A bumper moon shines over the harvesters: Spooning lovers and farmers watch out] The evening sky can fool your eyes - at any time of the year. Heather Couper and Nigel Henbest explain

LOVERS have always spooned under the moon, but it has a more practical significance for farmers. When it is full, the moon provides enough light for farm hands to work after sunset - well into the night if need be. And never is that help needed more than in September, the traditional month for the harvest. 'By the moon the reaper weary, piling sheaves in uplands airy', overhears the song of Tennyson's Lady of Shalott.

According to folklore, the harvest moon is nearer, bigger and brighter than any other full moon. It is none of those things. But there is a good reason why this month's full moon will help farmers - and anyone else who has reason to be out in the pale moonlight.

The harvest moon is simply the full moon that happens to fall closest to the autumnal equinox, the date when day and night are of equal length. The equinox is on either 22 or 23 September, so the harvest moon comes in September or the first week of October. This year's equinox is on 22 September and the harvest moon is on 12 September.

The full moon will certainly look large as it rises that night. But this is not unusual. It is a result of the 'moon illusion', which always makes the moon look bigger when it is near the horizon, and smaller when it is high in the sky.

To test whether the moon's changing size is fact or illusion, find an object - a pencil, perhaps - that will just cover the rising moon when you hold it at arm's length. Wait a few hours, until the moon is higher in the sky, and hold the pencil in front of the moon again. You will find, of course, that the moon is exactly the same size as it was when it rose.

The moon illusion occurs because we compare the rising moon with a distant skyline. Our minds put the moon beyond the trees and buildings on the horizon, and so interpret it as being very big. But once it climbs higher in the sky, we have nothing to compare it with, so the moon looks nearer to its real dimensions. In fact, it is surprisingly small: it would take 350 full moons side by side to stretch across the sky.

The moon's size in the sky actually does change slightly during the month, as it goes around its oval-shaped orbit. But this is not linked directly to its phases. The harvest moon this year happens when the moon is almost at its furthest from the earth, and so is comparatively small.

What marks out the harvest moon is that it pops up above the horizon at around sunset for several successive nights. Every full moon rises at sunset - it has to, because a full moon always lies exactly opposite to the sun in the sky, so one rises as the other sets. But this month it will look as if we have a full moon for almost a week.

As the moon plods along its orbit around the earth, it rises later on each successive night, by almost an hour. But this is very much an average. The moon's tardiness would be a constant 52 minutes only if it followed the celestial equator - an imaginary line running across the sky above the earth's equator. In fact, the moon's orbit is tipped, so it travels north and south in the course of a month. This makes a big difference to the times at which it rises and sets.

Because we live in the northern hemisphere, we get a better view of anything that lies in the northern part of the sky. The further a star lies north of the celestial equator, the longer it appears in the sky between rising and setting: a northern star rises earlier and sets later than its counterpart to the south. This is taken to extremes with constellations such as the Great Bear (or Plough) which lie near to the north pole of the sky, and never set.

This effect of geometry is the key to the harvest moon. At the time of the autumnal equinox, the full moon is moving rapidly northwards in its tipped-up orbit. On the evening of the full moon, it rises just as the sun sets. The next evening, the moon is several degrees further north in the sky: and just like a more northerly star, rises earlier than we expect. Instead of rising 52 minutes after sunset, it pops up only 15 minutes after the sun has gone down. Unless you are a careful clock- watcher, it looks like a replay of the previous evening.

Something similar - but less pronounced - happens with the next full moon, on 11 October this year. It is traditionally known as the hunter's moon.

But what the autumn moon gains on the swings, the spring moon loses on the roundabouts. At the time of the spring equinox, the full moon moves southwards in the sky, and moonrise comes an hour and a half later each night.

The advantages of our perspective on the sky are lost as you move closer to the equator. Farmers in the tropics have no harvest moon. But nature is even-handed. In the southern hemisphere, the southward-moving moon produces a harvest moon in March - just in time for the autumn reaping there.

The planets

This is a two-planet month. From Britain you will not see Mercury, Venus or Jupiter. They are all so close to the sun in the sky that they are lost in the twilight glow.

Look southwards during the evening to spot Saturn, glowing with a steady yellow light and matching the brilliance of the brightest stars. It lies in the rather dim constellation of Capricornus, tho sea-goat, and outshines all the stars nearby. Use a small telescope to catch a glimpse of its rings.

This will be our best view of the rings for several years. Our perspective on Saturn is gradually changing, so that we are seeing the thin rings more obliquely. They will look less spectacular with each passing year until 1995-96, when we will view them exactly edge-on. After that we will begin to see the other side of the rings.

Mars is rising before midnight in the north-east. It is the twin of Saturn in brightness, but has a distinctly red hue. At the moment, Mars is moving from Taurus into Gemini. This part of the sky contains several stars as bright as Mars, but you can easily pick out the planet because it does not twinkle.

The stars

Several constellations associated with myths about flight are moving across the southern sky. As a signpost, first pick out the large Summer Triangle, with its corners marked by the first-magnitude stars Deneb, Vega and Altair. This triangle is not a constellation as such; it comprises the brightest stars of three traditional star-patterns.

Deneb ('tail' in Arabic) marks the rear of Cygnus, the swan. The cross-shape of the rest of the constellation sketches out the creature's wings and elongated neck. In Greek mythology, this swan was the form that Zeus took when he seduced the beautiful Leda. The union resulted in the birth of the hero Pollux, also immortalised in the stars as one of the twins making up the winter constellation Gemini.

Below the swan flies the eagle, Aquila. Its brightest star, Altair,

is easy to spot, as it is flanked

by a faint star to either side.

This was the bird, according to

the Greeks, that carried the handsome lad Ganymede to

the amorous Zeus on another occasion.

Lyra is, strictly speaking, the lyre played by Orpheus during the voyage of the great ship Argo to find the Golden Fleece. But other cultures have seen this small star-grouping as an eagle, a vulture, a falcon, or even a goose. The name of its brightest star, Vega, comes from the Arabic for 'swooping eagle'.

To the left of the Summer Triangle is a more unusual winged creature: Pegasus, the flying horse. He sprang from the waves when drops of blood from the severed head of Medusa, the hideous Gorgon, fell into the sea. Pegasus became the steed of the hero Bellerophon when he slew another monster, the Chimera. The main stars of Pegasus form a large square, now in the south-east in the evening sky.

Diary (all times BST)

3 11.39pm moon at first quarter

12 3.17am full moon (harvest moon)

15 Mercury at superior conjunction

17 Jupiter at conjunction

19 6.53pm moon at last quarter

22 7.43pm autumnal equinox

26 11.40am new moon