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Why the Sun chases the Moon

Page history last edited by Andrew Alder 1 year, 2 months ago

Many ancient legends talk of the Sun chasing the Moon. In modern times, many amateur scientists and even some professional ones dismiss this ancient view.


But ironically, there is also a sense in which these modern purists are more than a century out of date. 




What we see

Every day, moonrise is about fifty minutes later than it was the previous day.  [1] Sunrise, on the other hand, is at about the same time. Sunrise varies a few minutes with the seasons, back and forth, but never so drastically as moonrise does on a daily basis.


Or does it? This is only because we have chosen to set our watches by the sunrise. But it's generally agreed to do this. It goes back to the invention of the sundial at least.  


And this led to the ancients concluding that the Sun was chasing the Moon through the sky. And if you believe as they did that the Earth is the centre of the Universe, that makes perfect sense.


Actually it's a bit more complicated than that. The Sun catches and overtakes the Moon every New Moon. And that is the other notable thing about the Moon of course. It has phases.


How the Sun chases the Moon

The Sun chases the Moon for about half of the month. At Full Moon, the chase begins. As the Sun catches up, the Moon gets smaller... or seems to do so. After about a week it is a half moon which we call the third quarter because (it is generally agreed) the month starts at New Moon. This shrinking moon is called a waning moon.


(Or at least the lunar month still starts at New Moon. But lunar month is really a tautology because lunar and month both mean moon. It got complicated when various Popes started naming months after themselves, to give themselves equal status to gods such as Janus who already had months, the days of the week being taken already by other gods such as Thor. This why the ninth month of the Calendar is now called September which means seventh month and so on until the twelfth month is called December which means tenth. But that's another story!)


How the Moon chases the Sun

At New Moon things swap back. The Moon is now chasing the Sun but losing. As the Sun gets further ahead, the Moon becomes a crescent, then a half moon which we call first quarter, and about two weeks after the New Moon we have another Full Moon.


And the whole time, the Moon points to the Sun. If you think of the crescent before and after New Moon as an arrowhead, in both cases the (fairly blunt) point is in the direction of the Sun. Similarly, the flat side of a half moon is always away from the Sun, and the curved side towards it... another blunt arrowhead if you like.  


This growing moon is called a waxing moon.


Slowly it builds to a full moon. A full moon rises at about sunset and sets at about dawn. And it then becomes a waning moon again, getting smaller until it is a half moon again (called the third quarter as we saw above... does that now make sense?) pointing the opposite way in the sky but again towards the Sun as the Sun catches it again. The third quarter moon rises at about midnight and sets about midday.



These deserve a mention too.


Sometimes at full moon the moon or part of it disappears. We now know that this is because the Sun, Earth and Moon line up and so the Earth shades the Moon from sunlight. It doesn't happen very often because the planes of the orbit of the Earth around the Sun (called the ecliptic plane) and that of the Moon around the Earth are not quite the same. But it does happen regularly and predictably.


We call this a lunar eclipse.


And similarly, sometimes they line up in the order of Sun, Moon and Earth, and then in some parts of the Earth the Sun is partly or completely hidden. We call this a solar eclipse and if the Sun is completely hidden it's a total solar eclipse.


Lunar eclipses take place at full moon, and solar eclipses at new moon. Think about why this is so, and then see below for why this is very relevant here!


While a lunar eclipse looks the same from all parts of the Earth that can see it at all, the appearance of a solar eclipse varies enormously. This is because the moon casts its shadow on only part of the Earth's surface. So in some parts it will be a partial eclipse, an some parts it may be total, and in others it will not occur at all. The experience of totality is an eerie and unforgettable one.     


The ancient view

The ancients watched this relatively predictable cycle in the heavens and decided that there was a perfection in the heavens that the weather on Earth did not possess {often to their discomfort as to ours!) and that the Heavens were the place of the Gods where things were under better control. Astronomy became one of the earliest sciences.


The current view

We now know that this cycle is the result of two cycles. The Earth rotates on its axis, causing the sunrise and sunset. And the moon revolves around the Earth, but far more slowly, causing the time of moonrise and moonset to continually shift, and the phases of the Moon.


That's the modern view. Or most of it. It's called the Heliocentric Universe. And for many purposes it's more useful than the Geocentric one in which everything is centred on the Earth.


But there is a newer and far better and more generally accurate picture of the Universe. It's called Relativity and the idea is probably not as new as you think. It means you use whatever frame of reference makes the sums easiest. And for some purposes, thinking of the Sun as chasing the Moon is the easiest way by far to understand what is going on and to get it right.


Did you work out above how eclipses work? Wasn't that easiest to imagine by thinking of the Earth as the centre of everything?


So the ancients are in this sense vindicated. And as is normally the case, the purists are quite wrong.


And this will probably be an unpopular essay as a result.

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