The Stars in their Courses

How to follow the Planets from your Balcony or Garden

1. The Daily Cycle

Does your balcony or garden face North? You'll know because it won't be sunny even in the middle of the day. And if you can't see the sun from your balcony, then sadly you won't be able to see the planets either. That's because the sun, moon and planets all trace a similar path through the sky: rising in the East, culminating (which means, reaching their greatest height) in the South, setting in the West.

So let's suppose you have a view to the South. To see any planets, you have to wait until night-time. When the sun is up, its bright light outshines the relatively feeble, reflected light of the planets.

Sometimes, but not always, one or more of the planets is visible immediately after sunset. If so, the planets usually come out before the stars, because they are somewhat brighter. Much brighter, in the case of Venus, often known as the Evening Star (also the Morning Star - more on that below).

One month a planet may be high up in the sky in the middle of the night, at another time it may appear only briefly before sunrise, in the East. In sections 2,3 and 4 of this article we will explore the cycles of changes governing the time of day or night at which each planet appears.

At all times each planet goes through a daily cycle of rising in the East, culminating in the South, setting in the West, going round below the Northern horizon and rising in the East again, once every 24 hours.

2. The Synodic Cycle - Outer Planets

What time of day the planet can actually be seen depends on the position of the planet relative to the sun. In the course of time, this position changes, going through a second cycle, known as the synodic cycle. Each planet has its own synodic period (length of time taken to repeat a synodic cycle). This is just over a year for Saturn and Jupiter, just over two years for Mars, 1.6 years for Venus, 116 days for Mercury.

We have explained what happens in the daily cycle, what about the synodic cycle? This is different for the outer planets (Mars, Jupiter, Saturn) and the inner planets (Venus, Mercury). Let's take the outer planets first.

There is a moment in the synodic cycle when the planet cannot be seen at any time of day. This is when the planet is conjunct with the sun, i.e. it appears so near to the sun that its light is drowned out by the sun. When the moon is conjunct with the sun we speak of new moon. Around the time of new moon the moon disappears for a few days. It is the same with the planets, only they disappear for longer.

After it has been renewed, the moon reappears as a thin crescent in the evening sky, setting quite soon after the sun. It is the opposite with the outer planets. They reappear in the morning sky, just before sunrise, in the East. Then, as the cycle progresses, the planet will rise earlier in the morning, eventually rising at midnight, then even earlier, at sunset. At this point the planet is in opposition to the sun, i.e. it is directly opposite to the sun in the sky - analogous to a full moon. It rises when the sun sets, moves across the sky the whole night through, and sets when the sun rises.

After this halfway point in the synodic cycle, the planet is above the horizon at sunset. At first just above the Eastern horizon, then high in the South, then above the Western horizon. By this time it is behaving as an 'evening star' - visible at sunset, but setting soon after the sun. Then the planet as it were sinks into the sun at sunset and is lost to sight. We are back where we started: conjunction with the sun. The synodic cycle is complete.

It so happens that all five bright planets were more or less in conjunction with the sun in May 2000. Jupiter and Saturn were in opposition to the sun round about November 2000 (6.5 months later). At present [April 2001] they are both approaching the end of one synodic cycle and appear as evening stars. Mars meanwhile is less than halfway through its cycle (it still rises later than sunset). It will reach its halfway point (opposition) in June 2001.

3. The Synodic Cycle - Inner Planets

Now for the inner planets, Venus and Mercury. Mercury is rather difficult to see from Northern climes (it helps to have a cloudless sky, too!), so we will concentrate our attention on Venus. Venus never strays further than about 45 degrees from the sun. This implies that Venus is never visible in the middle of the night - only in the evening, soon after sunset, or in the morning, shortly before sunrise. Venus spends about 9.5 months as an evening star, then 9.5 months as a morning star; then the cycle begins again.

Venus is conjunct with the sun twice in each cycle: when it changes from morning to evening star and when it changes back again. Let's start with the first of these conjunctions, known as 'superior conjunction'. This happens rather slowly: Venus disappears for a long time. At last Venus emerges slowly into the evening sky. For several months it is still quite close to the sun and sets rather soon after the sun, but eventually it reaches its full height (i.e. its maximum distance from the sun, known as 'Western elongation') and is gloriously prominent as the Evening Star. Then it starts sinking again. This time everything happens rather fast. In what seems no time at all, Venus sinks down again into the setting sun ('inferior conjunction'), only to reappear rising in advance of the rising sun. Very soon it reaches its maximum elongation as Morning Star, thereafter declining slowly back into the rising sun. We're back at superior conjunction.

Going back to May 2000, at that time Venus was just approaching the beginning of the cycle as described above. In fact, superior conjunction took place in June of that year. Since then Venus came slowly into view as Evening Star, attaining great prominence in December, January and February. During March it went into its precipitate decline, going through inferior conjunction at the end of that month. During April 2001 it will be rising once again to prominence, as Morning Star. The next superior conjunction will be in January of 2002.

4. The Sidereal Cycle

Interweaving with the daily and synodic cycles, each planet has a third cycle, the sidereal cycle. The word 'sidereal' (from the Latin for star, sidus) should be taken to mean 'relative to the stars'.

There is a path through the background of stars along which all the various planets, and the moon, are seen to move. In fact they don't keep exactly to the path, but wander off to either side. They are allowed a certain 'latitude'. So the path is not an exact line, but there is a belt of constellations, known as the Zodiac, in which the planets are always to be found.

