) and its position in relation to the Sun and the Earth at any given time. Apparitions can last from just a few weeks (in the case of Mercury) to almost two years (for Mars) although a planet is normally seen at its best for only a part of this time.
Unlike galaxies and nebulae which are invariably faint, the naked-eye planets can easily be seen in light-polluted skies (i.e. where the night sky glows yellow-orange due to a multitude of poorly-directed lighting from streetlights and other illuminations).
The planets each have their own movement, brightness and colour characteristics. The ancient Greeks, around the sixth century BC, referred to them as 'planetes asteres' ('wandering stars') from which the word 'planet' is derived. They named each planet mainly according to its brightness and colour, the name given to them being mostly associated with heat and light. Hence Venus' brilliant white colouration earned it the name Phosphoros ("the light-bearing one") and Mars' orange-red colour was associated with fire, so it was given the name Pyroeis ("the fiery one") and so on. A complete list of the ancient Greek names of the five brightest planets is given in the Planet Appearance section below.
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Saturn in Leo, 2007 Golden-yellow Saturn is currently located in the constellation of Leo. For much of 2007, the planet appeared as an additional bright 'star' just ahead of the Lion's breast. In 2008 the ringed planet is located a short distance to the South-east of Leo's brightest star, Regulus. Saturn reached opposition on February 24th (when it was closest to the Earth and brightest for the year). The planet will remain in Leo, slowly moving South-eastwards along the ecliptic, through to mid-2009 (Move your cursor over the image to identify the stars, and click for a full screen picture). |
In the fourth century BC, the Greek philosopher Aristotle converted these descriptive names into the names of Greek gods (and one goddess), each planet's attributes roughly matching those of its chosen god. The Romans later acquired these Greek names and translated them into their own equivalent gods, and these are the names that we still use today; hence the Greek goddess Aphrodite became the Roman goddess Venus and the Greek god Ares became the Roman god Mars, etc.
When another three planets were discovered in more recent times - namely Uranus, Neptune and Pluto, they were also given the names of Graeco-Roman gods, continuing the earlier tradition. Uranus is the only one of the major planets which retains the name of a Greek god - this is because the Romans had no god which was equivalent to Uranus.
Pluto Loses its 'Planet' Status
At a meeting of the International Astronomical Union (IAU) in Prague, Czechoslovakia in August 2006, astronomers decided that Pluto will no longer be officially classed as a 'planet' in the true sense; instead, it is now part of a new class of Solar System bodies known as dwarf planets, together with Ceres (the largest of the asteroids) and the newly-discovered object called Eris (previously known as 2003 UB313 or 'Xena' - for more details, see below under the entry for Pluto).
Consequently, there are now officially only eight true planets in our Solar System.
The Days of the Week
We are subtly reminded of the naked eye planets, together with the Sun and the Moon, in our names of the days of the week. Saturday, Sunday and Monday are evidently named after Saturn, the Sun and the Moon respectively; they are Old English adaptations of the Roman names Saturnus, Sol and Luna. The remaining weekdays were named after the Anglo-Saxon and Norse equivalents of the Roman gods Mars, Mercurius, Jupiter and Venus - they are namely 'Tiw's Day' (after the Old English version of Týr, the Norse God of War), 'Woden's Day' (after the Anglo-Saxon version of Odin, the Norse God of War and Wisdom), 'Thor's Day' (after the Norse God of Thunder) and 'Friga's Day' (after Freyja, the Norse Goddess of Love and Beauty).
In the French language, the Moon and planets are also clearly evident in the days of the week: Lundi (Moon's day), Mardi (Mars' day), Mercredi (Mercury's day), Jeudi (Jupiter's day), Vendredi (Venus' day) and Samedi (Saturn's day). The month of March is also named after Mars.
