First or Last "Quarter" phase terminolgy

[Lurcher]

Hi All,

Sorry, this might be really pointless, but I wondered if anybody knows of the reason why the phase of the moon, when HALF of the moon facing us is illuminated is called the "First Quarter"?

It's quite clearly half of what we can see so surely should be called the first HALF, not quarter.

If it's because they're thinking of the moon as a whole, as we're only seeing a quarter of the whole moon, then surely we should also remember that the other quarter we can't see would also be illuminated, and it is still therefore HALF of the whole moon at that phase?

Further more, if we're thinking of the whole moon at that phase then perhaps we should refer to the full moon as the HALF moon?? It seems strange to me that we go straight from quarter to full with nothing in between.

If anyone can please explain to me why astronomers refer to the first half as the first quarter I'd be really grateful!

Thanks!

 

[clarkpm4242]

It is not a description of what you see.

When the moon is 1/4 the way through its 4 week/28 day orbit it is at 1st quarter and half illuminated as we see it.

WHen the moon is 3/4 the way through its 4 week/28 day orbit it is at 3rd/last quarter and half illuminated as we see it.

 

[vlaiv]

Don't know exactly but here are two plausible explanations:

- We see one quarter of whole moon facing us and being illuminated

- More plausible - in complete cycle there are four distinct phases - new moon (or lack of moon, since the side facing us is not illuminated), first quarter, full moon, last quarter. Hence each phase is quarter of full cycle. First and last being - first that is visible and last that is visible.

[LightBucket]

Half of the moon is what we see, so half of the half is a quarter....

[Lurcher]

22 minutes ago, LightBucket said:

Half of the moon is what we see, so half of the half is a quarter....

But on a full moon we see half the moon so half of a full is a half!! Half moon not full moon!??

 

35 minutes ago, vlaiv said:

Hence each phase is quarter of full cycle.

I can kind of see it that way. Thank you!! Yes that makes sense! Got it!

 

42 minutes ago, clarkpm4242 said:

It is not a description of what you see.

When the moon is 1/4 the way through its 4 week/28 day orbit it is at 1st quarter and half illuminated as we see it.

WHen the moon is 3/4 the way through its 4 week/28 day orbit it is at 3rd/last quarter and half illuminated as we see it.

I knew you guys would put my feeble brain out of it's misery! Thanks! I was indeed thinking purely of what I could see. Thanks.