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Lunar Phases

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The Moon
Lunar Phases | Lunar Eclipses | Full Moon Names | The Moon and the Tides
Moonbows | The Giordano Bruno Crater
Last-quarter moon.
From invisible at new to Cheshire cat grin impersonation through to half-moon, then full, and back again to waning crescent, it waxes and wanes over roughly 29.5 days with a reassuring constancy.
– An h2g2 Researcher

Beauty is in the eye of the beholder, but few would disagree that catching a glimpse of a slim sliver of the crescent moon, or the glory of a full moon turning night into day is indeed a beautiful sight. The Moon is always in the sky but as it passes through its monthly cycle of changing from the slim crescent of a new moon to the brilliant full moon and back again to a crescent, it is clearly much more obvious on some occasions than at others. So, just why does the Moon have this ever-changing appearance?

It is easier to appreciate what is happening if we make a few assumptions that, although not strictly correct, they are as near as makes no difference to the actual. The Moon revolves around the Earth at a distance of a quarter of a million miles, and the Earth and Moon together orbit the Sun at a distance of 93 million miles. Let us assume that their orbits are circular and that the Earth is right in the centre of the Moon's orbit and that the Sun is central to the Earth/Moon pair.

Sunlight can only illuminate one side of the Earth and the Moon at any one time. Their other sides, those that are away from the Sun, are consequently in near-total darkness. To the earthbound observer the position of the Moon in its orbit directly governs the amount of the sunlit lunar landscape that can be seen. Consequently, it is the amount of the Moon's illuminated hemisphere that is seen from Earth that determines its phase. There are four primary phases of the Moon, and many intermediate steps as they progress.

New Moon

Imagine a straight line that stretches from the Sun through the centre of the Earth and beyond into deep space. If the Moon happens to be on that line and, at the same time, between the Sun and the Earth, it will present its unlit hemisphere to the earthly observer to whom it will be all but invisible. This is the point of the new moon.

Observant observers may be aware that the apparent size of the Moon is, coincidentally, almost exactly the same as that of the Sun. So it is that when the Moon is exactly on our imaginary line on both the north-south and the east-west axes we experience one of the most awe-inspiring events in nature, that of the solar eclipse. This only occurs when the Moon's path takes it across the face of the Sun in a direct line with Earth so that its projected shadow falls on the Earth's surface. This doesn't occur on every orbit because on the majority of passes it crosses either north or south of the line.

Starting from this point in its orbit, the Moon takes 29.5 days to make one complete orbit around the Earth and get back to the new moon position. This period is the is 'synodic month' or a 'lunation'. On the first day after new moon, the Moon moves along the path of its orbit and away from our imaginary line. This means that a slight sliver of the illuminated hemisphere comes into view to the earthly observer. Just after sunset as the sky begins to darken it may be possible to observe this slight crescent, but sightings of the one-day-old moon are in fact rare. Theoretically, a sighting is possible at only 14.5 hours using an optical aid of some sort.

It is important to realise that looking for the Moon at this early stage is a dangerous occupation. At this time an angular distance of only 7.5° separates Moon from Sun. Catching sight of the Sun through binoculars can result in permanent damage to the eye's retina. It is an absolute prerequisite that when looking for a crescent moon you should not search for it until the Sun has set. It is also wise to leave searching for sub-24 hour moons to the expert.

As each day passes the Moon continues on its way, increasing its angular distance from the Sun. Each day at sunset the Moon is higher in the sky and the sunlit crescent gets wider as the dividing line between light and dark moves across the Moon's face. This line is known as the Terminator, and during the lunation is going to cross the complete surface of the Moon. It is here where the best observations of the topography are made. The shadows created by the light striking the surface at an acute angle brings every feature into stark relief. In the first few days the thin crescent becomes wider each day and the crescent is commonly known as the New Moon.

It is also at this time that one of the most beautiful of the Moon's phases becomes apparent. It is usually best seen at the time of the two-four day old moon in the winter months. The crescent is readily apparent but looking closely the remainder of the Moon can also be dimly seen. It is not directly illuminated but by twice-reflected light. Sunlight is reflected from the Earth's surface to the Moon and then back to Earth. Leonardo Da Vinci explained the mechanics of it in his Codex Leicester, written in the early 16th Century. This soft, ghostly 'Earthshine' can reveal the dark seas and the lighter highlands of the moon's terrain. It is a beautiful sight, often against a slightly red-tinged sky, low to the horizon. There is an old saying which describes it equally beautifully: The new moon in the arms of the old. Earthshine, also known as the Moon's 'ashen glow', is most intense from northern latitudes during April and May.

First Quarter Moon

Seven and a half days after new, the Moon has completed a quarter of its revolution around the Earth. Looking down from space onto the Earth's north pole, the Sun, Earth, Moon alignment forms a right-angle and we can now see that exactly half of the Moon is illuminated and appears from Earth as a sphere cut in half from top to bottom. To most people this is probably the most familiar phase of the Moon. It is easy to observe as it takes place at the convenient time of late evening when the Moon is relatively high above the horizon and the Sun has long since set. This half-phase when a quarter of the Moon's surface is revealed and it has completed a quarter of its orbit is known as 'First Quarter'.

From this point on the Moon will no longer have the crescent shape. As the Terminator creeps on its way it will appear to grow a pot-belly and is said to become gibbous1. During this second phase some of the Moon's great Maria will be revealed.

