What can you do to participate?

• visit the IYA2009 official website and make it happen!
• visit your national website and look for events near you; the Australian node is here
• meet real astronomers at the Cosmic Diary
• listen to the podcast, and maybe record your own

We have a long and exciting year ahead of us; let's make it a good one!

Last week's episode, published on 19 December, was very heavy on astronomy content: it featured an interview with Astronomy Cast's Dr Pamela Gay, plus a review by Tiffany Day, from the Macquarie Skeptics, of Dr Phil Plait's new book, "Death from the Skies". Strongly recommended.

Enjoy!

I know, not quite an astronomical image. Fun nonetheless.

The orbit of the Earth around the Sun has one major, immediate effect: the position of the Sun on the sky (against the background of stars) changes as the year goes by; the Sun will complete a full circle of the sky in a year, which means that its position changes by about one degree a day (360 degrees in a circle, 365 days in a year) — one degree is about twice the apparent diameter of a full moon. Of course, this is not directly visible: we can't see the stars around the Sun during the day. What we will notice is that the field of stars that is visible at night will subtly change from day to day — the whole field will seem to move westwards by about one degree a day.

One important thing to notice is that this apparent movement of the Sun relative to the stars (or of the stars relative to the Sun) is not parallel to the Equator: it is, in fact, tilted by approximately 23.5° (the exact angle changes over time; nowadays it's more like 23.44°, or 23°26'). This tilt has one very significant effect on Earth, but that's the subject of our next article: the four seasons.

The apparent path of the Sun on the sky has a special name: it's called the ecliptic. This is also the path followed (with varying degrees of precision) by the other planets in their path on the sky, as we'll see in the future. The ecliptic goes through several constellations — to be more precise, twelve. These are called the zodiacal constellations, or the constellations of the zodiac (from the Greek meaning "circle of animals" — most zodiacal constellations are represented by animals). One can see how twelve constellations resulted not only in the twelve astrological signs, but also in the twelve months of the year.

There are four special points on the ecliptic that are worth mentioning:

• the solstices: these are the points of maximum north or south drift (or declination) of the Sun; the northern solstice happens in June, while the southern solstice happens in December
• the equinoxes: these are the points of minimum declination, that is, where the Sun crosses the celestial Equator; they are called the autumnal equinox (which happens in September), when the Sun crosses the Equator moving northward, and the vernal equinox (in March), moving southward.

And that's it for today. As I mentioned, in the next article we'll explore in more details the effects of the tilt in our orbit and the significance of the solstices and equinoxes.

As at any good Thanksgiving dinner, this one has touching stories, heated discussions (with several bloggers expressing differing opinions on the past existence of oceans on Mars) and even tips about the upcoming winter (northern hemisphere winter, of course — although, with the snow now falling in the mountains a few hundred km from here, it's feeling like winter in Melbourne as well...). Don't miss it!

Dave Moshen Mosher, at the Discovery Channel's Space Disco blog, has a good post about the mission with links to many pictures and videos from the mission. It's a shame that the microphone installed on the probe ended up never working...

There's a slim chance that the probe might come back to life in the next summer, after the polar ice cap melts back and it gets enough sun light. It is very unlikely that this will happen, though — even though the probe is named "Phoenix"...

A "test podcast" was already published, but the feed will actually go live on the 1st of January (for some time zone; it may be on the 31st of December or the 2nd of January depending on where you are...), and the project will depend heavily on contributions from interested listeners/readers — that means you. It doesn't matter that you don't have a podcast, or that you've never produced audio material: you can still participate. Go to the website, read the information that is there and volunteer to help in any way you can. Even if it is by just mentioning the project in your own blog.

2009 will be an interesting year!

Tonight it is Jupiter's turn, with the Moon (now considerably less thin) very close to it and both setting around midnight. Jupiter is not as bright as Venus but it's no less spectacular (especially considering that the sky will be darker as it sets later). The sky is fully clouded here, I hope people have better luck in other locations...

(the times above are for Melbourne, and things will look different — that is, the Moon closer or farther away from Jupiter — depending on where you are)

To a first approximation, most of the objects on the sky appear to move as if they were all fixed to the inside of a gigantic sphere with the Earth at its centre (the aptly named "celestial sphere"); the notable exceptions are the planets, the Sun, the Moon and a few other sporadic visitors such as comets and asteroids. For today we'll ignore those, though, and focus first on the simpler cases.

That (apparent) movement is the result of the (very real) movement of the Earth in relation to the rest of the universe; one can, of course, take the " geocentric" view and talk about celestial objects moving, if that makes things easier to understand — as long as it remains clear that the movement we see directly is apparent, not real.

First, some introduction. From any given point on the surface of the Earth, it's possible to see the "projection" of the Earth's pole onto the celestial sphere (as described in the last article); the point where that projection falls is the celestial pole (north or south, depending on which hemisphere you happen to be on). If you measure the angle between the celestial pole and the horizon, you'll find out that it's identical to your latitude. In other words: if you're in Melbourne, at a latitude of approximately 37 degrees south, the south celestial pole will be approximately 37 degrees above the southern horizon.

Similarly, from any given point on Earth you can see the projection of the equator onto the celestial sphere: that's, obviously, the celestial equator. It will describe a great circle across your local sky, tilted with relation to your horizon by an angle that equals 90° minus your latitude (because the equator is perpendicular to the poles); from Melbourne, the celestial equator is tilted by 53° with relation to the horizon, and the highest point in the line of the equator is 53° above the northern horizon.

As we all know, the Earth rotates around its axis, towards the east, once every approximately 24 hours. The result of this movement is that the objects we see on the sky will seem to rise from the eastern horizon, move across the sky and set on the western horizon some time later. This apparent movement is actually an "orbit" around the celestial poles, parallel to the celestial equator, and this has some interesting effects.

The first is that the actual path of an object across the sky will depend on your local latitude. If you're standing exactly on the equator, objects will rise straight up from the eastern horizon and set straight down on the west. If you happen to be on one of the poles, on the other hand, objects will not rise or set: whatever objects are visible will remain visible, circling the horizon always at the same distance from it. At any other latitude, objects will describe a path that is tilted with relation to the horizon by 90° minus your latitude — that is, tilted in the same way as the equator (and this is useful in navigation: measuring the angle of the path of stars equates to measuring your latitude). The picture above shows this movement from a mid-latitude, with the celestial equator running very visibly through the middle of the image (I'll leave as an exercise to the reader to determine where the picture was taken from the angle between the equator and the horizon).

The second effect is that, if the declination of a star is larger than 90° minus your local latitude, that star will never set. Why is that? Well, if the declination of the star is larger than 90° minus your local latitude, the distance in degrees between that star and the nearest pole is less than your local latitude, and thus less than the distance between the pole and the horizon, and that means that the whole path of the star across the sky is visible. The star is then said to be a circumpolar star from your location (being on the pole, at latitude 90°, is a special case of this — every star visible from the pole is circumpolar). The picture on the left, a long exposure image showing the movement of the stars over several hours, shows this and the location of the southern celestial pole very clearly (photo taken in Brunswick by Michael Efford - click for larger version and more info).

There is, of course, the opposite case: from any location but the equator, some stars will be permanently below the horizon. They are, say, anti-circumpolar: they circle the opposite pole to the one that is visible. Going again to our observer in Melbourne, the constellation Crux, at a declination around 60° south, will be always visible on the sky, at every hour of every day, while Ursa Major, at around 55° north, will never be visible.

And that's it for today. Next week, we'll start looking into the slightly more complex movements that result from the Earth orbiting the Sun.