How Can You Tell if the Moon is Waxing or Waning?
Question posed by a couple of Mice.
First of all, if you need to brush up on what "waxing" and "waning" mean you might like to read this post on the Moon's phases before carrying on with this one.
All sorted? Great.
The answer to this question is different depending on whether you're viewing the Moon from the Northern or Southern Hemisphere of the Earth. As I know the Mice who asked the question are mostly to be found in the Northern Hemisphere, that's where we'll start.
From the Northern Hemisphere
From our point of view, that is, with Earth's North Pole pointing directly up, the Moon orbits us in an anti-clockwise direction. The rotation of the Earth confuses the Moon's apparent motion a bit, so lets imagine we're hovering above the North pole, making sure we're continually facing the Sun (though definitely not looking directly at it because that would be very bad), with the Earth turning beneath us.
The views we'd see at different points throughout the month would look something like this:
When the Moon is in line with us and the Sun, the other side of it is lit, and the side facing us is in complete darkness. That is, as you know, a New Moon (not to be confused with a lunar eclipse).
As the Moon progresses in its orbit, moving to the left from our point of view, we start to see the sunlit side revealed to us, first of all as a tiny sliver of light along the right-hand edge of the Moon. As the Moon progresses further, this sliver grows into a band of light. Eventually we see the Moon in its First Quarter phase - in which the right-hand half of the Moon is lit, from our point of view, and the left-hand half is in shadow.
Hang on a sec: if we see the Moon as "half and half", why is it called the "First Quarter"? Because when it looks like this to us, it has completed one quarter of its orbit around us! That means, that in our position floating above the North Pole and facing (but not looking at) the Sun, we'd have to turn our heads 90 degrees to the left to see the Moon.
As the Moon continues its orbit, starting to slip behind us, now, we see more and more of the Moon's face lit up from our point of view, reaching Full Moon when it is directly behind us (half way through its orbit). After this, there's only one way to go so if we turn to look at the Moon as it continues we see its shaded face starting to come into view, and the shadow appears to creep across its surface from the right-hand edge (in just the same way that the sunlit side slid into view during the first half of the orbit).
If, instead of facing the Sun for the duration of the orbit - taking the occasional glance toward the Moon - we made sure that we turned to face the Moon the entire time this astonishing video, put together by NASA, shows us exactly what we would see:
In short, then, for anyone in the Northern Hemisphere, if you look up at the Moon and see that it is lit on its right-hand side, it is waxing. If it's lit on its left-hand side, it is waning.
From the Southern Hemisphere
The picture from the Southern Hemisphere is exactly the same, except it is turned upside down, so left and right are swapped: If you read the section for the Northern Hemisphere but swap every "left" for "right" (and vice-versa), and every "North" for "South" you'll get the idea.
In short, then, for anyone in the Southern Hemisphere, if you look up at the Moon and see that it is lit on its left-hand side, it is waxing. If it's lit on its right-hand side, it is waning.
If you could hover above the South Pole, making sure that you turned to face the Moon the entire time this astonishing video, put together by NASA, shows us exactly what you would see:
And this illustration should give you some idea what the situation would look like if you hovered just above the South Pole, facing (but not looking directly at the Sun:
If you need to brush up on what "waxing" and "waning" mean you might like to read this post on the Moon's phases now you've finished this one.
Question posed by a couple of Mice.
But wait! There's more!
Thanks to Ian on Facebook for reminding me of this bit...
I've been concentrating on what would happen if we were hovering above the poles of the planet, and forgot that most of us aren't doing that. Instead, we're standing on the surface of the planet and therefore subject to its rotation. The Earth rotates in the same direction as the Moon orbits (anti-clockwise), but does so much faster (the Earth takes a day to complete a rotation, whereas the Moon takes a month to complete an orbit).
Remember that the New Moon occurs when the Moon is in the same direction as the Sun (the only time we actually "see" a New Moon properly is when there's a Solar eclipse, and it's only a true, exact New Moon if it happens at midday from our vantage point). As the Moon moves further along in its orbit we have to wait a little bit longer in the Earth's rotation before we are directly "underneath" it.
The diagram below shows rough timings for when the Moon will be visible each day depending on which phase it is in: Green sections are when the Moon is waxing; red sections are when the Moon is waning. The background has been coloured to indicate whether over the course of a month a particular time of day sees more of the Moon either waxing (light red) or waning (light green):
This means:
- if we can see the Moon in the evening, it is probably waxing, and
- if we can see the Moon in the morning, it is probably waning.
Learn more about how the Solar System is measured in this free course from Alison.
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