[thedoc (OP)]

Rachel asked the Naked Scientists:
   Hi my daughter and I live in Tasmania and were wondering if we have the sun and moon in the sky what are they seeing on the other side of the world?? we presume its night but do they have a moon if we can still see it??
What do you think?

[Janus]

It depends on where the Moon and Sun are in the sky.  If they are high in your sky, they won't be seen in the sky for someone on the opposite side of the Earth for you until the Earth rotates enough to bring them into view.
However, there is a situation where you and a person on the opposite side of the Earth could both see the Moon and Sun at the same time. This is just before one of them sets while the other has just risen.  You will see each just above the horizon and so will the person on the opposite side. The reason for this is that our atmosphere refracts light passing through it and causes the light to bend so that while either body is actually physically just below the horizon, you will see it as being just above the horizon. 

[evan_au]

Overall, the Sun and Moon are visible from about half of the world at any time, provided:
- There aren't any trees or mountains blocking the horizon
- Ignoring clouds
- Ignoring the few days per month on either side of a "New" Moon, where the dark side of the Moon is lost in the glare of the Sun
- And most of the world sees the same view at some point in the next 24 hours, as the Earth rotates under the Sun and Moon.

The amount of time that you see the Sun on a given day is related to the tilt of the Earth's axis. So when there are short, cold, snowy days around Christmas in the Northern hemisphere, Australians are sweltering with long, hot days.
- When people from the northern hemisphere realize that Australia has opposite seasons, I am often asked questions like "Do Australians celebrate Christmas? How?". And the answer often relates to the beach or other outdoor activities like a BBQ...