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Mars in Gemini, 2007
Approximately every two years, Mars makes a close approach to the Earth and it then appears particularly bright in our night skies. This last happened in late December 2007; this photograph was taken about a week before it reached peak brightness, when it shone at magnitude -1.5. The pale-orange planet was situated a short distance to the West of the star Mebsuta (Epsilon Geminorum) as the planet retrograded (moved 'backwards', i.e. from East to West) heading towards opposition on 24th December. The Red Planet's next close approach to the Earth will take place in early 2010. (Move your cursor over the image to identify the stars, and click for a full screen picture). |
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2008 December 1st Sofia, Bulgaria (42º 41'.0 N, 23º 20'.8 E) Venus magnitude -4.2; Elongation 42º E of Sun |
Vanishing Venus
Whenever the Moon passes directly in front of a planet during its monthly course around the zodiac, the event is known as a lunar occultation. A lunar occultation can only be seen from a specific region of the world, depending upon the precise geometry of the Moon, the planet and the Earth at the time. This particular occultation will be visible from Europe and North-west Africa after (or around) local sunset (for details, see the IOTA website - note that twilight or daylight will interfere with observation the further West the observer is situated within the visibility zone). The graphic is an animated simulation of how an observer in South-eastern Europe will see the event. It is locked on Venus and runs in real time (the image updates at 1-minute intervals). Times are shown in UT (Universal Time, equivalent to GMT) and EET (Eastern European Time). At Sofia, Venus disappears behind the waxing crescent Moon at 16:53 UT (6:53 pm local time) and reappears at 17:11 UT (7:11 pm local); a total duration of 18 minutes. Adding to the spectacle will be the planet Jupiter (at magnitude -1.8) which will be positioned some 2º to the North of the pair. Also shown are the direction of the event (South-west throughout), the altitude of Venus (i. e. the angle above the horizon; it is low down throughout) and - during the occultation itself - the time elapsed since the start of the event (in hours:mins). |
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The Current Positions of the
Planets in the Night Sky
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Where are the Planets Now? |
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Today's Planets At a Glance The Sun and the five brightest naked-eye planets are shown here in their current positions (a star map identifying the constellations is shown below). The overlay grid marks intervals of 10º in celestial longitude and latitude. The graphic was produced (and is kindly made available for public use) by David Colarusso (note that the graphic requires the Shockwave Flash Player plug-in to display correctly). Detailed star charts showing the paths of the superior planets (i.e. from Mars to Pluto) are given under each planet entry in the main section below. |
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| Star map of the constellation patterns shown in the above 'planets at a glance' graphic (move your cursor over the image - or click on the image - to reveal the constellation names in their abbreviated three-letter form - the full names are listed here). Zodiac constellations are labelled in green and non-zodiac constellations in grey. A Southern hemisphere view of the labelled constellations can be seen here. Also marked on the chart are the ecliptic (the apparent path of the Sun through the constellations, which the planets and the Moon follow very closely) and the celestial equator. To see whereabouts on the horizon the Sun and the planets will rise and set today for your own latitude, note down the constellation in which they are currently positioned and then refer to the Zodiac Constellation Rise/Set Direction Table. The significant solar positions marked on the chart apply to the Northern hemisphere and are as follows: VE = Vernal (Spring) Equinox, SS = Summer Solstice, AE = Autumnal Equinox and WS = Winter Solstice. The numbers along the sides of the chart (Right Ascension and Declination) are co-ordinates of celestial longitude and latitude which astronomers use to locate the position of a celestial body in the night sky. Printer-friendly versions of the above chart are available for the constellation patterns only, the labelled constellations and the Southern hemisphere view. |
Visibility of the Planets in 2008
When are the Planets currently on view?
The following table provides an indication of the best times of the day in which to view the planets in 2008 (Pluto is not included because it is solely a telescopic object). The table entries can be read horizontally by planet (to track the changing visibility of a particular planet during the year) or vertically by month (to determine the visibility status of all the planets in any single month). Visibility tables can also be previewed for the years 2009 and 2010.
Note that when two or more planets are listed as being visible at 'Dawn' or 'Dusk' in any particular month (e.g. if Mercury, Venus and Mars all appear listed under 'Dusk' in a single month) they are likely to appear close together in the sky during this time, and planetary conjunctions (when the planets are at the same celestial longitude) are most likely to take place. For a list of specific dates of planetary conjunctions, see Wikipedia.
