Moon phases and the tides

Half the Moon is always lit

The Moon gives off no light of its own; it shines by reflecting sunlight. Being a sphere, it always has a bright half facing the Sun and a dark half facing away. Its shape changes not because the bright half grows or shrinks but because, as the Moon orbits the Earth, the angle from which we see that bright half changes. When the Moon is on the same side as the Sun we see only the dark side and it is almost invisible; when it is opposite the Sun we see the whole bright side, a full moon. In between we see part of the bright side, a crescent or a half moon.

The order through the month

The Moon returns to the same shape about every 29.5 days. Lunar calendar months of 29 or 30 days follow this cycle. Seen from the Northern Hemisphere, the Moon fills in from the right and wanes from the right; in the Southern Hemisphere it looks mirrored. Knowing the order lets you guess where tonight's Moon will be and when it will rise.

• New moon: same direction as the Sun, nearly invisible
• Crescent: briefly in the west after sunset
• First quarter: right half lit, southern sky in the evening
• Full moon: rises around sunset, visible all night
• Last quarter: left half lit, southern sky before dawn

When and where to look

The Moon orbits in the same direction the Earth spins, so after a day the Earth has to turn a little further before the Moon is back in the same place. That is why moonrise comes on average about 50 minutes later each day. A full moon rises in the east around sunset and sets in the west around sunrise, so it is visible all night. A first-quarter moon rises during the day, stands high in the southern sky in the evening and sets around midnight. A last-quarter moon rises around midnight and is seen before dawn. This is also why the Moon is so often visible in daytime; the idea that it only comes out at night is a misconception.

Why we always see the same face

From Earth we always see the same side of the Moon. It is not that the Moon doesn't rotate; it takes the same time, about 27.3 days, to spin once as to orbit the Earth once. Over a long time the Earth's gravity slowed the Moon's spin until the two matched, an effect called tidal locking. The phase cycle of 29.5 days is longer than the 27.3-day orbit because the Earth also moves around the Sun in that time, shifting the Sun's direction, so the Moon has to go a little further to reach the same phase. The far side is often called the dark side, but it gets just as much sunlight as the near side.

Not the same as an eclipse

The idea that the Earth's shadow changes the Moon's shape is a common misconception. The Earth's shadow falling on the Moon is a lunar eclipse, which happens only at full moon, when Sun, Earth and Moon line up. A solar eclipse, where the Moon blocks the Sun, happens only at new moon. You might then expect eclipses every month, but the Moon's orbit is tilted about 5 degrees to the Earth's orbit, so in most months the shadows miss. During a lunar eclipse the Moon is not cut cleanly like a half moon; the curved edge of the shadow passes across it, and it may glow reddish.

Why the tides come and go

The Moon's gravity pulls harder on whatever is closer. Water on the side facing the Moon is pulled more strongly than the Earth's centre, and water on the far side is pulled less, so it is relatively left behind. The result is two bulges in the ocean, one towards the Moon and one on the opposite side. As the Earth turns once a day, any given spot passes through both bulges, so many places get two high tides and two low tides a day. Just as moonrise comes about 50 minutes later each day, high tide shifts later each day too, with an average of about 12 hours 25 minutes between high tides.

Spring tides and neap tides

The Sun also pulls on the oceans, but its tidal effect is a little under half that of the Moon. Around new and full moon, the Sun and Moon line up and their effects add, so high tides are higher and low tides lower; these are spring tides. Around half moon the two act at right angles and the range shrinks; these are neap tides. Actual tide heights and times, though, depend heavily on the shape of the coast and the sea floor. That is why Korea's west coast has a very large tidal range while the east coast has a small one, and why high tide falls at different times in different places on the same day.

• Spring tides: near new and full moon, large range
• Neap tides: near half moon, small range
• Actual size and timing vary greatly with local geography

Safety on tidal flats and shores

The most practical reason to understand tides is safety. On gently sloping tidal flats, once the water starts coming in it covers a wide area much faster than you expect, and it can rise first along tidal channels and cut off your way back. Before mudflat walks, shore fishing or crossing a causeway to an island that floods at high tide, check the local tide table published by your national hydrographic or tide authority and come out well before high tide. Fog makes it easy to lose your bearings, so don't go alone, and in an emergency call your country's maritime emergency number at once.