If you go out at night and observe the stars carefully, you will notice that they are forever on the move. It appears that the stars are attached to a great sphere, with you (the observer) at the centre, and that the sphere is slowly rotating on an axis, passing though the centre of the sphere, and also through the Pole Star - the only star that doesn't move (or to be more accurate, hardly moves).

You should think of the Zodiac as a belt going right round this sphere (at its widest point). The belt joins onto its own tail: it has no obvious beginning or end. It cycles round for ever and ever.

To make matters more complicated, the plane of the belt is not perpendicular to the axis of rotation. It is not like the equator of a spinning globe, but like a great circle set at an angle to the equator.

There is in fact a well-defined line going down the middle of the Zodiac, but this is one step removed from direct observation. This is the line followed by the sun as it moves relative to the background of stars. This can't be observed directly, because when the sun is out, the stars cannot be seen. However, if you observe the stars that come out after sunset, near to where the sun has just gone down, you notice that these are stars in or near the Zodiacal belt, and that they are forever changing as the year progresses, going though a whole cycle of the Zodiac in one year.

From this one deduces that the sun too moves along the Zodiacal belt, but the idea that there is an exact line along which the sun moves comes from another direction: the phenomenon of lunar eclipse. The moon is eclipsed when it passes into the shadow cast by the sphere of the earth. This is only observable at night time, so it happens against a background of stars. From it one gets the idea of the earth's shadow as a (usually) invisible celestial body with a well-defined position relative to the stars. It only becomes visible when the moon passes through it - rather as a beam of sunlight in a mostly dark building such as a barn only becomes visible when dust is stirred up and floats into the sun-beam.

To sum up, one cannot observe directly the path of the sun relative to the stars, but one can, over time, using lunar eclipses, build up data on the path taken by the centre of the Earth's shadow against the backgound of the stars. Hence the name given to this path: the ecliptic.

Further careful observations reveal that this line is indeed straight (it deviates neither to the right nor the left), and that the sun moves along it at almost constant speed - roughly one degree per day. (Three points of the sky are in a straight line if you can hold up a ruler to them and they all lie on the edge of the ruler. But thinking of them as points marked on the celestial sphere of the stars, they all lie on a so-called great circle - a circle of maximum diameter, alternatively a circle defined as the intersection with the sphere of a plane passing though the centre of the sphere.)

Now for the planets. The steady motion of the sun along the ecliptic, combined with the synodic motion of the planets relative to the sun, results in a somewhat complicated motion of the planets relative to the backgound stars. At times they race forward through the Zodiac (when they are in conjunction with the sun - superior conjunction in the case of the inner planets). At other times they slow down, stop and even go backwards (at or near opposition - inferior conjunction in the case of the inner planets). This is the well-known 'retrograde motion' of the planets (not always good from an astrological point of view!).

But in the long run they move forwards through the Zodiac, cycling round it to eternity. During one of its synodic periods

(Note: a 'sign' of the Zodiac is simply a one-twelfth division of the circle of the ecliptic. Since the whole circle contains 360 degrees, each sign contains 30 degrees. Each sign has a name - Aries, Taurus, Gemini, etc. - the names originally taken from the names of the constellations of the Zodiac.)

Apart from its observable motion against the background of stars, a planet's sidereal cycle has another important consequence for observation of the planet.

Recall that each day a planet follows a path through the sky, like the daily path of the sun. But this is not quite precise, because the sun follows a very different path in the summer as compared to the winter. Now we can be more exact in the matter. If Mars, say, is in the sign of Capricorn, then it follows the same path as the sun when it is in Capricorn. Now the sun is in Capricorn in the middle of winter, so if Mars is in Capricorn its path will be low across the sky, and it won't be up in the sky for long. This is quite independent of where Mars is in its synodic cycle.

Remember, the synodic cycle governs the time of day or night at which the planet appears. The sidereal cycle governs the path taken by the planet across the sky, be it high or low.

Sign Dates between which the sun is in the sign (on average)
Aries 20 March - 19 April
Taurus 20 April - 20 May
Gemini 21 May - 20 June
Cancer 21 June - 21 July
Leo 22 July - 22 August
Virgo 23 August - 22 September
Libra 23 September - 22 October
Scorpio 23 October - 21 November
Sagittarius 22 November - 20 December
Capricorn 21 December - 19 January
Aquarius 20 January - 17 February
Pisces 18 February - 19 March

You will notice that the sun lingers in the summer signs (Gemini, Cancer and Leo) for slightly longer than in the winter signs (Sagittarius, Capricorn and Aquarius) - for about 31.5 days and 29.5 days respectively. This is because the sun is actually closer to the earth in winter (so its angular movement is greater) than it is in summer - contrary to what many people naively suppose (we're talking Northern Hemisphere here!).

Just to reiterate, if Jupiter is in Taurus, then its path across the sky is the same as the path followed by the sun in May (which will give you an idea of how high it goes, how long it spends above the horizon, and so on). The system of signs has been abandoned by modern astronomy. These days, to specify where a planet has got to in its sidereal cycle, you just give a plain number (the ecliptic longitude, which is the number of degrees measured round the ecliptic starting from the beginning of the sign of Aries). This is quite a recent development, however. In the time of Newton and Halley the signs were still used. Each part of the ecliptic still had an image, and with it an imagined quality. What a perfect illustration of the spirit of modern science, some would say: the images and qualities just had to be eliminated, in favour of pure numerical abstraction!

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