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General visibility times of the planets in 2008 Visibility times of the planets are shown here in general terms for the current year; for a description of each entry, see the box below. With the exception of Mercury, the times are based on the planets' positions at mid-month. Note that, although a planet may be listed as being visible at a particular time in a month, this does not necessarily mean that it will be visible from all locations on Earth. The observer's latitude and the local season can affect whether a planet is seen or not; this is particularly so for the 'Dawn' and 'Dusk' entries and especially so in the case of Mercury (see under Mercury's description for more details). Since the planets' viewing times can change in the course of a month (eg. from Dawn to Morning sky visibility) the entries inevitably contain an element of imprecision; consequently the table should only be used as a general guide to assess the best times at which to view the planets. |
The Moon's Current Phase and Position This Java Applet shows the current phase of the Moon, the percentage of the lunar disk illuminated, the Age of the Moon (ie. the number of days elapsed since the moment of the last New Moon), its current distance from the Earth (1km = 0.6214 statute miles) and the dates of the next New Moon, First Quarter, Full Moon and Last Quarter phases. Also shown are the dates when the Moon next reaches perigee (its closest point to the Earth) and apogee (its furthest point from the Earth). The diagram shows the Moon orientated with North up and East towards the left (i.e. the Northern hemisphere view); Southern hemisphere observers will need to invert the image to obtain a correct view. To find the Moon's current position in the zodiac (to within about ±8º) refer to the 'Age of the Moon' shown in the box (it will lie somewhere between 0 days 0 hours and 29 days 20 hours). Take the hour number and divide it by 24; add this to the day number (eg. '8d 20h' will be 8d + (20h/24) = 8.833d). Multiply this by 12.2; the result will be the approximate angle through which the Moon has moved Eastwards since the last New Moon, measured relative to the Sun (in this example, 8.833 x 12.2 = 107.762 degrees). Now look up the current ecliptic longitude of the Sun in the Solar Ecliptic Longitude Table (the ecliptic longitude is the longitude of a celestial body measured Eastwards along the plane of the ecliptic from the 'First Point of Aries', the 'zero point' from which celestial longitudes are measured). Add the Sun's ecliptic longitude to the angle calculated for the Moon's current position. The result is the current ecliptic longitude of the Moon, and it should have a value lying between 0 and 360 degrees; if it is greater than 360, subtract 360 to bring it into the correct range. Now refer to the ecliptic longitude star chart, in which ecliptic longitudes are marked at 10-degree intervals along the yellow curve (the ecliptic) [a Southern hemisphere version appears here). The Moon will be located in the immediate vicinity of the calculated longitude (note that the Moon may not be positioned exactly on the line of the ecliptic because the Moon moves above and below it by up to 5 degrees during the course of its orbit). Printer-friendly versions are available for the Solar Ecliptic Longitude Table and ecliptic longitude charts for the Northern and Southern hemisphere. Example It is 6:25 pm on January 10th and the box shows the Moon's Age to be 14d 06h 03m. We first convert the Moon's Age into decimal form by dividing the hour by 24 and adding it to the day number, ie. 14 + (6/24) = 14.25d. Now we multiply the Moon's Age by 12.2, so we have (14.25 x 12.2) = 173.85 degrees (for these purposes we need only take the result to the first decimal place, ie. 173º.8).
Referring to the Solar Ecliptic Longitude Table, we see that the Sun's ecliptic longitude on January 10th is 289.5 degrees. We add this to the angle which we have determined for the Moon, so we get (289º.5 + 173º.8) = 463.3 degrees. Now the Moon's longitude must lie within the range 0 to 360, so in this case we must subtract 360, hence the Moon's ecliptic longitude at 6:25 pm on January 10th is approximately (463º.3 - 360º) = 103.3 degrees. Looking at the ecliptic longitude star chart, we see that an ecliptic longitude of 103º.3 places the Moon in the constellation of Gemini, several degrees South-west of its two brightest stars, Castor and Pollux. The Moon should be seen in the night sky within 8º to the East or West of this point, and within 5º to the North or South of the ecliptic line. Now that we know the current location of the Moon among the stars, we can see whereabouts on our local horizon it will rise and set using the Zodiac Constellation Rise/Set Direction Table. |
The Appearance of the
Planets in the Night Sky
The following images of the planets were taken by the author using a video camera or a digital camera, and they give some indication of their naked eye appearance (or in some cases, their appearance with optical aid). The planets are listed in approximate descending order of their average apparent magnitude (brightness) when seen from the Earth. The brightness values listed for Mercury, Venus, Mars and Jupiter are the brightest and dimmest magnitudes the planet can attain throughout its orbit (the brightest magnitude is listed first); those for Saturn, Uranus, Neptune and Pluto are the average brightness values for when the planet is closest to the Earth.
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Following each description is an abbreviated table of planet positions (an ephemeris) for the period from 2005 through to 2010 (inclusive); an explanation of the abbreviations used in the table can be seen here. For the superior planets (i.e. from Mars to Pluto) star maps are included showing the positions of the planets to help in locating them in the night sky (maps are provided for both Northern and Southern hemisphere views). Click on a starmap thumbnail to see the full-size image and an accompanying description (these will open in a new window).
A brief discovery and/or observational history concludes each planet's entry.
Position your cursor over each image to see a photograph of the planet taken by NASA's Hubble Space Telescope (HST) or by other planetary spacecraft missions, with some additional 'pop-up text' information (if Javascript is disabled in your browser, click on the image to load the picture in a new window).
Click on a planet to be taken directly to its entry:-
Links to Selected Planetary Spacecraft Missions
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Starry Night Enthusiast 6.0 (Planetarium Software) Imaginova |
RedShift 6 Premium Edition (Planetarium Software) Maris Technologies |
The Night Sky Companion: A Yearly Guide to Sky-Watching 2008-2009 Tammy Plotner |
The 50 Best Sights in Astronomy & How To See Them Fred Schaaf |
Search for Star Atlases at |
What's Out Tonight? 50-year Astronomy Field Guide 2000-2050 Ken Graun |
Music Of The Spheres Mike Oldfield (Music CD) "The Planets Suite of the 21st Century" |
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