I wrote previously about blue moons when we last had one in December 2009, and two years later we are going to have another one on Saturday night.

Even though blue moons are fairly rare, this one will be even more exceptional because it will be a full lunar eclipse, starting late Saturday night for us in Western Australia.

Actually, this lunar eclipse is even rarer because it is the second one this year, the last being in June. You don't often get two eclipses in one year.

This is a full eclipse so it promises to be quite spectacular. I will be out there enjoying the view.

What would you say is the difference between the lowest and highest point on the Moon? We know it has low flat plains (the Mares) and it has higher areas where higher is, umm, I don't know, some sort of hand-waving number. Craters are in there somewhere, too.

NASA has released this topographic map of the Moon. Look at the range. Lowest is about −9,000m and highest is +10,000m, a range of roughly 20,000m or 65,000ft! Who'd a thunk it.

Click for a larger resolution image.

My guess would have been around 5,000ft at the most. I would have been very wrong.

Had a lovely moon ring last night. It was about 11pm and I went out to the courtyard and sat down to see how close Jupiter was going to come to the almost-full Moon. It was a little crisp for late Spring, but not too bad, still t-shirt and shorts weather, albeit chilly.

Looking up, I was surprised to see a bright, sharp moon ring, enormous in the light sky. Jupiter was hanging aglow nearby. It really was quite spectacular and unexpected given I had wandered out for quick look-see at the Moon.

Moon rings are caused by the light refracting from ice crystals in cirrus cloud high up in the atmosphere. Light from the Moon streaming down to Earth hits the crystals and is refracted in various directions. An observer will see all the light from crystals where the angle from the Moon to the cloud to their location is the angle of the internal reflection of the crystals.

This diagram shows what happens with just two rays of light. Some of the light is     [continued...]

I jumped in the car and headed out last night, ready for an hour or two of astronomy. As always, am feeling a bit tired this morning.

The thing that sparked me up was an article in yesterday's paper about Venus transits. The next one is in June 2012, less than a year away, and the one after that is not until 2117. It got me thinking about astronomy in general and particularly around Perth.

The motivation was M31, the Andromeda galaxy. M31 is about 2.5 million light years and is magnitude 3.4, visible to the naked eye in a moderately dark sky. Melbourne was too far south to see it, so I have always wanted to see it once I got to somewhere further north. Now I am in Perth where M31 gets up to about 20° above the horizon at a certain time of the year, and right now is that time.

I don't know my way around the dark areas outside Perth, so I figured I would just head north. North is easy for me, it is basically a freeway to the outskirts and then jump     [continued...]

XEphem is a well known astronomy and ephemeris program for unix and Mac (and Windows on Cygwin or an emulator), and scrotwm is a dynamic tiling window manager for the same set of operating systems, including Cygwin/Windows.

Since scrotwm is a tiling window manager, its default behaviour is to open new windows in a full size tile. XEphem opens quite a few new windows; almost any task opens a dialog window so you can specify parameters or choose various options. This means that using XEphem and scrotwm together results in a bunch of open tiles, some of which are clearly meant to be small dialog windows and others which are meant to be quite large, such as the view of the sky. We want to change this so that the windows open as they were designed to open (and which they would if they were on a normal desktop).

Scrotwm lets you override the default behaviour of any window by using its quirks mode. You specify the window class and name and the behaviour you want. This is clearly demonstrated in its man page. Unfortunately, you can't use a wildcard, so you can't say     [continued...]

Wolfram Alpha has a cool web page that shows satellites overhead, where overhead is defined as visible, i.e. above the local horizon.

I was very surprised to find I have 1,685 satellites overhead right now. If I had to guess, I suppose I would have said 3 or 4, not 1,685!

On the other hand, Wolfram Alpha define a satellite as anything, including junk. Even so, when we restrict it to non-debris satellites it is still a big number, 670 to be exact.

You can click on each satellite and get the orbital elements, range, azimuth and a bunch of other stuff including a map showing the orbits.

It is pretty cool and worth a visit to play around.

Check out this image of a big coronal mass ejection a few days ago.

Click for full size

Awesome.

The white circle represents the Sun, hidden by the coronagraph's occulting disk. Eye-balling the image using my Mk-1 thumb, the ejection was 2 or 3 times the Sun's diameter.

The instrument is part of the SOHO project, the Solar and Heliographic Observatory, and the actual instrument is the Large Angle and Spectrometric Coronagraph Experiment (LASCO).

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Today is Neptune's first birthday. It was discovered on 23rd September 1846 and it takes 164.79 years to complete one orbit, which is today.

Happy birthday, Neptune.

Bob (no, I don't know anything more about him) has a good in-depth article, and Wikipedia has more.

NASA's Image of the Day shows some stunning photos occasionally, and this is one of them.

Endeavour docked to the ISS, taken from Soyuz (click for bigger)

Did you catch the planetary alignment this morning? It was spectacular. I got up early just before 6am and drove to a nearby hill to get a full view, and it was awesome. The sky was crisp and sharp and the three brightest planets seemed to hang there glowing brightly in the low eastern sky. You could easily imagine our early ancestors several thousand years ago being gobsmacked by such a spectacle and wondering what those lights were. Heck, even today, when we know what it is. And all this on Friday the 13th! Marvellous.

Here is the view from the Southern Hemisphere; people in the north probably saw something similar flipped left-to-right.

top to bottom: Jupiter, Mercury, Venus and Mars

Venus and Mercury are in the inner Solar System, while Mars and Jupiter are in the outer. How can we see all four when two of them are "behind" us? The answer is that they aren't behind us at all. We all are travelling in a glorified circle around the Sun, so sometimes we can see across the Solar System, past the Sun, all the way across to the outer Solar System. It's obvious when you see the diagram:     [continued...]