FAQ

"I understand in the logical sense how the rotation of the moon around the earth and the rotation of the earth around the sun and their consequent alignment and the reflection of light results in the crescent moon phases but I cannot understand why in the first and last quarters the moon appears to be divided in half by a pretty straight line. A curve yes but it is a straight line and I am intrigued."

You're right that the line dividing the Moon's illuminated side from its dark side (which in astronomy is called the 'terminator') is curved. That's because the Moon's surface is itself curved. However, at first and last quarters, our view from Earth is exactly in the plane of the curve, and so it looks like a straight line. The best way to visualise this is to hold a tennis ball or a similar spherical object in the sunlight, and look at it from different angles. You'll easily find a position in which the 'terminator' looks like a straight line from your viewpoint - that's exactly the situation with the Moon at first and last quarters.

"Why is Pluto not considered a planet anymore?"

Its current status is that it is in a special class of dwarf planets called 'plutoids', of which there are three more known (Eris, Makemake and Haumea). The fifth dwarf planet, Ceres, is not a plutoid as it's in the main asteroid belt, rather than the distant Kuiper belt. So the bottom line is that Pluto is definitely not regarded as a planet.

There's a lobby to get it reinstated as a planet, led by astronomers in the USA - perhaps because Pluto is the only (former) planet to have been discovered in the USA! In my view, that would be a mistake, since Pluto is certainly not a planet as we normally understand the word.

There's a really good article on all this in the current New Scientist, which you can read online at:
http://www.newscientist.com/article/mg20327181.600-is-pluto-a-planet-after-all.html?full=true

Check out the diagram linked in the text - it sets things out really clearly, and I'm sure your class will love it. (1 Astronomical unit, by the way, is the mean distance from the Earthto the Sun, 150 million km.)

"I read that the recent major Sendai earthquake moved Japan by 2.4 metres and changed the tilt of the earth by 16 cm². I guess the latter comment means that if your are standing at the north pole, then the point from which the rotational axis emerges has just moved 16 cm in one direction relative to the ecliptic (to tilt the earth further or to stand it up straighter from the 23.45 degrees I am not sure. Is this angle called ³obliquity²?). I think there was a minute change in day length too. I guess the 2.4 metre surface movement might have all sorts of interesting implications for land based surface surveys in Japan which depend on exact lat/long coordinates and GPS, satellites for the latter remaining at defined places in the sky but new places relative to Japan. But what about astronomy? If the earth has gone even a little bit off its tilt, then any astronomical instruments looking at the sky will now be pointing slightly in the wrong direction towards the ³fixed² heavens. I assume that where extreme pointing precision is required, all the possible variations of tilt, precession, orbital ellipse changes etcetera can be accommodated in the software need to point the instruments. But can you confirm it please?"

Your comments are all correct, except that there has been no change in the tilt of the Earth's axis. That can only be done by external gravitational forces, and they would need to be very large to shift the rotational axis of a body the size of our planet even by a tiny angle.

What has changed is the position of the Earth's axis of figure - which has moved, as you say, by about 17cm. That's simply a consequence of the redistribution of mass changing the position of the Earth's centre of gravity very slightly. In fact, over the course of a year, this moves around randomly by tens of metres as mass shifts within the mantle and fluid core.

Have a look at
http://www.space.com/11115-japan-earthquake-shortened-earth-days.html for further details.

"We lived in an isolated farmhouse at Lerida between Gunning and Collector, north of Canberra. One evening, when we arrived home about 9pm, we saw what we believed to be the aurora - faint mostly green lights in the southern sky. It was a magic moment. Would this be the farthest north an aurora australis has been seen? Fred's article in Australian Geographic mentions Tasmania but we were along way from there. He also mentioned an 11 year cycle. Our sighting was slightly outside this. I'd be interested to hear your thoughts on our sighting."

... Very nice to hear about the aurora sighting, and while it's unusual that far north, it's certainly not a record. I've seen a few aurorae from Coonabarabran. And in times of high solar activity, they can be seen much nearer to the equator.