Full Moon

At last, at 14 days the complete celestial refulgence of the full moon is revealed. The Moon shines like a beacon in the midnight sky, bright enough to cast a shadow or to read a newspaper by. It is a truly wonderful sight. In winter especially, ice particles high in the upper reaches of the Earth's atmosphere can create a magical halo around it, or it can play hide and seek behind lower scudding clouds while illuminating them from behind.

The Sun, Earth and Moon are now once again on our imaginary line, but this time the Moon is on the opposite side to the Sun having travelled half-way around its orbit to get to the other side of the Earth. Looking from Earth the whole of the Moon's illuminated hemisphere is now visible and it appears as a complete sphere. It is at this time that if the alignment is exact the Moon can pass through the Earth's shadow and observers experience a lunar eclipse, darkening the Moon to a blood-red shade as it passes through.

There is another phenomenon apparent, more frequently, at this time. The Moon's albido is surprisingly low and only 5-8% of sunlight is reflected from its surface. Its reflective quality has been likened to that of dark tar macadam used for road surfacing. But the amount of light reflected by the Moon when full is disproportionately higher than when at part-phase. The full moon is several times brighter than at any other phase. Why this is so was one of the minor mysteries solved by the moon landings of the Apollo program.

Analysis of the soil samples returned from the moon missions showed a significant proportion of the lunar regolith consists of minute glass-like spherules. These were formed from volcanic fire fountains when volcanism was still active on the lunar surface. The spherules reflect light directly back towards its source in the same manner as the common 'cats-eye' used in the demarcation of roads. So when Sun, Earth and Moon are in line, light reflected by the spherules is reflected directly back at Earth.

Easter

The Spring full moon usually coincides with Holy Week in the Christian calendar because Easter is calculated by the lunar cycle. In 2008 it occurred on Good Friday and in 2009 it fell on the Thursday (Maundy Thursday).

Third Quarter Moon

During the course of this first half of a single orbit, as the terminator advanced across the moon's surface, it is said to be 'waxing' as it grows in apparent size. At full Moon it is at its biggest and brightest, but from day 15 onwards the edge of the dark hemisphere begins to show itself where the bright crescent began at new. The Moon is now 'waning' as the darkness replaces the light.

But as the sunlight now streams to the Moon from the opposite direction to that when it was new, a useful tool for the moonwatcher emerges. All the features that were observed during the Moon's waxing will now be seen with the sunlight illuminating them from exactly the opposite direction. Very often this highlights features that were all but invisible when the terminator crossed them before. Rupes Recta, otherwise known as the 'Great Wall' is one such feature that is typical of this. It is a 110 km long fault line situated about halfway between the Moon's apparent centre and its south pole. It becomes illuminated on day eight and shows as a long straight line in black shadow. But after full moon sunlight comes to it from the other side and as the lunation wears on, the illuminated slope shines brightly white.

At day 22 the Moon now reaches the point in its orbit where it forms another right angle with the Earth and Sun, but viewed from the same vantage point above the north pole this right angle is the mirror image of that of the first quarter. This phase is also known as 'Last Quarter', and it only has a quarter of its orbit to go before arriving back at our starting point. Here at last quarter the terminator has reached the central point of the Moon and again cuts it directly in half, but it is the opposite side to first quarter that is illuminated.

The last quarter Moon is not so easily seen. It is approaching the Sun and rises around midnight. Very often it can be seen after dawn and through the morning leading the Sun across the sky, but is rarely noticed in daylight. As it gets closer to the Sun it resumes its crescent shape but becomes more difficult to spot as it closes in to the Sun's glare and sets before the Sun. By day 29 and a bit, it crosses our start/finish line and embarks on its next journey.

Southern Hemisphere

Wherever in the world you happen to be looking from, the phase of the Moon at any one moment will be the same. Full moon over western Europe will still be a full moon a few hours later when the Earth has rotated on its axis to allow North America to catch sight of it. It will also still be a full moon when seen from the Earth's Southern Hemisphere. The only difference there being that it will be inverted, and to a person used to the view from the Northern Hemisphere, it would seem to be 'upside down'. In southern climes during summer the Sun will be low and the Moon high, and that reverses in winter. Meanwhile the opposite is happening in the Northern Hemisphere. Earthshine can be witnessed from the region too.

Full Moon Names

Over the years of human history, lore and legend have been attached to the full moon. Various romantic and portentous names and stories have associated themselves: Harvest Moon, Hunter's Moon and the Strawberry Moon are but a few of the specific monthly names. If you would like to learn more try the h2g2 Entry The Full Moons - What's in a Name.

Try an Experiment

Demonstration is worth a thousand words, so you might like to try a small experiment. Provide a light source in an otherwise darkened room, preferably a torch that provides a beam of light. Now stand on the opposite side of the room holding a tennis ball-sized sphere of some sort at arm's length and place it between your eye and the light. You will not see any part of the ball illuminated. Move it slowly to the left and watch as the shadow line retreats and a sliver of the illuminated half of the ball comes into view replicating new moon. Now move it to a point opposite your left ear and observe that half the ball is illuminated. The remainder of the circle around your head should illustrate fairly clearly what has been described earlier. Better still, make an appointment with a three-day-old Moon and just revel in the glory of one of nature's free attractions.

1Gibbous comes from the Latin word meaning 'a hump' or 'bulging'.

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