h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: UPDATE for Lunar Phases A346673 - A44336856
Author: Galaxy Babe - U128652

Update for A346673


GB

This whole thing is a bit sparse and some of the information isn't really relavent to the subject of 'Phases'. Really it requires more introduction into the reasons why we on Earth see phases. Namely the relationship between Sun/Moon/Earth, their orbits and the Sun and the point of view of the observer on Earth . If ever a guide entry needed a diagram this is it.

The first para needs rewording as it is misleading.
>The Moon rotates on its axis once for every time it completes its journey around the Earth. This is caused by the strong tidal coupling between the Earth and Moon.<

The statement is partly correct in itself, but it has little to do with phases. Phases are not caused by the ‘tidal coupling’ between Earth/Moon. The tidal coupling is a gravitational effect which has caused the same side to be presented towards Earth all the time. We would still see the same phases if the moon was rotating at any speed, or even not at all.

>However, we see slightly more than half of the Moon's surface due to a slight wobble in its orbit, allowing us to see 59 percent of the lunar surface from Earth.<

Not so. The moon does not have a wobble in its orbit. There are a couple of reasons for being able to see more than 50% and which is called libration. It has an elliptical orbit and varies in distance from the Earth Its distance varies from 406,700km at apogee to 356,400km at perigee, a difference of 50,000 odd km realised each month.

The greater of these optical libration effects is due to the angular velocity of the moon in orbit when it is travelling faster when it is closer to Earth than it is at its furthest away, in keeping with Kepler’s laws of planetary motion. But it still remains revolving on its own axis at the same rate, the consequence of which is that these two unsynchronised speeds favour a view of the eastern limb on one side of the orbit and the western on the other.

There is also a north-south libration where the moon appears to ‘nod its head’. This comes about from the axial tilt of the moon which is not perpendicular to its orbital plane. Consequently from earth we can see over the northern pole at one side of the orbit and under the southern pole at the other side.

There are other reasons which have lesser effects, but none of these are really the subject of ‘Phases‘.

>>New moon is the position and time when the Earth, Sun and Moon are roughly aligned. The new moon rises at dawn and sets at sunset. A solar eclipse can only occur during new moon.<<
‘New moon’ is the point at which the centres of the Sun/Moon/Earth are ‘precisely’ aligned. When that alignment happens to occur on the Earth’s orbital plane, then a solar eclipse occurs. What is commonly called ‘new’ is when the moon becomes visible, but really is some one/two/three days after the event.

>>The first-quarter moon rises at noon and sets at midnight local time.<<
>>The full moon rises at sunset and sets at dawn<<
>>Last-quarter moon occurs a week after full moon, …The last-quarter moon rises at midnight and sets at noon.<<<
Well, this might happen if the moon’s orbit was circular and the Earth was central to the orbit. But it isn’t as explained above in libration. Approximately you could say, but you wouldn’t want to set your clock by it.

'Full Moons' section.
I don’t think this is needed here as it is repeating information given in the Full Moon Names EGE.

Deke

Did you read my update, or your original?
I merely repeated what was in the original EGE, except I edited out the errors.

I read A44336856 which is the later one I believe.

I didn't write the original. I contributed one sentence.'The old moon in the arms of the new'. That's one of the reasons I don't any longer like to take a credit on a piece which I've only had a small part in.

Deke

I apologise. I thought you had written the original, which is why it was practically the same. All I added was "Earthshine" and removed the Full Moon stuff, directing instead to the newer EGE on Full Moon names.

I will update this with your suggested alterations.

Tomorrow.

Goodnight, Deke

No need for apologies.

If you want any assistance just shout.

Deke

I've started to try to put together an explanation of 'Phases' which I hope will be a bit clearer than the original was. I'll let you know when it;s done to see what you think. Should only take a day or so.
Deke

Thanks Deke, this won't move on until you've approved it

GB

I've written a bit about phases here: A12460790

Feel free to take as much or as little as you want.

Hope this helps

Deke

A bit?

I've used all of it, and just a teeny bit of what I wrote, and added a few appropriate links

Let me know if you like how it's looking

It looks fine to me except I seem to have left in a couple of typo's. I'll go through it later and pick them out.

Did you not want to include the previous para concerning 'Names of Full Moons? at the end? I see that you have a link but the bit you had amended miight serve better.

Deke

Oh I deleted that because you said in posting two:
<<'Full Moons' section.
I don’t think this is needed here as it is repeating information given in the Full Moon Names EGE.>>

and I don't have a copy of what I wrote

No matter, The link will do the job.

Could you make the following corrections please. Sorry but I've just read it too many times and am just missing stuff:

>.. north-south and the east-west axis we experience..<
Change axis for 'axes'.

>This half-phase when a quarter of the Moon's surface is revealed and it has completed a quarter of its orbit is also known as 'First Quarter'<
Take out the word 'also'.

>The full moon is several times brighter than when at any other phase<
Take out the word 'when'.

>tennis ball-sized sphere of some sort at arms' length<
Shift apostrophy 'arm's'

Thanks Deke

When you talk about the times the moon rises and such, does this apply wherever you are in the world? Also, if it's full moon in the UK, is it also full moon everywhere else in the world?

They may be daft questions, but it's something I was wondering about!

Well asked Cyzaki. When I'm giving training seminars, early on I always tell the participants that the only daft question is the one you didn't ask.

Indeed, we're going through a period of training everyone at work at the moment, and I've been saying 'there's no such thing as a stupid question' until I'm blue in the face!

Cyzaki - yes, local time (I can elaborate but I have to go offline now, sorry)

Deke - I am unable to find this word anywhere, and I don't know what you mean, so I can't replace it:

<<...towards its source in the same manner as the common 'cats-eye' used to demark road lines on Earth.>>

demark?

The moon rises at different times in different locations though, like the sun does, doesn't it? I'm sure when I was in Namibia the crescent moon was a different way up, too. Am I mis-remembering things?

No it's not a daft question by any means.

The phase of the moon isn’t dependant on what time of day it is on Earth. Only on where the moon is in its orbit. So yes, if it’s full moon for the UK then it will still be full moon in the US (say) six hours later. Admittedly that’s a bit of a simplification, but by and large correct.

I tried to stay away from actual ’times’ because as you rightly state the point in time of UK’s midnight will be different to the US’s. Six or so hours ahead in fact.

If you can imagine full moon and see that it is illuminated by the sun completely, there is nothing here on Earth that is going to effect that.. You can imagine that the Earth is a revolving stage and you are observing from a moving platform. In the UK the full moon comes up over the horizon around sunset. It appears to pass overhead and sets around dawn. Relative to the backdrop of stars it has moved very little, only a few degrees across the sky. Meanwhile someone in the US sees it rise six hours later when their sunset occurs. It’s still the same full moon but they are just watching it for a period which starts some six hours later.

The times are not exact because they are effected by a lot of other factors. For instance winter/summer when the ecliptic, which theNo it’s not a daft question by any means
moon follows is higher in summer and effects the moonrise/set times.

Preview is my friend. Repeat 100 times.


If you are observing from south of the equator then the moon will appear 'upside down'.

In northern climbs during summer the sun will be high and the moon low. It reverses in winter. Meanwhile the opposite is happening in the southern hemisphere. Their sun is high in our winter and moon low.

GB
Demark. As in demarcation. Another of those words that I seem to have been using all my life but no one else knows. It went through the spellchecheck OK. But change it for 'marks' as I can't substantiate it

Deke