• Choose a month

  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Luna-see II: Frame it!

For closely related links, see:

The purpose of this little exercise is to drive home the significance of the Earth being tilted on ita axis by 23.5 degrees.  That fact accounts not only for our seasons, but for the rapidly changing, north-south position of the Moon in our sky – changes that many sky watchers are either unaware of, or can’t explain if they are aware of them. But it’s not that complicated – it’s all in the way we lean, and we can see this plainly if we make ourselves a simple window frame with which to view the heavens.

To do that simply click on this text to download and print a proper frame. Then cut out the center portion with scissors. It’s a minor exercise, but it gets you past one those counter-intuitive realities where things go up in your view when you think they might go down.

We’ll use this frame first to illustrate the movement of the Sun in our sky at different seasons. Pick a small object a reasonable distance away and pretend that object is the Sun.

Now:

1. Hold the frame by the sides with thumb and forefinger of both hands.

2. Stretch out your arms in front of you so you’re looking through the frame and your eyes are level with the center of the frame.

3. Center your “Sun” in the frame. Your horizon is down, the zenith up.

 

4. Being careful to keep the center of the frame in line with your eyes, tilt your head back - raising your arms to keep your eyes in the center of the frame as you tilt your head. Did the Sun appear to move up or down when you did this? That is, did it appear to get closer to your horizon, or closer to the zenith?

When you did Step 4 you were simulating the relationship between Earth and Sun in the northern hemisphere winter.  It is low in our southern sky as it crosses from east to west each winter day.

 

5. Repeat Step 4, but this time tilt your head forward. Does the Sun move up or down?

When you did Step 5 you were simulating the relationship between Earth and Sun in the northern hemisphere summer. Then it crosses high in our sky.

In the starting position – with your head level – you are simulating the relationship of the Sun in spring and fall when it  rises pretty much due east, sets due west,  and crosses at a midway point half way between the low extreme of winter and the high extreme of summer.

There’s one problem with our little exercise though – and it’s critical to understand.  We changed our view  by leaning forward or leaning back. The Earth never changes the tilt of its axis. It is always leaning the same way. What changes, of course, is we’re going around the Sun. So when we’re on one side of the Sun, we in the northern hemisphere are leaning towards the Sun – and when we’re on the other side of it, we’re leaning away from the Sun. It’s not because we change the way we lean. it because we moved from one side of the Sun to the other.

The lesson this exercise should drive home is how the way we are leaning changes the apparent height of the Sun in our sky. That is, how far above our southern horizon it appears to be when it reaches the highest point in its east-west path across the sky. Or simply put, how high it is at noon.

Framing the Moon

If you are comfortable with how our leaning changes the position of the Sun in the sky as we go around the Sun, now consider the Moon.

We are not going around the Moon.

It is going around us – every month!

But the effect is the same as when the Earth goes around the Sun – when the Moon is on one side of us, we are leaning towards it. When it is on the other side of us, we are leaning away from it.

We haven’t changed our leaning. The Moon has simply gone around us. But the impact is the same as with the Sun. From our perspective, the Moon appears to be highest in the sky when we are leaning towards it – and lowest when we are leaning away from it.

So the motions we observe with the Sun – the way it gets lower in our sky in winter and higher in summer – are duplicated by the Moon – EVERY MONTH.

Simple? Yes, but . . .

Here’s where it gets a little complicated.

At the same time that the Earth goes around the Sun, the Moon is going around the Earth – but much quicker. It makes 12 trips for our one. So to really understand where the Moon is going to be on any given night, you have to combine your knowledge of the Earth’s relationship with the Sun, the Moon’s relationship with the Earth, and how the Moon goes through phases.

Don’t let the changing west-to-east position of the Moon confuse you. As the Moon goes from New Moon to First Quarter it is higher in our sky at sunset each month. That is because of the west-to-east movement (counterclockwise) of the Moon around the Earth each month.   When we say “higher”  or “lower” in what follows we are not talking about this west-east movement. We are talking about how far north or south the Moon is in our sky – for northern hemisphere observers we are talking about  how high it is above our southern horizon when it crosses the central meridian, the highest point it gets to each 24 hours. You can think of that as how high it is at high noon! (Or maybe we should say “high Moon! 😉

It’s not hard to understand if you take it one season at a time, but it does help if you have built the Lunar-See model and made one important addition to it – you should mark the north pole of the Earth with a dot using pen or pencil so you can set the Earth in the center of your model  and lean that pole in the correct direction.

If the Earth on your Lunar-See model is wood, put a black-dot to indicate the northpole. If it is clay, stick a piece of a tooth pick in it to indicate the axis of the Earth .

Let’s try winter.

Black dot on Earth on this "Luna-See" model indicates the north pole, so in these images the Earth is tilted to the right. Notice that this tilt doesn't change from New Moon to Full Moon. What changes is the position of the Moon. At New Moon the northern hemisphere of the Earth is leaning away from it, and at full Moon it is leaning towards it. (Click image for larger view.)

In winter the Earth is leaning away from the Sun, so the Sun appears to be low in our sky.  That means at new Moon, the Moon will be between us and the Sun, and like the Sun, will be low in our sky – to the southwest.

At Full Moon the opposite will be true. The Moon will be opposite the Sun in our sky. So while we are leaning away from the Sun, we are leaning towards the Moon.

And that is why the full Moon in December rises in the northeast, appears to go nearly overhead, and then sets in the northwest.

Summer? It’s just the opposite. In June the Full Moon rises very low in the southeast as the sunsets opposite it in the northwest.  Like the winter Sun, the summer Full Moon  never gets very high above the southern horizon, and sets in the southwest.

It’s all for one simple reason – we are leaning on our axis 23.5 degrees! And how high the Moon or Sun is in our sky depends on whether we’re leaning towards it or away from it.

What about the First Quarter Moon? That’s a favorite of many lunar observers because the terminator cuts across very interesting areas and the shadows in those areas are especially long  at that time making them stand out in dramatic relief.  So when will the First Quarter Moon be highest in our sky? It will be highest when we are leaning towards it, of course. And for northern hemisphere observers that happens in March near the Spring Equinox. At that time the Earth is leaning towards the Moon just as it reaches the First Quarter phase – so that First Quarter Moon gets very high in our sky.  Of course this means the Last Quarter Moon is low in our sky in spring because when the Moon gets around to that phase it is on the other side of us and thus we are leaning away from it.  If you want to get the best look at the Last Quarter Moon, that will come in Septmber when it is highest in our sky.

So now when you look at the Moon you should be able to get all the dynamics clear in your mind. Ask yourself first, where is the Sun at that particular moment – what direction is it coming from to strike the Moon? Then ask yourself how is the Earth leaning? Towards the Moon? Away from it? Or somewhere inbetween.  You may find this a lot to swallow in the abstract – even with models to move around – but if you make the effort to put it all together when you’re out under the night sky, you’ll gain a greater sense of just where you are and what the various players in this magnificent medley of motions, are doing at that moment. If you;re really lucky yu may get one of those special, “aha!” moments when you feel like your part of the action and it all make wonderful sense in an ineffable way.



For closely related links, see:

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: