- These are all real questions Iâve been asked, so I thought Iâd write them down. Hope it helps.
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. If itâs a new crescent Moon here, then it will also be a new crescent Moon there, when you can see it. It doesnât necessarily look the âsame way upâ, though; its orientation, in a different observerâs sky, will probably be different - but if itâs visible, it will be the same shape. The same is true of lunar eclipses - they look the same, so long as you can see them, from wherever on Earth you happen to be watching. The only differences are locally-variable things like: how high in the sky the Moon is, which way up it is, local weather conditions, and the local time zone - the time shown on local clocks will of course depend on the time-zone to which they are set.
Itâs because we can only actually see the part of the Moonâs surface thatâs lit up by the light from the Sun. The Moon is shaped roughly like a solid ball, and is mostly made of rock - it doesnât give out any light by itself. The only reason we see it is the sunshine reflecting off it. Because itâs solid, the sunlight can only reach the half of the Moon thatâs 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 âlimbâ 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 weâre 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 âCâ. Weâll sometimes see the crescent Moon almost like a letter âUâ, but very rarely an ânâ, because that would put the Sun above the Moon in the sky, and it would probably be too bright to see the Moon!
The Moon cannot be seen at the moment of New Moon - as printed on the calendar for instance - itâs much too near the Sun (as seen from Earth - itâs not actually nearby the Sun, but itâs in almost the same direction from Earth). You might see it the following evening, but the crescent will still be so thin and in the glare of the Sun that it usually canât be spotted unless youâre in a really ideal location. If youâre in the tropics, or if itâs springtime in your hemisphere, then the new crescent will be relatively high in the sky, and easier to see. In other places - even if the sky is clear of clouds - it can be hard to spot the new crescent Moon, as it can be very low in the sky and set very soon after the Sun. Itâs fun to try to spot it though! Usually you wonât be able to see it until two or three days after the New Moon time. It helps to have a good prediction of exactly where it should be - some planetarium-type software can be helpful. The first actual sighting of the new crescent marks the beginning of a new calendar month for Muslim people who follow their traditional lunar-based calendar.
This phrase has sometimes been used - a bit archaically - to describe the side of the Moon thatâs turned away from Earth. Itâs 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. Itâs probably better to call it âthe far sideâ. 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 because the Moon is not exactly symmetrical, and its heavy side is 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 itâs 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 âday lengthâ (from one sunrise to the next) is a month long - about 29.5 normal Earth-days - so there is no side of the Moon thatâs always in darkness (except the inside, perhaps!).
Perhaps strangely, the times of Full and New Moons are always defined in terms of only the celestial longitudes of the Moon and the Sun. That is, we say itâs New Moon when the Moonâs celestial longitude (i.e. its position around the ecliptic, i.e. the zodiac) is the same as the Sunâs. The Moon isnât always exactly on the ecliptic plane, though, and even in an eclipse, it doesnât exactly line up with the Sun. When the Moon is near the ecliptic (during an eclipse, for instance), its apparent motion against the fixed stars (as it orbits the Earth) is at an angle to the ecliptic of about five degrees (the inclination of its orbit). 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 âclosestâ as seen from Earth. The former is the official ânew Moonâ 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.
The time when the Moon culminates in the sky (i.e. reaches its highest point in the sky, halfway between rising and setting) is, on average, about 50 minutes later each day. The times of Moon-rise and Moon-set are of course similarly later each day, but these times are also affected by the Moonâs declination (how far North or South of the equator it is that day). When the Moon is further North, then in the Northern hemisphere itâs in the sky for longer, just like the Sun is in summer.
So, for instance, in the Northern hemisphere, looking at the full Moon nearest the autumn equinox, in late September, when the Sun is entering Libra, the full Moon will be close to Aries zero, and so will have a declination of around zero, but is moving Northwards day-by-day. This means that the change in its rising time from one evening to the next is quite a bit less than 50 minutes, because each evening its declination is more Northerly than the night before (so itâs in the sky for longer). So as the Moon approaches full in September (or, say, the full Moon nearest the autumn equinox) we have several evenings in a row where the Moon comes up very nearly just as the Sun goes down, and the evenings are light enough to continue working in the fields, bringing in the harvest. This, apparently, is the origin of the idea of the âHarvest Moonâ, which shines brightly on, night after night, as the harvest is brought home.
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 nodes, as the monthly maximum declination of the Moon varies over an 18.6-year cycle, as described on the âMoon, Earth and Sunâ page. This has quite a noticeable effect, especially when itâs near its maximum value.
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 relatively high in the sky, making the biggest angle with the
horizon. The opposite is true in Autumn. So if you want to see a
really thin crescent fairly high above the horizon in a dark sky,
look for a New Moon in Spring, or a very Old Moon (just before
dawn) in Autumn.
Note: this only applies if youâre not
too close to the equator; if youâre in the tropics, the ecliptic
is always quite high in the sky.
As in the section above, the Moonâs celestial latitude (North or South of the ecliptic) will slightly increase or decrease this effect, so the position of the nodes will affect the exact details. In a year thatâs about half way from major to minor âstandstillsâ (in the 18.6 year node cycle described on the âMoon, Earth and Sunâ page), the ascending node will be around zero degrees of Capricorn, so Spring New Moon will be particularly far North!
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 oâclock (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 donât 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 about 29½ days weâre back to the same phase, so on average the tides are about 24/29½ hours - nearly 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 (smaller) tidal effect, which either adds to or cancels some of the moonâs effect, and the direction of their 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 Earthâs landmasses, as far as I can tell.) Just after the quarter moons, the tidal range is at a minimum (approx. fortnightly); these tides are called Neap tides.
The tidal deformation of the waters of the Earth by the Moon, has a ârugby-ballâ shape (a.k.a. a âprolate ellipsoidâ). It seems intuitive that there should be a âtidal bulgeâ on the Moonâs 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, itâs 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 Moonâs gravity drags these two bulges round and round the Earth.