Expert: Tom Whiting - 5/31/2009

Question
Hi,
Just a curiosity really, but when the UK can see the moon the daytime, what does Australia see at the same time during their night? Do they see the same phase of the moon as we do or the opposite?

Answer
Hi Hannah,
Everyone sees the same phase of the same moon, but not quite
at the same time.  If you see a first quarter moon, (a moon 7.0
days past new moon, like tonight), then when the moon rises in Australia they will be seeing a 7.5 day old moon, just slightly past first quarter, about 12 hours later.
(And since the moon moves roughly 12 degrees eastward every 24 hours,
the Australian 7.5 day old moon will be roughly 6 degrees east of
where you were seeing it in the sky, relative to the starry background, 12 hours before, or one fist-width at arm's length, eastward of where you were seeing it).

So the moon, like the sun (and every star in the equatorial region
of the sky) is above your horizon for roughly 12 hours, and below
your horizon for 12 hours, no matter what the phase the moon is in.

And that's true for all the other observers on the other side of the Earth too.  {And in the USA, since we're roughly 6 hours west of you,
(or roughly 90 degrees of longitude west of the UK...we see the same moonrise 6 hours after the UK observers see the moon rise}.

As you observe the first quarter moon rising in the east around noon UK time, the Australians are seeing it set in their west at midnight Australia time. When you see the first quarter moon setting in the west around midnight UK time, then Australians are seeing the first quarter moon (+0.5 day)...or more specifically the 7.5 day old moon... rise in their east at about noon Australian time.
So see, you both see the moon for 12 hours above your horizon,
and then it's not visible for about 12 hours below your horizon.
When the moon rises the next day in the UK, around 1 pm UK time, then it's an 8 day old moon, or one day past first quarter.
(Called a gibbous moon).

6 days after that, On Sunday night June 7th, the FULL moon rises at sunset in the UK (as it's setting in Australia in the west at their sunrise)...then you see the full moon set in the west at UK sunrise, while it's just rising, 12 hours PAST full moon, in the east at sunset (or technically, about an hour after sunset)...in Australia...Still nearly full, but not exactly full....and that's the way it works....all the time, for the past 3 billion years or so.

We see a moon in our daytime sky when it's bright enough TO be
seen at that time, generally a moon at or past first quarter during
the afternoon hours in the eastern sky, and a moon for about 7 days past full moon, in our early morning western daytime sky. During the smaller crescent phase moons, it's there too in our daytime sky, it's just not bright enough to easily detect it, plus it's nearer the sun, making it even more difficult to see in the daylit hours.
But it's still there.  12 hours up, and 12 hours...below your horizon.

Hope this helps,
Clear Skies,
Tom Whiting
Erie PA  USA