Frequently asked questions
(involving the moon)

Is the Moon the same shape when seen from different places on Earth?

Yes. At the same moment, the Moon looks pretty much the same shape, if you can see it, wherever you are on Earth. The difference made by travelling a few thousand miles to the other side of the world is very slight because the Moon is dozens of times further away than that. It doesn't necessarily look the 'same way up', though. Its orientation, in a different observer's sky, will probably be different - but if it's visible, it will be the same shape. So, Full Moon happens at the same moment everywhere on Earth - although of course the time shown on local clocks will depend on the time-zone to which they are set.

What makes the Moon change shape?

It's actually because we can only see the part that's lit up by the light from the Sun. The Moon is shaped roughly like a solid ball, and is mostly made of rock - it doesn't give out any light by itself. The only reason we see it is the sunshine reflecting off it. Because it's solid, the sunlight can only reach the half of the Moon that's facing the Sun at that moment. If you get a light-coloured ball, and a good torch, and a very dark room, you can get a similar effect. When we look at it from the side, we can only see half of it lit up. As the Moon orbits the Earth, the angle between the line-of-sight of an observer on Earth looking at the Moon, and the sunshine hitting the Moon, changes, and the apparent shape of the Moon changes.

You can notice, too, that the bright 'limb' of the Moon - the semicircular lit-up edge - always points towards the Sun. So, for example, when we see a New Moon, just after sunset, if we're in the Northern hemisphere, North of the tropics, the Moon will be a bit to the left of the Sun (and up a bit), so its bright limb will point to the right, and downwards. The very old crescent Moon, on the other hand, will be orientated more like a letter 'C'. We'll sometimes see the Moon like a letter 'U', but never like an 'n', because that would put the Sun above the Moon in the sky, and it would be too bright to see the Moon!

What is the 'dark side' of the Moon?

This phrase has sometimes been used - a bit archaically - to describe the side of the Moon that's turned away from Earth. It's not necessarily actually dark there - the sun does shine on it sometimes, as it does on the rest of the Moon - but the word dark in this context really means hidden from us. It's probably better to call it 'the far side'. It happens because the Moon and Earth have been so closely involved, gravitationally speaking, for so long, that the spin of the Moon has settled down to match its orbital speed around the Earth - in other words, the Moon always keeps more-or-less the same side facing the Earth. It naturally does this simply because the Moon is not exactly symmetrical, and its heavy side is naturally pulled more toward the Earth. It still has a bit of oscillation (wobble) remaining, but by and large we only ever see the same side of the Moon, from Earth. The far side had never been seen at all by humans until it was photographed, by one of the first automatic space probes sent from Earth, in 1959.

The Sun shines on the far side of the Moon when we have a New Moon - and a fortnight later, when it's Full Moon, the Sun is shining directly on the near side of the Moon, so the far side really is dark. On the Moon, the 'day length' (from one sunrise to the next) is a month long - about 29.5 normal Earth-days - so there is no side of the Moon that's always in darkness (except the inside, perhaps!).

Why is the time of an eclipse not exactly the same as the time of the new or full Moon?

Perhaps strangely, the time of Full and New Moons is always defined in terms of only the celestial longitudes of the Moon and the Sun. That is, we say it's New Moon when the Moon's celestial longitude (i.e. its position around the ecliptic, i.e. the zodiac) is the same as the Sun's. The Moon isn't always exactly on the ecliptic plane, though, and even in an eclipse, it doesn't exactly line up with the Sun. The Moon's apparent motion against the fixed stars at that point (because of its obiting the Earth) is at an angle of about five degrees to the ecliptic. So the moment when the Sun and Moon have equal ecliptic longitude, is not quite the same as the moment when the Moon and Sun are 'closest' as seen from Earth. The former is the official 'new Moon' time; the latter is the actual moment of greatest eclipse. An exactly similar situation happens at lunar eclipses and Full Moon - when the Moon is opposite the Sun.

Why is the new crescent Moon really high in the sky sometimes?

It must be Spring! The New Moon is visible only for a while after sunset. At sunset on the Spring equinox, the line of the ecliptic is really high in the sky. The opposite is true in Autumn. So if you want to see a really thin crescent in a dark sky, look for a New Moon in Spring, or a very Old Moon (just before dawn) in Autumn.

Why is the full Moon really high in the sky?

It must be Winter! The full Moon is opposite the Sun, and as the Sun gets higher in the sky in Summer, so the full Moon will be lower in the sky at that time of year. In Winter the opposite is true.

The exact height that the Moon reaches will also be affected by the position of the Moons nodes, as the maximum declination of the Moon varies over an 18.6-year cycle, as described on the 'Moon, Earth and Sun' page. This has quite a noticeable effect, especially when it's near its maximum value.

When's high tide?

It does depend on where you are and on the phase of the Moon, but for any particular place, you will find the the phase of the Moon can tell you quite usefully when the high and low tides are. Check it out for your favourite beach! In South-West Wales, for example, the high tides are about 6 o'clock (morn and eve) on a New or Full Moon, and near noon and midnight on a waxing or waning Quarter. This will only ever be a first approximation though, so don't expect it to be spot-on!

Each day, the tides are a bit later than the day before, just as the Moon rises later on successive days. After 29½ days we're about back to the same phase, so on average the tides are about 24/29½ hours - about 50 minutes - later each day. In fact, the change in time from high tide one day to the next can be as little as 25 minutes just after new or full moon, and well over an hour just after a quarter-moon. This is because the Sun also has a tidal effect, which adds or cancels some of the moon's effect, and the direction of the combined effect changes more quickly at the quarter-moons.

When the tidal effect from the Sun adds to that of the Moon, at New or Full Moon, the tides are biggest. (That is, the tidal range is biggest: the tide comes in higher at high tide, and also goes out further at low tide). In fact, the biggest tides tend to occur a day or two after the new or full Moon, presumably because nature never does anything in a hurry. These are called Spring tides, though they happen twice a month! However, the Spring tides near the equinoxes (i.e. in Spring and Autumn) are the very biggest of the whole year. (The reason for this seems to be to do with the shape of the Earth's landmasses, as far as I can tell.) Just after the quarter moons, the tidal range is smallest; these tides are called Neap tides.

Why are there two tides a day?

The tidal deformation of the waters of the Earth by the Moon, has a 'rugby-ball' shape (a.k.a. a 'prolate ellipsoid'). It seems intuitive that there should be a 'tidal bulge' on the Moon's side of the Earth, but there is also another on the opposite side of the Earth! The gravitational force that keeps the Earth and Moon orbiting their common centre of mass, is less strong on the side of the Earth furthest from the Moon. On the near side, it's stronger, and a tidal bulge is formed. The Earth in the centre is pulled less than this water, but more than the water on the far side, so it leaves a second tidal bulge on the far side. The Earth spins, and the Moon's gravity drags these two bulges round and round the Earth.